An Introduction to Digital Scanning

An Introduction to Digital ScanningDigital Color Prepress volume four This Agfa scanning guide is intended to be both an introduction for beginners, and useful resource material for more experienced operators. The techniques and terminology relating to scanning for prepress and other applications are clearly explained and extensively illustrated. Dramatic increases in computer power and software capabilities, combined with the proliferation of affordable image-capturing devices now offer image-manipulation facilities to us all. The colour knowledge and skills of an experienced scanner operator are, however, far less easily acquired. Printed image quality is highly dependent upon the accuracy and correct colour and tonal balance of the initial scanning process. If significant image details and tonal ranges are absent from the start, even the most skilful image retoucher will find it difficult to produce an acceptable result. Simple rules are provided in this guide to allow the best possible results to be obtained when scanning any original, taking into account the intended output device. Potential scanning or image-processing errors are illustrated for easy identification. A section is also included for those about to purchase image-capturing equipment, giving advice on the suitability of currently available devices for specific applications. Although a guide of this size could never be an exhaustive description of imagecapturing technology and techniques, we have aimed to provide the reader with sufficient general information to obtain the best possible image quality from a wide range of equipment. Used in combination with our other Digital Colour Prepress guides, which deal mainly with the requirements for successful prepress and printing, a comprehensive knowledge of the complete process from original to print will be gained. The terms printed in bold throughout this guide can also be found in the glossary. 1 CONTENTS SETTING THE SCENE An overview of current imaging technologies 1 2 4 6 8 10 11 12 14 16 18 20 22 24 26 28 29 30 32 34 36 38 CHOOSING YOUR INPUT DEVICE SENSING TECHNOLOGIES COLOUR BASICS Advice on the suitability of image capturing devices for specific tasks A comparison of the two methods presently used to read image data Perception of colours in nature, on computer monitors and on the printed page THEORY OF OPACITY AND DENSITY JUDGING YOUR ORIGINALS PICTURE ELEMENTS RESIZING BITMAPS OUTPUT BASICS Understanding the principles of measuring film and paper densities Deciding whether images will require special treatment during or after scanning The composition of digital images, with an introduction to binary numbers The effects of increasing or decreasing image size and resolution A summary of output possibilities and an explanation of resolution terminology LINE-ART RESOLUTION Advice on scanning black-and-white originals, plus simple resolution rules GREYSCALE RESOLUTION COLOUR RESOLUTION Advice on scanning greyscale originals, plus simple resolution rules Advice on scanning colour originals, plus simple resolution rules HISTOGRAMS AND TONE CURVES How to check and adjust the tonal ranges of scanned images LINEAR AND NON-LINEAR TONE CORRECTIONS SCANNER DENSITY CONTROLS Various methods of changing image brightness and contrast Use of automatic and manual controls to obtain optimum image density GLOBAL AND SELECTIVE COLOUR CORRECTIONS SHARPEN YOUR IMAGE COLOUR DEFINITIONS Removal of unwanted colour casts and the selective modification of colours The explanation of a sharpening process, highlighting its potential pitfalls Precise descriptions of colour for accurate measurement and communication COLOUR MANAGEMENT Automatic systems that ensure consistent colour matching from original to print FILE FORMATS AND FILE STORAGE GLOSSARY Popular formats, compression techniques, size calculations and storage methods SETTING THE SCENE 2 Technological advances in photography have enabled us all to cheaply record picturesque scenes such as the illustrated stone arch, for distribution to a wider audience. However, image reproduction using light-sensitive film or paper requires time-consuming and exacting processing techniques. Multiple photographic copies are expensive and final results often differ widely in colour from the original scene. Ink-based printing processes, such as offset lithography, allow high-volume image reproduction of photographic or other originals at a reduced cost per copy. These processes require images to be separated into cyan, magenta, yellow and black components (CMYK), the four process ink colours used in printing presses. In the past, CMYK separation methods employed either a large-format reprographic camera (1) equipped with coloured filters or a combined drum scanner and recorder (2). Repro camera operators used RGB filters to record the red, green and blue components of colour images on black-andwhite (monochrome) films. Many intermediate positive and negative films had to be created before CMYK separations were obtained. The more productive drum scanner recorder employed three RGB signal amplifiers known as photomultiplier tubes (PMT’s) to read RGB colour values from an original that was mounted on a rotating drum. These values were translated into CMYK colour separations and directly exposed onto monochrome film attached to a second rotating drum. Both methods produced film separations from which printing plates were made. New digital scanning and recording methods have now eclipsed these highly specialised and costly systems, opening the world of image processing to us all. U Y I An alternative to in-house image-capturing facilities is the use of professional scanning services to transfer film-based images onto compact disks (CD’s). Computer-linked CD players (9) give rapid access to these potentially massive digital-image databases. 3 Digital output The conversion of pixels into printing pigments is dealt with in the “Output basics” section but a summary of available digital output devices is included here. Interactive multimedia presentations require either a computer-driven projection system (10) or a colour monitor (11) with audio facilities to reach their audience. Digital printing devices have proliferated to cater for the increasing use of computerbased publishing and image manipulation programs. Film recorders (12) expose digital data onto colour transparency film for use in slide presentations, or to obtain any number of second originals (high-quality copies of an original photographic image). This digital photo-imaging allows digitally-created, modified or restored originals to be output on positive or negative film for convenient photographic distribution or storage in image banks. Multiple black-andwhite paper copies are produced by laser printers (13), which rely on the xerographic copier process (dry toner). Paper output from tabletop colour printers (14), using technologies such as thermal wax transfer or dye sublimation, is restricted to proofing and low-volume printing, due to high costs and slow speeds. Digitally-driven xerographic colour copiers (15) offer slightly faster printing speeds but costs remain high. Monochrome film separations for ink-based colour printing processes are output by high-resolution imagesetters (16). Some of these devices can now expose directly onto printing plates (direct-to-plate), avoiding the need for intermediate films (17). Efforts are being made to transfer digital data directly to special offset litho press rollers (18), eliminating the film and plate-making processes (direct-to-press). The most exciting development in low to medium volume digital colour reproduction is the introduction of high-speed duplex (double-sided) web presses, based on improved xerographic imaging technologies (19). These “computer-to-paper” systems produce low-cost colour copies in any quantity without requiring costly and time-consuming press preparation or clean-up time between jobs. Q W R E T r O t { P Digital input Modern digital input techniques allow images to be manipulated and retouched on a computer, with precise control and great flexibility. Final results are easily copied, any number of times, without loss of quality. In contrast to fragile photographic and hand-drawn original images, multiple digital copies stored on magnetic tapes or other media ensure reliable data integrity. The main disadvantage of digital images is that their quality is generally matched to the intended output size and printing process. Unforeseen changes in application may require new digital input from the original source. Digital images consist of a grid of small squares, known as picture elements or pixels. RGB input devices reduce the visible colour range to a limited palette. Each pixel is allocated the palette colour that most closely matches the original image. The bigger the palette, the more accurately an original image is described. Palette size is specified in bits, which are explained in the following “Picture elements” section. y u e w q } Scanners are used to convert photographic or hand-drawn originals into digital data. Recent drum scanners (3) incorporate traditional PMT sensors, but are designed to provide only digital data. Adaptations have also been made to earlier drum scanners (4), which enable them to supply digital data instead of directly exposing films. The PMT sensor technology is not easily implemented in compact flatbed scanners (5) or digital cameras so a new technology has evolved. Charge-coupled devices (CCD’s), consisting of thousands of minute, light-sensitive receptors (elements) convert varying light levels into digital signals. Modern still digital cameras (6) use a two-dimensional CCD array or matrix to instantly record ‘snapshots’. Data is either down-loaded directly to a computer or it is stored on a removable disk. Detachable CCD matrix camera backs are also available to adapt professional still cameras (7). Digital video cameras or camcorders (8) use a CCD matrix to record consecutive frames, which are either transferred directly to a computer, or recorded onto high-quality magnetic tape (video). Flatbed scanners normally employ a linear CCD array instead of a matrix to record successive lines of data as an image is scanned into the computer. CHOOSING YOUR INPUT DEVICE 4 The varying designs of image-capturing devices make them more suited to some tasks than others. A good way of deciding which device best fits your requirements is to establish what type of originals must be handled and how the captured data will be used. Are the originals flat or three-dimensional? Will flat originals flex or are they rigid? How large are the originals? By what proportion will captured images need to be increased in size? Are the images on transparent materials (film) or reflective materials (paper)? Do they consist solely of hardedged black-and-white areas or lines (line art)? Are they continuous tone images (contones) such as photographs, containing smoothly blended grey tones or colours? Have originals been printed as halftones? The descreening option for smoothing out the dots in halftone originals is not available with all image-capturing devices. Many other factors may influence the final decision, including ease of use, versatility, software features, robustness, reliability, efficiency of service and credibility of the manufacturer. The ability to read a wide range of tones, especially in shadow areas, is particularly important when scanning colour transparencies. Well-designed software interfaces may offer advanced features such as the conversion of colour negatives to positive images, and the direct separation of RGB data into CMYK for ink-based printing processes. To read printed text into a wordprocessor, a hand-held scanner with Optical Character Recognition (OCR) software may be adequate. Simple black-and-white flatbed scanners capture greyscale images or line art. The ability to significantly enlarge detailed colour images without noticeable deterioration calls for a large number of closely-spaced readings to be taken from them. This high resolution is provided by professional flatbed or drum scanners. Input device resolutions are quoted in pixels per inch (ppi), whereas the maximum resolution of output devices is the number of dots they are able to print or record per inch (dpi). The true optical resolution of a CCD input device is determined by the quantity of CCD cell readings taken per inch and by the optical system. When comparing input devices, check whether their optical resolutions have been increased by software enhancement (interpolation). This process avoids visible pixels in enlarged images but captures no additional detail. Black-and-white flatbed scanner Colour flatbed scanner Transparency scanner Professional flatbed scanner Drum scanner Digital camera Video camera Hand-held scanner Drum scanners The photomultiplier tubes (PMT’s) used in drum scanners to sense RGB colour values are capable of producing very highquality results. Early drum scanners were complex devices, requiring skilful operators in order to reach their full potential. They remain the most expensive image-capturing devices on the market, although prices have begun to fall with the introduction of desktop models. Only flexible originals may be mounted on the transparent acrylic cylinders used in drum scanners, which is a time-consuming task. Negative or positive, transparent or reflective originals are normally accommodated. 5 Main use Office, OCR, FPO, B/W printing Office, FPO B/W and colour printing Office, OCR, FPO, B/W printing B/W and colour printing Digital photo-imaging B/W and colour printing Digital photo-imaging Audio/Visual communication B/W and colour printing Audio/Visual communication Office, OCR FPO Originals Rx flexible, rigid pos. 3-D objects Rx, Tx flexible, rigid pos., neg. 3-D objects Tx flexible pos., neg. Rx, Tx flexible, rigid pos., neg. 3-D objects Rx, Tx flexible, pos., neg. Rx, Tx flexible, rigid pos. 3-D stationary scenes Rx flexible, rigid pos. 3-D scenes Rx flexible, rigid pos. Digital cameras Some digital CCD cameras are designed solely to record digital data, having no provision for loading traditional films. Others are adapted from standard film cameras by the addition of a digital back unit. With portability in mind, the dedicated digital cameras record data onto removable disks. The capacity of these disks and number of elements in the CCD matrix limits the resolution of captured images. A suitable application is journalism, where digital images are immediately relayed via modem or satellite for publication in newspapers. Standard film cameras with digital backs tend to capture higher resolution images, feeding data via a cable to fast hard disks. RGB primary colours are scanned simultaneously or in three passes, requiring the camera to be mounted on a stable tripod and allowing no movement in the subject matter. Although more suited to capturing 3-D objects, digital back cameras could be used as an alternative to flatbeds. Quality Can be high Can be high High High to very high High to very high Medium Low to medium Low Video cameras Moving image sequences or individual stills can be directly captured by special computer-based frame-grabbing systems from CCD video cameras or recorded tapes. Multimedia presentation software is able to utilise and edit clips of these moving images, although the high volumes of digital data restrict image quality, size and duration. When a video clip is the only record of an important event, it is possible to increase the resolution of individual frames by the resampling process described in the “Resizing bitmaps” section, so that they can be printed with reasonable quality. Operation* Simple, often automatic Productivity* Medium to high Cost* Low to medium Medium Medium to high Medium to high Medium to high Medium to high Low to medium Low Medium to high High High Medium to high Medium to high Medium Medium Simple, often automatic Simple, often automatic Simple, often automatic Complex, often requiring specialist knowledge Simple, often automatic Simple Simple but inconsistent Hand-held scanners These low-cost CCD devices are manually passed over flat black-and-white or colour originals. They are not designed for transparency scanning and their maximum width is normally less than A4 format. Although some hand-held scanners have a quoted resolution of 800 ppi, their ability to produce acceptable results is limited. Applications for these scanners include OCR, and rapid capture of contone images used for position only (FPO) when developing page-layout concepts. Flatbed scanners CCD-based flatbed scanners are the most popular imagecapturing devices for desktop publishing (DTP) and professional prepress. They can normally be operated from within standard image-editing programs. The more advanced software interfaces used to drive flatbeds require minimal operator training because optimal colour balance and image density are automatically determined. A wide range of flatbed scanners is available, from the lowcost black-and-white variety to high-quality, professional colour devices. High-end flatbeds scan both reflective and transparent originals but an optional unit may need to be purchased to scan transparencies on mid-range devices. * Although these factors are determined partially by the design and construction of each device, the quality and versatility of software interfaces will significantly affect overall performance. Abbreviations OCR: Optical Character Recognition FPO: For Position Only Rx: Reflective Tx: Transparent pos.: positive neg.: negative Professional flatbeds are generally cheaper than traditional drum scanners, although they are capable of producing scans of similar quality. Another advantage of flatbeds over drum scanners is that images on rigid substrates of any thickness can be scanned, such as books or card-backed page layouts (mechanicals). Transparency scanners These CCD-based devices are dedicated to scanning films at high resolutions. They are popular with service bureaus, trade shops, newspaper and magazine publishers. When the film format is not restricted to 35mm, a range of standard film holders is usually included. Automatic loading and batch or gang scanning of framed transparencies is supported by some models. Focusing, colour control and calibration of image density may also be automated. SENSING TECHNOLOGIES 6 Both photomultiplier tubes (PMT’s) and charge-coupled devices (CCD’s) convert different brightness levels into continuously varying or analogue voltages. These are chopped into a specific number of steps or levels by an analogue-to-digital (A/D) converter, in a process known as sampling. The purity of small analogue signals is easily affected by electrical interference, producing noise or inaccurate readings. Good signal-to-noise ratios are important in the design of sensors and associated circuitry. Most light sources and analogue electrical devices require a period of time to reach a stable operating temperature or condition. It is therefore advisable to wait a few minutes after switching on any scanner before making the first scan. PMT drum scanning Mirror Lens Dichroic mirrors Original Colour filters Photomultiplier tubes (PMT‘s) for red, green and blue Light source To A/D converter and output processing PMT sensors Traditional drum scanners use a xenon or tungsten-halogen light source, which is focused onto a small area of the original by fibre optics and condenser lenses. Transparencies are lit from inside the drum and reflective materials from outside. Transmitted or reflected light from a minute point on the image enters the sensor unit travelling along the outside of the rotating drum. The light is directed onto semi-transparent or dichroic mirrors, which are angled at 45° to the beam. Some light is reflected from each mirror, whilst the remainder is transmitted to the next mirror. Reflected light passes through either a red, green or blue filter and then into one of three optical amplifiers known as photomultiplier tubes. A/D converters then chop the analogue voltages into digital data. A fourth PMT may provide image-sharpening information, although software-applied sharpening after scanning offers greater flexibility. PMT technology is capable of registering a wide density range, but its complexity causes manufacturing and maintenance costs to be higher than those for CCD devices. Extensive manual controls in most PMT scanners require the knowledge and practice of an expert to procure the best settings. Only flexible originals are supported, the mounting of which is a time-consuming operation. Rigid originals must be reproduced on flexible material. Precious flexible originals may need to be duplicated to avoid possible damage during the mounting and rotational scanning process. Filtered light Cathode The photomultiplier Dynode A PMT is a vacuum light sensor in which electrons are multiplied by secondary emission. Light (photons) falling on the photocathode releases electrons. These are conducted to the dynodes. Each dynode releases more electrons by what is known as secondary emission. Several dynode layers are needed to convert a small quantity of light into a usable electrical signal. Variations in electrical current are measured at the anode. After amplification, the analogue signal is converted by an A/D convertor into a digital signal. Anode A/D converters 255 time 0 Analogue Digital Continuously varying analogue voltages are sampled into a series of steps or levels, each having a specific numerical value (binary). The number of levels depends upon the design of the A/D converter. An 8-bit A/D converter samples 256 levels, 10 bits give 1024 levels, 12 bits provide 4096 levels and a 14-bit converter allows 16384 unique levels to be counted. If signal-to-noise ratio is poor, additional samples of a smaller size will not necessarily improve image quality. Bits and binary numbers are explained more fully in the following “Picture elements” section. CCD flatbed scanning Original Greyscale scanners take a single set of light intensity readings from originals. Colour scanners capture three sets of readings from colour originals by the use of red, green and blue filters. Scanners that incorporate a single linear CCD array sometimes rotate an RGB colour filter wheel in the lens unit before each of the three separate passes of the original are made. Single-pass scanners may use three linear CCD arrays, which are individually coated to filter red, green and blue light. The same image data is focused onto each array simultaneously. Mirror 7 Light source Mirror RGB-coated CCD chip Although three-pass scanning is slower than single-pass scanning and registration between colours is more critical, there are a number of advantages. CCD sensors are less sensitive to blue light than to green light and are most sensitive to red light. Red, green and blue light come into sharp focus at slightly different points from each other. Some three-pass scanners optimise both the scanning speed and lens focusing for the specific colour being read. Well-designed and integrated CCD sensors are able to read an image density range similar to that of PMT sensors, once certain characteristics are taken into account. The individual CCD elements in an array have a slightly different sensitivity from each other, and may also give a small reading when no light is falling on them (dark current). Some devices compensate for these anomalies by precisely calibrating each CCD element. When an excess of light falls on a CCD element, its charge can spread or bloom into neighbouring elements, causing incorrect readings. The solid-state design of CCD sensors makes them far more compact and mechanically simple than PMT sensors. They are also cheaper, more stable and require considerably lower voltages than PMT’s. Lens To A/D converter and output processing Light-capturing CCD elements CCD sensors Flatbed scanners employ a linear CCD array, which consists of several thousand charge-coupled device elements arranged in a row on a single silicon chip. Originals to be scanned are placed on a glass plate. Transparencies are evenly lit from above and reflective originals from below by a fluorescent or halogen light source. Lengthwise movement of the light source together with a mirror directs consecutive lines of image data onto the static CCD array, via a second mirror and a synchronously focused lens unit. The full width of an image is read simultaneously as a line. Light of a specific colour and intensity falling on each CCD element creates a proportional electrical charge within it. This analogue charge is systematically passed along chains of cells to an A/D converter, where it is sampled into digital data. The CCD is now clear to receive the next light-induced charge. COLOUR BASICS 8 Our perception of colours in nature is determined by three factors - the type of light source, how substances change the reflected or transmitted light, and the sensitivity of our eyes to the resulting light. The sun radiates a wide variation of electromagnetic waves, each having a different wavelength. The human eye is sensitive to only a small range of these wavelengths, known as white light. Rainbows are created when white light is split up by droplets of water. Passing a beam of white light through a glass prism produces a similar effect. The shorter wavelengths are bent (refracted) more than the longer ones, splitting the white light into its component spectrum of visible colours. Each colour causes a specific reaction in the eye’s red, green and blue cones or receptors. Yellow is perceived by both the red and green cones, for example. The spectrum colours are the basic building blocks of a much wider range or gamut of colours. When selections of these pure wavelengths are mixed or added together in differing proportions, thousands of different colour sensations can be perceived. The visible spectrum Violet Gamma Röntgen Ultra violet Visible spectrum Infra red Microwave/Radar T.V. Orange AM radio Red Sound Electromagnetic spectrum Spectrum colours White light (visible spectrum) Spectrum colours Yellow Green Dark blue Blue Natural image Subtractive colours All substances absorb, transmit or reflect specific wavelengths of white light. When an object absorbs some light, only the remaining mixture of reflected or transmitted wavelengths is detected by our eyes. An opaque white material reflects all wavelengths, whilst a black one absorbs them. Translucent or transparent materials absorb or subtract certain wavelengths of white light and transmit the others. All of the spectral colours can be produced from a white light source by passing it through single or pairs of translucent CMY filters. This is a subtractive process since the transmitted light will be less intense than the light source. A cyan filter, which transmits blue and green light but subtracts red light, followed by a magenta filter, which subtracts green light, results in only the blue light being transmitted. Weakening the cyan filter allows some red light to be transmitted, producing violet light. Colour photography materials incorporate variable density, subtractive CMY dyes, which filter light to reproduce life-like images. In printing techniques such as offset lithography the density of the CMY process inks cannot be continuously varied across an image, so a range of colours is produced by a halftone technique, where CMY dots of variable size are printed in overlapping grids. The smaller the dot, the less light it will absorb, decreasing apparent density by increasing the amount of reflected light. Process ink pigments are less pure than photographic dyes, so pure black cannot be obtained by overprinting solid CMY inks. For this reason, black (K) ink is printed in addition to, or instead of dense CMY combinations. Process ink impurities, combined with the incomplete reflectance of printing paper generally result in a smaller colour gamut than photographic materials. 9 Additive colour Blue Monitor reproduction Red Green Additive colours Colour monitors and TV’s mirror the function of the eye by emitting red, green and blue colours (RGB) - the three primary colours of light. All other colours can be composed by adding these primaries in different proportions and intensities, giving rise to the term additive mixing. Green and blue light result in cyan (C), red and blue light make magenta (M), and red and green light form yellow (Y). C, M and Y are known as the secondary colours of light, or the primary colorants when referring to pigments. White light is produced when red, green and blue are added in similar proportions, whereas black results from their total absence. In reality, the black displayed on colour monitors is likely to be a dark green or brown grey due to stray light emissions. The gamut of colours that can be displayed on a monitor is smaller than that seen in nature because it is limited by the characteristics of the phosphor screen coatings that emit the light. The merging of light emitted from coloured light sources is an additive process. All spectral colours and white light can be created by adding red, green and blue light. Monitors display a colour gamut that is smaller than the visible spectrum. Subtractive colour Print reproduction Yellow 45 ° 75° 90° 105° Cyan Magenta Halftone colour printing normally employs four overlapping grids of dots (CMYK), which subtract differing amounts of RGB light in proportion with dot size. Printing inks also produce a smaller colour gamut than the visible spectrum, but this is not the same as the monitor gamut. Cyan, magenta and yellow filters or pigments subtract varying quantities of red, green and blue from white light to produce a limited gamut of spectral colours. THEORY OF OPACITY AND DENSITY 10 Exposing a film to increasing light levels raises the opacity (O) or blackness of its emulsion after development. The opacity of a film is defined by the total amount of light falling on it, divided by the amount transmitted through it. A clear film that transmits 100% of the light falling on it has an opacity of 1 (100% / 100%), although in practice a small proportion of light is always absorbed. When only 50% of light is transmitted by a film, its opacity is 2 (100% / 50%). A 10% transmission indicates an opacity of 10 (100% / 10%). Exactly the same calculations can be applied to reflective materials. If a printed area on paper gives 50% light reflectance, its opacity is 2. Opacity is the inverse of transmission (T) and is also the inverse of reflectance (R). An opacity of 2 results in a transmission or reflectance of 1/2 of the incident light. When the thickness (mass) of a filtering dye or exposed film emulsion is increased in equal steps, its opacity rises at a progressively faster rate. For this reason the term density was introduced, which directly corresponds with the thickness of a filtering layer. Density is proportional to the logarithm of opacity, as explained in “Density in depth”. JUDGING YOUR ORIGINALS B/W original Before beginning to scan any original, it is worthwhile checking whether it contains a restricted tonal range or unusual colour balance. Try to locate the most dense shadow area (Dmax). Photographic images without dark shadows could be intentionally high key, or may have been incorrectly exposed or processed. Conversely, predominantly dark images may be intentionally low key. When the lightest image area (Dmin) is a bright, reflected specular highlight, it will probably contain no detail at all. An image containing bright highlights and deep shadows has a high contrast. This is more noticeable when few midtones are present. A small tonal range (DmaxDmin), lacking extreme highlights and shadows, indicates a low contrast image. When any of these characteristics are intentional, the automatic exposure controls offered by some scanner interfaces may need to be bypassed or adjusted manually to prevent unwanted changes to images. The black-and-white reflective original of a man in sun glasses contains a wide tonal range. A white circle on the sun glasses indicates the position of Dmax. The black circle on his normal glasses shows Dmin. The colour transparency of a woman has an intentional colour cast throughout the image. Automatic location of Dmin finds the pixel having the brightest, combined colour values. In this case, Dmin in the woman’s eye will not be a neutral tone due to the colour cast. Likewise, Dmax in her hair is not a neutral shadow area. Certain colours, such as deep reds, are more difficult to scan than others. Transparent and reflective originals 11 Dmax 0% 10 0% 10 0% 10 Dmin O=1 0% 10 O=2 % 50 O = 10 % 10 10 0% 50 % O=1 O=2 O = 10 Dmin For an image-capturing device to faithfully reproduce a transparent or reflective original, it must be able to register virtually the complete density range present. This range is the difference between the most dense area and the least dense area, which is Dmax - Dmin. Transparencies generally have a Dmax of about 3.3 and a Dmin of 0.3, giving a density range of 3.0 D. Reflective materials may have a density of 2.0 D, but in most cases the range is nearer to 1.7 D. The fact that a transparency film contains a tonal range ten times wider than reflective materials requires devices intended for transparency scanning to be far more sensitive. 10 % 0% 10 0% 10 0% 10 Colour original Dmax Colour negative original Density in depth If a light source of 2000 units is shone onto an exposed film of opacity 10, there will be 200 light units transmitted (2000 / 10). Adding a second film of the same opacity cuts the transmitted light again by a factor of 10, leaving 20 units (200 / 10). The overall opacity of the two films is 10 x 10 = 100. Another way of writing this is 102, which is “ten raised to the power of two”. The total opacity of three identical films is 10 x 10 x 10 = 1000 or 103, which will transmit only 2 units of light. Density indicates the thickness or mass of a filtering dye or exposed film emulsion. It can be seen from the simple calculations above that doubling the thickness of a filter does not double its opacity but instead raises it to the power of two. Density is proportional to the power by which opacity is raised, or in other words the logarithm of opacity. A film with a density of 1.0 D has an opacity of 101 or 10, transmitting 10% of incident light. A 2.0 D film has an opacity of 102 or 100, giving a transmission of 1%. A 3.0 D film has an opacity of 103 or 1000, which means that it transmits only 0.1% of the light falling on it. An increase of 0.3 density doubles the opacity range. A 3.3 D film contains twice as many tonal variations as a 3.0 D film (103.3 is close to 2000). Density formulas Transmission (T) = Reflectance (R) = Opacity (O) = Density (D) = log (Opacity) = Amount of light transmitted Total light source Amount of light reflected Total light source 1 T log 1 T 1.3 20 5% Colour negatives contain a strong orange mask, which is removed before the image is reversed to a positive version by some advanced scanner drivers. Screened originals need to be descreened during the scanning process by use of a software blur filter or by defocusing the scanner optics. This avoids moiré patterning and colour shifts in printed output. or 1 R = log 1 R 3 1000 0.1% 3.3 2000 0.05% Density Opacity Clear film (carrier) Emulsion D = log O O= T R 1 T 0 1 100% 0.3 2 50% 1 10 10% 2 100 1% Transmission or reflectance PICTURE ELEMENTS 12 A scanned digital image is composed of a matrix or bitmap of touching pixels (picture elements), which are small squares of solid black, white, varying grey tones or colour. Bitmaps are either square or rectangular. Every digital or bit-mapped image has four basic characteristics: resolution, dimensions, bit depth, colour model. When an image is scanned, the number of samples or readings to be recorded in a given distance must be specified. This is known as the scanning resolution, which is normally specified in pixels per inch (ppi) or samples per inch (spi). The use of metric resolutions is increasing “Res 12” means 12 pixels per millimetre (305 ppi). The physical size of pixels changes according to the chosen resolution. Appropriate resolutions are indicated in the following “Line-art, Greyscale and Colour resolution” sections. Bitmaps always consist of whole numbers of pixels, so although dimensions may be given in inches or centimetres, measurements are more simply stated in pixels. Division of the number of pixels in the height and width of a bitmap by its resolution provides the physical size. For example, if an image is scanned at 300 ppi and the width and height are 900 pixels, the physical size is three inches square (900 / 300). When the resolution is changed to 150 ppi, the physical size will be six inches square (900 / 150). The number of pixels has not changed but they are now four times as big (double the width and height). Bit depth (also known as pixel depth) offers 256 different grey levels (including black and white). This is normally sufficient levels to reproduce a smooth gradation from black to white without seeing tonal jumps or bands. 13 Colour models In order to record coloured pixels, tonal information is required for individual primary colour channels. RGB images normally use 24-bit depth (3 x 8 bits) and 32-bit depth is needed for CMYK images (4 x 8 bits). When each colour channel is defined by an 8-bit number, 256 brightness levels per channel are possible. The combination of 256 levels of red, green and blue allows more than 16 million colours to be described. Supersampling Most colour scanners are able to differentiate 256 tonal levels for each of the RGB primary colours. Some are designed to record many more levels, extending the bit depth to 10, 12, 14 or even 16 bits per colour. This additional or supersampled data is rarely used by output devices, but it allows a wider range of shadow details to be captured and subsequently heightened. When scanning high-density transparencies this is particularly important and it provides flexibility when RGB images are converted to CMYK. Some image-editing programs are now able to work internally with 16-bit data, providing greater flexibility in colour correction prior to down-sampling to 8-bit data for output. The fact that a scanner records a higher number of bits per colour does not necessarily mean that it can differentiate additional tonal levels. If sensors and electronic circuitry are poorly designed, the scanner may incorrectly register the same numerical value for various tones. RGB colour 3 x 256 colours 3 x 8 bit Bilevel Black and white 1 bit Greyscale 4 grey levels 2 bit Binary number system Digital computers use millions of linked electronic switches to make calculations and process all data. Each switch is either on or off, representing a value of one or zero respectively. In order to count in ones and zeros it is necessary to use the binary number system. With the standard decimal system, each digit increases from zero to nine before it is reset to zero and the digit to the left is incremented (09 becomes 10). Binary digits, known as bits, only increase from zero to one before the next digit is incremented. A 2-bit binary number (22) has only four possible values: 00, 01, 10, 11 (representing 0, 1, 2 and 3 in decimal values). An 8-bit binary number (28) provides 256 different values. defines how many tones or colours every pixel in a bitmap can have. In other words, the depth of information recorded during the scanning process is limited by the chosen bit depth. If an image is scanned digitally to a depth of one bit, each pixel can have only two states - black or white (zero or one). Images reduced to pure black and white pixels are called bilevel images or flat bitmaps. When more than one bit is used to describe each pixel, a range of grey tones or levels can be placed between the black and white. A depth of two bits adds two grey tones to the black and white, giving four levels in total. Eight-bit data Greyscale 256 grey levels 8 bit Bitmaps and file size Dimensions, resolution, bit depth and colour model all affect the digital file size of an image, determining the disk space required to store it. File size also has a direct relationship to the calculation time used by a computer's processor during image editing. If the resolution of an image is doubled, the file size will increase by a factor of four since there will be twice the number of pixels in both width and height. A 32-bit CMYK file is 32 times as large as a 1-bit line-art version of the same image. RGB colour 3 x 65536 colours 3 x 16 bit RESIZING BITMAPS 14 Any bitmapped image has a specific resolution or number of pixels per inch. If an image is enlarged without adding extra pixels, the size of each pixel must increase. This means there will be less pixels per inch, thus the resolution is decreased. Although the pixels appear bigger, their description in the file is identical, so the file storage requirement remains constant. When images are enlarged too much, individual pixels become clearly visible. The resulting staircasing or aliasing in diagonal lines is particularly disturbing (jaggies). The opposite happens when an image is reduced in size without removing pixels. Pixels become smaller, so the resolution increases. Visually this is not a problem but the resolution could become unnecessarily high when compared with output requirements. Keeping image resolution in the correct relationship to the intended output device minimises file sizes and ensures efficient processing and printing. Appropriate resolutions are indicated in the following “Line-art, Greyscale and Colour resolution” sections. If an original will need to be resized, the scanning resolution should be adapted accordingly. For example, a 5 cm x 5 cm photograph has to be scanned and enlarged to a size of 20 cm x 20 cm. This is a sizing factor of 4 (20 cm / 5 cm), meaning that the adapted scanning resolution needs to be four times as high as the desired final image resolution. When a final resolution of 200 ppi is required, the original photograph must be scanned at 800 ppi (200 x 4). Some scanner interfaces allow output size and resolution to be specified, avoiding the need to calculate sizing factors. outline pms witlijn pmslijn zwartvlak Enlargement without resampling Resampling witlijn pmslijn pmslijn witlijn zwartvlak witlijn pmslijn pms outline Original size Scale: 200% Pixels have four times the area of those in the original image. If an image needs to be changed in dimension and it is not possible to rescan it, pixels must be added or removed to maintain the same resolution. This process is called resampling. The removal of pixels, known as down-sampling, is a relatively simple calculation, often achieved by pixel skipping. When too great a reduction is made, staircasing will be visible in diagonal lines and fine details will break up. Resampling an image upwards by the addition of pixels is known as interpolation. Image-capturing devices may incorporate interpolation software to enhance their maximum optical resolution. Whilst increasing resolution by interpolation helps to reduce visible staircasing, it adds no extra detail to images. In fact excessive interpolation will result in a blurred, out-of-focus appearance. Subsequent application of unsharp masking (USM) will resharpen the image to a certain extent. witlijn 15 Enlargement with resampling outline pms witlijn pmslijn zwartvlak witlijn pmslijn pmslijn zwartvlak witlijn pmslijn pms outline Original size Pixels remain the same size as those in the original image. outline pms witlijn pmslijn zwartvlak witlijn pmslijn pmslijn witlijn Reduction with down-sampling When an image is disproportionally resized, more in one direction than the other (anamorphic distortion), new pixels must be interpolated or redundant ones removed. This also applies when images are warped, sheared (skewed), or placed in perspective. Whenever possible it is best to avoid resampling by scanning images at the correct resolution. If the scanner is not able to reach the resolution required to allow for enlargement, interpolation is the only solution. zwartvlak witlijn pmslijn pms outline Original size Pixel size remains the same but the image becomes smaller. Resolution adaptations for resizing witlijn Interpolation pmslijn witlijn pmslijn Adapted scan resolution = Original scan resolution x Sizing factor Interpolation programs determine where new pixels must be added throughout an image to achieve an increased resolution. They then normally use one of three methods to decide what colour the new pixels should be. Nearest neighbour interpolation is the quickest but least accurate method where each new pixel takes on the colour of the closest pixel. Bilinear interpolation averages the colours of two pixels either side of the new pixel, giving a more accurate result. The most accurate but time-consuming method is bicubic interpolation. In this case, all pixels surrounding each new pixel are averaged to determine its colour. Sizing factor = Desired size Original size Sizing factor x 100 witlijn witlijn pmslijn pmslijn Sizing (%) = OUTPUT BASICS 16 This section briefly covers output techniques, in order to explain their relationship with scanning resolution. Further output information is available in Agfa’s “Digital Colour Prepress” guides. Scanning resolution is determined by the number of samples or pixels per inch (ppi) that are recorded. Output devices produce a hard copy from digital image information, either by applying small dots of pigment to a substrate such as paper, or by using an intermittent light source to expose dots in a light-sensitive emulsion. The resolution of an output device is the number of dots it is able to reproduce within an inch (dpi). In most cases, scanning resolution is not the same as output resolution, so the bitmapped image is sampled to produce a new output grid. Line art consists of black and white pixels, which are easily reproduced by adjoining dots of pigment or exposed emulsion. If line-art output resolution is too low, staircasing occurs on angled edges and fine details are lost. One method of outputting the 256 tonal levels in an 8-bit greyscale image is to produce a grid or raster of varying size dots. This is also known as a halftone screen. Viewed from a distance, halftone dots and white substrate merge to create different grey tones. The larger the dots, the darker the tone becomes. Screen ruling or screen frequency is the distance between lines of halftone dots, which is normally quoted in lines per inch (lpi) or lines per centimeter (lpcm). Since most output devices use a fixed dot or “spot” size, varying numbers of these spots are grouped together to produce larger halftone dots. The spot size of a 2400-dpi imagesetter is around one hundredth of a millimetre. Imagesetter resolution may be quoted in recorder elements or rels per inch (rpi) instead of dpi. A minimum of 64 grey levels are needed to print smooth tonal gradations on a laser printer. This means that each halftone dot must comprise at least an 8 x 8 matrix of spots (64 spots). Using a 400-dpi laser printer, a screen ruling of 50 lpi is the maximum that will allow an 8 x 8 matrix (400 dpi / 8 spots = 50 lpi). Specifying a 100-lpi screen ruling for this device reduces the possible grey levels to 16, creating tonal banding in gradations. Variations in screen angle change this ruling calculation slightly. In summary, the maximum resolution of halftone output devices limits both the clarity of details and the number of grey levels that can be reproduced from bitmapped images. Colour images can be printed by creating separate halftone screens for the CMY primary pigment colours. The addition of a black halftone increases contrast and reduces the quantities of more expensive CMY pigments. Each halftone screen must be printed at a specific angle to prevent a disturbing interference pattern called moiré occurring. Adjacent or overlapping CMYK halftone dots merge together when viewed from a distance, creating a wide range of colours. Original Line art Low resolution High resolution The recently introduced stochastic or frequency modulated (FM) screening technique uses much smaller CMYK halftone dots than the traditional method, retaining more image detail. Tonal variations are obtained by changing the number rather than size of dots. Scans to be output with FM screening can be of lower resolution than those made for traditional screening, increasing productivity without loss of quality. An alternative to the halftone reproduction of greyscales is the application of translucent pigments in different thicknesses to produce continuously variable tones without separate dots. This continuous tone, or contone printing is achieved in ink jet printers by changing the CMYK spray durations at any given position to vary ink densities. Thermal sublimation devices incorporate an array of minute heating elements, which vaporize varying quantities of wax-based pigment from a carrier film, depositing it smoothly onto a special substrate. Although the 300-dpi resolution of many of these devices seems low, the blending of pigment between dots gives the impression of a much higher resolution. Photographic methods of reproducing images allow much higher resolutions than pigment-based systems. Imagesetters employ one or more fine beams of intermittent light to expose adjoining dots or spots. Film recorders expose the red, green and blue components of an image in three separate passes. Resolutions up to 5000 dpi are used by some film recorders, allowing high-quality second originals to be created. 17 300 dpi 1200 dpi Halftone greyscale Low screen ruling: 50 lpi High screen ruling: 175 lpi Halftone colour Low screen ruling: 100 lpi High screen ruling: 175 lpi Contone greyscale Contone printer: 300 dpi Film negative Contone colour Contone printer: 300 dpi Film positive LINE-ART RESOLUTION 18 Resolution rule 19 Scan res.* = Output device res. x Sizing factor *1200 ppi is the upper scanning resolution limit (assuming no resizing), since the improvement gained by higher resolutions is insignificant. Note: Although not definitive, Agfa has established these rules through practical experience. When an original image consists of lines and solid areas of flat black or dark tones, it can be scanned as line art. Any colour or grey tones present will be reduced to pure black and white, creating a bilevel image. Pen or pencil drawings scanned as line art may be converted to contours for further manipulation in drawing programs (vectorisation). Objects drawn with a single flat colour, such as logos, are also referred to as line art. Four factors are important when scanning line art: sizing factor between original and output formats; output resolution; sharpening; black-white threshold value. The examples given below assume one-to-one sizing. Scanning resolutions must be multiplied by a sizing factor if the output format differs from the original. The resolution at which line art is scanned depends upon its intended application. If it is to be converted into contours for use in a drawing program, the highest resolution available on the scanner could be used. When line art will be output without conversion, the scanning resolution should match the output device resolution, unless this is higher than 1200 dpi. It can be seen from the output samples made at different resolutions that there is minimal visible difference between scans made at 1200 ppi and 2400 ppi. There is, however, a considerable difference in file size, which makes handling more difficult and storage more costly. No benefit will be gained by scanning an image at 1200 ppi instead of 300 ppi when output will be made with a 300-dpi laser printer. If the maximum resolution of a scanner is not high enough, a greyscale scan could be resampled, sharpened and then converted to line art in an image-editing program. The conversion of grey tones to either black or white is determined by reference to a threshold value. Pixels that are lighter than the threshold value will be converted to white, and darker pixels will be changed to black. To retain details in a greyscale original, the threshold should be set at around the middle of the tonal range present. Sharpening prior to conversion may also improve results. Different threshold settings for line-art scanning may allow line thicknesses to be varied. Image resolution: 2400 ppi File size: 2130 Kb Lacking shadow detail Image resolution: 1200 ppi File size: 547 Kb Good shadow detail Image resolution: 600 ppi File size: 137 Kb Shadow detail too open Image resolution: 300 ppi File size: 35 Kb Sharpening and threshold Some scanners allow varying levels of sharpening to be applied to an image, increasing the definition of details in dark and light areas, before conversion to pure black and white. This is important when edges or fine details are slightly blurred, since these may otherwise be lost. The threshold setting determines how much detail is retained in light areas and whether or not small highlights in shadow areas fill in. Highlights too dense Good highlight detail Lacking highlight detail GREYSCALE RESOLUTION 20 Resolution rule: Conventional halftone printing Scan res. = Screen ruling x Quality factor (qf) x Sizing factor* qf = 2 if screen ruling ≤ 133 lpi qf ≥ 1.5 if screen ruling > 133 lpi 21 Resolution rule: Stochastic halftone printing Scan res. = Comparable screen ruling x Quality factor (qf) x Sizing factor* For stochastic screening, a scanning resolution equal to a conventional halftone screen ruling gives comparable print quality. qf ≥ 1 for stochastic screening Resolution rule: Contone paper output Scan res. = Output device res. x Sizing factor* *Sizing factor = Desired size Original size Note: Although not definitive, Agfa has established these rules through practical experience. To scan greyscale images for halftone reproduction, four items must be considered: output screen ruling (frequency); sizing factor between original and output formats; correct tonal range; sharpness. Output device resolution instead of screen ruling is important for contone printing. When a greyscale image is converted to a halftone, grey pixels are changed to black dots of varying size or number. In the case where scanning resolution is similar to screen ruling, pixel positions may not always coincide with dot positions, causing incorrect dot densities to be chosen. If more than one pixel is available to define the dot density at any position, better results are obtained. Scanning resolution is therefore equal to output resolution multiplied by a quality factor (also known as halftoning factor). When scanning images to be converted into halftones of above 133 lpi, a quality factor of 1.5 is required. Images containing geometrical subject matter, including straight lines or repeated patterns and textures, benefit from an increased quality factor of 2. Halftones of 133 lpi and below need a quality factor of 2, because incorrect output dot densities will be more noticeable at low screen rulings. Restricting the quality factor to a minimum keeps file sizes small whilst ensuring the best possible printed results. An image to be output using a 175 lpi at 75% of its original size should be scanned at 197 ppi (Scan res. = Screen ruling x Quality factor x Sizing factor = 175 lpi x 1.5 x 0.75 = 197 ppi). Reflective contone printing devices give best results when image resolution is the same as the device (after any resizing). Scanning resolutions required for output to film recorders are calculated as described in the following “Colour resolution” section. The full range of tones present in an original should be captured in the scanned image, and these must be correctly distributed between black and white to ensure good contrast and brightness, whilst retaining maximum image detail. These aspects are explained fully in the sections covering scanner density controls, histograms, tone curves, linear and non-linear tone corrections. Sharpening applied either during the scanning process or in an image-editing program will increase apparent image detail by heightening the contrast at object edges. Further information is given in “Sharpen your image”. Screen ruling: 175 lpi / Image res.: 263 ppi File size: 210 Kb Screen ruling: 133 lpi / Image res.: 266 ppi File size: 263 Kb Screen ruling: 85 lpi / Image res.: 170 ppi File size: 88 Kb Screen ruling: 50 lpi / Image res.: 130 ppi File size: 52 Kb Stochastic screening technique Image res.: 300 ppi / File size: 273 Kb COLOUR RESOLUTION 22 Resolution rule: Conventional halftone printing Scan res. = Screen ruling x Quality factor (qf) x Sizing factor* qf = 2 if screen ruling ≤ 133 lpi qf ≥ 1.5 if screen ruling > 133 lpi 23 Resolution rule: Stochastic halftone printing Scan res. = Reference screen ruling x Quality factor (qf) x Sizing factor* For stochastic screening, a scanning resolution equal to a conventional halftone screen ruling gives comparable print quality. qf ≥ 1 for stochastic screening Resolution rule: Contone paper output Scan res. = Output device res. x Sizing factor* Resolution rule: Contone film output Scan res. = Output device res. x Sizing factor* Maximum addressable pixels Output device res. = Longest side of output film *Sizing factor = Desired size Original size Note: Although not definitive, Agfa has established these rules through practical experience. The four criteria described for greyscale images apply also to scanning colour originals, but two additional items should be considered. Colour and grey balance vary from one image to another, often requiring some modification. RGB scans need to be correctly converted to CMYK separations for halftone output. Just as for greyscale images, the scanning resolution for printing four-colour halftones at above 133 lpi should employ a quality factor of 1.5. Images containing geometrical subject matter, including straight lines or repeated patterns and textures, benefit from an increased quality factor of 2. Halftones of 133 lpi and below need a quality factor of 2. Scanned RGB files require three times the storage space needed for greyscale images and CMYK files use four times as much space. It is therefore even more important to keep the quality factor to the lowest effective level. Scanning resolutions must be multiplied by a sizing factor if the output format differs from the original. Sharpening of both colour and greyscale images increases apparent detail by heightening the contrast at object edges. Further information is given in “Sharpen your image”. Screen ruling: 100 lpi / Image res.: 200 ppi File size: 485 Kb Screen ruling: 175 lpi / Image res.: 263 ppi File size: 838 Kb Screen ruling: 150 lpi / Image res.: 225 ppi File size: 613 Kb Screen ruling: 133 lpi / Image res.: 266 ppi File size: 858 Kb Contone film output (digital photo-imaging) A3 water-colour original directly scanned on an Agfa flatbed CCD scanner. Reflective contone printing devices give best results when images are scanned at identical resolution, taking into account the sizing factor. The resolution of film recorders is typically quoted as being 2K, 4K, 8K or 16K. This measurement refers to the maximum number of addressable pixels that can be exposed on a film, regardless of its format. The number of pixels per inch must be calculated from this to indicate scanning resolution. For example, when a 4K recorder (4096 pixels) is used to produce a 35mm slide (defined as 1.5” x 1”), the resulting resolution is 2731 ppi (4096 / 1.5). A 4”x 5” slide produced on an 8K recorder (8192 pixels) requires a scan resolution of 1638 ppi (8192 / 5). Colour and grey balance are affected by the automatic or manual selection of the darkest and lightest neutral tones in an image, known as black and white point setting. Colour corrections can be made either at scan time or later in an image-editing program. It is advisable to obtain the best possible result during the scanning process, since more data is available at this stage. If the scanned image does not contain the necessary information, it is very difficult to create it later. The sections dealing with scanner density controls, histograms, tone and colour corrections explain how to obtain a well-distributed tonal range with correct colour balance. Stochastic screening technique Image res.: 300 dpi / File size: 1060 Kb HISTOGRAMS AND TONE CURVES 24 The histogram of an 8-bit greyscale image contains 256 vertical bars (0 to 255), each representing a specific grey level. Bar heights are proportional to the number of pixels per grey level. For RGB images, a combined histogram indicates overall brightness but separate histograms for each primary colour may also be viewed. The distribution of pixels in a histogram, especially at its extremities, provides a guide for tonal corrections. The top-left scanned image is low in contrast, having virtually no pixels at the black (0) and white (255) ends of the histogram. Stretching the data out to fill the histogram, as shown in the top-right image, increases the contrast but causes gaps to open up. This absence of pixels in a number of consecutive grey levels creates posterisation or tonal banding, which may only become obvious when additional corrections are applied. Using incorrect highlight and shadow settings to scan an image that has a wider tonal range will result in a histogram containing very high values at both ends. Shadow detail (a) in the middle-left image has been forced or clipped to black and highlights (b) are clipped to white, as indicated by the circles. Scanners that provide automatic density control create internal histograms after a prescan, from which they determine correct shadow and highlight settings. The final scan then captures the full tonal range without posterisation or clipping. An unevenly distributed histogram does not necessarily mean that the image is incorrect. The intentionally high-key image at the bottom-left contains few shadows, as indicated by its histogram. Conversely, the histogram of the low-key image is weighted towards the dark end. Redistribution of these histograms would destroy the intended effect. The earlier “Judging your originals” section should help in assessing the need for corrections. Histogram A histogram shows the distribution of pixels throughout the tonal ranges of an image, highlighting irregularities. 0 255 Low contrast Posterisation Clipping Correct tonal range (a) (b) High-key image Low-key image Tone curve Input values prior to changes are shown on the horizontal axis. Modified output values are read from the vertical axis. Dark and light quarter tones are indicated (3/4 and 1/4) together with the midtone position (1/2). Some programs show reversed black and white positions. The 45° line leaves output values unchanged. Any other curve causes tonal changes, which are evident when input and output greyscale wedges are compared. Modifying histograms with tone curves Highlights 25 1/4 tones Output values Midtones One method of redistributing tones in a histogram is to use tone curves, which allow smooth changes to be applied to specific tonal ranges. A single curve modifies overall brightness levels in colour images, whilst separate tone curves change primary colours individually. Normally, the horizontal axis of a tone curve indicates the tonal ranges of an image prior to changes (input values) and the vertical axis shows the effect of tone corrections (output values). Tone curve modifications cause the vertical line from a specific input value to cross the curve at a new point. A horizontal line from this point shows what the output value will become. The top-left image is lacking in shadow details, indicated by densely packed histogram bars of similar height in that area. Drastically raising three-quarter and midtones expands input shadows across a wide range of output grey levels, amplifying subtle tonal variations (topright image). This compresses input highlight and quarter tones, losing details in these areas. Excessive expansion of 8-bit data in an image-editing program causes posterisation. The bottom-left image has a more moderate lift of three-quarter tones, with a slight drop of highlight quarter tones, improving shadow detail without unduly sacrificing highlight detail or producing disturbing posterisation. Automatic correction in some scanners applies this type of curve. The facility to download user-defined tone curves to a scanner by means of its software driver, allows tone corrections to be made on super-sampled data at greater than 8-bit depth, avoiding posterisation and retaining smooth tonal gradations in the final image (bottom-right). 3/4 tones Shadows Input values No correction Excessive tone correction with posterisation Acceptable tone correction with reduced posterisation Improved tonal correction using scanner tone curve LINEAR AND NON-LINEAR TONE CORRECTIONS 26 Linear tone corrections Selectively modifying points in a tone curve to affect some output ranges more than others is known as non-linear correction. A more basic variety is linear correction where overall changes to image brightness and contrast are made by simply changing the position of a straight tone curve. Both linear and non-linear tone corrections to prescanned files throw away some grey level information in order to expand or move other areas. Linear corrections remove data less intelligently than non-linear ones, so they should be used with caution. Multiple tonal corrections reduce information at each stage. This is another reason for making corrections during the scanning process. Original image 1/4 tones Highlights Midtones Shadows 3/4 tones Brightness Darkening an image causes the 45° linear tone curve to be shifted to the right, or white end of the input axis. Input shadow detail is completely lost because values are clipped to black. No white or bright highlights will be present in the image now, meaning that overall contrast has been reduced. Brightening the image moves the tone curve to the left, or black end of the input axis, sacrificing all highlight detail by clipping it to white. Black and dark shadows are totally removed, resulting in a smaller tonal range. Brightness adjustments Linear darkening of an image throws away shadow details by clipping them to black. Increasing brightness linearly removes highlight details by clipping them to white. Contrast When the overall contrast of an image is increased, its tone curve is rotated so that the midtone input is expanded to fill the complete output range. Any input shadow detail is clipped to black and highlights are clipped to white. Posterisation may be visible if the curve is too steep. Reducing overall contrast rotates the tone curve in the opposite direction, compressing the full input range into only the output midtones, whilst removing light and dark tones. Increasing contrast linearly clips highlight details to white and shadow details to black. Linear reduction of contrast compresses the full input range into a smaller output range. Contrast adjustments Original image Increased shadow detail Non-linear tone corrections Gamma correction is another term used 27 Shadow detail is lacking in the original image. Raising midtones and quarter tones emphasises decoration in the poorly lit church dome. for non-linear tone corrections. Derived from the photographic industry, high gamma specified a film with high contrast. Image-manipulation programs use different methods to edit gamma or tone curves in a non-linear fashion. Some offer a freehand drawing tool, which is difficult to control precisely, although smoothing may be possible. Others allow the curve to be split into a number of control points, which can be moved to new positions manually, or in some cases numerically. Sliders may also be provided. In colour images, tone corrections are normally carried out before any colour corrections are made. Tone curves can sometimes be saved for use with other images. The ability to download tone curves to the scanner allows the extra, oversampled data to be utilised during tone corrections that involve the expansion of tonal ranges, such as shadow areas. A dark original is improved by lifting the quarter tones and midtones, increasing shadow details whilst brightening the image. Highlight details are virtually absent, so compression of this area is not a problem. Original image Increased midtone contrast The midtone area of interest is lacking in contrast. By lowering the shadow three-quarter tone and raising the highlight quarter tone, details in the bone are more pronounced. Original image Highlight/shadow detail Lowering the shadow three-quarter tone and raising the highlight quarter tone produces an S-shaped curve that will give a low-contrast image more “snap”. Midtone contrast and detail is increased, whilst highlight and shadow details are compressed but not lost entirely. A high-contrast image containing few midtones may be improved by lifting the shadow quarter tone and dropping the highlight quarter tone, expanding these areas into the midtone output range. Compression of the few midtones present is a worthwhile sacrifice. The high shadow density of transparencies is normally expanded by gamma correction to retain additional detail. Raising the shadow three-quarter tone and lowering the highlight quarter tone increases detail at both ends of the tonal range. Patterning in the dark church door and texture in the light walls become evident. SCANNER DENSITY CONTROLS 28 During a low-resolution prescan, some flatbed and drum scanners employ automatic density control to calculate specific exposure settings for varying density originals, prior to making the final scan. The lightest tone (Dmin) and darkest tone (Dmax) are automatically located, indicating the density range present. This automatic setting is good for originals with bright, neutral highlights and dark shadows but some originals will benefit from alternative light and dark points being selected elsewhere in the image. The manual selection of a new white point and black point can be used to retain the intentionally limited tonal ranges of high-key and low-key originals, or to correct contrast and brightness levels. These modifications have a similar effect to the linear tone corrections described in the previous section. Automatic white point location will be too dark in a low-key image, causing dark shadows to be lightened unrealistically. In a high-key original, the automatic black point will be too light, resulting in an unwanted darkening of the image. When images contain a restricted tonal range, it may be necessary to scan a greyscale tone wedge next to the image to enable white and black points to be set correctly. Some scanner interfaces include a tone wedge for this purpose. Incorrect automatic settings Manually-adjusted settings On-screen densitometer Many interactive scanner interfaces provide an on-screen densitometer, allowing exact pixel colour values to be displayed. A few provide CMYK values in addition to RGB, giving an indication of results after separation. These readings identify any colour casts, which may not otherwise be noticed. A neutral grey normally contains equal quantities of magenta and yellow, with a slightly higher percentage of cyan. Specular highlights If an image contains direct light sources or specular highlights such as reflections from metal or glass surfaces, these should remain as pure white. An automatic white point will generally be placed incorrectly in specular highlights, causing other highlights and midtones to be darkened. The aim should be to set a white point in a bright, neutral highlight where some detail is still visible. Setting the white point to light grey causes all lighter greys to be burnt out to white (clipping), washing out remaining tones in the process. Black point setting is less critical than the white point. The darkest neutral tone is selected where detail should still be visible. Any darker tones will then be clipped to pure black, removing all details. Automatic location of the white point in specular highlights may cause other highlights and midtones to be too dark. In this image, the white point has been moved to a highlight that contains unwanted tone. When this is pulled to white, the image is lightened, and specular highlights are burnt out correctly. Scanning artwork To ensure that correct colours are retained when reproducing artwork, a colour reference card is often included in photographs, which can be measured during the printing stage. The picture shown above has been scanned directly on a flatbed with a colour reference and tone wedge placed beside it. These enable white and black points to be set precisely, and colour casts to be avoided. GLOBAL AND SELECTIVE COLOUR CORRECTIONS Global colour correction Global colour corrections Unreal colour casts in originals can also be removed (or added for artistic reasons) by the appropriate choice of white and black points. To remove the overall magenta cast from the lake photograph, the white point has been repositioned in a magenta highlight. All colour present at the selected point is removed, pulling it to white. This lightening process is applied in diminishing quantities towards the black point, reducing the red and blue (magenta) values more than the green to eliminate the magenta cast. Tonal imbalance at the black point is less visible, so an on-screen densitometer may be provided to view precise colour values. If the black point is placed in a dark shadow where red and blue (magenta) are predominant, green will undergo the greatest darkening when all three colours are pulled to black, reducing the magenta cast in diminishing proportions as the white point is approached. The RGB colour space has been used for this description, but densitometer readings in CMYK could also be available. In some cases, one or more additional grey points may be indicated to provide greater control over irregular colour casts. Any colour imbalance at the chosen grey point is averaged to a neutral tone and this change blends with those introduced by setting white and black points. 29 The magenta (red and blue) cast in this original is removed by setting white, grey and black points in areas where magenta is predominant. The tone curves show how red and blue are lightened in relation to green. Red Green Blue Selective colour correction: changing emphasis The shirt of the saxophone player was violet in the original photograph. Most of the magenta has been removed, and the cyan has been lightened to simulate blue denim. Selective colour corrections There are some occasions when it is useful to apply local corrections to specific colour ranges in an image, either to increase impact or to totally change colour relationships. Numerous methods of making selective colour corrections exist, the more complex ones involving masking techniques. A simple way is to select some pixels within the range to be changed, and then to specify how many additional colours should be affected by setting the width of the range. These corrections are normally carried out in the CMYK colour space to observe the impact on printing processes. Selective colour correction: adding impact Magenta has been added to the guitar so that it appears more orange, whilst the red shirts of the people in the back row have had yellow removed and magenta increased. SHARPEN YOUR IMAGE 30 Unsharp originals are given the impression of being sharper by the application of unsharp masking (USM). This process does not add detail but heightens the contrast at edges of objects to make them more visible. Image-manipulation programs and most modern scanners carry out USM by modifying scanned pixels completely in software, although a few drum scanners use a fourth PMT for this purpose. The USM technique is similar to traditional photographic methods in that it combines the unsharp image with an even more blurred copy (mask) to produce the sharpening effect. For each stage of the USM process, an enlarged view of the text on a ship’s hull is shown next to the original image. Grey level charts indicate the changes that occur at the edge of one of the painted characters. The first chart shows that there is a tonal jump of 18 grey levels between the background colour and the light characters. This jump is unsharp because it is blurred across 2 intermediate pixels. A copy of the unsharp image is further blurred by a defocusing filter such as a Gaussian blur filter. Tonal changes at object edges are now spread across 8 pixels (9 steps), as shown in the second chart. When the tonal values in the blurred copy are subtracted from those of the unsharp original, the result is a set of positive and negative tonal values, represented by the green areas in the third chart. When these green areas are rearranged along a horizontal line, a sharpening mask (c) is produced. The final stage, shown in the fourth chart, is to add the mask values to the unsharp original. This produces two tonal blips or peaks, increasing the original tonal jump. Observed closely, these peaks will appear as adjacent light and dark lines around edges of objects. Greater control is provided by softwarebased USM than the photographic method. Peak width determines the number of modified pixels. Kernel size and radius are terms used for peak width. When the width is too great, disturbing haloes are created around objects, modifying or destroying image details in the process. 80 74 Grey levels 255 98 This chart shows the original step in grey levels between the edge of a light character and the dark ship’s hull in the above images. The vertical arrow (a) in the chart indicates the tonal jump, which needs to be larger than the threshold setting for USM to be applied. (a) 80 0 80 98 Grey levels (b) 255 98 80 0 The first step in USM is to make a copy of the original and to apply a blurring filter to it, causing the tonal jump to be spread across additional pixels. The horizontal arrow (b) shows the kernel size, which determines the number of pixels affected by USM. 80 98 (c) Grey levels 255 98 80 0 The mask (c) is in reality a series of positive and negative tonal values obtained by subtracting the blurred copy from the unsharp original. Negative tonal values cannot be visualised, so the zero line has been shifted to the midtone grey level (128) in order to illustrate the contents of the mask. 80 98 (c) Grey levels 255 104 Positive and negative tonal values in the mask (c) are added to, or subtracted from the original image. The light and dark peaks thus formed exaggerate the original tonal jump, making it appear sharper. 74 0 104 98 The height of the peaks or amount of lightening and darkening is changed by a strength setting. When the strength is set too high, tonal peaks reach the limits of pure black and white, producing an artificial appearance. USM is only applied when tonal jumps are greater than the number of grey levels specified by a threshold setting. A high threshold restricts USM to large jumps in tone such as the white text on the ship’s hull, preventing it from being applied to areas that should remain smooth gradations. Most scanned images contain subtle tonal variations in flat areas of colour. If USM is applied to these variations, an unpleasant texture called mottling occurs. Dark shadows sometimes contain a few isolated lighter pixels, caused by noise during scanning. USM emphasises these pixels, producing speckling. Mottling and speckling are avoided by raising the threshold setting. Some scanner interfaces allow USM to be switched off in shadow areas or in specific colours such as skin tones. The USM process exaggerates the staircasing or aliasing along angled edges. This will only be apparent when the image resolution is too low in relation to the final output resolution. Applying USM to images that contain fine textures or patterns may produce unexpected results. These subject-related problems are more difficult to control. 31 Unsharp masking: another viewpoint The action of USM at object edges is simply explained by a 3-D view of the combined grey level charts from the previous page. The purple surface represents the original, unsharp tonal step. This is blurred to produce the red surface. Tonal values lying between the original purple and blurred red surfaces are flipped to the outside of the purple surface to create the sharpened green surface. By this means, the light tones in the original have been lightened near the object edges, and the dark tones have been darkened, increasing the overall tonal jump. Unwanted effects of sharpening Halo Mottling Descreening halftone originals Subject-related problems Speckling Haloes occur when kernel size is too great. Mottling and speckling are avoided by raising the USM threshold setting. Sharpening fine textures or patterned contents may produce unacceptable results. Screened originals may suffer from moiré patterning and colour shifts when reprinted. Defocusing during the scan, or blurring after the scan will help avoid this. Sharpening will definitely not improve scans of halftone originals because haloes will occur around each dot. Conversely, they must be blurred by a software filter or by defocusing the scanner optics to avoid moiré patterning and colour shifts during output. Without descreening With descreening COLOUR DEFINITIONS 32 The ability to precisely measure and define colours is essential in the reproduction of images. All visible colours can be defined by the three factors described below. Alternative terms are shown in brackets. Hue - the colour perceived when one 3-D colour model or two of the three RGB colours of light predominate (colour). Saturation - the extent to which one or two of the three RGB colours predominate. As quantities of RGB equalise, colour becomes desaturated towards grey or white (chroma, purity, intensity, vividness). Visible spectrum Hue Lightness Saturation Pure wavelengths of light lie on the curved edges of the triangular gamut of visible colours. The lower straight edge represents the colours obtained by mixing red and blue wavelengths from both ends of the spectrum. Although distances between colours in this model do not correspond to perceived colour differences, it allows us to indicate the relative gamuts of RGB monitors, and different sets of printing inks. Inks in the Pantone Matching System (PMS) provide a much larger colour gamut than CMYK process inks, such as the Standard Web Offset Press set (SWOP). A special fifth ink is sometimes used to extend the CMYK gamut. The non-linear CIE Yxy colour model was mathematically transformed in 1976 to the uniform CIE L*a*b* model, in which distances between colours more closely match those perceived. All colours of the same lightness lie on a circular flat plane, across which are the a* and b* axes. Positive a* values are reddish, negative a* values are greenish, positive b* values are yellowish and negative b* values are bluish. Lightness varies in the vertical direction. SWOP-CMYK Pantone® 33 Lightness - the strength or amplitude of the RGB wave forms activating the eyes’ receptors (luminance, brightness, value, darkness). Frequently associated terms for these three factors are HSV (hue, saturation, value), HSL (hue, saturation, lightness), and HVC (hue, value, chroma). CIE Yxy model Colour gamuts 520 These characteristics can be illustrated by a three-dimensional model consisting of stacked “disks”. Circular movement around each disk varies the hue. Upwards movement from one disk to another increases the lightness. Radial movement from the centre of each disk outwards increases saturation. The model is irregularly shaped because the eye is more sensitive to some colours than others. 0.8 510 530 540 0.7 550 Monitor 560 0.6 500 570 CIE L*a*b* model 580 0.5 590 600 White L* The standard observer In 1931 the “Commission Internationale de l’Eclairage” (CIE) precisely defined three primary colours, or tristimulus values, called X (red), Y (green) and Z (blue) from which all other colours visible to a “standard” observer could be created. More recently, the CIE Yxy colour model was introduced. All colours having the same lightness lie on a roughly triangular flat plane. The horizontal x axis in the illustration of the CIE Yxy model shows the redness of colours and the vertical y axis indicates the amount of green in colours. The Y axis representing the value or lightness of colours can only be shown in a 3-D view of the CIE Yxy model, since it comes out of the page. 0.4 0.3 610 490 620 630 650 700-750 Samples from the CIE L*a*b* colour space (more simply known as CIE LAB) are used to create industry standard IT8 transmissive and reflective reference charts, against which the gamuts of input and output devices can be compared and calibrated by use of the Colour Management Systems (CMS) described in the next section. Spectrophotometers or colorimeters make precise colour measurements, normally quoting values in both CIE Yxy and CIE LAB colour models. 0.2 480 Yellow +b* 0.1 Green -a* 470 460 450 400-380 Red +a* Blue -b* 0 0 IT8.7/2 colour reference 0.1 0.2 Pantone® 0.3 0.4 Monitor 0.5 0.6 0.7 SWOP-CMYK Black COLOUR MANAGEMENT 34 Matching colours in printed output to those in scanned originals is no simple task, due to the number of variable factors in the reproduction chain. Imagecapturing devices output different values when reading the same original. Adjustments to monitor controls cause wide colour variations. Gamut differences between monitors and printing processes mean that unprintable colours can be introduced during image retouching. The conversion of scanned RGB data to CMYK separations differs from one program to another. Proofing devices vary wildly in colour rendition, due to pigment and substrate characteristics. Viewing proofs and printed matter under non-standard lighting conditions introduces errors of judgement. Ink-based press adjustments permit wide variations in ink densities. Alternative ink sets and paper types affect colour rendition. Paper coating and texture affect dot gain, which modifies colours. Attempting to compensate for all these colour variations by trial and error is too expensive in time and materials. Colour management systems (CMS) solve the colour mismatch between input and output devices. These systems vary in their method of application but ideally, the gamut of each device in the colour reproduction chain is related to a standard colour space such as CIE LAB. Variations from the chosen standard are recorded in a device-specific tag or profile. Future input or output from each device is then matched by use of its profile, resulting in device independence or portable colour. The input characterisation process requires industry standard colour reference targets of transmissive (IT8.7/1) and reflective (IT8.7/2) varieties to be scanned using normal settings. These reference targets contain 264 patches of colour and neutral greys, representing the complete gamuts for the media used to create them. The CMS relates scanned readings for each patch to colorimetric readings of the IT8 reference, which have been measured by a spectrophotometer. Output device characterisation is achieved by printing an IT8.7/3 reference file, which contains more colour patches than the IT8.7/2 used for input devices. Colour mismatch between devices Input and output devices produce unique colour gamuts. Lack of compensation for these differences causes unreliable results. Device characterisation Device-specific profile Scanned IT8 values related to reference IT8 values provide a device-specific profile. Device-specific profile Measurements of printed IT8 values related to reference IT8 values create output device profiles. Matching colours throughout the colour chain Using the device-specific profiles, colour matching between all devices in the colour chain is guaranteed by the underlying CMS. Final prints reliably match original images. CM S Matching output on different devices Original Thermal transfer process The results are accurately read with a spectrophotometer or a colorimeter and are fed back into the CMS to create unique profiles. A range of profiles may be made for a device that uses more than one pigment or paper type. Varying levels of dot gain on ink-based systems can also be profiled. Having made colour profiles for scanners, proofers and printing devices, final results should now reliably match original images, assuming that no colours are judged and corrected on a monitor display. The final link in the chain is accurate monitor calibration. Profiles for specific brands of monitor may be created and supplied as digital data by the CMS manufacturer. These allow approximate calibration but settings vary between monitors. On-screen colours may be visually matched to standard colour patches, although some monitors now include their own calibration sensor, providing automatic adjustment to match CMS profiles. Monitor calibration completes the chain, permitting reliable on-screen colour corrections to be carried out. The success of any CMS relies upon the colour stability and correct calibration of all devices in the reproduction chain. Keeping printing press results identical to their measured reference targets is a difficult task, although modern ink-based presses are fairly stable once correct colour balance for a specific job has been achieved. Due to increasing demand for low-cost, high-quality colour printing, manufacturers of computers, input and output devices are working together with software developers to implement highlybeneficial colour management facilities throughout the design and production chain. Computer operating systems have already been modified so that all resident colour-aware programs can access a common CMS. 35 Sublimation process CMYK offset reproduction IT8 colour reference targets The industry standard IT8.7/2 reference target for input characterisation Midtones Shadow tones Highlight tones CMYK colours RGB colours Skin tones and other frequently occurring colours in nature The industry standard IT8.7/3 reference target for output characterisation A B C D E High total ink amount (TIA) patches to check ink trapping 1 2 3 4 5 1 2 3 4 Saturated colour patches with no black Saturated colour patches with 20% black A B C D E F 1 2 3 4 5 6 G H I J K L M N Solid CMY patches to check density IT8.7/3 A B C D E F 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13 Shadow tones CMYK density wedges to establish dot gain Neutral greys printed with CMY (and K in some cases) to check grey balance FILE FORMATS AND FILE STORAGE 36 Image files may be stored in a wide variety of formats. The choice depends largely upon how and where images will be used. Will they need to be imported into other programs for page layout or image manipulation? Do they need to be saved in a compressed format to reduce storage space or transmission time via a network or telephone lines? Would a loss in image quality caused by high compression factors be noticed in the chosen output medium? The tables on this double-page spread list the resolution requirements for various printing applications, together with alternative file formats and corresponding file sizes. Native formats that are proprietary to one program manufacturer may be extremely efficient in their own environment but they provide little or no compatibility with other systems. A few image formats have become universally supported by the graphics industry, due to their versatility and openness. The TIFF format is capable of describing bilevel, greyscale, RGB and CMYK images with more than ten compression techniques available. This versatility has one main drawback. Programs designed to read TIFF files need to be equally versatile to understand any data contained within them, which is unfortunately not always the case. The EPS format is more comprehensive than TIFF, being able to describe both vector and image data, together with page-layout details. Its greater complexity results in larger files than the TIFF format but image compression techniques are also available. EPS files are intended to be included in other PostScript files so they normally contain a lowresolution, bit-mapped image for fast onscreen manipulation. Both RGB and CMYK composite EPS files exist. In order reduce the quantity of CMYK data loaded into page-layout programs, the Desktop Colour Separations (DCS) format was developed. Also known as EPS 5, this consists of four files containing full-resolution CMYK data and an additional fifth master file, which contains a low-resolution preview image. Only the master file is loaded into layout programs to reduce memory requirements and increase operation speeds. At output time, the high resolution files are automatically used in place of the master file. Line art Original: 16 x 20 cm Output: as above Reproduction quality Resolution requirements dpi Image resolution ppi TIFF Normal TIFF Compr. EPS Composite PICT Composite JPEG Non-lossy JPEG Lossy Size in Kb 560 2195 8729 35726 446 842 2129 5965 594 2228 8762 37924 513 1749 5825 32586 Image quality: Image quality: excellent good Compression: fair Compression: good The PICT 2 format, developed by Apple, also caters for vector and bit-mapped data but it has little support on other platforms. Numerous methods of file compression exist to reduce storage and transport problems. Compressed files need to be decompressed before they can be edited, although this process may be carried out automatically when a file is loaded. Compression techniques fall into two categories - lossy and non-lossy. Lossy compression means that data is permanently removed during the compression process, causing a loss in quality. When coarse screen rulings and low-quality paper are being used this loss is not noticeable, allowing considerable reductions in file size. LZW compression is a non-lossy technique, which is particularly effective when repeated patterns of pixels are present in an image. JPEG compression offers both lossy and non-lossy versions. The lossy version is very efficient, requiring only one bit per pixel instead of eight to reproduce an image that is virtually indistinguishable from the original. A special utility may be needed in addition to standard programs, to enable decompression of JPEG files. 37 Laserprint Laserprint Print Print 300 600 1200(1) 2400(1) 300 600 1200 2400 n/a n/a n/a n/a n/a n/a n/a n/a EPS saved with 1-bit preview Greyscale Original: 16 x 20 cm Output: as above Reproduction quality Halftone Screen frequency lpi Quality factor qf Image resolution ppi TIFF Normal TIFF Compr. EPS Composite PICT Composite JPEG Non-lossy JPEG Lossy Size in Kb 1403 3515 2516 3437 4373 842 1636 1171 1599 1832 1607 3722 2664 3639 4637 1353 3303 2364 3230 3993 Image quality: Image quality: excellent good Compression: fair Compression: good Newspaper Magazine Magazine Art magazine Art magazine 85 133 150 175 Stochastic 2 2 1.5 1.5 screening(2) 170 266 225 263 300 462 835 598 816 957 231 364 260 356 463 File sizes and storage Traditional photographic reproduction methods require the storage of films and plates for possible future use. Digital storage methods take up less space and the data can be repeatedly reproduced with identical quality. EPS saved with 8-bit preview RGB colour Original: 16 x 20 cm Output: as above Reproduction quality Halftone Screen frequency lpi Quality factor qf Image resolution ppi TIFF Normal TIFF Compr. EPS Composite PICT Composite JPEG Non-lossy JPEG Lossy Size in Kb 5818 10283 7358 10056 13085 2821 4739 3491 4622 6097 7989 13943 10043 13637 17674 5324 9172 6669 8996 11525 Image quality: Image quality: excellent good Compression: fair Compression: good Newspaper Magazine Magazine Art magazine Art magazine Contone output 100 133 150 175 2 2 1.5 1.5 200 266 225 263 300 ppi 1677 2951 2087 2776 3985 208 351 260 345 436 Stochastic screening(2) dpi Size in Kb 13085 146500 6097 27510 17674 198778 11525 131154 3985 6737 436 2228 Paper print (proofing) Film positive (second original) 300(3) 1600(4) 300 1600 In most cases, images will need to be transferred between systems in order to be manipulated and eventually output. The use of a local area network (LAN) to frequently transfer large image files may prove impractical because it will slow down other network activities. Modem transfers via a normal telephone line are impractical unless files are heavily compressed, since the transfer rate on a reasonably good line (9600 baud) would be about 15 minutes per megabyte (Mb). One solution to file transport is to install devices that use removable storage media on all systems needing to share images. For back-up purposes, tape drives are ideal because of their large media capacity (8 gigabytes in some cases) and relatively low media cost. Access to files is slow, however, and they must be loaded onto a disk before they can be used. Removable hard disks of the Syquest variety allow images to be modified in situ. Access to data is not quite as fast as a fixed hard disk. These are available in a variety of capacities. Magneto-Optical Disks (MOD’s) are gaining popularity because of their robustness and low cost. Small diameter disks contain 128 Mb, whilst the larger format disks hold 650 Mb (325 per side), 1.3 Gb or 2 Gb. Access times for MOD’s are currently slower than fixed hard disks. New data formats and capacities are being introduced, as technology rapidly chagnes. Those mentioned within this section are just a selection of currently available media. EPS saved with 8-bit preview in binary format CMYK colour Original: 16 x 20 cm Output: as above Reproduction quality Halftone Screen frequency lpi Quality factor qf Image resolution ppi TIFF Normal TIFF Compr. DCS JPEG Non-lossy Image quality: excellent Compression: fair JPEG Lossy Image quality: good Compression: good Size in Kb 7755 13715 9815 13410 17440 4550 7599 5610 7300 9393 9000 14800 11000 14500 18400 Size in Kb 17440 9393 18400 Newspaper Magazine Magazine Art magazine Art magazine Contone output 100 133 150 175 2 2 1.5 1.5 200 266 225 263 300 ppi 2581 4433 3179 4121 5623 429 712 520 676 865 Stochastic screening(2) dpi Paper print (proofing) 300(3) 300 5623 865 DCS saved in binary encoding, with 8-bit (72-dpi) preview in the separate CMYK master file (1) There is minimal visible difference between scans made at 1200 ppi and the higher resolution of 2400 ppi. (4) Both (2) Stochastic screening quality is comparable with conventional screening when scanning resolutions match conventional screen rulings. These do not exceed 300 lpi so a maximum scan resolution of 300 ppi gives excellent results for stochastic screening (assuming no sizing factor). (3) Contone output devices normally give best results when scanning resolution is the same as output device resolution. This example uses a 300-dpi dye sublimation printer. input and output sizes are 4"x 5" (for this example only). The “Colour resolution” section (“contone film output”) explains how the resolution is calculated. GLOSSARY 38 A/D converter A device used to convert analogue data to digital data. Analogue data is continuously variable, whilst digital data contains discrete steps. clipping The conversion of all tones lighter than a specified grey level to white, or darker than a specified grey level to black, causing loss of detail. This also applies to individual channels in a colour image. dichroic mirror A special type of interference filter, wich reflects a specific part of the spectrum, whilst transmitting the rest. Used in scanners to split a beam of light into RGB components. FPO For Position Only. A low resolution image placed in a document to indicate where the final version is to be positioned. additive primaries Red, green and blue are the primary colours of light from which all other colours can be made. frame-grabbing system digital Data or voltages consisting of discrete steps or levels, as opposed to continuously variable analogue data. A combination of hardware and software, designed to capture individual frames from video clips for further digital manipulation, or consecutive replay on computer platforms. CMS Colour management system. This ensures colour uniformity across input and output devices so that final printed results match originals. The characteristics or profiles of devices are normally established by reference to standard IT8 colour targets. aliasing Visibly jagged steps along angled lines or object edges, due to sharp tonal contrasts between pixels. direct-to-plate Direct exposure of image data onto printing plates, without the intermediate use of film. gamma correction The correction of tonal ranges in an image, normally by the adjustment of tone curves. analogue Continuously variable signals or data. CMYK Cyan, magenta, yellow and black are the base colours used in printing processes. CMY are the primary colorants of the subtractive colour model. direct-to-press Elimination of intermediate film and printing plates by the direct transfer of image data to printing cylinders in the press. batch scanning Sequential scanning of multiple originals using previously-defined, unique settings for each. gamut The limited range of colours provided by a specific input device, output device, or pigment set. Dmax baud Bits per second. A measurement used in data transfers via telephone lines. colorimeter A light-sensitive device for measuring colours by filtering their red, green and blue components, as in the human eye. See also spectrophotometer. The point of maximum density in an image or original. gang scanning Sequential scanning of multiple originals using the same previouslydefined exposure setting for each. Dmin The point of minimum density in an image or original. bilevel A type of image containing only black and white pixels. gigabyte (Gb) 1,024 megabytes, or 1,048,576 kilobytes of digital data. colour cast binary number system A counting system used in computers consisting of only 1’s and 0’s. An overall colour imbalance in an image, as if viewed through a coloured filter. down-sampling The reduction in resolution of an image, necessitating a loss in detail. grey balance The balance between CMY colorants required to produce neutral greys without a colour cast. dpi bit Binary digit. The smallest unit of information in a computer, a 1 or a 0. It can define two conditions (on or off). compression The reduction in size of an image file. See also lossy and non-lossy. Dots per inch. A measurement of output device resolution. See also lpi. grey levels drum scanner (and recorder) An image scanning device in which originals are attached to a rotating drum. Early drum scanners separated scans into CMYK data, recording these directly onto film held on a second rotating drum. Discrete tonal steps in a continuous tone image, inherent to digital data. Most CT images will contain 256 grey levels per colour. contone (CT) An abbreviation for continuous tone. A colour or greyscale image format capable of illustrating continuously varying tonal ranges, as opposed to line art. bit depth The number of bits used to represent each pixel in an image, determining its colour or tonal range. greyscale A continuous tone image comprising black, white and grey data only. bitmap A digitised image that is mapped into a grid of pixels. The colour of each pixel is defined by a specific number of bits. DCS Desktop colour separation. An image format consisting of four separate CMYK PostScript files at fullresolution, together with a fifth EPS master for placement in documents. dye sublimation A printing process using small heating elements to evaporate pigments from a carrier film, depositing these smoothly onto a substrate. halftone An simulation of continuous tones by the use of black or overlapping process colour dots of varying size or position. black point A movable reference point that defines the darkest area in an image, causing all other areas to be adjusted accordingly. decompression The expansion of compressed image files. See also lossy and non-lossy. EPS Encapsulated PostScript. A standard format for a drawing, image or complete page layout, allowing it to be placed into other documents. EPS files normally include a low resolution screen preview. halftoning factor See quality factor. halo A light line around object edges in an image, produced by the USM (sharpening) technique. densitometer byte A measurement unit equal to 8 bits of digital information. The standard measurement unit of file size. See also kilobyte, megabyte and gigabyte. A measuring instrument that registers the density of transparant or reflective materials. Colours are read as tonal information. See also colorimeter and spectrophotometer. EPS 5 Another term used for DCS. high key A light image that is intentionally lacking in shadow detail. CCD Charge-coupled device. An integrated, micro-electronic light sensing device built into some image-capturing devices. density The degree of opacity of a light absorbing filter, pigment or exposed photographic emulsion. film recorder Used in reference to colour transparency recording devices, and sometimes also to imagesetters. highlight The lightest tones in an image. A spectral highlight is a bright, reflected light source. descreening CIE The “Commission Internationale de l’Eclairage”. An organisation that has established a number of widely-used colour definitions. Removal of halftone dot patterns during or after scanning printed matter by defocusing the image. This avoids moiré patterning and colour shifts during subsequent halftone reprinting. flatbed scanner Any scanning device that incorporates a flat transparent plate, on which original images are placed for scanning. The scanning process is linear rather than rotational. histogram A chart displaying the tonal ranges present in an image as a series of vertical bars. hue The colour of an object perceived by the eye due to the fact that a single or pair of RGB primary colours predominates. low key A dark image that is intentionally lacking in highlight detail. optical resolution In the scanning context, this refers to the number of truly separate readings taken from an original within a given distance, as opposed to the subsequent increase in resolution (but not detail) created by software interpolation. rel Recorder element. The minimum distance between two recorded points (spots) in an imagesetter. 39 lpi/lpcm Lines per inch or per centimetre. Units of measurement for screen ruling. res A term used to define image resolution instead of ppi. Res 12 indicates 12 pixels per millimetre. imagesetter A device used to record digital data (images and text) onto monochrome film or offset litho printing plates by means of a single or multiple intermittent light beams. Colour separated data is recorded as a series of slightly overlapping spots to produce either solid areas of line-art or halftone dots for printing continuous tones. LZW The Lempel-Ziv-Welch image compression technique. PICT/PICT 2 A common format for defining images and drawings on the Macintosh platform. PICT 2 supports 24-bit colour. resampling An increase or reduction in the number of pixels in an image, required to change its resolution without altering its size. See also down-sampling and interpolation. matrix This often refers to a 2-dimensional array of CCD elements. pixel Picture element. Digital images are composed of touching pixels, each having a specific colour or tone. The eye merges differently coloured pixels into continuous tones. megabyte (Mb) interpolation In the image manipulation context, this is the increase of image resolution by the addition of new pixels throughout the image, the colours of which are based on neighbouring pixels. 1,024 kilobytes or 1,048,576 bytes of digital data. RGB Red, green and blue are the primary colours of light perceived by the eye. midtone The middle range of tones in an image. rpi pixel skipping A means of reducing image resolution by simply deleting pixels throughout the image. Rels (recorder elements) per inch. A measurement of the number of discrete steps that exposure units in imagesetting devices can make per inch. modem IT8 Industry standard colour reference target used to calibrate input and output devices. Modulator/demodulator. A device required to convert digital computer data into modulated analogue data for transfer via non-digital telephone lines. PMT Photomultiplier tube. The light sensing device generally used in drum scanners. sampling The process of converting analogue data into digital data by taking a series of samples or readings at equal time intervals. jaggies See aliasing. moiré A repetitive interference pattern caused by overlapping symmetrical grids of dots or lines having differing pitch or angle. posterisation The conversion of continuous tone data into a series of visible tonal steps or bands. JPEG Joint Photographic Experts Group. An organisation that has defined various file compression techniques. saturation The extent to which one or two of the three RGB primaries predominate in a colour. As quantities of RGB equalise, colour becomes desaturated towards grey or white. ppi/ppcm monochrome Single-coloured. An image or medium displaying only black-andwhite or greyscale information. Greyscale information displayed in one colour is also monochrome. Pixels per inch or pixels per centimetre. Units of measurement for scanned images. kernel size The number of pixels sampled as a unit during image manipulation and sharpening processes. primary colour A base colour that is used to compose other colours. screen frequency The number of rows or lines of dots in a halftone image within a given distance, normally stated in lines per inch (lpi)or lines per centimetre (lpcm). A frequency of 200 lpi would only be used in high-quality printing. kilobyte (Kb) 1,024 bytes of digital data. mottling A texture similar to orange peel sometimes caused by sharpening. It is particularly visible in flat areas such as sky or skin. process ink colours CMYK pigments used in printing processes, chosen to produce the widest range of colour mixtures. LAN Local Area Network. A wire or optical fibre link between computers installed on a single site for data transfers. screen ruling noise In the scanning context, this refers to random, incorrectly read pixel values, normally due to electrical interference or device instability. profile The colour characteristics of an input or output device, used by a CMS to ensure colour fidelity. Another term used for screen frequency. laser printer Although a number of devices employ laser technology to print images, this normally refers to blackand-white desktop printers, which use the dry toner, xerographic printing process. second original High-quality, contone reproduction of an image, intended to be identical to the original. quality factor non-lossy Image compression without loss of quality. A multiplication factor (between 1 and 2) applied to output screen ruling to calculate scanning resolution for optimum output quality. This is also known as the halftoning factor. secondary colour Colour obtained by mixing two primary colours. Although known as primary colorants, C, M and Y are the secondary colours of light. Red plus green produce yellow for example. line art Images containing only black and white pixels. Also known as bilevel images. The term line art is sometimes used to describe drawings containing flat colours without tonal variation. OCR Optical Character Recognition. The analysis of scanned data to recognise characters so that these can be converted into editable text. quarter tones Tones between shadow and midtones are known as 3/4 tones and those between highlight and midtones are known as 1/4 tones. offset lithography A high-volume, ink-based printing process, in which ink adhering to image areas of a lithographic plate is transferred (offset) to a blanket cylinder before being applied to paper or other substrate. shadow The darkest area of an image. lossy Image compression that functions by removing minor tonal and/or colour variations, causing visible loss of detail at high compression ratios. raster A synonym for grid. Sometimes used to refer to the grid of addressable positions in an output device. speckling Isolated light pixels in predominantly dark image areas, sometimes caused by incorrect readings or noise in the scanning device. 40 spectral highlight A bright reflection from a light source containing little or no detail. x<y x is less than y. x≤y spectrophotometer An extremely accurate colour measurement device using a diffraction grating to split light into its component wavelengths, which are then measured by numerous light sensors. x is less than or equal to y. x>y x is greater than y. x≥y x is greater than or equal to y. staircasing See aliasing. substrate The base material used to carry or support an image, for example paper or film. subtractive primaries Another term for primary colorants. supersampling The capture of more grey levels per colour than is required for image manipulation or output. This additional data allows shadow details to be heightened, for example. tag See profile. threshold The point at which an action begins or changes. The threshold setting used in scanning line art determines which pixels are converted to black and which will become white. The threshold defined in the USM process determines how large a tonal contrast must be before sharpening will be applied to it. thermal wax transfer A printing process using small heating elements to melt dots of wax pigment on a carrier film, which are then transferred to paper or transparent film by contact. This differs from the dye sublimation process in that individual dots do not fuse together, so thermal wax transfer appears to be of a lower resolution. TIFF Tag Image File Format. A popular image file format supported by the majority of image-editing programs running on a variety of computer platforms. tone curves Also known as gamma curves. These are used to smoothly adjust the overall tonal range of an image, or the individual tonal ranges of each colour channel. USM Unsharp masking. A process used to sharpen images. white point A movable reference point that defines the lightest area in an image, causing all other areas to be adjusted accordingly. OTHER EDUCATIONAL AND REFERENCE PUBLICATIONS FROM AGFA An Introduction to Digital Colour Prepress A fundamental reference for anyone interested in PostScript colour. Basic concepts are explained in a clear, objective, and highly visual way. Now in an updated fifth edition, with over 300000 copies in print in eight languages. Also available as a scripted slide presentation (English only). 41 Digital Colour Prepress – volume two The essential complement to “An Introduction to Digital Colour Prepress”. This booklet provides a more advanced look at the topic of PostScript colour, with a special emphasis on reproducing colour pages in print. Now in print worldwide in eight languages. Also available as a scripted slide presentation (English only). Working With Prepress and Printing Suppliers Digital Colour Prepress – volume three The latest volume in the acclaimed “Digital Colour Prepress” series (Spring, 1994), this booklet clearly explains key elements in the working relationship between document creators and their most important service providers. Contains useful time-saving tips to help ensure successful transition of projects from design to film output to final print. PostScript Process Colour Guide This 52-page reference contains over 17000 electronically created CMYK process colour combinations (on coated and uncoated stock), intended to help predict how colours on the screen will look in print. Also includes production tips, instructions for use, and special colour viewing templates. Available in U.S. (SWOP) and a multilingual European print standard version. FOR PRICING AND ORDERING INFORMATION, CONTACT: North America, Agfa Prepress Education Resources, P.O. Box 7917, Mt. Prospect, IL, 60056-7917, Tel.: 1-800-395-7007, Fax: 1-708-296-4805. United Kingdom, Agfa Prepress Education Resources, P.O. Box 200, Stephenson Road, Swindon, Wilts, SN2 5AN, Tel.: (0793) 707099, Fax: (0793) 705745. In other countries, contact your local Agfa subsidiary. Agfa also offers a complete range of electronic and photographic prepress solutions. For more information on these and other products, contact your local Agfa dealer or sales office, listed on the back of this publication. Halftoning Software Scanners PostScript Imagesetters Film Recording Systems Colour Management Systems PostScript Type on CD-ROM Photographic Prepress Systems PostScript Raster Image Processors AGFA and Agfa rhombus are registered trademarks and Agfa CristalRaster is a trademark of Agfa-Gevaert AG, Germany. FotoLook and SelectSet are trademarks of Agfa-Gevaert N.V., Belgium. Adobe, Photoshop, Illustrator and PostScript are trademarks of Adobe Systems, Inc. which may be registered in certain jurisdictions. QuarkXPress is a registered trademark of Quark, Inc. Pantone and PMS are registered trademarks of Pantone, Inc., for colour reproduction and colour reproduction materials. Apple and QuickDraw are registered trademarks of Apple Computer, Inc. All trademarks have been used in an editorial fashion with no intention of infringement. Credits Project management Marc Pollaris, Jan Tas, Martine Vandezande (Agfa-Gevaert N.V.) Technical direction Jan Tas, Rudy Van Hoey (Agfa-Gevaert N.V.) Patrick Gypen (Image Building bvba, Antwerp, Belgium) Art direction, design, illustration and prepress: Patrick Gypen, Bart Van Put, Jean Oppalfens (Image Building bvba, Antwerp, Belgium) Scanning Jan Tas, Mireille De Baere (Agfa-Gevaert N.V.) Copywriting and glossary Tangent Design sc, Meerbeek, Belgium Photography Richard Cox Roger Dijckmans Karel Fonteyne Patrick Gypen (all above: Antwerp, Belgium) Korff & Van Mierlo, Eindhoven, The Netherlands Water-colour painting Ever Meulen Special thanks Paul De Keyser, Gaby Herken, Eugene Hunt, Dirk Kennis, Viviane Michels, Koen Van de Poel, Kris Vangeel, Paul Vinck This publication copyright © 1994 by Agfa-Gevaert N.V. All rights reserved. No portion of this book may be reproduced in any form without expressed written permission from the publisher. Production notes The images in this booklet were scanned on Agfa CCD scanners, using the FotoLook driver software from within Adobe Photoshop. Images were manipulated in Adobe Photoshop, and placed into QuarkXPress as EPS/DCS files. Illustrations were drawn using Adobe Illustrator, saved as EPS files and placed into the same QuarkXPress document. All pages were output as film positives at 150 and 175 lpi or with Agfa CristalRaster stochastic screening on an Agfa SelectSet 7000 PostScript imagesetter. Agfa CristalRaster™ screening was used for the following images: cover; p. 11 (all images); p. 16 (all "original" images except line art); p. 17 ("Contone" images); pp. 20-21 (background image and stochastic screening image); pp. 22-23 (background image and stochastic screening image); p. 35 ("Original"); All other images and line art were output using Agfa Balanced Screening Technology. The book was printed on Profistar paper with CMYK colours plus PMS 421 and varnish. Agfa subsidiaries English literature Argentina, Agfa-Gevaert S.A., Tel.: 54-1-981-0200, Fax: 54-1-953-4304 Australia, Agfa-Gevaert Ltd., Tel.: 613-264-7711, Fax: 613-264-7890. Denmark, Agfa-Gevaert A/S, Grafiske Systemer, Tel.: 45-43-96-6766, Fax: 45-43-96-3955. Finland, Oy Agfa-Gevaert Ab, Graafiset järjestelmät, Tel.: 358-0-88781, Fax: 358-0-8878278. Greece, Agfa-Gevaert A.E.B.E., Tel.: 30-1-53333-200208, Fax: 30-1-574-4900. Hong Kong, Agfa-Gevaert (H.K.) Ltd., Tel.: 852-5-55-9421, Fax: 852-5-55-2480. Ireland, Agfa-Gevaert Ltd., Tel.: 353-1-506733, Fax: 353-1-519613. Japan, Agfa-Gevaert Ltd., Tel.: 81-3-5704-3072, Fax: 81-3-5704-3083. New Zealand, Agfa Division, Tel.: 649-443-5500, Fax: 649-443-5487. Norway, Agfa-Gevaert A.S., Grafiske Systemer, Tel.: 47-2-76-8941, Fax: 47-2-76-0753. Portugal, Agfa-Gevaert Lta, Tel.: 351-1-419-5558, Fax: 351-1-419-8165. Singapore, Agfa Division, Tel.: 65-261-3389, Fax: 65-266-4866. South Africa, Agfa Division, Tel.: 011-921-5563, Fax: 011-921-5548. South Korea, Agfa Korea Ltd., Tel.: 82-2-275-7181, Fax: 82-2-275-7187. Sweden, Agfa-Gevaert AB, Tel.: 46-8-793-0100, Fax: 46-8-793-0171. Taiwan, Agfa Division, Tel.: 886-2-503-9123, Fax: 886-2-504-4819. U.K., Agfa-Gevaert Ltd., Business Group Graphic Systems, Tel.: 081-560-2131, Fax: 081-234-4957. U.S.A., Agfa Division, Miles Inc., Graphic Systems, Tel.: 1-800-685-4271, Fax: 1-508-658-4193. Literature in other languages Austria, Agfa-Gevaert GmbH, Tel.: 43-1-89112.0, Fax: 43-1-89112204. Belgium, Agfa-Gevaert N.V., Verkooporganisatie Benelux, Tel.: 32-3-4509711, Fax: 32-3-4509898. Canada, Agfa Division, Miles Inc., Tel.: 1-416-241-1110, Fax: 1-416-241-5409. Chile, Agfa-Gevaert Ltda., Tel.: 56-2-2383711, Fax: 56-2-2384507. France, Agfa-Gevaert S.A., Tel.: 33-1-40-99-7991, Fax: 33-1-40-99-7990. Germany, Agfa-Gevaert AG, Grafische Systeme, Tel.: 49-221-57170, Fax: 49-221-5717130. Italy, Agfa-Gevaert S.p.A., Tel.: 39-2-30741, Fax: 39-2-3074429. Mexico, Agfa Division, Tel.: 52-5-250-2055, Fax: 52-5-203-95227. Netherlands, Agfa-Gevaert B.V., Tel.: 31-70-3-110591, Fax: 31-70-3-903175. Spain, Agfa-Gevaert S.A., Tel.: 34-3-207-5411, Fax: 34-3-458-2503. Switzerland, Agfa-Gevaert AG/SA, Tel.: 41-1-823-7111, Fax: 41-1-823-7376. Venezuela, Agfa-Gevaert S.A., Tel.: 58-2-238-2922, Fax: 58-2-239-0477. Agfa-Gevaert N.V. Septestraat 27 B-2640 Mortsel Printed in Belgium (577/EM) Published by Agfa-Gevaert N.V., B-2640 Mortsel-Belgium NCUAV GB 00 1994 04 The complete picture.