nderstanding light and colour is at least as important to the serious digital designer as it was to his traditional counterpart. It could even be considered more important, in the sense that the digital designer is virtually ‘painting with the colours of light’. The diagram below summarises the process by which the digital designer captures the image of an object for inclusion in a composition, works on the composition while viewing it on a monitor screen and then outputs the result to a printer. Understanding this colour reproduction process, with all its limitations and conversions, is important if unexpected and disappointing results are to be avoided.
Colour Reproduction
Camera
A key element of many graphic projects is a photographic image captured with a conventional optical camera. Light reflected from the object or scene passes through the camera’s aperture and lens system and impinges on thesurface of the light-sensitive film placed in the camera’s focal plane. Colour film has three layers of emulsion on a cellulose acetate base. Each of the three layers is sensitive to only one of the primary colours red, green or blue. The emulsions are thin, gelatinous coatings containing light-sensitive silver halide crystals in suspension. When exposed to light, each emulsion reacts chemically, recording areas where its particular colour appears in the scene and forming a latent image on the film. When the film is developed, particles of metallic silver form in areas which were exposed to light and each emulsion releases a dye which is the complementary colour of the light recorded – blue light releases yellow dye, green light releases magenta dye and red light releases cyan dye. Complementary colours are used because they reproduce the original colour of the scene when the film negative is processed to produce the final colour print or transparency. Because the sizes of the silver halide particles in the film emulsion and the silver particles formed during the development process are very small, the resolution of detail in the final image is very high. To the unaided eye, the image appears to have continuous tone, with colours blending smoothly from one to another. Only when the image is considerably enlarged does the ‘graininess’ of the particles become visible.
A conventional optical camera
Scanner
The principles underlying the operation of drum, flatbed, sheet feed or hand-held scanners are essentially the same. To use a typical flatbed scanner, the photograph or transparency to be scanned is placed face down on the scanner bed or transparency attachment and the cover is lowered on top of it. A light source inside the scanner, running the full width of the bed, then traverses the image. Light reflected from the image passes via a lens and a series of mirrors on to an array of CCD (Charge Coupled Capacitor) devices which also span the full width of the bed. A CCD is a semiconductor chip – usually silicon – the surface of which has been | doped to make it light sensitive. The light reflected from the source image impinges on the surface of the chips and is converted into electrons in numbers proportional to the intensity of the light beam. The resulting changes in voltage across the chip are then amplified and converted to an analogue ‘picture’ of the image. In order to detect the colour information in the image, rather than just the intensity variations, the reflected light is sampled, in turn, via red, green and blue filters, so that intensity variations are recorded separately for each of the three primary colours. After RGB separation, an analogue to digital converter converts the analogue picture of the image to a digital one before passing it to the PC. A black and white scanned image is considered to be only ‘1 bit’ deep because all the information (on or off, black or white, 0 or 1) to describe each of the dots in the image can be stored in a 1-bit number (21). A greyscale image is considered to be 8 bits deep because to store the information to describe 256 (28) levels of grey, 8 bits of information must be stored for each dot. A full colour scan requires 8 bits for each of the three primary colours red, green and blue and is therefore 24 bits deep, i.e. the scanner records 24 bits of information for each dot (224).
A flatbed scanner
Scanner operation
The role of the conventional scanner is likely to be taken over increasingly by the fast evolving digital camera which operates using the same CCD technology as the scanner, but receives and digitises light from a scene via a conventional optical camera ‘front end’. Digitised images are saved to an internal disk and can be downloaded directly to a PC for processing. Prices are still high for cameras capable of producing high resolution images, but will undoubtedly fall rapidly as the technology is applied to the consumer market.
Digital camera
Monitor
A colour monitor has a screen coated internally with three phosphors capable of emitting red, green or blue light when excited by an electron beam. The phosphors are laid down in bands (trinitron tubes) or patterns (shadow mask tubes). To illuminate the phosphors and produce spots of colour, the cathode ray tube contains three electron guns – one for each of the three phosphors. As the three electron beams track across the screen (from left to right and top to bottom, as in a normal TV tube, they cause red, green and blue light to be emitted fromphosphor dots so close together and so small that the colour seen on the screen is the addition of light from all three dots. Instead of seeing this moving dot of coloured light, persistence of vision deceives the viewer’s eye into seeing the coloured screen image built up by the moving spot. To create colours such as orange or yellow, the three ‘primary’ colours are mixed together in varying degrees by independently controlling the intensities of the electron beams and, therefore, the intensity of the light emitted by the phosphors. As the intensity of each beam can be varied in steps from 0 to 255, the number of possible colour combinations for the combined spot = 256 256 256 = 16.7 million – a palette which our artistic predecessors would have killed for!
For serious graphics work, at least a 17" – and preferably a 21" – non-interlaced monitor is recommended. The ability of a monitor to display colours depends critically on the graphics adapter card which drives it. To display graphics at an ideal working resolution of 1024 768 in 24-bit ‘photorealistic’ colour on a 21" monitor requires a card with 4 Mb of on-board video memory and appropriate software drivers. For many graphic design tasks, however, an acceptable compromise is a 17" screen operated at a resolution of 800 600 in 16-bit colour.
Desktop Printers and the Offset Press
Colour printing systems are based on the subtractive colour model, mixing the subtractive primaries, cyan, yellow and magenta, to produce other colours. Unfortunately, the reflective properties of printing inks are affected by impurities and experience showed that printing black, which should theoretically be possible by combining cyan, yellow and magenta, produced instead a muddy brown. To overcome this, most colour printers include black ink as a fourth ‘colour’ in the print process. As well as allowing correct printing of black, this results in improved shadow density and overall contrast. The nineteenth-century discovery of the halftone process showed how the juxtaposition of small enough dots of cyan, yellow, magenta and black inks could produce an image which, to the naked eye, would appear to produce continuous tones, colours being produced not by the physical mixing of the inks, but in the optical mixing of primary colours by the viewer’s eye. The majority of modern low resolution desktop printers use this principle, laying down dots in various ‘dither’ patterns to produce colour output ranging from crude – with the dot pattern clearly visible – to a quality verging on photorealistic.
Halftone dot pattern
In the offset process, the dot pattern is created by photographing the original artwork through a halftone screen. To separate a full colour image into yellow, magenta and cyan, it is necessary to photograph the copy three times, through filters which are the same colour as the additive primaries -red, green and blue. When the copy is photographed through the red filter, green and blue are absorbed and the red passes through, producing a negative with a record of the red. By making a positive of this negative we will obtain a record of everything that is not red, or more specifically, a record of the green and blue. The green and blue, as we have seen earlier, combine to produce cyan; therefore, we have a record of cyan. The process is repeated, using a green filter to produce a record of magenta and using a blue filter to produce a record of yellow. As each filter covers one-third of the spectrum a record of all the colours in the original copy has been created. Finally, to improve shadow density and overall contrast, a black separation is made by using a yellow filter. When printed with the subtractive colours – cyan, yellow and magenta – plus black, all the colours and tones of the original are reproduced.
Please see Chapter 3 for a more detailed description of printer types and techniques.
Separation of image into CMYK components
As the above summary shows, scanners and colour monitors use a different colour model to describe colour from that used by cameras, desktop printers and offset presses As colours move from the original image through the camera and transparency via the scanner to the computer screen and then on to the desktop proofing printer and, finally, to the printing press, they are converted from one colour model to another several times.
Colour Depth and Colour Modes
Colour depth, sometimes called bit depth, refers to the maximum number of colours which can be stored in an image file. A 1-bit file stores two colours (usually black and white) and can be described as 1-bit deep since all the information required to specify each of the dots making up the image can be stored in a 1-bit number (0 for black or 1 for white). A 2-bit file stores four colours, a 4-bit file stores 16 colours, an 8-bit file stores 256 colours and a 24-bit file stores 16 million colours. A greyscale image is an 8-bit file, with 254 shades of grey plus black and white. The greater the colour depth of an image, the more space it takes up on disk. A number of applications now use 32 bits to specify the colour of each pixel in an image. The extra 8 bits are used to describe the transparency of the pixel in 256 steps from completely transparent to completely opaque.
1-bit scan
8-bit scan
24-bit scan
A colour mode determines the colour model (see below) used to display and print compositions. The most commonly used modes are Greyscale, for displaying black-and-white documents, RGB, for displaying colour documents on the screen and for printing slides, transparencies, and RGB colour prints, CMYK, for printing four-colour separations and L*a*b for working with Photo CD images. Other modes are Bitmap, and Indexed colour.
Colour mode is specified when a new painting or photo editing process is started, but can be altered midway through the task or when saving or exporting the finished work. If the original image has many colours, and it is converted to a lower colour depth (e.g. 24-bit RGB colour to 256 colours), the file will create a palette of colours and use combinations of these to simulate the original colour of each pixel. The colours in the palette will be derived from the colours in the original image. Indexed colour files are much smaller and easier to manipulate than 24-bit files and can provide a very good colour approximation if the number of colours in the original is limited. Indexed colour images are widely used for multimedia animation applications. Another common reason for changing the mode of an image – from RGB to Greyscale – would be to preview the work before printing to a monochrome printer.
| SHADES VERSUS BIT DEPTH | ||
| 1 bit | 21 | 2 shades (B or W) |
| 2 bit | 22 | 4 shades |
| 4 bit | 24 | 16 shades |
| 8 bit | 28 | 256 shades |
| 16 bit | 216 | 65 536 shades |
| 24 bit | 214 | 16 777 21 6 shades |
Monitors and graphic display cards vary widely in their capacity to display colour. At the most basic level, a monochrome display and its card can only vary the moving spot on the screen between black or white, displaying a 1-bit image. A low-end colour monitor/card combination can display images 8 bits deep, i.e. made up of 28 or 256 colours. Moving up the range, an image which approaches ‘photorealistic’ colour requires 8 bits of information for each of the three primary colours, red, green and blue, making it 24 bits deep. If converted to CMYK format, the same image becomes 32 bits deep, as 8 bits are required now for each of four colour channels.
Colour Models
Each device type is associated with a specific colour space – an imaginary three-dimensional space enclosing all the colours which the device is capable of reproducing and defined by means of a coordinate system. There are several digital colour models which can be used to define these colour spaces. Such models, which, like colour matching systems, are supplied with most drawing and painting applications, provide an interactive means for the designer to explore colour space and to specify colours for a project with great accuracy. Two of them (the RGB and CMYK models) also describe the means by which mechanical devices reproduce colour.
The RGB Model
An additive colour model in which three primary colours of light (red, green and blue) are combined in varying intensities to produce all other colours. An additive colour model is used for any colour system which mixes light to generate colours, including monitors, desktop scanners and film recorders.
RGB model
The CMYK Model
A subtractive colour model producing colour when light is reflected off an object or surface. The reflected light determines what colour we see when we look at that object. A perfectly white surface reflects all wavelengths of light. A black surface absorbs all wavelengths. The three primary colours in the subtractive colour model are cyan, magenta and yellow. In theory, combining all three primaries produces black. In practice, impurities in the ink pigments degrade the black to a muddy brown, as mentioned earlier. To resolve this, black is added to the model. The K designation represents the black component of the CMYK model. This is the model used for colour systems which use reflected light to generate colours, including desktop printers and the printing presses.
CMYK model
The HSB Model
This mode approximates the way in which the human eye perceives colour. Colour is defined by three components – hue, saturation and brightness. Hue refers to the name of the colour, for example red. Saturation defines the intensity of the colour, i.e. how vibrant the colour is. Brightness defines the lightness or darkness of the colour.
The HLS Model
Similar to the HSB model, the HLS model contains three components – hue, lightness and saturation. The lightness component is similar to the brightness component in the HSB model. Hue and saturation are the same as in the HSB model.
L*a*b Model
Based on the original CIE (Commission Internationale de l‘Eclairage) model, the L*a*b model is based on the way the human eye perceives colour. It contains a luminance (or lightness) component (L) and two chromatic components – the ‘a’ component (green to red) and the ‘b’ component (blue to yellow).
The Digital Palette
For many centuries, a tedious but essential part of the preparation of the traditional artist before beginning a new project was the mixing of paints, from the limited number of pigments available. Once mixed, the colours could be applied directly to the canvas or other medium selected for the work in hand, or could be first blended, lightened or darkened on the surface of the artist’s palette. Testing how a colour, or blend of colours, would appear on canvas was a simple matter of applying paint to a test sample.
Custom Pallete
Colour blending dialogbox
Color mixing Pallete
By comparison, the digital artist is spared the tedium of mixing colours and enjoys the advantage of having literally millions of choices, selectable from any of the colour models described above. Specific colours selected from any of these models can also be stored on a custom palette which can be saved with the project in which the selected colours have been used; this ensures ready availability of the correct colours if further work on the project is necessary, or if the same colour set is required for use on a related project. Such palettes can also include selected spot or process colours dragged from any of the colour matching systems described below, as well as colours mixed using either a colour blender or mixed by hand. While the colour blend is limited to a maximum of any four colours in the blend, the mixing area is unlimited in the number of colours used. The mixing area emulates an artist’s palette on which colours can be blended using a brush tool. By varying the blend setting in the mixing area, extremely subtle variations in colour can be achieved. Any bitmap can be loaded into the mixing area, permitting further choice of colours from photographs or drawings.
For projects being composed for the screen – for example, those to be transmitted via the Internet for viewing on the screens of other users – the designer can be reasonably confident that the colours seen on the screens of the other viewers will closely approximate those used to create the original (such variations that do occur will be caused by slight variations in phosphors used by different screen manufacturers, variations in brightness and contrast adjustments from screen to screen, variations in background lighting conditions and so on).
For projects being composed for printing to a desktop printer, or for separating and printing on a four colour offset press, the situation is very different. As we have seen, within the digital publishing process, colour is device-dependent, i.e. the output colour at each stage of the process depends on the device (scanner, monitor, printer, or press) which produces it, and device colour output is based on different models (scanner and monitor colour output being additive, while printer and press colour output are subtractive). To make matters worse, the devices involved have progressively diminishing colour-reproduction capabilities. The human eye discerns a wide colour spectrum, while a colour monitor displays only a fraction of those colours, and a desktop printer or printing press reproduces even fewer. The colour gamut -the range of colours which can be reproduced – of each device is provided by the manufacturer in a file called a device profile. Colour publishing, therefore, presents the designer with something of a challenge!
Fortunately, help is at hand and the challenge can be met in different ways, depending on the nature of the design project, by using colour matching systems and/or colour management systems, which are explained below.
Comparision of spectral ranges
Pantone spotcolour dialogbox
Focoltone spotcolour palette
Colour Matching Systems
Spot Colours
The principle behind colour matching systems is most easily explained by using a simple example like spot colour. Spot colours are opaque printing inks created by ink manufacturers like Pantone in an assortment of hundreds of predefined, pre-mixed hues. The designer (and client, where appropriate), can select spot colours to be used in a project from a swatch book of samples provided by Pantone. The designer then simply selects the agreed colours on screen from a proprietary Pantone Spot Colour Matching System (supplied with most drawing or painting applications). By selecting and assigning a spot colour (e.g. Pantone 507 CV) to elements of a composition and by specifying the corresponding Pantone 507 CV ink for colour printing the same elements in the final work, the designer can be assured that the colour will print as it appears on the swatch, regardless of how it appears on screen or on the output of a proofing desktop printer. (Note that the palette dialog box displays the CMYK values corresponding to the spot colour selected, which implies that the same result could be achieved by four colour printing, but overlaying opaque spot colours can produce unpredictable results and is not recommended.) Once a spot colour is selected in the palette dialog box, a tint of the colour can be selected by choosing a percentage in the Tint window. The range of spot colours includes some – e.g. gold or silver -which could not be reproduced by combining the four basic CMYK inks.
FOCOLTONE is an alternative spot colour system which provides a range of spot colours built from the process colours, cyan, magenta, yellow and black. The colours in the palette are organised so that the user can easily choose pairs of colours with at least 10% of a process colour in common, minimising the need for trapping and making it an ideal system to use for colour separation.
Swatchbook
Pantone process colour dialog box
Process Colours
Process colour is the collective name given to those colours which can be created by combining the four standard printing inks – cyan, magenta, yellow and black. Process colour inks are largely transparent, incident light passing through them and then reflecting back off the paper into the eye of the viewer. This transparency is what makes four colour printing possible and predictable. Like spot colours, process colours can be pre-selected from a paper swatch and then specified by the designer, using a process colour matching system. When a colour is selected from the Pantone System palette – e.g. Pantone S146-2 – the CMYK percentages which will be used to print the colour are displayed below the colour name. Other proprietary colour matching systems based on process inks include the following.
TRUMATCH (the palette of which organises colours according to the principles of the Hue, Saturation, Brightness model)
DuPont’s SpectraMaster, with colours based on the L*a*b colour space, printed by means of colours available through the DuPont solid colour library
Toyo, which consists of colours available through the Toyo Colour Finder system. These colours are defined using the L*a*b colour space and are shown as CMYK for display. The colours offered include those created using TOYO process inks
Dainippon’s DIC palette, which is arranged into categories – gay and brilliant, quiet and dark, greys and metallics, basic – available through the DIC Colour Guide and created by mixing DIC brand inks
So, when the designer is preparing a four colour printing job, specifying colours from a process colour matching system will ensure that these colours print as expected during the subsequent print run. The use of specified process colours ensures that a chosen colour is never out of gamut and helps to ensure consistency of colour reproduction within a publication and from one publication to another.
Colour Management
While a colour matching system gives the assurance that selected colours will print correctly on an offset press, it does nothing to help with the problem of those colours appearing quite differently on screen or when printed from a desktop proofing device. Neither does it do anything for the accurate colour reproduction of bitmapped images – e.g. scanned photographs or images created in a painting program – to be included within a project. A desktop system typically includes a scanner, a monitor and one or more printers. As stated earlier, these devices do not reproduce colour consistently from one to the next, each device reproducing or displaying a limited set of colours called its colour gamut. Also devices from different manufacturers will display different colours for the same digital colour data; even two devices of the same model may display subtle colour differences using the same colour data.
Colour Management Systems
Colour management systems are designed to address the problem of device variability, adjusting the colour relationships between devices to ensure consistent colour throughout the publishing process. A CMS translates colours from the colour gamut, or colour space, of one device into a ‘neutral’ device-independent colour space, and then fits that colour information to another device’s colour gamut by a process called colour mapping. The CMS obtains the colour characteristics of each device from its device profile. In one method, the relationship between colours is preserved as they are shifted into the device’s colour gamut. In another method, only the out-of-gamut colours are replaced by colours that the device can produce, without preserving the relationships between the colours.
Profiles for the most popular devices are usually supplied with the CMS software and those matching the devices on the user’s system are installed at the same time as the CMS software. Profiles for other devices are usually supplied with the device installation software. Manufacturer’s device profiles are based on a particular set of calibration settings for a given device. To use a colour management system effectively, devices first have to be calibrated to match the expected performance defined in the device profile. The quality of the final result depends on how well the devices match their profiles.
Calibration
There are various methods and techniques used for calibration of devices, the necessary instructions and software often being bundled with a CMS or with individual application programs. The following provides an overview of the calibration process.
Since a scanner’s light detectors are affected by prolonged use, the RGB output signals will vary over time, affecting colour balance and linearity. This means that updating the scanner’s profile is needed, from time to time, by recalibration of the scanner. Scanner calibration requires a target sheet of colour swatches and a data reference file, both supplied by a vendor. The target sheet is first scanned to produce a TIFF file of the scanner’s output, and software then compares the values in the TIFF file with the values in the data reference file. Any differences which are detected are then used to update the profile for the scanner.
loupe – useful for close inspection of printed colour
Scanner target sheet
Monitor calibration dialog box
Like the output of CCDs in a scanner, the output characteristics of a monitor’s phosphors can change with time. Monitor output is also subject to a second set of variables related to environmental conditions, like the characteristics of ambient lighting. Calibration of a monitor involves adjust-ment of the gamma values for red, green and blue, and the white point value. The chromaticity may also need to be adjusted, but this is generally not required. Adjustments can be made by sight, comparing output to a target photograph, or can be made with the use of measurement devices such as a colorimeter or a spectrophotometer.
During normal use, the output of a printer will vary as colorants – for example, inks or toners – are consumed. Colorants also vary slightly in their purity from batch to batch. To compensate for these factors, occasional recalibration of the printer is needed to keep its profile up to date. The simplest calibration method involves printing a target file and scanning it through a calibrated scanner, providing the information needed to update the printer profile. A more accurate method involves printing a target sheet and then measuring each colour swatch on the sheet with a spectrophotometer. The results, which are typed into a measurements file, are used to adjust the TAC (Total Area Coverage) and K-curve (black Keyline curve). Software controls adjust the amount of ink transferred to the paper (TAC) and the amount of grey component replacement or undercolour removal (K-curve shape).
Spectronic Genesys spectrophotometer
The Work Environment
The way colour appears on a monitor or on the output from a proofing printer is influenced by factors in the work environment. For average, day-to-day colour projects, these factors are not critical, but if high quality, accurate and consistent colour reproduction is to be assured, then certain precautions need to be taken.
Control of workstation environment is important in ensuring consistency of results
The walls and ceiling of the work area should be painted in a matt, neutral coloured emulsion such as pale grey to minimise any interference by the background with the perception of either monitor colour or printed colour.
Ambient lighting should be controlled. Changing sunlight through windows will change the way colours appear on screen. Artificial lighting – ideally 5000 K lighting – provides a more consistent ambient, eliminating the yellow cast from normal fluorescent lighting. The light intensity should be comparable with that of the monitor.
Operating systems for the PC or the Mac permit the use of patterns and bright colours on the virtual desktop. Use of these should be avoided as they may interfere with accurate perception of the colours in a working project.
Installing a Colour Management System
The simplest procedure for installing a CMS involves simple step-by-step guidance such as that provided by Corel’s CMS ‘Wizard’. Prompted by a series of Wizard screens, the user has only to respond to step-by-step guidance:
CorelDRAW’s CMS Wizard provides help with the calibration process
Scanner caliberation
Monitor caliberation
Printer caliberation
Choose one from three alternative colour mapping methods. (i) Photographic mapping which maintains the relationship between colours and is recommended for printing photographs and illustrations with continuous tone, (ii) Saturation mapping which expands or contracts the source gamut to fit the destination gamut and is recommended for printing business graphics, (iii) Automatch, which automatically detects the type of image to be printed and selects Photographic or Saturation mapping accordingly.
Choose a scanner profile from a dropdown list to match the scanner in use. The scanner profile is needed so that the CMS can measure the variance between the scanner’s output and a set of fixed reference values. This is essentially software calibration of the device to a standard.
Choose the appropriate monitor manufacturer and model number from the lists displayed in order to select the corresponding monitor profile.
Finally, choose the appropriate output printing device from a list displayed to set up the correct printing profile. After this final choice is made, the CMS proceeds to set up a system profile based on the choices made.
In circumstances where more than one scanner or more than one output device is being used, different system profiles can be created for each combination. The appropriate profile is then selected at the start of each new project. After creating a publication using a CMS, original photographs, proofs and the final printed publication can be used as references to assess how well the process has worked and to indicate the need for any further fine tuning. (At each stage of the process described above, the CMS provides interactive means of making adjustments to the individual device profiles.)
CMS and Kodak Photo CD
One of the most important sources of high quality photographs for use by the graphic designer is the Photo CD, which is based on a process developed by Eastman Kodak. The process converts 35 mm film negative or slides into digital format and stores them, in the Photo CD Master format, on a CD in a range of five different resolutions (the Photo CD Master Pro format has six) ranging from Poster size – 2048 3072 pix els – through Large (1024 1536 pix els), Standard (512 768 pixels), Snapshot (256 384 pix els) to Wallet (128 192 pix els). Using a utility such as Corel’s Photo CD Lab, once an image has been loaded into a viewing area, it can be ‘pre-processed’ by making selections from a menu covering rotation angle, resolution, number of colours and format, e.g. BMP EPS, PCX or TIF.
Kodak Photo CD dialog box
Corel’s Photo CD Lab dialogbox
Picture Publisher’s Photo CD dialog box
Because of the popularity of the format, many applications now include a CD reader which can access Photo CD files directly. In the Picture Publisher reader, for example, a dialog box allows specification of parameters for the image to be opened. The Corel reader provides two alternative colour correction methods – Gamut CD and Kodak – to permit colour correction to a Photo CD ROM image before importing it into PHOTO-PAINT or CorelDRAW. GamutCD uses gamut mapping to enhance the colour fidelity and tonal ranges of the image which ensures that the colours in the image can be reproduced by a printer. Kodak Colour Correction allows adjustment of brightness, contrast, colour tints and colour saturation.
Before importing a CD, most readers provide the option of applying colour management to the image to ensure that it will print correctly to the specified output device. Many Photo CDs come with a device profile for the scanning device used to create the bitmap images on the Photo CD. This profile should be specified as the CMS source when importing.
Applying Colour
As we have seen, digital designers and artists have at their disposal a vastly greater choice of colours than were available to their traditional counterparts. As the earliest digital applications emerged, the objective of the developers was to provide tools and techniques which allowed application of colour in ways which mimicked the traditional pencil or pen, the only variation possible being the thickness of the stroke. From these early beginnings, the ingenuity of developers – and the healthy competition which exists between them – has extended the range of tools and techniques dramatically in the matter of only a few years, as the following summary shows.
Applying Colour in Drawing Applications
Lines
After using a line tool to create a straight, freehand or Bezier line and setting its width, colour can be selected from any of the spot or process colour matching systems described earlier or indeed from any of the colour models or mixers. Using the HLS model, tints, shades or tones can then be applied. Macromedia Freehand makes the job of selecting tints even easier by providing a convenient tint option within its colour mixer box; the tint required can be applied by simply be dragging and dropping the required tint swatch on to the line to be tinted. As well as providing means for colouring and tinting lines (a specific tinting dialog box is provided for Pantone spot colours), Micrografx Designer also allows the application of vector hatching, gradient, bitmap textures or object line fills to any line.
Fills
As with lines, any closed shape can be filled with solid colour selected from a matching system, model or mixer; tints, shades or tones can be applied if required. In addition, CorelDRAW provides fill options in the form of two colour bitmap patterns, full colour bitmap patterns, vector patterns, textures, gradients or PostScript fills, but the designer has to be aware that these fills do not rotate with the object filled. Micrografx fills – as described for lines above – do rotate when the object filled is rotated. Freehand provides a selection of patterned, graduated and PostScript fills, but only the graduated fills rotate with the object; additionally, Freehand provides a tiled fill using any object copied to the clipboard as the basis of the tile.
Blends
Colour blends can be created between open or closed paths, using the same fill and stroke type for each path. For example, a path using a graduated fill will not blend with a path using a radial fill. It is important to use a valid colour combination; if two spot colours or a spot colour and a process colour are blended, intermediate colours will print successfully on process colour separations. Choice of the number of steps used in a blend depends on the printer resolution (higher resolution, more steps) and on the colour change between the two selected paths. As well as being useful as a technique for transforming one object into another, blending is a very effective way of creating ‘spot’ highlights and shadows to give objects depth.
CorelDRAW′ Special fill dialogbox
Using Blend to create highlights
Photoshop’s Scratchpad
Applying Colour in Painting Applications
Lines
Before a line is created in a photoediting or painting application, using any one of a variety of line tools, colour is first selected from any of the spot or process colour matching systems or colour models described earlier. Alternatively, a scratchpad – such as the one provided with Photoshop – can be used for mixing a custom colour before application. Using the HLS model, tints, shades or tones of a hue can be can selected before application. The opacity of a stroke can also be defined by adjusting an opacity slider. Using a pressure sensitive stylus, pressure can be set to control stroke width, colour or transparency, or a combination of these, creating strokes with smoothly changing properties.
Picture Publisher’s Brush Style Dialogbox
Painter’s Brushes
Experimenting with brush looks in Painter
Applications like PHOTO-PAINT and Picture Publisher offer a wider range of stroke types for applying colour than does Photoshop. Picture Publisher, for example, includes styles like Brushed Oils, Chalk, Colorizer, Crayon, Distort, Dots, Marker, Oil Pastels, Oil Paint, Scatter, Smudgy Marker and Watercolour, but the application offering the graphic artist the widest range of colour application styles is still Fractal Painter. As well as emulating the widest range of drawing and painting tools, Painter makes it possible to apply colour and texture simultaneously to the working surface. A range of variables can be applied to the tools to produce a wide range of different effects. A dialog box contains sliders for control of brush size, opacity and percentage ‘graininess’ of each stroke. Another Painter dialog box provides the means of creating and saving special brushes. Using Painter’s Nozzles feature, it is even possible to paint with images!
Adjusting brush parameters in Painter
Painting with images – in this case, the image of a leaf
Fills
A standard Fill tool provides the means of flooding a bounded area with any of the range of colours, tints, tones or shades selected, in the same way as for lines. The boundary of the enclosed area can be created with a pencil or brush tool, or defined by a mask. Reducing the opacity setting before applying a fill to an object allows underlying objects to show through. Gradient fills are also offered by most painting applications. Photoshop provides several options – linear or radial fills from foreground to background colour, with controls to adjust the gradient midpoint (linear fill) or offset (radial fill) as well as clockwise or counterclockwise gradients between two points on the colour wheel.
Adjusting gradient options in photoshop
More complex gradients can be created in Photoshop, of course, by joining two or more two colour gradients back to back, but some applications, like MetaCreations’ KPT Gradient Designer or Painter, provide a library of more complex gradients which can be used as they are, or edited, using tools provided. Using an interesting feature provided by Painter, an image can be filled with a gradient in such a way that the image’s luminance values are replaced by the gradient’s luminance values. Painter also allows the user to create new gradients by capturing colours from an existing image – e.g. a photograph of a sunset – or by using a range of colours produced with Painter’s tools and colour sets.
KPT Gradient Designer dialogbox
Painter’s Gradient fills
Linear sepia gradient applied using image luminance in Painter
In addition to colour fills or gradient fills, some applications provide the facility to fill specified areas with patterns or textures. Preset patterns may be provided via a menu, like that of Painter’sWeaves or Picture Publisher’s Textures, but more often the user selects an image or part of an image, using a mask to create a pattern ‘tile’, and then fills a specified area with tiles.
Selecting Weaves in Painter
Painter’s Patterns
Medical research shows that creative brain activity occurs in the right hemisphere of the brain
1. Defininga file in Photoshop
2. Painting with the tile
Colour Combination Techniques
The effects which can be produced by the various line and fill tools described so far in this section on applying colour have parallels in the realm of traditional graphic design and are largely intended to allow the digital designer to mimic the traditional style of working. The use of colour combination techniques, on the other hand, represents a crossing of a boundary into the domain of digital manipulation, in which the intrinsic properties of digital colour are exploited to produce effects which would be difficult or even impossible to achieve by traditional methods. While the design process predominantly takes place on the right side of the brain – the creative side – certain aspects of digital design, such as the use of colour combination techniques, requires also the use of the left, or logical, side. Each pixel in a composite RGB colour image is defined by the values assigned to each of its three channels, with these values varying between 0 (black) and 255 (white). It is the ability to alter the values in these channels in a precisely controlled way which opens the door to a range of new possibilities.
By applying mathematical ‘operations’ to the channels in an image, the way in which the applied colour interacts with the underlying colour can be controlled. In Picture Publisher, for example, these operations – called Merge Modes -allow the user to combine, or mix, colours using additive or subtractive colour theory. An image can also be changed selectively according to hue, saturation, or lightness, while other modes make modifications to the red, green, or blue channel of an image.
Using the Additive mode, the applied colour is mixed with the underlying colour according to the additive colour model. Painting a green image – R(0), G(100), B(0) – with a blue brush – R(0), G(0), B(100) – produces cyan in the image – R(0), G(100), B(100) – as a result of the additive mixing of green and blue.
Using the Subtractive mode, painting on a cyan image with a magenta brush produces blue, according to the subtractive colour model.
The If Lighter mode is used to edit an image based on the lightness values of the image and the lightness value of the applied colour. Lightness refers to the ‘L’, or lightness value, in the HSL colour model. If the applied colour has a lightness value equal to or higher than that of the image, the applied colour is transferred to the image. If the lightness value is less than that of the image, no change occurs.
The If Darker mode is used to edit an image based on the lightness values of the image and the lightness value of the applied colour. If the applied colour has a lightness value lower than that of the image, the applied colour is transferred to the image. If the lightness value is not lower than the image, no change occurs.
The Multiply mode multiplies the value of the image by that of the applied colour. The resulting colour is always darker. The effect is analogous to placing a coloured transparent film over the underlying image.
The Filter mode uses a combination of Additive and Multiply to create a filtered effect.
The Difference mode subtracts the value of the applied colour from the value of the underlying colour to produce a new colour.
The Texturize mode converts the paint colour to greyscale, then multiplies the greyscale value by the image colour.
Creating texture on the right side of this image using Texturize mode
The Colour mode is used to replace the colour of an image with the hue and saturation values of the applied colour, leaving the lightness value unchanged.
The Hue mode is used to replace the hue value of an image with the hue value of the applied colour, leaving saturation and lightness values unchanged.
The Saturation mode is used to replace the saturation value of an image with the saturation value of the applied colour. Using this mode, painting with white or black (which have zero saturation) alters the underlying colours to their equivalent greyscale values.
The Luminance mode is used to replace the lightness value of an image with the lightness value of the applied colour.
Replacing the saturation value on the right side of this image using magent a as the applied colour in Saturation mode
The Red mode is used to replace the red channel (using the RGB colour model) of an image with the red value of the applied colour. Only the red channel is affected.
The Green mode is used to replace the green channel of an image with the green value of the applied colour. Only the green channel is affected.
The Blue mode is used to replace the blue channel of an image with the blue value of the applied colour. Only the blue channel is affected.
The Invert mode is used to reverse the colours of an image. A black-and-white image reverses to look like a photo negative. A colour image reverses using additive colours.
Invert mode has been used to invert the right side of this image
PHOTO-PAINT provides a set of Merge modes, similar to those of Picture Publisher, while Photoshop provides a set of Mode Options, some of which are similar to Picture Publisher’s, while others differ, offering results sometimes difficult to predict, so that experimentation is recommended:
The Screen mode multiplies the inverse brightness values of the pixels in both channels. The resulting colour is always a lighter colour.
The Overlay mode performs a combination of multiplying and screening. Applied colours are overlaid on the existing pixels but the highlights and shadows are maintained.
The two channel pixels are mixed to reflect the lightness or darkness of the original colour.
Applying Soft Light mode
The Soft Light mode multiplies or screens the pixels in the two channels. It produces the effect of shining a diffused spotlight on the image.
The Hard Light mode multiplies or screens the pixels in the two channels. It produces the effect of shining a harsh spotlight on the image.
Any of the above modes can be applied, by means of editing tools such as a paintbrush or fill tool, to modify an underlying image. Alternatively, they can be used when combining two images to produce a third image. This is achieved in Photoshop by selecting Calculations from the Image menu. The effects produced by Photoshop’s Calculations can be summarised as follows: When working with composite images, Adobe Photoshop calculates the pixel values in each set of colour channels and then combines them into a single channel in a third, composite, image. The combining process can be applied while using any of the merge modes described above.
Image I
Image 2
The images combined using Photoshop’s Hard Light mode
Photoshop’s Calculations dialog box
Colour Editing
Colour editing techniques are found mainly in bitmap painting applications, as these applications give the user access to individual pixels within an image. The same is not true of vector images, which are defined in terms of lines and shapes; however, thanks to the ingenuity of software developers, even vector applications are beginning to offer colour editing techniques, some of which are described below.
Colour Editing in Vector Drawing Applications
Lenses
CorelDRAW provides a number of ‘lenses’ which can be used to create interesting colour effects.
Applying the Transparency lens to an object effectively fills the object with a tint of the colour selected in the Lens dialog box, while at the same time making the object partially transparent (transparency increases with the Rate percentage selected in the dialog box).
CorelDRAW’s Lens dialog box
When the Brighten lens is applied to an object and the object is placed on top of another object or bitmap image, it brightens the underlying object to an extent determined by the Rate percentage. Because of the accuracy with which vector objects can be drawn and positioned, this can be an effective method for moderating the brightness of targeted parts of a bitmap.
The Invert lens works like the Brightness lens, but in this case the colours in the underlying objects are inverted, red becoming cyan, green becoming magenta etc. This lens simulates the effect of a colour filter on a camera.
The Colour Limit lens filters out all colours under the lens except black and the colour specified in the Colour dialog box. For example, if a blue lens is placed over an object, it filters out all colours except blue and black within the lens area. The strength of the filter is set by the value specified in the Rate box. A rate of 100% would only allow blue and black to show through. A lower setting would allow tints of the other colours to show through.
Applying the Invertlens
The Colour Add lens mixes the colours of the lens and objects underlying it.
Applying the Colour Add lens
The Tinted Greyscale lens causes the colours of objects under the lens to be mapped from the lens colour to an equivalent tonal colour of that lens. For example, a green lens over a light-coloured object creates light green, while the same lens over a dark-coloured object creates dark green.
The Custom Colour Map lens maps underlying object colours to colours using a colour range specified in the dialog box. The Heatmap lens maps underlying object colours to colours in a pre-defined Heatmap palette, creating a (rather garish) heatmap or infrared look.
Freehand’s Eyedropper tool
Freehand provides a tool which is very useful when a project requires matching colours from different sources, e.g. matching the colour in line art created in Freehand with colours in an imported bitmap. Using the tool simply involves dragging the required colour from the source to the destination.
Freehand’s Xtra Tools
Using the Eyedropper tool to drag a colour from a bitmap in to a vector star object
Colour Editing in Bitmap Painting Applications
Colour masking
Detailed and sophisticated editing of colour images is made possible by tools which enable the user to select portions of an image based on the colour similarities of adjacent pixels. The Magic Wand tool, found in most bitmap applications, provides a simple way of achieving this. Using it simply involves clicking the tool in the toolbox, entering a tolerance value in the Magic Wand Options palette (a low tolerance value to select colours very similar in colour value to the pixel clicked and a higher tolerance value to select a broader range of colours) and clicking the target colour in the image.
Setting the Magic Wand tolerance
The ubiquitous Magic Wandicon
Photoshop’s Color Range command selects a specified colour within a selection or within an entire image. A colour can be chosen from a preset range of colours, or a selection can be built by sampling colours from the image using the eyedropper tool in the dialog box. Alternatively, highlights, midtones or shadows can be selected. An initial selection can be modified by clicking OK and then reopening the Color Range dialog box. In the central window of the dialog box, Greyscale displays the selection as it would appear in a greyscale channel, Black Matte displays the selection in colour against a black background, White Matte displays the selection in colour against a white background, while Quick Mask displays the selection using the current Quick Mask settings. The range of colours selected can be adjusted by using the Fuzziness slider or by entering a value in the Fuzziness text box. Increasing fuzziness increases the range of colours selected. Plus or minus eyedroppers in the Color Range dialog box can be used to add or delete colours from the selection.
Photoshop’s Color Range dialog box
Picture Publisher’s Color Shield operates in a similar way to Photoshop’s Color Range. Clicking the Color Shield dialog button opens the dialog box shown, containing eight ‘shields’, each with its own colour range. Clicking the Color Select button alongside the first shield activates it and displays an Eyedropper tool which is used to click the first image colour to be shielded. Further colour ranges can be added by sequentially clicking additional shields and selecting additional image colours.
Picture Publisher’s Color Shields
Once a colour range has been selected, it can be colour-edited using a brush tool or fill tool and any of the colour application methods described above.
In any masking operation, a challenge for the designer is to produce changes which appear natural, without sharp edges when the mask is removed. This can be accomplished by feathering the edges of the mask and Picture Publisher’s Chroma Mask provides the facility to combine colour masking and feathering operations. Using the Chroma Mask dialog box, areas of an image can be masked as described for the Color Shield, while the Fade setting at the bottom of the dialog box determines the smoothness of the edges of the mask, creating a natural blending between the masked object and the background.
Picture Publisher’s Chroma Mask dialog box
Corrective Colour Editing
The most common corrective editing task is the removal of a colour cast from an image. A colour cast – an imbalance between the red, green and blue components of an image -can result from several causes such as photographing a subject under coloured lights or due to the fading of colours in an old photograph by the effects of sunlight. An alternative cast removal method provided by Photoshop is called Variations, which may be selected from the Image/Adjust menu. Colour adjustments can be applied sequentially to a thumbnail preview of the image and assessed visually until an acceptable result is obtained. A cast can be removed by selecting the appropriate red, green or blue channel in the Levels dialog box and adjusting the gamma setting. If a set of images with similar casts is to be corrected, the dialog box settings can be saved and reapplied to the other images.
Using Variations toremove colour casts
Another common colour editing task is the removal of ‘red eye’ – the red colour which appears in the eyes of a photographed subject, caused by red light from the camera’s flash being reflected from the eye back through the camera lens. Usually, the part of the eye that reflects red should be black and can easily be corrected by choosing a high image magnification and using single pixel sized editing brush to paint the appropriate pixels black.
Photoshop’s Levels dialog box
Removing red eye
Creative Colour Editing
As well as methods for correcting colour defects in images, bitmap applications provide a range of techniques for more creative colour editing. Picture Publisher’s Hue Shift command, based on the Hue, Saturation and Lightness (HSL) colour model, allows all the hues in an image to be manipulated. Hue is specified by a angular value ranging between 0° and 360°, corresponding to the colours on the colour wheel. When an angle is specified in the Hue Shift dialog box, all hues in the image are rotated by the same amount, effectively changing all the colours in an image. The dialog box also includes sliders for adjustment of the saturation and lightness of the image.
Hue Shift in Picture Publisher
Also using the Hue, Saturation and Lightness (HSL) colour model, Picture Publisher’s Hue Map command allows selected ranges of hues in an image to be changed. For this purpose, the HSL colour wheel is divided into twelve ranges, each range representing 30 of the 360 hues. A range is shifted by moving its corresponding slider. Hue Shift is useful for changing a single colour in an image without affecting other colours. Hue Map can also be used to colorise a greyscale image; after converting the greyscale image to RGB mode, the skin area was masked (leaving out the eyes and mouth) and the Hue Map was opened. On the top and bottom of the sliders are colour swatches. The lower swatch is the original hue, while the upper swatch is the new hue. Setting the saturation level to +20%, the first hue slider was dragged down until the masked area became flesh coloured. The dialog box’s Saturation and Brightness sliders were then used to make fine adjustments.
Picture Publisher’s Hue Map
The Colorize option in Photoshop’s Hue/Saturation dialog box can be used to convert all the colours in the image to the 0° point on the colour wheel (red), with a saturation of 100%, while preserving the lightness value of each pixel. Dragging the hue slider then cycles the hue around the colour wheel. For example, if the hue slider is dragged to 120° then the image takes on a green cast since green is the colour located 120° degrees in the clockwise direction from red.
Using Hue Map to colorise a greyscale image
Monotones, duotones, tritones and quadtones can be created in Photoshop. Monotones are greyscale images printed with a single, non-black ink, while, duotones, tritones and quadtones are greyscale images printed with two, three and four inks, respectively. In these types of image, different coloured inks are used to reproduce different levels of grey rather than to reproduce different colours.
Photoshop’s Hue saturation dialogbox
Colorising an image using the settings shown in the dialog box above
A typical offset printing press can reproduce only about 50 levels of grey per ink, therefore duotones are often used to increase the tonal range of a greyscale image, using a black ink for shadow detail and a grey ink for the midtone and highlight areas. Duotones may also be printed using a coloured ink for the highlight colour, producing an image with a slight tint and significantly increased dynamic range. Duotones can be used to extend the range of graphic possibilities for inclusion within a two-colour print job. Tritones and quadtones may be used to introduce even greater tonal range of a greyscale image or to add even more subtle coloured tints. For the designer in a hurry, Photoshop also includes an extensive library of presets which can be applied to any greyscale image.
Converting the monotone image on the left to the tritone image on the right using the settings in Photoshop’s dialog boxshown below
Both Photoshop and Painter provide the means of adding the effect of coloured lighting to a composition. Photoshop provides sixteen different lighting presets, selectable from the Style menu at the top of the Lighting Effects dialog box. Sliders are provided for adjustment of the light intensity and spread, and clicking on the swatch to the right opens a colour dialog box for selection of a colour for the light. Clicking the Preview button (bottom left of the dialog box) shows the effect of the chosen light parameters on the image. An ellipse shows the spread of the light and handles on the ellipse can be dragged to alter the light’s spread and position. Using the four lower sliders, the ‘properties’ of the light can also be adjusted to relate correctly to the nature of the object or scene being illuminated.
The Gloss property determines the reflectance of the surface on which the light is shining, varying from Matte to Glossy.
The Material property determines whether the light or the object colour has more reflectance. Plastic reflects the colour of the light, while Metallic reflects the object colour.
The Exposure property lightens or darkens the light, positive values adding light, negative values subtracting light.
Applying Lighting effects in Photoshop
The Ambience property diffuses the light as if it were combined with other light in the room, such as sunlight or fluorescent light. The slider varies the ambience from Positive (increasing the effect of the light source) to Negative (diminishing the effect of the light source).
The colour of the ambient light appears in the colour swatch and can be altered by clicking on the swatch. An additional light or lights can be added to the scene by dragging the small lightbulb icon at the bottom of the dialog box into the preview area. Once positioned, the parameters of the additional light can be adjusted as required. Quite complex effects can be built up by using a combination of lights and by using masks to control the areas of the image affected.
Painter’s Apply Lighting dialog box
Painter offers thirteen lighting presets, selectable from its Apply Lighting dialog box. Each light is represented in the preview window as a line, indicating the direction of the light, with circles at each end. Dragging the large circle moves the light source; dragging the small circle changes its direction. An additional light can be created by clicking in the preview area. Sliders provide control over brightness, distance, elevation, spread and exposure of the selected light. (Photographic principles apply to editing lighting. For example, if light intensity is increased then exposure may need adjustment.) The Ambient slider controls the surrounding light in an image. As in Photoshop, light or colours may be changed by clicking on the appropriate colour swatch.
Opening an image in Picture Publisher in Low Resolution mode
Working with High Resolution Colour Images
A standard feature found in both drawing and painting applications allows the designer to specify the size of the ‘page’ or ‘canvas’ before work begins and even to increase the size while work proceeds. A very important difference between the two types of application emerges, however, when such a size increase takes place. The file size of a drawing created at, say, standard A4 remains the same when the drawing is scaled up to a new page size of, say, A2, since the mathematical information needed to describe the components of the drawing are independent of size, and – even more significantly – the resolution or sharpness of the drawing remains the same when it is scaled. By contrast, the file size of a colour A4 painting scaled to A2 size would increase by a factor of four – in proportion to the area increase – as the new size would require four times as many pixels to describe it. Also, when a drawing image prints, it does so at the resolution of the printing device, while the printed resolution of a painted image depends on the resolution at which it is created – the usual rule of thumb being to create the painting at a resolution equal to double the line screen to be used for printing, e.g. at 300 dpi for a line screen of 150 lpi. This difference in behaviour creates a major challenge for the designer working with large colour bitmapped images such as scanned photographs or paintings. Even an A4 sized RGB image scanned at 300 dpi equates to a file size of 29.7 Mb. Manipulating such an image on the screen and applying effects to it – such as filters – is beyond the capabilities of the average desktop system. Fortunately, however, help is at hand!
Even importing a multimegabyte colour image into a painting program is a highly RAM-intensive process. To allow users with limited RAM to open large files, Picture Publisher offers a Low Resolution mode which allows the user to open a TIFF image at a lower resolution than it was saved at. For example, an image to be used only as part of a screen presentation can be opened at the resolution of the monitor screen. Choosing the Low Resolution option opens a dialog box for choosing the lower resolution. This dialog box displays the file size for each resolution chosen. Even when the system is powerful enough to work with the full resolution image, a low resolution version of the file can be used to test general changes such as adjustments to hue and saturation. Because the file resolution is low, processing such changes is faster. When the required changes have been established, a macro (a script file containing details of the change) can be recorded and then applied to the larger original file. Low resolution files also speed up the printing of a proof on a low resolution printer, as a printer wastes processing time discarding data above its resolution.
Picture Publisher’s FastBits dialog box
Using FastBits to apply different effects to different areas of an image
Applications which use the above Low Resolution option, in which a 72 dpi file is used to represent the larger, high resolution file, are often described as ‘proxy’ systems. The disadvantage of working with a proxy is that it is not possible to zoom in and examine effects at pixel level. In addition, it is difficult to carry out precise masking work as the proxy file lacks the detail of the actual file.
An alternative trick offered by Picture Publisher – called FastBits mode – displays a preview representation of a TIFF image from which the designer can choose a segment to open for editing. The mouse pointer is first dragged to superimpose a variable size grid on top of a preview of the image displayed in the FastBits dialog box. Clicking on a segment of the grid opens just that part of the image corresponding to that segment. When the image is saved, the segment – including any edits – is recombined with the rest of the image. Using this method, a large image can be edited step by step on a system with limited memory. A macro can be used to apply the same edits sequentially to different parts of the image. The FastBits technique can also be used as an efficient way to apply different effects to different segments of an image.
While modes like those described above offer a way of working around the problems of manipulating large colour files, recent innovative applications like Live Picture and Macromedia’s xRes take a more radical approach to the problem. Macromedia xRes, for example, offers two quite distinct modes of working; in Direct Mode, which is used for images up to about 10 Mb in size, operation is similar to that of conventional bitmap applications like Photoshop; in xRes Mode, which is used for images greater than 10 Mb in size, the way in which images are created, modified and saved is quite different, the processing principle used being analogous to that found in 3D applications, in the sense that the time-consuming final rendering of an image is delayed until the design operations have been completed.
Macromedia’s xRes allows the user to work on large images atlow resolution and later render the changes at high resolution
To illustrate the principle involved, let us suppose that a Motion Blur filter is applied to an image which is 4000 3000 pix els in size. In Direct Mode, the filter is applied immediately to all pixels in the 12 million pixel image. In order for the processor to manipulate 12 million pixels, it needs about 100 Mb of RAM. Even with sufficient RAM and a powerful processor, the operation could take many minutes to complete (and in many cases would simply cause the designer’s system to crash!). In xRes Mode, the filter is only applied to the pixels currently visible on screen at the selected zoom level. For example, if the image is being viewed at the 1:8 zoom level and an area of 400 300 pix els is being viewed, the Motion Blur filter would be applied only to 400 300 pix els, or 120 000 pixels in total – only 1% of the full 12 million pixels in the image. Even with a small amount of RAM, the filter can be applied in just a few seconds. If the image is now exported as, say, a TIFF file, xRes performs the processing it has delayed in order to produce the final file, applying the filter effect to all pixels (a process described – somewhat misleadingly – as rendering).
In general, operations which would take many minutes to apply in Direct Mode take only seconds to apply in xRes Mode, as the speed of the operation in xRes Mode is not dependent on the size of the file. The selective processing used in xRes Mode achieves this rapid speed of operation by requiring the system processor to do only the work necessary at the selected zoom level and processing only the area of the image visible on screen. Use of xRes Mode does not involve the compromising of a proxy system, as it is possible to zoom in to the actual pixel data in order to evaluate the result of an applied effect. Editing, painting and masking are all possible at single pixel level.
xRes’s LRG document format is designed specifically to address the problems of saving very large files (larger than 10 Mb). The data within an LRG document is stored in a series of tiles, making it easy to access the image data rapidly. When a file is converted into the LRG format, up to seven different zoom levels of document are made at varying resolutions. Each level is composed of several tiles of data.
By way of illustration, imagine a 4000 4000 pix el document converted to the LRG format. The lowest level of the LRG file is 4000 4000 pix els, the same size as the original document and representing the 1:1 zoom level (although it is not an exact copy of the image as it is arranged in rectangular tiles, as opposed to lines of pixel data). The second level in the LRG file is 2000 2000 pix els, representing the 1:2 zoom level. This level is one-quarter of the size of the original document. The third level is 1000 1000 pix els – the 1:4 zoom level – now only one-sixteenth of the size of the original document, and so on up to seven zoom levels ranging from 1:1 to 1:64. Organising data in this manner allows it to be processed selectively. When an operation is performed, it is applied to only one of the seven zoom levels.
In the years ahead, we can expect to see further developments in methods of handling large colour files efficiently. A consortium of companies, including Kodak, Hewlett-Packard, Microsoft and Live Picture are working on a revolutionary technology called FlashPix, which has already been demonstrated at computer shows. FlashPix is a highly optimised way of handling graphics which allows the designer to load several 50 Mb-plus image files into an ordinary Macintosh or Windows PC with standard disk and RAM, and manipulate them rapidly and safely. The technology works by including in the file a number of versions of the image, all at different resolutions, from a full photographic quality version with 16.7 million colours down to a thumbnail used for previewing. The format is said to be capable of handling the staggering number of 232 pixels, managed in tiles of 64 pixels each. The file also accumulates all the edits made since the image was first created, allowing multiple levels of undo and, because the changes made don’t affect the original, screen updates can be made quickly, as only a small part of the image needs to be recreated. When imported, only the appropriate part of the file is loaded, so for display on screen only the 72 dpi image will be used, while printing on a standard office laser printer would use the 300 dpi image, with all edits applied.
Summary
As we have seen in this chapter, working with digital colour presents disadvantages, but also offers advantages, when compared with working in traditional media. Some of these are listed below.
Disadvantages
Problems of maintaining consistency of colour from device to device (camera to scanner to monitor to printer or press)
Limited colour gamuts of digital devices
Problems of manipulating large images
Advantages
Precision and consistency in specifying and replicating colours, shades, tints and tones
Range of colours to choose from
Ability to experiment on screen with different colours before committing to a final choice
Range of application methods – stroke types emulating oils, pastels, charcoals etc. and fill types such as gradients, blends and patterns
Ability to combine colour and texture
Special effects such as use of lenses, modes, lighting effects, duotones etc.
Range of editing methods – colour masking, colorising, H/S adjustment etc.
Ease of importing and combining coloured objects and images
Only a few years ago, the hardware and software available to the digital designer could produce only the crudest simulation of work done by traditional techniques, which had been developed and fine-tuned over the centuries. Now, as we have seen, work is going on continuously to find ways of reducing or eliminating the remaining areas of disadvantage, while the advantages multiply as the cost/performance of hardware continues unabated and the ingenuity of software developers not only provides closer and closer emulation of traditional methods, but also offers an increasing range of exciting colour techniques which are purely digital in concept↗