a system for wysiwyg colour communication

A system for WYSIWYG colour communication Peter A Rhodes and M Ronnier Luo

Despite the development of more powerful and cheaper colour imaging devices, computer users often spend considerable effort creating colours on the screen only to find that when printed out later they appear to be a completely different shade. This is a problem which computer-aided design and colour management systems are only just beginning to address properly. More generally, if high colour fidelity is desired, there is cause for concern whenever attempts are made to reproduce the same colour across any different media. In the past, such problems have tended to be overcome by elaborate, tedious, iterative and usually manual matching processes. With rapid response times becoming increas- ingly important, the ability to communicate colour both quickly and accurately is highly desirable. This paper follows on from an earlier one in this journal which described a prototype system which was used successfully to demonstrate the feasibility of accurate colour fidelity and communication. The system, Colour- Talk, has since been further refined and the most recent developments are discussed here. This system illustrates a new way of communicating colour using the techniques of device indepen- dency and colour appearance modelling, both of which are essential to achieving WYSIWYG colour.

Keywords: colour fidelity, colour communication, colour order systems, WYSIWYG, colour appearance, colour management systems

Colour fidelity and accurate colour communication remain a problem for industry. Despite advances in computer- controlled colour imaging technology, software and practice have failed to keep pace, leading to disappointing colour reproduction across different media and very limited scope for colour interchange between different workers. Recent developments in colour management systems (CMS) have attempted to solve these problems. Many of these are necessarily concerned with device gamuts and, in order to

Design Research Centre, University of Derby, Britannia Mill, Mackworth Road, Derby DE22 3BL, UK

maximize usage of the limited colour range available, scale colours in some way using a technique known as gamut compression or gamut mapping. Unfortunately, this leads to a lack of direct correspondence between screen and print (this being the most common application of a CMS). While the overall relative look of an image may well be acceptable, this type of transform is disastrous for those users requiting an absolute colour match. One example of such a require- ment is in the textile industry, where designs may be created on screen and the same designs may have to be subsequently reproduced on paper or fabric with high colour fidelity. Typically, 'what you see is what you get' (WYSIWYG) colour is required for applications involving relatively few colours, as opposed to more complex photo- graphic imagery.

With the ultimate goal of achieving WYSIWYG colour in mind, a consortium comprising Coats Viyella, Crosfield Electronics and the LUTCHI Research Centre began a research project in 1990. The CARISMA Project j looked into a number of relevant areas including: device characterization, colour appearance modelling, recipe formulation and the integration of colour notation systems. The research results were incorporated into a prototype computer-based system for colour communication named ColourTalk ~ directed primarily at textile designers and aimed at demon- strating the feasibility of the approach used. Since then, the ColourTalk system has been further enhanced and developed and these changes will be discussed in detail here.

W Y S I W Y G C O L O U R

Achieving WYSIWYG colour across a range of media requires a combination of device characterization and colour appearance modelling 3'4. (It is the latter of these two that the majority of CMS software fails to consider.) The purpose of device characterization is to derive an objective representation of the colour produced by a given set of device primaries. Device primaries characterized in this way include the RGB levels used to drive a CRT

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A system for WYSIWYG colour communication: P A Rhodes and M R Lug

display or the amount of printing inks applied to paper. The actual techniques employed in ColourTalk will be detailed in the next section. This is necessary because the same primary values on two different devices seldom yield the same colour. Here, the CIE system of colorimetry 5 is used to describe colour in an objective, device-independent manner.

Unfortunately, this representation is also viewing condition- dependent, meaning that merely reproducing the same set of CIE XYZ values does not guarantee that colours will match. In particular, parameters such as illumination, background, texture and viewing geometry all modify the way in which colours are perceived. Therefore, it is necessary to incorporate a model of human colour vision which can take such factors into account. Device characterization and colour appearance modelling are used in a pipeline process known as the four-stage transform 3 and illustrated in Figure 1. In this scheme, device-independent CIE XYZ values are related to device-dependent primaries via forward and reverse device models. The XYZ values are, in turn, transformed into viewing condition independent appearance terms such as lightness, chroma and hue (LCH). The Hunt colour appearance model 6 is used by the ColourTalk system for this purpose. This model has been extensively verified and refined 7-~° using experimental results produced by both the CARISMA Project and its predecessor, Alvey Research Project MMI/146. The results form the LUTCHI Colour Appearance Data, which is one of the largest sets of colour appearance data produced anywhere to date. Using these data, it was found that the Hunt model could consistently predict colour appearance with the same performance as a typical human observer.

D E V I C E C H A R A C T E R I Z A T I O N

As mentioned earlier, the key to achieving WYSIWYG colour in the ColourTalk system is the four-stage transform (Figure I), which maps colours between source and target devices. The source devices in the system include colour measurement instruments and the display itself. Target devices include the same display and colour hardcopy peripherals.

The display employed is a Barco Calibrator monitor which includes an external light sensor and internal micro- processor. A routine calibration process was developed to ensure that the display remains stable over time. To characterize any display involves the derivation of a mathematical

transform 11 from the display's device-dependent RGB intensities to a set of device-independent CIE tristimulus values, and vice versa (the device model in Figure 1). In the prototype system 2, the Log-Log model (also known as gamma correction) was used. At a later stage, an extensive study 12 was carried out as part of the CARISMA project to compare the performance of a number of different characterization models in predicting a set of nearly 1000 measured test colours. In this study, it was found that the Log-Log model gave poorer colour accuracy for darker and highly saturated colours. It was also established that the PLCC (piecewise linear interpolation assuming constant chroma- ticity coordinates) model showed a significant improvement for those colours, and hence it was adopted in the new ColourTalk system.

The colour measurement device used to characterize the display is a Bentham telespectroradiometer (TSR) which is calibrated against a standard illuminant A lamp, itself traceable to the NPL standard. For display purposes, ColourTalk simulates the viewing conditions of a VeriVide viewing cabinet using TSR measurements of the cabinet's interior when viewed under the illuminant D65 simulator. In addition, three further light sources (tungsten, TL84 and white fluorescent) are also used in the viewing cabinet and the corresponding appearance of colours when viewed under these was also simulated. The ColourTalk system hardware is shown in Figure 2.

THE PROTOTYPE SYSTEM

The prototype ColourTalk system was detailed in Reference 2; however, a brief summary of its capabilities and limi- tations is given here. The system was centred around the task of fashion palette creation and communication in a textile environment and was produced in response to an initial series of interviews held with a number of textile design groups. To support their tasks, colours could be selected from one of three computer-based colour atlases (CIELAB, Munsell and NCS). The palettes themselves consist of a fixed 6 x 4 arrangement of colour patches with an additional two rows used for colour adjustment. Colour appearance changes according to its context and so additional tools were provided to support the visualization of colour.

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oordlnatesJ tConditionsJ tCondition, LCoordinatesJ

Figure 1 The four-stage transform Figure 2 ColourTalk system hardware

214 Displays Volume 16 Number 4 1996

A system for WYSIWYG colour communication: P A Rhodes and M R Luo

These included the ability to overlap and resize colour patches (to examine simultaneous colour contrast and viewing geometry) and to predict the effects of metamerism and colour constancy due to changes in light source.

Throughout, colours and palettes are treated as objects which may be exchanged between each application's window via a common 'work surface' area which acts as an intermediate store of such objects. Figure 3 illustrates logically how objects are exchanged using the work surface. Once created, palette data could be saved to the system disk or alternatively written to floppy disk. The latter was exploited as a means of communicating colours between systems and was specifically used to send and receive data from a separate Coats Colour Physics System (CPS). The CPS was able to generate recipes independently for the chosen colours, which could subsequently be used to dye actual samples. The CPS could also be used as an alternative indirect means of inputting colour into the prototype system via its attached spectrophotometer. As an aid to production, further facilities for colour tolerance were present, enabling users both to specify and visualize the degree of colour difference that was to be permitted during the production of colours.

The prototype system successfully demonstrated the feasibility of computer-based WYSIWYG colour fidelity however it also suffered from a number of shortcomings outlined below:

• No direct means of interrelating between different colour notation.

• Notation systems other than CIELAB support only a fixed number of colours and all systems use CIELAB for fine colour selection.

• Only a limited number of colour notation systems are provided.

• Exchange of colours via the work surface is inefficient. • Lack of direct colour measurement or hard copy facilities. • No design capabilities.

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Picker I " co/our

Figure 3 The prototype system

All of the above points were addressed in the final ColourTalk system developed by the end of the CARISMA Project.

T H E N E W S Y S T E M

Following the encouraging success of the prototype in proving the feasibility of WYSIWYG colour fidelity and communication, a new computer-based system having the same two goals was developed. The new ColourTalk system also aimed both to demonstrate and to integrate the research results of the CARISMA project. This system was redesigned from scratch using the lessons learned through the development of the prototype. In terms of achieving colour fidelity, the same underlying approach was taken. However, as mentioned earlier, the PLCC calibration model was adopted for display characterization after a comparative study of a number of alternative methods t2 revealed its superior performance. There were three main areas of change involved: better support for colour notation systems, improvements in the user interface and the ability to measure and print colour (colour input/output).

User interface changes As already highlighted, the user interface was particularly

cumbersome when it came to exchanging colours between each of the tool windows. Previously this had been done using a large intermediate storage area known as the work surface. In the new system, this was replaced by a new mechanism which will be immediately familiar to any user of modern word processing or desktop publishing software: cut and paste. Using this paradigm, colours can be 'cut' or 'copied' from one window and 'pasted' into another. These actions directly mimic the real world activities of using scissors and glue to manipulate objects. All ColourTalk applications support this mechanism and it has proven very powerful. Furthermore, because colour data are represented as textual information they can be pasted into non- ColourTalk applications, such as electronic mail software, thereby enabling colours to be conveniently and precisely communicated to other users around the world.

A number of other changes were made to simplify the display. The functionality of the prototype system's colour palette was broken up into a separate palette storage and colour adjustment tools. For informational purposes, colours could be tagged with descriptive names. The colour adjustment tool, like its predecessor, allows either individual colours or groups of shades to be adjusted according to their CIE metric tightness, chroma and hue (LCH) attributes. For textile designers, this might be of benefit when colours from a previous year's fashion palette come to be reused. If, for example, the prevailing trend is for pastel shades, these can be created by increasing the lightness and decreasing the colourfulness of existing shades (Figure 4). As colours are being adjusted, the colours' relative posi- tions are plotted in CIELAB space to provide the user with feedback concerning their changes. Also, for reference, the original (unmodified) colours are presented in the row above the adjusted colours.

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Figure 4 Adjusting three colours simultaneously

When it comes to real designs, colours are seldom used in isolation and when they are combined they often do not appear the same owing to the phenomenon of simultaneous colour contrast t3. ColourTalk provides a colour viewer tool (Figure 5) to help the user visualize such effects; this permits colours to be resized and overlapped. In addition, the tool allows individual colours to be adjusted in terms of LCH in place making it possible for users to correct for any undesirable effects due to colour contrast.

The prototype system's data management function was quite basic and files were presented to the user as lists of names. In the new system, this was abandoned in favour of an icon-based file manager (Figure 6). The file manager presents all files and directories as a series of icons. Each object type (individual colour, colour palette, design or folder) has a different icon. Once again, these may be

Figure 5 Examining simultaneous colour contrast using the colour viewer

Figure 6 The file manager and colour palette

manipulated using the 'cut and paste' operations described earlier to achieve file copying, deletion or movement. Two file system types are supported: the system's hard disk and a PC-compatible floppy disk. The latter of these can be used as another medium for long-distance colour communication since it is possible to exchange such disks through the post.

Other more cosmetic changes include the ability to resize many windows. While the majority of users seemed to prefer larger colour patches to make their colour judgement tasks easier, this tends to slow things down a little and makes it difficult to both locate other tool windows and to compare colours contained in different windows. Although it was never the goal of this research to develop a complete CAD system, it was decided that supporting basic design would be useful for showing off the system's potential. To this end, some existing software for 2D line art design was modified and extended to allow users to import ColourTalk palettes (Figure 7). The palettes were deliberately stored separately from the form of the design because designers often want to show their clients the same design in different co!ourways.

Colour notation systems

The original motivation behind providing multiple computer-based colour notation systems is that there is no one universal system in use by industry and no convincing argument why any one system should be adopted over another for all applications. Colour order systems based on physical samples are prone to fading or becoming dirty and invariably comprise relatively few actual colours. A computer atlas suffers from none of these drawbacks and can use interpolation techniques to produce a range of 'in between' colours which would be prohibitively expensive to reproduce physically. Furthermore, it is possible rapidly to navigate the colour space defined by such systems as an



Figure 7 Lineart design

aid to visualization and training. The original prototype's three systems were extended to include TekHVC, PAN- TONE, HLS, RGB, CMY, NCS (Natural Colour System), Munsell, CIELAB and CIELUV (Figures 8-11). Since internally colours are represented by their CIE XYZ coordinates, even users of the device-dependent RGB or CMY colour spaces are able to exchange data in a device- independent manner.

Each of these systems was implemented as a bidirectional mathematical mapping between the system's own attributes and CIE xIrz as depicted in Figure 12. (In the case of CMY and HLS, this is done indirectly via screen RGB). Apart from the obvious difficulty in deriving such mappings,

another problem to be solved was that certain colour systems are defined in terms of measurements made under dissimilar conditions. For example, NCS is generally defined for illuminant C whilst PANTONE might be measured under Ds0. As already stated, colorimetry is inappropriate for such circumstances and so again the four-stage transform introduced earlier was used to 'normalise' each of the colour notation systems to ColourTalk's viewing conditions (i.e. D65, 2 ° observer).

Unlike the previous system, each colour order system was mathematically defined and as such could use interpolation to display a micro space in each system's own colour space. This is primarily of use for the fine adjustment

Figure 8 Natural Colour System colour picker



could easily exceed 5AE CMC(I : I ) units owing to the uneven distribution of PANTONE colours. Details of these conversions are completely transparent to the user. For example, to convert an NCS colour to its equivalent Munsell notation simply involves copying the colour from the NCS window displayed on the screen and then pasting this into the Munsell window (Figure 11). This ability, in particular, has proven to be both an easy to use and powerful tool.

Figure 9 The Tektronix TekHVC colour system

of colours. The only exception to this was PANTONE. The PANTONE system arranges colours undimensionally according to a reference number. Hence, it is not possible to form a three-dimensional colour space without changing the inherent nature of the notation system. In addition, the reverse mappings (i.e. from colour notation to XYZ) were used to interrelate colours from one system to another. In the case of PANTONE, the closest colour matching the target is located. The closest colour is defined as that giving the smallest colour difference from the target (according to the CMC (1:1) colour difference formula). Such differences

Colour input and output

A Macbeth MS2020 + spectrophotometer was connected to the new system. This allows users to measure surface colours as a direct means of inputting colours into the system. As with the TSR, a calibration procedure was established. The MS2020 calibration, however, used a white tile (rather than the standard lamp used to calibrate the TSR) supplied by the manufacturer and which was traceable to the NPL standard. However, it was found that a somewhat poor agreement occurred between the TSR and MS2020. This is due to the common problem of instrument metamerism which arises as a result of a number of factors, including large differences in the optical design (i.e. il- luminating and viewing geometry), bandwidth, light source, measurement range and interval. To overcome this, an empirical mathematical transform was derived to correlate the two sets of instrumental results.

For hard copy purposes, a Mitsubishi G650 thermal wax colour printer was also linked into the new ColourTalk system. This permits screen colours to be presented on paper media. A study was also carded out in the CARISMA project t to compare the accuracy of various mathematical

Figure 10 Interrelating PANTONE and NCS



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Figure It Interrelating NCS and Munsell

colorant. Instead, reflectance curves were generated from the real colorants that were used to produce samples as opposed to arbitrary or synthetic curves. This helps to ensure that both a very close first match can be produced and a similar degree of colour constancy can be obtained in comparison with the reproduced colours. Different formulation algor- ithms were also developed as part of the CARISMA project to predict ink, textile and paint colorant recipes. These enable users of the system to obtain a set of reflectance curve data corresponding to a given screen colour for sub- sequent reproduction onto different physical materials.

Figure 12 Interrelation between different colour notation systems

models in computing the amounts of CMY inks required to reproduce a given set of CIE tristimulus values. The third- order masking model that was implemented in the new system gave a reasonably accurate representation of screen colours; a mean value of around 1.1 CMC(I : I ) AE units was attained for those colours tested, which is likely to be adequate for colour proofing purposes.

One limitation of the system is that any colours created using either the colour notation systems or the colour adjustment function are defined only in terms of CIE tristimulus values. While this is usually not a problem, occasionally it may be necessary to describe colours in terms of their spectral reflectance. One example of such a case is in sending colours to a colour physics system which requires these values to help ensure colour constancy for its predicted recipes. Although it is possible to measure the actual spectrum of light emitted by the monitor's phosphors, a spectral match is pointless as it would be highly meta- meric with any physical materials produced using real

SYSTEM PERFORMANCE

As part of the analysis of ColourTalk's performance, it was used by designers for a real task. Using the system, a colour palette was created containing a combination of existing shades and new colours. These palettes were subsequently reproduced across different media (Figures 13 and 14). In addition, some of their sketches for designs and labels were scanned into the system and these were coloured using the palette colours (Figure 7). Again, this artwork was reproduced on various (paper) media (Figure 15). A more detailed report which quantifies the accuracy of this colour reproduction is given in Reference 1. Com- pared with the designers' conventional way of working, this was quite an improvement. Not only were they able to work with the correct colours on the screen, but also the colours chosen could be accurately communicated to the dye house without the ambiguity surrounding the use of fabric swatches or thread samples. The end result was an acceptable level of colour reproduction without the trouble and expense of rematching.



by performing gamut mapping or compression, However, unlike the solution given here, no existing system has applied colour appearance techniques to this process. In any case, there are still many applications requiring high- fidelity absolute colour matching across multiple media and future advances in colour imaging technology are likely to broaden the gamut of available colours. Hence, WYSIWYG colour is likely to become both more practical and more desirable.

Figure 13 Palettes reproduced on paint and cotton

C O N C L U S I O N

The outcome of this work has successfully demonstrated the practicality of WYSIWYG colour reproduction and communication. The ColourTalk system that was produced points the way forward for future colour management systems and colour-critical CAD/CAM workstations. Such systems can achieve cross-media high-fidelity colour through the application of colorimetry and colour appearance modelling. In addition, effective colour communication requires the ability to interrelate multiple colour notation systems.

The ColourTalk system is being further developed at the Design Research Centre based at the University of Derby by the same team of researchers. Part of this ongoing work includes the production of an industrial system to be used for colour communication within the Coats Viyella group.

Figure 14 Polyester thread, Cromalin and cotton media

Figure 15 Printed label and matching T-shirt

It must be said, however, that WYSIWYG is not ideal for all applications. This is mainly due to the limited colour gamut of real coloration processes. For complex, photo- graphic imagery containing many thousands of colours, it is inevitable that some of these colours cannot be reproduced using a particular imaging device. In this case, it may be desired to maximize the use of the available gamut while still retaining a pleasing appearance (e.g. the sky remains blue and flesh tones look natural). Conventional colour management systems already take care of such eventualities

A C K N O W L E D G E M E N T S

This work was conducted under project number IED4/1/ 1201 of the UK Information Engineering Advanced Tech- nology Programme (IEATP) supported jointly by the DTI and SERC (contract GR/GO2321). The project involved a collaboration between Crosfield Electronics, Coats Viyella and the LUTCHI Research Centre. The authors acknowledge the efforts of Dr John Xin and Mr Haishan Huang in the research work.

Further information regarding ColourTalk and colour research at the Design Research Centre is available elec- tronically on the Internet from http:llziggy.derby.ac.uklcolourl via the World Wide Web.

R E F E R E N C E S

1 Luo, R, Rhodes, P, Xin, J and Scrivener, S. 'Effective colour communication for industry.' J. Soc. Dyers Colourists, 1992, 108(12), 516-520

2 Rhodes, P A, Scrivener, S A R and Luo, M R. 'ColourTalk--a system for colour communication.' Displays, 1992, 13(2), 89-96

3 Yousif, W S and Lao, M R. 'An image process for achieving WYSIWYG colour.' Proc. 14th Worm Congress on Computation and Applied Mathematics, Voi. 4, IMACS 91, Dublin, July 1991, pp 1841-1843

4 Berns, R S. 'Color WYSIWYG: a combination of device colorimetric characterization and appearance modeling.' S/D 92 D/g., 1992, 549-552

5 CIE. Colorimetry, 2nd Edn., Central Bureau of the CIE, 1985 6 Hunt, R W G. 'An improved predictor of colourfulness in a model of

colour vision.' Color Res. Appl. 1994, 19(1), 23-26 7 Luo, M R, Clarke, A A, Rhodes, P A, Schappo, A, Scrivener, S A

R and Tait, C J. 'Quantifying colour appearance. Part I. LUTCHI colour appearance data.' Color Res. Appl. 1991, 16(3), 166-180



8 Luo, M R, CLarke, A A, Rhodes, P A, Schappo, A, Scrivener, S A R and Tait, C J. 'Quantifying colour appearance. Part II. Using LUTCHI colour appearance data.' Color Res. Appl. 1991, 16(3), 181-197

9 Luo, M R, Gao, X W, Rhodes, P A, Clarke, A A and Scrivener, S A R. 'Quantifying colour appearance. Part III. Supplementary LUTCHI colour appearance data.' Color Res. Appl. 1993, 18(2), 98-113

10 Luo, M R, Gao, X W, Rhodes, P A, Xin, H J, Clarke, A A and Scrivener, S A R. 'Quantifying colour appearance. Part IV. Transmissive

media.' Color Res. Appl. 1993, 18(3), 191-201 11 Post, D L and Calhoun, C S. 'An evaluation of methods for producing

desired colors on CRT monitors.' Color Res. Appl. 1989, 14, 172-186 12 Luo, M R, Xin, J H, Rhodes, P A, Scrivener, S A R and MacDonald

L W. 'Studying the performance of high resolution colour displays.' CIE 22nd Session--Division 1, Melbourne, 1991, pp 97-100

13 Luo, M R, Gao, X W and Scrivener, S A R. 'Quantifying colour appearance. Part V. Simultaneous contrast.' Color Res. Appl. 1995, 20(1), 18-28


a system for wysiwyg colour communication

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