Extending the CARISMA Gamut Mapping Model

High quality reproductions made by prepress operators were studied in order to derive gamut mapping strategies. Ektachrome transparencies were reproduced on gloss coated and newsprint hard copy media, and the implicit mapping strategies in dimensions of hue, lightness and chroma were analyzed. The results supported some elements of the CARISMA gamut mapping algorithm, including a shift in hue towards the colorant primaries of the reproduction media and (for gloss coated media only) linear lightness scaling from the minimum and maximum L* lightness of the image to those of the reproduction media. Compression of lightness and chroma towards the achromatic axis was found to be non-linear, and to have a variable convergence point. A gamut mapping algorithm incorporating these features (Graphic Arts Media Mapping Algorithm or GAMMA) was compared to other mapping methods in a transparency-to-newsprint workflow. The experiment compared different methods of determining the achromatic convergence point in simultaneous lightness-chroma compression, and compared linear compression against a nonlinear distance-weighted compression. Non-linear compression methods and variable, lightness and chroma dependent convergence points performed better than those with linear compression and fixed convergence points.

My research interests are in colour imaging generally, and specifically in colour management, colour difference, colour appearance, colour metrolology, device characterization, colour gamut mapping and colour image quality. Colour imaging is an extremely active area of research across the world, but there remain many outstanding problems and each advance in technology throws up new research and development opportunities.My current research projects include: metrics for quantifying the smoothness of colour transforms; development of a reference printer architecture for colour management; scalability of colour gamut mapping algorithms; colour stability of printing processes; colour tolerances across different media and different cultures; psychophysical methods for assessing image quality in large sample sets; effect of surround and background on colour appearance; and colour reproduction on chromatic substrates.I am interested in the implementation of colour transforms, and have developed the Colour Engineering Toolbox, which provides a comprehensive set of tools in the Matlab language for this purpose.At LCC we have outstanding laboratory facilities in colour measurement and colour imaging, and I and my students have been able to participate in numerous national and international research projects coordinated by bodies such as CIE and ICC. We have also worked on projects with researchers at large companies such as HP and IBM.