Optimizing gamut mapping: Lightness and hue adjustments

This article describes an investigation of several chroma compression algor ithms with constant and modified lightness and hue. While the rescaling of differing lightness ranges is not considered here, gamut-mapping algorithms with different mapping types and mapping directions are evaluated. Among t hese are the methods of mapping colors towards a focal point as well as the newly developed relative lightness change (RLC) technique. The latter maps colors along curved lines and enables the direct use of cylindrical coordi nates lightness, chroma, and hue for the mapping, regardless of the mapping direction. Moreover, the RLC method is straightforward and does not need a ny iteration or complex cross sectioning, thus it is well suited for real t ime gamut mapping. Issues of gamut mapping such as the mapping space, the c oordinate system within the space, and gamut boundary description are discu ssed. Psychophysical experiments are described which were conducted to eval uate the reproduction of monitor images on a device with a clearly smaller color gamut. All images were viewed on the monitor. The algorithms were dep endent on the image gamuts. The experiments showed that the RLC method perf orms superior compared to the focal mapping algorithms. Moreover, the optim um mapping direction was RLC50 that means a slight adaptation of lightness. The best mapping type was pure clipping, regardless of the mapping directi on. Notably RLC50 outperformed other, recently published algorithms. While the found algorithms may still require some fine-tuning, a thorough underst anding of the mechanisms of gamut mapping was acquired.