The gamuts of input and output colour imaging media

摘要

The colour gamuts of colour imaging media are important parameters in the reproduction of colour images between them and their assumed magnitudes directly influence the degree to which colours are modified. In spite of this, the determination of gamut boundaries is often done in a way that ignores some basic implications that follow from the definition of colour gamuts. This is partly due to the fact that some of these implications are not understood and partly due to the fact that if they are understood their magnitude is underestimated. Hence, the approach that is taken in this paper is to first discuss the theoretical implications of what colour gamuts are and subsequently to illustrate them by experimental means. Firstly, colour imaging media can be divided into two categories which have very different characteristics in terms of colour gamut. The determination of gamuts of input colour imaging media, for example, introduces a number of problems that do not arise for output media as it involves the determination of the range across which they can capture colour information. This results in significant difficulty from a practical point of view as it necessitates the availability of stimuli from a larger gamut than that of the input gamut to be determined. As this latter gamut is to be determined and hence as yet unknown, the former gamut needs to be very large so as to be usable in general. Secondly, the most crucial factor that is commonly ignored is that viewing conditions are intrinsic to colour gamuts - i.e. that one can only talk about the colour gamut of a set of stimuli if the corresponding viewing conditions are specified. The consequences of this will be shown by looking at the gamuts of commonly used output colour imaging media under a range of viewing conditions with the aim of establishing how the resulting gamuts change in terms of colour appearance. Amongst other things, this will show that the gamuts of different media can be affected to different degrees across the range of illumination levels used in this study. The magnitude of changes ranged from virtually no difference for some LCD displays up to a six-fold change for some projected media, Hence it will be shown that instead of a medium having a single gamut, it has a multitude of them. Describing a colour reproduction medium using a single gamut boundary inevitably leads to mismatches between what that gamut boundary suggests and how the gamut of the medium is seen under different conditions. While one solution to the problem is to generate a number of gamut boundaries for each medium - viewing condition combination, this would result in an explosion of gamut boundary descriptors and in some degree of inflexibility. Alternatively and preferably this relationship could be modelled and this would result in the possibility of having a single reference gamut per medium which could then be modified to suit particular viewing conditions. Understanding the nature of colour gamuts is of significant practical as well as theoretical importance as it can often be the source of errors in cross-media colour reproduction applications.