Color Space Conversion with Perceptual Rendering Intent: ProPhoto RGB to sRGB
Matrix vs Cluts (Part 1)
My recommended approach for perceptual color space conversion from ProPhoto to sRGB is less straight forward with compare to direct RelCol conversion, as additional steps are involved.
Using my method, the image with ProPhoto profile is converted to jc1RGB color space, and to sRGB_jc1 subsequently, and finally to standard sRGB. The initial step is similar to converting to sRGB with straight RelCol. The main difference is that by converting to an intermediate color space first, the " color distortion" due to RGB channel clipping that could have been introduced is significantly reduced, compare with directly convert with RelCol to sRGB. This loss is further minimized with jc1RGB being selected as the intermediate color space.
The matrix conversion has its advantages, not only that its profile size is relatively smaller and portable, color transformation can be carried out within the known CIE XYZ color space and its transformation behavior is very well predictable. The shortfall is, either sRGB, Adobe RGB, ProPhoto or other proposed color spaces, which are known to be Absolute color space, are not associated with mapping functions. When conversion takes place from a wider color gamut space to a smaller gamut space, colors that are OOG (out-of-gamut) will be clipped and that imply that color information pertaining to the OOG will be lost completely.
In V2 (icc profile specification version 2), color rendering with perceptual rendering intent in CMM can be carried out with output type profile such as printer profile. Output profile uses Cluts ( Color Look Up Tables). ProPhoto has 2 of its primaries extended outside the visible locus and hence problem could arise if the image with ProPhoto has colors that are not within the visible locus. Are these invisible colors exist? The fact is, if they are non existence, why do we care?
Related information: Perceptual Conversion Made Easy
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