Implement absolute color conversions and chromatic adaptation #139
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tomcur
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This looks great, thanks! I tried to find a nit just to prove I actually read it, but failed to do so.
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This is useful to test the current implementation of absolute conversions (added in #139), but the main motivation is to have tests in place as preparation for manual implementations of `ColorSpace::{to_linear_srgb_absolute, from_linear_srgb_absolute}` for the non-D65 color spaces.
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This builds on #139, which added absolute color conversions (conversions without chromatic adaptation). The default method implementations added there use the chromatically-adapting methods and undo adaptation through additional matrix multiplies. For efficiency, we can do the absolute conversions using matrices that do not include adaptation in the first place. This PR does that for ProPhoto RGB. More to follow.
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This adds absolute color conversions and chromatic adaptation, and is, more or less, a prerequisite for #137.
Conversion methods are added to
ColorSpace,ColorSpaceTag, andDynamicColor, that convert between color spaces while keeping the same absolute color (i.e., if that color were to be reproduced, it would be the same physical color, but a perceptually different color under the intended reference white of the color space). These methods are suffixed_absolute. This also adds achromatically_adaptmethod for manual chromatic adaptation (useful for, e.g., "white balancing" pictures).The white points are represented as CIE
xychromaticities. Calculation of chromatic adaptation matrices isconstwhere possible.A follow-up to this would be to manually implement
ColorSpace::{to_linear_srgb_absolute, from_linear_srgb_absolute, convert}for the color spaces we provide. Those can in most cases just lift directly from their non-_absolutecounterparts, with the exceptions being XYZ-D50, ACEScg and ACES2065-1. Those methods will get somewhat simpler matrices, as the{D50, ACES}<->D65adaptation transforms can be dropped.