Re: [w3ctag/design-reviews] CSS Color: lab(), lch() (#488)

@imkremen wrote:

> Why Lab proposed as main perceptual uniformity color space for color 4?

Because Lab is the industry standard. It is widely implemented; it is the Profile Connection Space for International Color Consortium (ICC) profiles, it is used in most books and articles 

> And why proposed to use Lab instead of Luv?

Because no-one uses Luv. No commercial instrument reports it. It has a terrible chromatic adaptation built in (XYZ scaling) and does not have a distance metric. Most books mention it once, in passing, when the introduce Lab.

The 1976 quote (from when the CIE standardized both Luv and Lab) is classical standards-body equivocation and relates more to the backgrounds of the input to the standards body; but in the 40-odd years since, everyone uses Lab unless they also have the data to construct a color appearance model.

Why not more modern CAM 16 or JzAzBz (especially for HDR)?

It's interesting that you mention the CIECAM color appearance models (97, 02, 16) , and it is reasonable to ask why CSS Color 4 and 5 are defined in terms of colorimetry (Lab and LCH) instead of color appearance. The reason that color appearance (which was considered) could not be used in CSS is because of the inherent nature of CSS. Style rules from various origins (author, reader, user agent) are combined via specificity and cascading to yield an eventual result. Thus, all colors are specified at a very granular level on individual elements in the document tree. There is thus no notion of the overall visual field or surroundings in which colors will take on an appearance. Certain aspects of that (the background color or image for a specific element, the colors of nearby elements) could in theory be tractable to analysis by a CSS processor. Others (the colors of other windows that are visible in addition to the browser window) are not available (and must not be, for security and privacy reasons); the overall room luminance, the current white point and the degree of user adaptation to that white point are unknown to a CSS processor and thus cannot be used as input to a color appearance model.

CIECAM is often used in the literature for perceptual gamut mapping of images, but in that case all the colors are available and the (assumed homogeneous, monochromatic) values for the immediate and distant surround and the lighting conditions for the print  can be measure or specified and input to the model.

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