[cairo] Spot colors (and CMYK)

Russell Shaw rjshaw at netspace.net.au
Thu Feb 18 17:22:05 PST 2010

Bill Spitzak wrote:
> Russell Shaw wrote:

>> Storing negative values in sRGB is clearly a nonsense hack (square roots
>> and all that).
> True, and this is the biggest problem with saying "unclamped sRGB". 
> Microsoft with their scRGB definition have said that the conversion of 
> values greater than one follow the gamma curve, and negative numbers are 
> encoded as -f(-x). From previous experience with Microsoft standards I 
> am sure they thought about this for about 10 seconds, but unless we can 
> find a persuasive argument for different math I think using Microsoft's 
> definition is now the safest.
> It is also certainly true that composites are useless for these 
> out-of-gamut colors. Conversion to linear is needed before compositing 
> math. This is exactly what Nuke does. However I can warn you that there 
> are serious consequences that are going to hit a library used for vector 
> graphics like Cairo much worse than they hit Nuke. The problem is that 
> an awful lot of compositing relies on the "perceptually uniform" math 
> being done, the most obvious is the result of drawing the same-thickness 
> lines (or letters) in black or white on the opposite background. If the 
> antialiasing is done "correctly" in linear space, the user thinks the 
> white lines are far thinner or fainter than the black ones. But wrong 
> antialiasing done in sRGB looks "correct". Also users do not expect 
> their carefully painted partially-transparent overlay from Photoshop 
> will produce visible artifacts when composited.
> I do not have a solution to these problems. I wish I did. Linearizing 
> only outside the 0-1 range kind of works but the discontinuous slope at 
> 0 and 1 produced artifacts.

Discarding mathematical linearity for the sake of "perceptually uniform"
is a high price to pay.

If the most common case is for light letters on a dark background,
i'd hint the font renderer to change its rendering. For other cases
such as thin lines in a drawing program, users should just accept
the result as a fact of nature much like objects under flourescent
light look different than in sunlight.

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