On 11.08.2010 19:19, Rupert Weber wrote:
> =================
> While writing the Lab/LCH layer mode stuff, I wondered so far why the
> result is still slightly different from the current GEGL implementation
> of Color/Hue/Saturation/Value modes.
> Now after taking a quick peek at the babl source, it seems that
> GEGL/babl assumes a PAL/SECAM RGB space as source (just like the
> Decompose plug-in.)

that sounds a bit strange to me. I thought PAL uses something like luminance 
plus 2 chroma channels?  (See section 10.1 in [1] for a definition of PAL)

Possibly you've run into a linear light RGB vs. gamma-corrected RGB issue?!?

> [..] -- barring actual color management the most
> reasonable assumption seems to be sRGB.

Me, too, thinks that sRGB is a reasonable assumption. If you want to Do It 
you will have to take the image's color profile into account. Since most, if 
not all
8-bit implementations of color operations are agnostic of the current color 
however, i think it's valid to postpone full color space support until GEGLized
processing takes over.

> D50 vs D65
> ==========
> Another question during transformation to Lab is, which illuminant or
> reference white to use.

As far as layer modes are concerned, you are actually free to choose.
If i understand correctly, you're on a chase for the 'best' 'color' layer mode
anyway (where 'best' refers to some subjective quality). All other things being
equal, it doesn't hurt to conform to the industry standard aka photoshop,
which uses D50 [2].

Explanation from [2]:
"" Lab values do not define absolute colors unless the white point is also 
"" Often, in practice, the white point is assumed to follow a standard and is 
"" explicitly stated

Which means, if you import or export Lab data you need to know the whitepoint to
interpret to the data correctly. Layer modes, in contrast, are used purely 
so the chosen whitepoint doesn't really matter here.


[1] http://www.poynton.com/PDFs/coloureq.pdf
[2] http://en.wikipedia.org/wiki/CIELAB

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