> Disclaimer: I am not a color buff. Anybody who actually *knows* about
> that stuff, please chime in.
> =================
> 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.)
> Considering that two different implementations use PAL/SECAM, I am
> wondering if there is a good reason for it and I just don't understand;
> or if maybe they just either copied from each other or happened to
> reference the same (limited) resource?
> After all, accurate color conversion information isn't abundant and was
> probably less so when that code was written.

sRGB has only been around since 1996.  I suspect that the gimp version 
dates from before that, or at least before sRGB came into common use.

> So far I'd say it's a bug -- barring actual color management the most
> reasonable assumption seems to be sRGB.

I don't think we want our color profiles to affect layer blending, so I
think it is best that we choose a single color space and stick with it.

My intuition(possibly wrong) is that when LAB is only used as a temporary
calculation space (as in layer blending) the RGB color space used will
make only a very small difference in results as long as the RGB and LAB
white points match up (ie RGB(255,255,255) => L*a*b*(100,0,0)).

This would leave speed, compatibility, and legacy support as the primary
reason to choose a particular RGB color space.

> D50 vs D65
> ==========
> Another question during transformation to Lab is, which illuminant or
> reference white to use.
> That part, I don't quite understand yet. Does that depend on the source
> data, or simply on how the monitor is calibrated?

You would not use the monitor white point, but you might want to use the
white point of the image's color space or D50 depending on what you are
trying to accomplish.

For a convenient color space to do math in use the source image's white
For data interchange use D50 (as photoshop and ICC profiles do).

Jay Cox

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