Re: [Gimp-developer] Attempt to summarize the discussion of my examples of what doesn't work in unbounded sRGB

On 04/21/2014 04:45 PM, Øyvind Kolås wrote:
On Mon, Apr 21, 2014 at 1:16 PM, Elle Stone
<ellestone ninedegreesbelow com> wrote:
On 04/21/2014 07:07 AM, Teo Mazars wrote:
Hmm, I understand, then the default black-to-white gradient would be non
perceptually linear, which is more surprising than the color-to-color
gradient. I think I am now convinced this is correct, but it will probably
be puzzling to use.

BTW, "gradient" is not such a good example because it's not related to
chromaticities, "Invert" would be a better one.

I don't understand what you are trying to say. How is drawing a gradient
from most saturated red to most saturated green in any given color space not
related to chromaticities?

Picking the gradient color stops (including start and end) colors can
be considered as separate from the color space the rendering process
and interpolation is done in.

The color space chromaticities are what determine the most saturated
possible colors in any given color space.

Aren't the negative RGB values required to draw a gradient in unbounded mode
sRGB from Rec. 2020 reddest red to Rec. 2020 greenest green exactly what
makes the gradient so horribly wrong when drawn in perceptual rather than
linear? It's the effect of the perceptual TRC that sends the "out of bounded
sRGB gamut" colors to strange places.

This might be an argument that do the _interpolation_ by default in
for instance CIE Lab instead of sRGB.

No. It's an argument to make gradients using linear light, not perceptual.

Drawing a gradient works just fine in unbounded mode sRGB as long as you convert the image to a true linear gamma sRGB, and most likely also just fine in default GIMP if gradients were able to be drawn using linear light.

I can't test that because for some reason someone decided that the right way to draw a gradient was to use perceptual instead of linear, for both linear and gamma precision. But in my modified babl/gegl/GIMP that has the babl conversions back and forth between linear and regular TRC disabled, in a true linear gamma color space drawing a gradient is one of the things that does actually work in unbounded mode sRGB.

On another note, if the answer to "it doesn't work in unbounded mode sRGB" is "do it in CIELAB instead", then you are turning GIMP into a LAB editor. LAB is a wonderful adjunct to RGB editing. But operations in LAB should never be a wholesale replacement for any RGB operation.

If drawing a gradient didn't work in unbounded mode sRGB (but it does if linear light is used) the answer wouldn't be "use CIELAB". The answer would be "use the source color space primaries, not the sRGB primaries".

GIMP is the best by far RGB image editor available to open source. Depriving GIMP of *any* RGB operation by replacing it with a LAB operation is a move in the wrong direction. *Adding* LAB functionality is a very good thing. *Replacing* RGB functionality with a LAB operation weakens the usefulness of GIMP as an RGB editor. Things mix differently in LAB. Color is XYZ/RGB first. LAB is derivative.


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