Re: [Gimp-developer] Gegl gaussian blur gamma error

[Elle Stone <l elle stone gmail com>]

Some day I might figure out how to use email. In the meantime, here's
the rest of what should have been part of the previous email:

> but the meta data
> passed around in terms of bablformats in the GeglBuffer will still
> state that this is sRGB data, and when gegl:gaussian-blur is blurring
> it that sRGB data will be converted to linear light data for the
> actual blurring.

Are you saying is that babl/gegl always acts as if any image opened by
Gimp is really in the regular sRGB color space, with sRGB's peculiar
"almost gamma=2.2" tone response curve? Or that if the image is
created by Gimp, even after a diffferent ICC profile is assigned, that
babl/gegl still assumes that the image is an sRGB image, with the
peculiar sRGB tone response curve?

What I would have assumed would happen, is that "behind the scenes"
gegl/babl creates (something like a) linear scRGB version of the image
(by literally converting the image *from* whatever RGB color space it
happens to be in *to* the scRGB color space) and then applies the
gegl/babl gaussian blur to the *converted* image (not to the original
image).

If what I just described is what really happened, then there would be
no gamma error during the gegl gaussian blur. The linear gamma image
would be converted to linear scRGB "behind the scenes", blurred
correctly, and converted back to the original linear gamma profile
upon exporting the image. And the regular gamma image would be
converted to linear scRGB "behind the scenes", blurred correctly, and
converted back to the original regular gamma profile upon exporting
the image.

But what really seems to be happening is that gegl/babl assumes all
images have the sRGB not-quite-gamma-2.2 tone response curve,
regardless of the image profile's actual tone response curve. In other
words, there doesn't seem to be any "behind the scenes" converting
*from* the ICC profile the image has upon opening, *to* the babl/gegl
internal working space, before the gegl operation is applied to the
image.

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