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

[Øyvind Kolås <pippin gimp org>]

On Sun, Aug 5, 2012 at 7:36 PM, Elle Stone <l elle stone gmail com> wrote:
> 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.

GIMP-2.9 is only partially on it's way through to be fully working
properly with GEGL/babl. ICC profiles should only need to be involved
upon loading files from disk and exporting to files used outside -
internally it is up to GIMP/GEGL/babl to assign appropriate fully
color managed color spaces to the raster storage of the buffers in the
layers; for 8bpc precision this will be sRGB and for higher bitdepths
this should be linear light/linear scRGB. GEGL through babl knows how
to _accurately_ and efficiently convert between the color spaces and
representations native to babl (without interpolated lookup tables),
this differs from any assumptions that might have been in 2.8 and
before wrt color management and the ability to assign color profiles
to images; I would not trust (and want GIMP to get rid of) any such
things in the UI.

The only part of this that is done is as far as I know the distinction
between sRGB in 8bpc and linear scRGB in >=16 bpc, nothing has been
done wrt ICC profiles before you started poking the lcms bits.

/Ø

PS: I try to make sure that babl and GEGL do what they promise; this
is already more than I have time or energy to deal with; thus I try
stay out of GIMP unless it is changes I have made to GEGL that
directly stops GIMP from building.)
-- 
«The future is already here. It's just not very evenly distributed»
                                                 -- William Gibson