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

[Elle Stone <l elle stone gmail com>]

On 8/5/12, Øyvind Kolås <pippin gimp org> wrote:

> 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.
>

I don't think I understand what the preceeding sentence says. I think
it says that ultimately, when all changeover to gegl/babl is complete,
Gimp should *not* allow the user the option of assigning one or
another ICC color space profile to an image, and *not* allow the user
to convert an image from one ICC profile to another. But that can't be
right, can it?


> 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.)

My apologies - where Gimp leaves off and gegl/babl take over is not
something that I thought about, and I'm still working on figuring out
what (and who!) does what.

I was hoping (wishful thinking?) that maybe the same 0.45 gamma error
that makes tif thumbnails look correct and png thumbnails incorrect
(too dark), whereas 16-bit tifs load incorrectly (too light), and pngs
load correctly, might also be related to the gegl incorrect blurring
of a linear gamma image, as if it had a gamma of 0.45 instead of 1,
plus the Gimp 2.8 gamma error of 0.45 reported in this thread:
https://mail.gnome.org/archives/gimp-developer-list/2012-July/msg00050.html