Re: [Gimp-developer] Some blend modes break in unbounded mode sRGB

[Øyvind Kolås <pippin gimp org>]

On Fri, Apr 11, 2014 at 11:54 PM, Elle Stone
<ellestone ninedegreesbelow com> wrote:
> On 04/09/2014 06:36 PM, Elle Stone wrote:
> > For Lighten only, Darken only, Multiply, Divide, and some of the other
> > blend modes, results are *highly dependent* on the color space in which
> > the blending is done. Removing clipping code doesn't fix the problem.

You seem to be under the impression that all processing whatever the
operation is done going to happen in one color space/pixel format a
"working space". In a GEGL processing world; it is the individual
operations that have working spaces; there is no global working space
that things happen in. (NB: having gamma toggles in blending modes of
GIMP is according to this model making things confusing, compositing
in different color spaces should be _different_operations_).

The operations you are identifying as not operating correctly with
linear data are GEGL operations that should not specify
babl_format("RGBA float") as their working space, but maybe
babl_format("R'G'B'A float"); or even Lab. Other operations you
mention, like gaussian blur which should work like optical focusing is
specifying both linear data, and pre-multiplied data in its input
pixel format since that makes it easiest to do computation that is
optically correct, and weighted correctly according to alpha. This is
the blur behavior you should get regardless of the pixel-storage
format of the individual layer, the same for compositing.

Some GEGL operations make most sense in linear color spaces others
make most sense in perceptual color spaces. For consistency in
behavior operations should always be made in the same space regardless
of whichever source space the images originated.

/pippin