[Gimp-developer] Code for editing in non-sRGB color spaces

[Elle Stone <ellestone ninedegreesbelow com>]

I've been looking at the relevant babl/GEGL/GIMP code for enabling GIMP 
to edit in color spaces other than sRGB. Here are two questions:

1. Code for retrieving colorant information from an ICC RGB working 
space profile embedded in an image is already in 
https://github.com/GNOME/gimp/blob/master/libgimpcolor/gimpcolorprofile.c

Can the following babl functions from 
https://github.com/GNOME/babl/blob/master/babl/babl.h be used to send 
RGB colorant information (a 3x3 matrix) from GIMP to various babl 
functions that use Y and XYZ information?

/**
 * babl_set_user_data: (skip)
 *
 * associate a data pointer with a format/model, this data can be 
accessed and
 * used from the conversion functions, encoding color profiles, palettes or
 * similar with the data, perhaps this should be made internal API, not
 * accesible at all from
 */
void   babl_set_user_data     (const Babl *babl, void *data);

/**
 * babl_get_user_data: (skip)
 *
 * Get data set with babl_set_user_data
 */
void * babl_get_user_data     (const Babl *babl);


2. Instead of full-blown support for editing in all RGB working spaces, 
would it be easier and maybe even better to start with hard-coding into 
babl/GEGL/GIMP an assortment of commonly-used RGB working space 
chromaticities, including:

sRGB
Rec.2020
ACEScg
ACES
AdobeRGB
ProPhotoRGB

and then pass a numerical identifier from GIMP to babl as the user 
switches from one image to another, so babl knows what chromaticities to 
use? This option of course would require that the user make an ICC 
profile conversion to one of the hard-coded RGB working spaces. But 
given the many ICC RGB profiles floating around that are not 
well-behaved and/or have odd TRCs, maybe this is the better option. 
Though support for "random RGB chromaticities" would be nice to 
eventually add in.

Best,
Elle