[Gimp-developer] Code for editing in non-sRGB color spaces (was Re: suggestions)

[Elle Stone <ellestone ninedegreesbelow com>]

On 04/05/2016 12:17 PM, Alexandre Prokoudine wrote:
> Anything in that section can be moved to e.g. 3.2 provided there is a
> developer working on any of the features listed there. At some point, the
> unified transform tool was there too. Then Mikael came, and now it's a
> v2.10 feature.

Two years ago I spent a lot of time trying to figure out how to pass RGB 
colorant information around GIMP and back to babl, but failed to make 
any progress.

I've been taking another look at the relevant babl/GEGL/GIMP code, but 
the task still seems to be way above my coding level. I suppose I should 
ask for help. So 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.

I don't think I understand the relevant portions of 
https://github.com/GNOME/babl/blob/master/docs/roadmap.txt. If I do 
understand, the plan in the roadmap seems to require rewriting major 
portions of babl/GEGL/GIMP.

If it were possible to use babl_set_user_data and babl_get_user_data to 
pass along colorant information, or else a numerical reference to a 
selection of hard-coded chromaticities for commonly-used RGB working 
spaces, this approach would seem to be a lot less invasive of the 
existing code.

Very likely I've overlooked major coding obstacles. And as I already 
noted, it's very likely that I simply don't have sufficient coding 
skills to add the ability to edit in non-sRGB color spaces to GIMP.

Best,
Elle