[Gimp-developer] Adding LAB support to GIMP

[Elle Stone <ellestone ninedegreesbelow com>]

Recently I was asked about LAB support in GIMP 2.9/2.10, and realized 
that GIMP already has a fair amount of support for editing in the LAB 
and LCH color spaces. So this email is an attempt to summarize what's 
already available and then make a list of what would be needed for full 
support. Comments, corrections, additions, etc are very welcome.

Given other coding priorities and limited developer time, better LAB 
support to GIMP might not make it into 2.10. But it would be nice to 
have an idea of what full support for LAB in GIMP might require.

Already it's possible to:

* Decompose to LAB and LCH

* Compose/recompose from LAB and LCH

* Pick colors using LAB and LCH

* Modify colors using the LCH Hue-Chroma tool

* Extract LAB and LCH channels as layers, eg for use as layer masks.

* Manipulate image colors in LAB/LCH using LCH and Luminance blend 
modes. For example:
   Using the Lightness blend mode allows to produce the exact same 
result as doing Curves, Levels, Brightness, Exposure, etc on the LAB 
Lightness channel, simply by doing the operation on a copy of a layer, 
and setting the layer blend mode to LCH Lightness.
   Often people want to raise and lower Colorfulness (Chroma divided by 
Lightness) concurrent with changes in tonality. For this, use Luminance 
blend for the layer with the modified tonality.

It seems to me that full LAB support would require:

* The option to open images that are in the LAB color space, keeping in 
mind that there are various LAB color space ICC profiles floating 
around, and some LAB images don't have any embedded ICC profile, but 
instead just have metadata that indicates the color space.
    Upon opening a LAB image, it could be immediately converted to RGB 
for processing, as the conversions from RGB to LAB or LCH for further 
processing in these color spaces are already handled by babl.

* The option to convert an image to LAB, including to a specific LAB 
profile from disk, just before exporting the image to disk in whatever 
file formats there are that support the LAB color space.

* The ability to do Curves and Levels operations on the LAB channels and 
perhaps on the CH channels. Having LAB/LCH Curves would allow, for 
example, to raise and lower specific points in the L/A/B/C/H channels 
without moving the endpoints of the Curves. Such edits are often used 
for color correction using the A and B channels. As far as I know, 
nobody offers C and H Curves/Levels, but it would be interesting to see 
what uses people might come up with.

* Adding L/A/B/C/H histograms to the Histograms dialog: We already have 
Luminance in the histograms dialog. Lightness is just the perceptually 
uniform transform of Luminance.

What else might be required to add full support for the LAB space to GIMP?

These two bug reports are about adding support for editing in the LAB 
color space:

   Add LAB color mode:
   https://bugzilla.gnome.org/show_bug.cgi?id=309485

   support other colorspaces in the Curves tool:
   https://bugzilla.gnome.org/show_bug.cgi?id=432933


These two bug reports are about fixing a couple of bugs in current 
LAB/LCH functionality:

   Decomposed to LAB images have the wrong ICC profile assigned:
   https://bugzilla.gnome.org/show_bug.cgi?id=765178

   "Extracted L" from "Tools/GEGL Operation/Extract Component/Component 
LAB L" isn't really "L":
   https://bugzilla.gnome.org/show_bug.cgi?id=755376

Best,
Elle