Decomposed to LAB images have the wrong ICC profile assigned

Submitted by Elle Stone (@ellestone)

Description

When an image layer is decomposed to LAB or LCH, currently the resulting grayscale layer stack is not assigned the correct grayscale profile. Instead:

If the RGB layer stack is at "Linear light" precision, the GIMP built-in Linear RGB grayscale color space is assigned.
If the RGB layer stack is at "Perceptual gamma" precision, the GIMP built-in sRGB-Gamma grayscale color space is assigned.

Regardless of the precision of the source RGB layer stack, when decomposed to LAB or LCH, the resulting grayscale layer stack should be decomposed to "Perceptual gamma" precision and should have a grayscale profile with the LAB companding curve assigned. Otherwise results of dragging and dropping the decomposed grayscale layers back to the RGB layer stack will be incorrect (http://ninedegreesbelow.com/photography/lab/gimp29/correct-vs-gimp29-decompose-drag-drop.jpg).

Composing/recomposing from the LAB layer stack back to RGB seems to work just fine regardless of what grayscale ICC profile is assigned to the LAB layer stack.

The above summary was copied from Description below was coopied from https://bugzilla.gnome.org/show_bug.cgi?id=756389#c20. Also see https://bugzilla.gnome.org/show_bug.cgi?id=755376 and https://bugzilla.gnome.org/show_bug.cgi?id=756389 comments 43 and 44.

Version: git master

Edited Jun 07, 2024 by Bruno