[Gimp-developer] Three questions about opening an image and converting it to linear light RGB

[Elle Stone <ellestone ninedegreesbelow com>]

Please bear with me while I ask some questions. I'm trying to clarify 
something that I might be completely confused about. The point of the 
first two questions is to be able to ask the third question. If I'm 
confused about the answers to the first two questions, what I'm asking 
about in the third question won't be possible.


First question:

The phrase "linear light RGB" is used a lot. When the topic is 
BABL/GEGL/GIMP, does "linear light RGB" mean Pippin's extended sRGB 
color space defined by the sRGB primaries and the linear gamma tone 
reproduction curve?


Second question:

Is Pippin's extended sRGB/BABL's "linear light RGB" the same as LCMS's 
unbounded mode sRGB?

In other words, are BABL's "linear light RGB" and LCMS's "unbounded mode 
linear gamma sRGB" two ways of describing the same thing, namely, after 
converting from some other color space to linear light RGB, otherwise 
out of gamut RGB values aren't clipped but rather carried along as RGB 
values that are less than 0.0 and/or greater than 1.0?


Background for the third question:

Right now when using GIMP from git, it's possible to open an image in, 
for example, the ProPhotoRGB color space, and edit the image while 
keeping it in the ProPhotoRGB color space. There's no automatic 
behind-the-scenes conversion to unbounded mode sRGB/extended sRGB/linear 
light RGB.

I've been assuming that before 2.10 is released this behavior will 
change. My understanding is that in the future:

1. The user will open an image with GIMP and maker sure the right ICC 
profile has been assigned.

2. Before actual image editing can begin, the image will be converted to 
extended sRGB/linear light RGB "behind the scenes" without the user 
necessarily even knowing that this has happened, though a little 
warning/user education/notes in the GIMP documentation might explain the 
sudden appearance of negative RGB values and/or RGB values that are 
greater than 1.0.

3. All subsequent image editing will be done in the extended sRGB/linear 
light RGB color space.

4. Upon exporting the image to disk, the use will be able to convert the 
image from extended sRGB/linear light RGB to their chosen ICC profile. 
They won't be forced to export an sRGB image to disk.

I asked almost the same question in a previous email 
(http://gimp.1065349.n5.nabble.com/Soft-proofing-and-the-GIMP-Display-Filters-and-Color-Management-settings-td42139i20.html). 
But Pippin made a distinction between "pipeline" and "workflow", and I'm 
not sure what the distinction might be in practice. So I'm asking again 
as explicitly as possible:

Will the image be converted to extended sRGB before image editing can 
begin?

Will the user see out of gamut (that is, out of the sRGB color space's 
gamut) RGB values expressed as RGB values that are less than 0.0 and/or 
greater than 1.0?


Here's the third question:

If what I just wrote is an accurate description of the way GIMP will 
eventually work, can the future workflow can be tested *now* by:

1.Promoting the non-sRGB image to 32-bit floating point

2. Doing an ICC profile conversion *from* whatever ICC profile color 
space it might be in (perhaps ProPhotoRGB), *to* the GIMP built-in sRGB 
profile

3. Editing the image as desired

4. Doing an ICC profile conversion *from* the GIMP built-in sRGB profile 
*to* the desired output ICC profile (perhaps again ProPhotoRGB) and then 
exporting it to disk.


Kindest regards,

Elle