Re: [Gimp-developer] Some blend modes break in unbounded mode sRGB

[Elle Stone <ellestone ninedegreesbelow com>]

On 04/10/2014 08:39 PM, Przemyslaw Golab wrote:
> Isn't that expected? You don't change color space, for it to have the
> same results.
>
> You choose best color space for the job and use it from beginning to the
> end,
> or if you know what you are doing convert it in middle of work to do the
> thing
> you want to do.
> If all color spaces look the same whats the point of using them.

I agree 100% with what you just said. When editing an image,
choose the best color space for the job.

But it seems that the current plan is that when GIMP 2.10 is released, 
all image editing will be done in the unbounded mode sRGB color space. 
Quoting from the post Gez mentioned:

Re: [Gimp-developer] Three questions about opening an image and 
converting it to linear light RGB
> On 04/02/2014 04:38 PM, Michael Henning wrote:>

> > On Wed, Apr 2, 2014 at 9:31 AM, Elle Stone
> > <ellestone ninedegreesbelow com> wrote:

> > 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.
>
> We will need a way for users to change the incoming icc profile on
> import, just in case it has the wrong embedded profile. There will be
> some level of user awareness, but I don't know how much.
>
> > 3. All subsequent image editing will be done in the extended sRGB/linear
> > light RGB color space.
>
> Yes. To clarify this point: Users will have the ability to choose to
> edit in different bitdepths and (probably) also the option between
> editing with the layers stored as a linear or a perceptual (sRGB TRC)
> color space (which only really affects the way layer modes work, along
> with things that depend on layer modes like painting).
>
> Also, gegl operations can request to do their work in different color
> spaces, so depending on the operation being performed, the editing may
> actually happen in other color spaces. This is transparent to the
> user.

My understanding is that the phrase "gegl operations can request to do 
their work in different color spaces" does *not* refer to different 
color spaces such as ProPhotoRGB, AdobeRGB1998 or Rec 2020. Rather it 
means "work in sRGB converted to CIELAB", "work in sRGB converted to 
HSL", "work in sRGB converted to CMYK", and so forth. I asked about this 
point in the previous email, but so far no one has clarified whether my 
understanding is correct or not. So I'm asking again. Please? Anyone? Is 
my understanding correct?

It seems that the plan for GIMP going forward is firmly in the direction 
of converting all images to unbounded mode sRGB before editing can 
begin. So I've been testing to see what it will be like to edit images 
in the sRGB color space instead of my preferred color spaces.

Some editing operations work already in unbounded mode sRGB, meaning 
these operations are actually *completely independent* of the linear 
gamma color space in which they are performed, assuming unbounded mode 
ICC profile conversions. Other editing operations will work once 
clipping code is removed.

For example I'm pretty confident that scaling, gaussian blur, unsharp 
mask, normal blend mode, Addition blend mode, and Subtract blend mode 
give exactly the same results regardless of what linear gamma (or "gamma 
corrected") RGB color space the operation is performed in, assuming 
clipping code is removed as appropriate.

However, some editing operations are *highly dependent* on the RGB color 
space chromaticities. These operations give odd, wrong, unexpected, 
unwanted and sometimes completely unuseable results after an image is 
converted from wider gamut color spaces to unbounded mode sRGB.

I'm not merely speculating about what "might work" and what "might not 
work". I've providing concrete examples.

I encourage everyone on this list who shoots raw to output your images 
in a larger color space like ProPhotoRGB, pick some of the more 
saturated images, and compare what happens when editing in the larger 
gamut color space vs editing in the unbounded mode sRGB.

Your testing will be made somewhat complicated by the fact that GIMP 
does background TRC conversions between the sRGB TRC and the linear 
gamma TRC. The workaround is to output your interpolated raw file images 
as ProPhotoRGB with the sRGB TRC. RawTherapee allows you to do this very 
easily. Just pick "RT_Large_gsRGB" as the "Output Profile".

Open the image with GIMP from git, and change the precision to 32-bit 
floating point linear precision.

Then go back to the unedited image, change the precision to 32-bit 
floating point linear precision, *convert to unbounded mode sRGB* (the 
built-in GIMP sRGB profile), and repeat the edits.

Compare the two resulting images and see whether you think editing all 
images in the unbounded mode sRGB color space will work for you.

Perhaps I'm making mistakes in my tests, but I don't think so. Perhaps 
there is a workaround for the examples I've posted, but I think the only 
realistic workaround is to not convert the image to the unbounded mode 
sRGB color space in the first place.

Elle Stone