Re: [Gimp-developer] [Gegl-developer] Don't make an architectural mistake based on a groundless premise

On Sat, Oct 11, 2014 at 1:18 AM, Elle Stone
<ellestone ninedegreesbelow com> wrote:
I really don't want to forget about LAB. The plan is that "sRGB as PCS" will
be used for "something". I'm trying to figure out what exactly "something"
covers. So I have two specific questions. The first question is about
converting to LAB. The second question (which I haven't asked yet) has to do
with Y.

We will try to do what makes sense and helps us achieve what we need
to achieve efficiently; while keeping what works well, working well.

"sRGB as PCS" is linear gamma unbounded sRGB. I think it's reasonable to ask
why a conversion to unbounded sRGB should be involved in a conversion from
User_RGB to LAB.

For the same reason XYZ is involved (or not, depending on how the CMM
does it) in an ICC workflow.

Existing code written with assumptions of sRGB, whether in GIMP and
GEGL that we control or in XPM, GTK, GDK, or elsewhere that has sRGB
assumed will continue to work. We take the existing architecture which
is following general guidelines of assuming sRGB where none is

Assigning sRGB to images with no embedded profile is done because you can't
display images in an ICC profile color-managed workflow until an ICC profile
is assigned. How images with no embedded profile are handled is irrelevant
to the question of why a conversion from User_RGB to LAB needs to involve
unbounded sRGB.

This is not about how images with no embedded profile are handled.
sRGB derived 8bit (and to a lesser degree 16bit) formats are used for
many other things than images with no embedded profile. These pixel
formats are crucical for integrating with existing file formats and
libraries; and we want these conversions to be as fast as possible -
even if it might have an impact on the conversion speed for CIE Lab,
though that does not have to be the case either. Choosing any PCS
*but* linear sRGB would tend to make these important conversions

I can move on to my question about Y and unbounded sRGB. But I still don't
understand why unbounded sRGB should be involved in a conversion from
User_RGB to LAB.

We will do what makes most sense and is neccesary when we get there, I
suspect each RGB model with have an associated Y and YA model. Due to
how the existing BablModels interact with components and vocabulary
used to address them in babl; potential support for different TRCs is
even vaguer; we'll know when we have more code.

Maybe we have more code by the time of LGM, if not that would be an
excellent place to elaborate; until then I prefer IRC.


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