[RFC wayland-protocols v2 1/1] Add the color-management protocol

[Pekka Paalanen]

On Mon, 4 Mar 2019 23:09:59 +1100
Graeme Gill <graeme2 at argyllcms.com> wrote:

> Pekka Paalanen wrote:
> 
> Hi,
> 
> > another thought about a compositor implementation detail I would like
> > to ask you all is about the blending space.
> > 
> > If the compositor blending space was CIE XYZ with direct (linear)
> > encoding to IEEE754 32-bit float values in pixels, with the units of Y
> > chosen to match an absolute physical luminance value (or something that
> > corresponds with the HDR specifications), would that be sufficient for
> > all imaginable and realistic color reproduction purposes, HDR included?  
> 
> I don't think such a thing is necessary. There is no need to transform
> to some other primary basis such as XYZ, unless you were attempting
> to compose different colorspaces together, something that is
> highly undesirable at the compositor blending stage, due to the
> lack of input possible from the clients as to source colorspace
> encoding and gamut mapping/intent handling.
> 
> AFAIK, blending just has to be in a linear light space in
> a common set of primaries. If the surfaces that will be
> composed have already been converted to the output device colorspace,
> then all that is necessary for blending is that they be converted
> to a linear light version of the output device colorspace
> via per channel curves. Such curves do not have to be 100% accurate
> to get most of the benefits of linear light composition. If the
> per channel LUTs and compositing is done to sufficient
> resolution, this will leave the color management fidelity
> completely in tact.

...

> > Meaning, that all client content gets converted according to the client
> > provided ICC profiles to CIE XYZ, composited/blended, and then
> > converted to output space according to the output ICC profile.  
> 
> See all my previous discussions. This approach has many problems
> when it comes to gamut and intent.

Hi Graeme,

ok, doing the composition and blending in the output color but linear
light space sounds good. I'm glad my overkill suggestion was not
necessary.


Thanks,
pq
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