[RFC wayland-protocols 1/1] unstable: add color management protocol

Graeme Gill graeme2 at argyllcms.com
Mon Mar 4 05:28:43 UTC 2019

Sebastian Wick wrote:
> A color space consits of three primaries, the white point and the EOTF/OETF.

That's not true in general. An RGB colorspace can be described (and is
describable using ICC profiles) in ways that cannot be decomposed
into 3 primaries and per channel curves, since some devices
are not additive. A cLUT encoding is such a description.

Sharma, Shashank via wayland-devel wrote:
> Well, this is slightly off the chart for me. If I understood this correctly, the cover
> letter talks about the intent is to focus on Colorspace only, but here we are
> differentiating enum values based on their EOTF HDR curve, which is the only difference
> between these three colorspaces. Then why not just mind the colorspace regardless of
> the HDR curve.
> On the other hand, we can accommodate the HDR specific information also, like the EOTF
> curve type, and the HDR metadata type and content for the frame, if we can add some
> additional filed in this same protocol. So the colorspace information will remain the
> same, and the presence of HDR metadata type / content filed can indicate if the current
> frame is HDR BT2020 frame or SDR BT2020 frame.

It is not normal in a color management context to use the term
"colorspace" to refer just to the gamut enclose by 3 additive
primaries, instead it usually encompasses the full multi-dimensional
(i.e. 3 dimensional in the case of RGB devices) response.

ICC profiles encompass a description of a devices color response,
but also typically contain information for the mechanisms to convert
between that colorspace and others (i.e. A2B and B2A tables). For
performance and flexibility reasons, this can and does include various
perceptual and gamut transformations chosen by the profile maker, so
a nominated ICC profile can encompass a variety of treatments or intent
within the formalized ICC Perceptual and Saturation intents. There often
is no formal scheme for such variations, just the name, description and
other notes that come with the profile convey the intention of the maker.

	Graeme Gill.

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