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

[Sebastian Wick]

On 2019-03-04 12:27, Pekka Paalanen wrote:
> On Mon, 4 Mar 2019 19:04:11 +1100
> Graeme Gill <graeme2 at argyllcms.com> wrote:
> 
>> Pekka Paalanen wrote:
>> 
>> > My failing is that I haven't read about what ICC v4 definition actually
>> > describes, does it characterise content or a device, or is it more
>> > about defining a transformation from something to something without
>> > saying what something is.
>> 
>> The ICC format encompasses several related forms. The one
>> that is pertinent to this discussion is ICC device profiles.
>> 
>> At a minimum an ICC device profile characterizes a devices color
>> response by encoding a model of device values (i.e. RGB value 
>> combinations)
>> to device independent color values (i.e. values related to device
>> independent CIE XYZ, called Profile Connection Space values in ICC
>> terms). A simple model such as color primary values, white point
>> and per channel responses is easily invertible to allow transformation
>> both directions.
>> 
>> For less additive devices there are more general models (cLut -
>> multi-dimensional color Lookup Table), and they are non-trivial
>> to invert, so a profile contains both forward tables (device -> PCS
>> AKA A2B tables) and reverse tables (PCS -> device AKA B2A tables).
>> 
>> Then there is intents. The most basic is Absolute Colorimetric
>> and Relative Colorimetric. The former relates the measured
>> values, while the latter one assumes that the observer is adapted
>> to the white point of the devices. Typically the difference is assumed
>> to be a simple chromatic adaptation transform that can be encoded
>> as the absolute white point or a 3x3 matrix. The default intent
>> is Relative Colorimetric because this is the transform of least
>> surprise.
>> 
>> cLUT based profiles allow for two additional intents,
>> Perceptual where out of gamut colors are mapped to be within
>> gamut while retaining proportionality, and Saturation where
>> colors are expanded if possible to maximize colorfulness. These
>> two intents allow the profile creator considerable latitude in
>> how they achieve these aims, and they can only be encoded using
>> a cLUT model.
> 
> Hi Graeme,
> 
> thank you for taking the time to explain this, much appreciated.
> 
> I'm still wondering, if an application uses an ICC profile for the
> content it provides and defines an intent with it, should a compositor
> apply that intent when converting from application color space to the
> blending color space in the compositor?

I think the correct approach would be to first convert from
application color space to the output color space using the intent and
then to blending color space. That way all colors in the blending
color space will fit in the output color space.

> Should the same application provided intent be used when converting the
> composition result of the window to the output color space?

When all blending color sources are in the output color space so is
the resulting color. No intent required.

> What would be a reasonable way to do those conversions, using which
> intents?
> 
>> So in summary an ICC profile provides device characterization, as well
>> as facilitating fast and efficient transformation between different
>> devices, as well as a choice of intent handling that cannot typically
>> be computed on the fly. Naturally to do a device to device space 
>> transform
>> you need two device profiles, one for the source space and one
>> for the destination.
> 
> Do I understand correctly that an ICC profile can provide separate
> (A2B and B2A) cLUT for each intent?

That's my understanding as well.

>> > What is the use for a device link profile?
>> 
>> Device link profile use was a suggestion to overcome the previously 
>> stated
>> impossibility of a client knowing which output a surface was mapped 
>> to.
>> Since this no longer appears to be impossible (due to 
>> wl_surface.enter/leave events
>> being available), device link profiles should be dropped from the 
>> extension.
>> It is sufficient that a client can do its own color transformation
>> to the primary output if it chooses, while leaving the compositor to 
>> perform
>> a fallback color transform for any portion that is mapped to a 
>> secondary output,
>> or for any client that is color management unaware, or does not wish 
>> to
>> implement its own color transforms.
>> This greatly reduces the implementation burden on the compositor.
> 
> Btw. wl_surface.enter/leave is not unambigous, because they may
> indicate multiple outputs simultaneously. I did talk with you about
> adding an event to define the one output the app should be optimizing
> for, but so far neither protocol proposal has that.
> 
> Niels, Sebastian, would you consider such event?

My proposal has the zwp_color_space_feedback_v1 interface which is
trying to solve this issue by listing the color spaces a surface was
converted to in order of importance.

> 
> Thanks,
> pq
> 
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