[RFC wayland-protocols] Add the color-management protocol

[Graeme Gill]

Daniel Stone wrote:

Hi,

> it. Some KMS devices can perform per-plane colour management
> (typically a variable-depth degamma LUT, 3x3 CTS matrix and re-gamma
> LUT), such that a non-fullscreen client can have correct colour
> conversion, independent of any compositor rendering.

there's certainly a lot of complexity in full utilizing
hardware color transform capabilities. Similar
capabilities exist in some higher end displays themselves,
although this basically boils down to a mechanism to
get the display to emulate a particular colorspace.

If the computer system driving the display is smart enough
though, such display capabilities can largely be ignored,
and the display simply run in a native, widest gamut mode,
making that capability available to applications.

(Exception - the display per channel LUTs may
be higher precision that what is offered by the
graphics card VideoLUTs, therefore it is desirable
to set calibration in the display. Unfortunately
such in display capabilities are largely hidden
by proprietary protocols. Typically some means of communicating
wit the display via the EDID is needed to support this.)

> However, another
> client could then come along and force your client off the plane, such
> that you land in the compositor's GPU pipeline before display: this
> rendering may be done in a 'lowest common denominator' colourspace,
> such that the most optimal / least lossy output from your colour-aware
> client would then _not_ correspond to your display device's native
> characteristics.

Some details make for possible disadvantages with color
critical applications. Is a "lowest common denominator" space
one with a gamut smaller than any attached display, ensuring
that every color can be displayed without loss, while not
allowing any use of a displays full gamut, or is it
a colorspace that has a larger gamut than any attached
display, meaning that colors will get clipped in
ways the application has no control over, or may
not want ? (i.e. the particular application may
want some other intent such as perceptual or saturation
gamut mapping).

Cheers,
	Graeme Gill.