A solution for gamma-adjustment support in Wayland

[Daniel Stone]

Hi Mattias,

On 14 December 2016 at 00:20, Mattias Andrée <maandree at kth.se> wrote:
>     There should also be a protocol for identifying whether a VDC have
> CLUT that can be edited, and whether the compositor supports cooperative
> gamma. The size and the depth of the CLUT should should also be
> retrievable. Since cooperative gamma is supported, the size and depth of
> the CLUT can exceed the controller’s CLUT for better results when output
> filters are chained together.

'The size and depth' and 'the CLUT' are no longer applicable. Colour
management units, incorporating two LUTs and a matrix (coarsely,
degamma-transform-regamma) are becoming rather common now. These units
can be present per-plane as well as per-output/pipe. Sometimes you
have to make tradeoffs.

For instance, on recent Intel hardware, you can have the matrix with
10-bit LUTs on either side with 512 entries each, or you can have the
matrix with only _one_ 10-bit LUT (either pre- or post-matrix) which
can now contain 1024 entries. The weighting for interpolation between
entries can be programmable, and the last entry in the post-matrix LUT
is used to deal with overshoot, as the matrix can produce values in
the range of [-3.0,3.0].

Tegra is pretty similar, except that its indexing into the LUT is very
much non-uniform; the tradeoff for simplicity is that it's a fixed
8-bit -> 12-bit LUT pre-matrix, and the reverse post-matrix; wider
colour modes are not supported. NVIDIA's desktop hardware is a black
box to me, but their technical marketing material (if nothing else)
very strongly suggests that they have something very similar. AMD
hardware (CIK, at least) has roughly the same similar functionality,
except it's not supported with the open driver.

The point is that I'm extremely wary of copying X11 by way of encoding
this into an API; it's a truly dizzying space to even enumerate, let
alone abstract.

Cheers,
Daniel