[compiz] color management spec

[Owen Taylor]

On Sun, 2008-06-15 at 01:38 +0200, Tomas Carnecky wrote:
> Owen Taylor wrote:
> > If the visual of a window is ARGB, it can't hold XYZ or Lab, or most
> > obviously CMYK data.
> 
> > difficulties of per-monitor specifics... if I have an image in Adobe RGB
> > with 8 bits per primary, it might be interesting to preserve that
> > colorspace and use that as the colorspace on my toplevel, but that's not
> > done to avoid having colorspace conversion code in the application: it's
> > done to avoid losing gamut when converting it to something else.
> 
> If you have an image in CMYK the best way to avoid losing gamut, just 
> like in your AdobeRGB example, is to let the CM convert it at the very 
> last stage. So it might be useful to be able to upload CMYK pixels into 
> the xserver and tag the data somehow. I don't see any conflict there as 
> long as the application and CM agree on the pixel format.

I think there is a significant difference between having the CM deal
with the exact interpretation of RGB in a RGB pixel format, and having
the CM take "random" data stuffed into a RGB pixel and convert it - does
the user still see something intelligible if the CM/app communication
goes wrong? If a CM isn't running? If the user takes a screenshot of the
window?

Also, of course, CMYK isn't a precisely defined color space like sRGB;
you'd have to accompany the window with a color profile.

> > Honestly, the only reason this is at all interesting is the limitations
> > of RGB 8:8:8. The long term solution here is support for something
> > like scRGB (http://en.wikipedia.org/wiki/ScRGB_color_space.)
> 
> First the server would have to support buffers with at least 6 byte per 
> pixel, which it does not currently. I would love to see support for 
> arbitrary pixel formats in the xserver. I would even go as far as having 
> the xserver create bare buffers and then tag them with the pixel format 
> / icc profile. After all, if a (OpenGL-based) CM is running the xserver 
> doesn't need to know anything about the windows besides the dimensions.

There's certainly significant work there at all layers. I believe that
there was discussion on the cairo mailing list of someone starting doing
work within pixman, which is certainly a good place to start.

As above, I'm not sure uninterpreted window buffers is a good idea... or
really necessary. Better to support a small set of things well than a
large set of things poorly.

- Owen