Full screen color management

[Richard Hughes <hughsient gmail com>]

First a little introduction if I may. if you're easily bored please
skip to the last paragraph.

An image is usually generated with a source profile of the devices
used to generate it, that's a lump of data that describes what kind of
"red" is actually red. This is encoded in the ICC profile, which is
normally in the image metadata. Images without metadata are usually
assumed to be sRGB.

A color server usually runs at session start and sets the _ICC_PROFILE
atom on the XRandR output (or root window) of the screen. This is
either the manufacturer generated profile, or the profile that the
user has lovingly created. The color server also sets up the color
lookup tables (gamma tables) if required.

An application that does "early color binding" takes the source
profile (e.g. "Hughsie's Nikon D60") and the destination profile ("T61
IBM Internal Panel") and converts one color gamut to another using
something like LCMS on Linux, or ColorSync on OSX. This means the
"red" that you saw in the viewfinder matches pretty much what you see
on the screen, modulo how crappy your laptop LCD panel is. It then
tags the Window which is displaying the image with an atom so that the
next part works (read on...). This only works for applications that
care about color, and only works if the window is entirely on one
output (as different outputs might have different output profiles).

A "whole screen" color manager is normally an optional plugin to a
compositing window manager. There is a pretty hacky proof of concept
plugin written for compiz, but that's doing far more stuff than I'm
proposing here. It tries to convert the whole screen to the
destination gamut, without affecting windows that have already been
early-bound. It can detect windows that should be ignored using the
atom that has been set on some windows (possibly none, or typically
one) and assumes the rest is sRGB and converts this to the destination
(monitor) profile.

Now, you might be wondering why the last part is required, as most
people say that LCD's are supposed to be sRGB anyway. Well, I would
disagree, and will disagree more as the years pass. More and more LCD
panels are being sold that are "wide gamut" and therefore can display
colors well outside of sRGB. Panels (like this T61) are also very much
smaller than sRGB and need to be corrected as best we can. On a wide
gamut monitor 100% red is piercing red (the sort of piecing red that
hurts your eyes) and needs to be controlled. Of course, we want to be
able to use the wide gamut features for photographs and HD movies, but
we really don't want to be color correcting window borders and [x]
controls.

If you've read this far, I'm impressed, and you're probably in the
minority. Well done.

What I'm proposing is a plugin for mutter that uses lcms to color
convert the whole screen, masking out the color-corrected regions.
Using mutter allows this to work with GNOME 2 and GNOME 3, and mutter
also seems to be the sweet-spot for this kind of display correction
without putting ICC profile decoding and gamut mapping into the X
server.

Is full screen color correction something that you think is a good
idea, and also would you allow me to create the required hooks in
mutter-plugin.h to make this possible?

Thanks,

Richard Hughes