[chongli]

Why is this something a Wayland compositor (a glorified window manager) needs to worry about? Apple figured this out back in the 1990s with ColorSync and they did it once for the Mac OS and any application that wanted colour management could use the ColorSync APIs.

[hedgehog]

Color management infrastructure is intricate. To grossly simplify: somehow you need to connect together the profile and LUT for each display, upload the LUTs to the display controller, and provide appropriate profile data for each window to their respective processes. During compositing then convert buffers that don't already match the output (unmanaged applications will probably be treated as sRGB, color managed graphics apps will opt out of conversion and do whatever is correct for their purpose).

[chongli]

Yes, but why is the compositor dealing with this? Shouldn't the compositor simply be deciding which windows go where (X, Y, and Z positions) and leave the rendering to another API? Why does every different take on a window manager need to re-do all this work?

[hedgehog]

Turning the question around, what other part of the system _could_ do this job? And how would the compositor do any part of its job if it doesn't have access to both window contents and displays? I'm not super deep in this area but a straight-forward example of a non-managed app and a color-aware graphics app running on a laptop with an external display seems like it is enough to figure out how things need to go together. This neglects some complicating factors like display pixel density, security, accessibility, multi-GPU, etc, but I think it more or less explains how the Wayland authors arrived at its design and how some of the problems got there.

[chongli]

I'm questioning the idea that people should be writing compositors at all. Why doesn't Wayland itself do the compositing and let everyone else just manage windows?

It's like going to Taco Bell and they make you grind your own corn for your tortillas.

[hedgehog]

Why? Probably better to ask the Wayland developers that. Maybe you're right. That said, whether everyone uses the same compositor and window management is modular, or not and shared code travels as libraries, I don't think the complexity of color management is much different.

[zahlman]

I mean, when I hear the word "compositing" I definitely imagine something that involves "alpha" blending, and doing that nicely (instead of a literal alpha calculation) is going to involve colour management.

[chongli]

That's on the Wayland team though. They drew up the new API boundaries and decided that all window managers would now be in the business of compositing.

If I wanted to put it most uncharitably, I'd say they decided to push all of the hard parts out of Wayland itself and force everyone else to deal with them.