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In a comment last year, Sir_Cmpwn wrote:

> We don't just paste pixels on the screen, we composite them. This involves alpha blending, projecting the scene onto an orthographic output projection, and more.

What would we lose, in usability and not just eye candy, if the "compositor" did just paste pixels onto the screen? Woudl such a compositor be as usable without GL as Xorg is?

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You lose any alpha blending and ability to retain the contents of window if the app responds slowly or stops responding at all. Some other features like hidpi scaling done by compositor would also be lost. Otherwise it seems like it might be doable (although I'm not 100% sure, Wayland protocol definitely wasn't designed with this in mind), but the implementation would be so different to the standard GLES mode that it should probably be just a separate project.

A perfectly usable GUI is possible without alpha blending, isn't it? If I'm not mistaken, Windows didn't have alpha blending until Windows 2000, and it was still a runaway success.

Alpha blending and retaining the contents of windows doesn't need a full-blown compositor...

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