[Mesa-dev] Fwd: Question about handling RGBA/BGRA in etnaviv driver
laanwj at gmail.com
Fri Feb 3 10:09:09 UTC 2017
Sorry for getting the list wrong again. Please reply to mesa-dev not
---------- Forwarded message ----------
From: "Wladimir J. van der Laan" <laanwj at gmail.com>
Date: Feb 3, 2017 10:56
Subject: Question about handling RGBA/BGRA in etnaviv driver
To: <dri-devel at lists.freedesktop.org>, <etnaviv at lists.freedesktop.org>
Cc: "Chris Healy" <cphealy at gmail.com>
With the Etnaviv driver we're running into an issue: The GPU can only
BGRA formats. It can however render *from* BGRA as well as RGBA textures.
I know that the OpenGL ES standard allows drivers to choose what order is
appropriate when being asked for "GL_RGBA" textures. So back when etnaviv
only BGRA, Mesa automatically picked that and everything was okay.
However a recent patch added support for RGBA formats in etnaviv_format.c.
Now, Mesa creates a real GL_RGBA texture when this is requested. This is all
and well for rendering, however for anything using FBO to render to textures
this is a problem.
Qt, for example, is assuming it can attach the GL_RGBA texture to a FBO.
fails now that GL_RGBA textures are really GL_RGBA, and it doesn't handle
error to fall back to something else so rendering issues ensue.
I'm not sure how to handle this:
- The quick fix would be to revert the RGBA formats patch, but the hardware
supports rendering *from* RGBA textures fine so this would be throwing
- Another way would be to try to fix Qt to cope with this, and try e.g.
when it wants to render to a texture. Burdening the client code with this
unintuitive to me.
- Another hack would be to implement shader variants, and swap R/B in the
shader to emulate rendering to RGBA.
Neither seems great. Does anyone have suggestions, do any of the other
(gallium) drivers have this problem?
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