[Mesa-dev] Question about handling RGBA/BGRA in etnaviv driver

[Wladimir]

Yes, but it seems suboptimal, incurring overhead on every rendered pixel.

Another way that I just realized would be to convert a texture to BGRA the
first time it's rendered to.

In contrast to the shader solution that has only a one-time overhead. Is
this a stupid idea for any reason?

Wladimir

On Feb 3, 2017 11:00, "Christian Gmeiner" <christian.gmeiner at gmail.com>
wrote:

> Hi,
>
> 2017-02-03 10:56 GMT+01:00 Wladimir J. van der Laan <laanwj at gmail.com>:
> > Hello,
> >
> > With the Etnaviv driver we're running into an issue: The GPU can only
> render *to*
> > 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 most
> > appropriate when being asked for "GL_RGBA" textures. So back when
> etnaviv supported
> > 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.
> This
> > fails now that GL_RGBA textures are really GL_RGBA, and it doesn't
> handle that
> > 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
> away a
> >   feature.
> >
> > - Another way would be to try to fix Qt to cope with this, and try e.g.
> GL_BGRA_EXT
> >   when it wants to render to a texture. Burdening the client code with
> this seems
> >   unintuitive to me.
> >
> > - Another hack would be to implement shader variants, and swap R/B in
> the pixel
> >   shader to emulate rendering to RGBA.
> >
>
> In my opinion shader variants are the way to go - almost every driver
> make use of them to
> 'fix' such issues.
>
> greets
> --
> Christian Gmeiner, MSc
>
> https://www.youtube.com/user/AloryOFFICIAL
> https://soundcloud.com/christian-gmeiner
>
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