Xvfb and number of bytes per pixel

[Adam Jackson]

On Wed, 2020-09-09 at 13:28 +0200, Marcin K wrote:
> Hello,
> I'm grabbing frames from Xvfb to use them as an opengl texture. I've
> noticed that there are 4 bytes per pixel, that suggest that there's
> an alpha channel, or something else. Is it possible to force Xvfb to
> store only RGB values in the shared memory?

Not without hacking the code, no. There's some code in the shadow
framebuffer logic that will downconvert from 32bpp to 24bpp, but it's
meant for super-low-end cards like the Matrox G200SE or Cirrus Logic
GD-5446 that really only have depth 24 at 24bpp. You could hook it up
to Xvfb, but you'd save no memory or performance to do so, since (in
xserver 1.20 and later) the server can't render directly to 24bpp, so
you'd be drawing to a 32bpp surface and then copying from that to
24bpp.

At which point, since basically no OpenGL-capable GPU since like the
Intel i810 actually has an RGB24 texture format in hardware, you'd
convert _back_ to 32bpp when you upload the texture, and that
conversion would also probably be in software.

You're almost certainly better off using a 32bpp XRGB texture in the
first place.

- ajax