[Mesa-dev] _mesa_texstore_argb8888 with GL_RGBA -> GL_RGB

[Tristan Schmelcher]

Hello,

I have an OpenGL app where I'm writing video frames to textures. The
video frames are semantically opaque but the alpha in the source
pixels is set to 0xff, so I have been using glTexImage2D with
format=GL_RGBA and internalFormat=GL_RGB to indicate that the input
has a valid alpha channel but that the OpenGL implementation could
discard it if it wanted to (e.g., if the hardware had native support
for GL_RGB which was faster than its GL_RGBA support, or something).

This has worked well for me on most systems, but today I found that it
results in poor glTexSubImage2D() performance on an Intel IGDNG_M GPU
(i965-based) with MESA 7.7.1 (using Ubuntu Lucid). Because
internalFormat is GL_RGB instead of GL_RGBA, _mesa_texstore_argb8888
takes a slow path by using _mesa_swizzle_ubyte_image instead of
memcpy_texture.

Wouldn't it make sense for _mesa_texstore_argb8888 to use
memcpy_texture in this case?

Or maybe it is right to not do that since there is no way for it to
know that the alpha component in the input is already set to 0xff ...
but in that case shouldn't it at least take the third path--the one
that uses PACK_COLOR_8888(0xff, ...) ?