Optimal texture formats
Reading around on the Internet there are a few tidbits I have come across several times, they are:
1. Modern GPUs don't have hardware support for uncompressed RGB textures. They will be converted to RGBA internally because the GPU doesn't like 3 component textures.
2. The best transfer format to use (especially on Windows?) is GL_BGRA. If you use GL_RGBA, the driver will have to swizzle your texture data when you call glTexImage2d, slowing the performance.
I've read both of those countless places it seems and so I decided to corroborate this by using ARB_internalformat_query2. I did the appropriate glGetInternalformativ calls with GL_INTERNALFORMAT_PREFERRED and GL_TEXTURE_IMAGE_FORMAT. What I got from this was different from what I expected considering the things I read.
According to glGetInternalformativ the internal format used when you ask for GL_RGB8 is GL_RGB8, the optimum transfer format for GL_RGB8 is GL_RGB and the optimum transfer format for GL_RGBA8 is GL_RGBA. So, is what I read outdated, or is my graphics driver lying to me?
I have a AMD HD 5850 and am using the latest drivers with OpenGL 4.3.
the driver will have to swizzle your texture data
Swizzling is essentially free
Internal formats are likely to change with every release of hardware - I would not focus too much time on trying to optimize the load of the texture other than not wasting space by for example using a 32-bit format when 8 bits will do.