Hardware color correction update, continued

You have heard of the case of the dog that plays checkers? It's not that he doesn't play to well, it's amazing that he can do it at all? It's sort of like that sometimes with graphics programming.

I can't believe that my previous code worked at all, as I was not doing some very basic things. In particular, I was not using the texture handles correctly. Before any call to glTexParameteri, glTexImage2D, glTexImage3D, glTexSubImage2D and so on, you need to make sure that you're working with the correct texture unit, so you have to call glBindTexture(flag, handle) Doing this correctly gave the program a chance of working, and now I can reload the 3D color lookup table at will. I also had not called cgGLEnableTextureParameter() after calling cgGLSetTextureParameter(). Again, I can't imagine how the program worked at all before.

But anyway, now it all works. I had trouble when I enabled GL_DITHER, but that's a bit of an anachronism, anyway (back to the old SGI O2's, where double-buffered images were only 4-bits (dithered) per channel).

I had hoped to get some performance benefit by going to RGB instead of RGBA textures (less data to transfer, less data to lookup) but whatever gain there is miniscule at best. RGB textures require you to pad out each scanline to a 4-pixel boundary, which is kind of annoying (and not documented in the glTexture2D man page, but it is documented other places.) Similarly, I saw no speedup by using glTexSubImage2D instead of glTexImage2D to reload textures for each frame of a moving sequence -- at least on my development machine. I still use the glTexSubImage2D call whenever the image doesn't change size, because it seems like the right thing to do.

One minor annoyance is that it appears that there are limits to the size of texture images -- looking at 8k x 4k images, for example, doesn't seem to work -- where using glDrawPixels worked fine at that (and much larger) sizes. This is, so far, not much of a limitation -- but it could become one as the film industry moves to 4K images.

In the end, I'm getting the required 24 fps at 2048x1200 images with a 3D lookup table. Barely.