OpenGL Programming Guide

Appendix AOrder of Operations

Pixel Operations

Pixels from host memory are first unpacked into the proper number of components. The OpenGL unpacking facility handles a number of different formats. Next, the data is scaled, biased, and processed using a pixel map. The results are clamped to an appropriate range depending on the data type, and then either written in the texture memory for use in texture mapping or rasterized to fragments.

If pixel data is read from the framebuffer, pixel-transfer operations (scale, bias, mapping, and clamping) are performed. The results are packed into an appropriate format and then returned to processor memory.

The pixel copy operation is similar to a combination of the unpacking and transfer operations, except that packing and unpacking is unnecessary, and only a single pass is made through the transfer operations before the data is written back into the framebuffer.

OpenGL Programming Guide