Accessing and modifying packed arrays

Loosely speaking, the code inside a shader has to be executed for at least every pixel in your screen. This is the reason why GPUs are highly optimized for parallel computing; they can execute multiple processes at the same time. This philosophy is also evident in the standard type of variables and operators available in Cg. Understanding them is essential, not just so that you can use the shaders correctly, but also to write highly optimized ones.