There are two types of variables in Cg: single values and packed arrays. The latter can be identified because their type ends with a number such as float3 or int4. As their names suggest, these types of variables are similar to structs, which means that they each contain several single values. Cg calls them packed arrays, though they are not exactly arrays in the traditional sense.
The elements of a packed array can be accessed as a normal struct. They are typically called x, y, z, and w. However, Cg also provides you with another alias for them, that is, r, g, b, and a. Despite there being no difference between using x or r, it can make a huge difference for the readers. Shader coding, in fact, often involves calculation ...
