grTexCombine() also keeps track of required vertex parameters for the rendering routines. GR_COMBINE_FACTOR_NONE is provided to indicate that no parameters are required. Currently it is the same as GR_COMBINE_FACTOR_ZERO.
Table Texture Mapping.4 Texture combine functions.
The rgbFunction and alphaFunction arguments to grTexCombine() can take on any of the values listed in the first column. The second and third columns show the computed color or alpha value for each choice. clocal and alocal represent the color components generated by indexing and filtering from the mipmap stored on tmu; cother and aother represent the incoming color components from the neighboring TMU (refer to Figure Texture Mapping.1).
