Parameters func Specifies the alpha comparison function.
This value is clamped to the range 0 through 1, where 0 represents the lowest possible alpha value and 1 the highest possible value. The default reference is 0. The alpha test discards fragments depending on the outcome of a comparison between the incoming fragment's alpha value and a constant reference value. The comparison is performed only if alpha testing is enabled. The incoming alpha value is compared to ref using the function specified by func .