With PVRTC there are six different variables[REF] stored in each block: Modulation data (32 bits), punch-through alpha flag (1 bit), color A (15 bits), color A opaque flag (1 bit), color B (14 bits) and color B opaque flag (1 bit).
Although in PVRTC the opacity flags can be set independently for the A & B colours, in PVRTC2, Color A and Color B must both be in same format (i.e.