> [Transparency] introduces some complexity in the rasterizer. To flip only one bit [of the colour value] we need to know the prior value of the pixel at this l

> [Transparency] introduces some complexity in the rasterizer. To flip only one bit [of the colour value] we need to know the prior value of the pixel at this location. This is like one of only three advantages to using planar graphics, like the Amiga did. With bitplanes you don't need to read back video memory: you just assume the top bit is for transparency effects only and blat out your spans. [The other two advantages are cool moire effects because you can move the planes relative to each other, and memory / bus bandwidth efficiency for awkward colour depths which don't fit nicely into a byte or a nybble, like 8 (3 bits per pixel) or 32 (5 bits per pixel)]