Blitter and sprites with transparency

[elliot]

If I was to copy a 4 bitplane sprite to a 4 bitplane "screen", how would you go about having a transparent colour? e.g you copy a 32x32 sprite to a destination but you want colour 0 of that sprite to NOT overwrite the background?

[Cyprian]

there are two methods:
A) two the BLiTTER passes: 1) mask AND destination; 2) sprite OR destination.
B) "compiled sprites" - where the CPU drives the BLiTTER fore every sprite's horizontal line. Endmask 1/2/3 is used for masking.

[Orion_]

if your sprite transparency color is 0, you can write a simple routine to generate the 1 bitplane mask by ORing all the 4 bitplanes together and store the result in a buffer.
then apply this 1 bitplane mask using AND operation to all of your 4 bitplane screen background, then you can OR your sprite on 4 bitplane