Sprites Transparency Index Register
|Short Description||Index into sprite palette (of "transparent" colour).|
Index into sprite palette, set to $E3 after reset.
When rendering sprites, the pixel index is compared before palette-offset (byte 3 of Sprite Attribute Upload ($xx57 / 87)) is applied to it. The transparency is index-based, not colour based (contrary to Layer 2/ULA/LoRes modes, where resulting pixel colour is compared).
The 4-bit sprite compares pixel-index to low 4 bits of this register, i.e. 4-bit pixel $3 (%0011) is treated as transparent, if this register still contains default $E3 (%1110_0011) value.