Sprite Transparency Index Register
From SpecNext Wiki
| Number | TBRegisterNumber::$4B |
|---|---|
| Readable | TBRegisterReadable::Yes |
| Writable | TBRegisterWritable::Yes |
| Short Description | ShortDesc::Index into sprite palette (of "transparent" colour). |
Index into sprite palette, set to $E3 after reset.
When rendering sprites, the pixel index is compared before palette-offset (byte 3 of {{#ask: PortNumber::$xx57 }} ($xx57{{#ask: PortNumber::$xx57 |mainlabel=- |headers=hide |intro= / |?NumberDec#- }})) 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.