sprite_palettes

Mod: Additional palette selection bits per sprite, unified sprite palette

The original Gameduino supports four palettes for 256 color sprites (1024 colors in total), but only two (separate) palettes each for 4 and 16 color sprites, respectively. This mod makes all sprites use palette colors from the same main sprite palette (RAM_SPRPAL), and allows 4 and 16 color sprites to choose between many more sprite palettes (sub-palettes of RAM_SPRPAL).

This mod builds on the 512 tiles mod, which introduced the ability to store an extra bit per byte in the tile map. This mod makes these ninth bits availble in the sprite records (RAM_SPRVAL) too, for a total of 36 bits per sprite record.

The four extra bits (eeee) in the sprite record are used for palette selection. If you want to use them, the recommended procedure for writing a sprite record is:

• First, write eeee to NINTH_WRITE
• Then, write the the four bytes that make up the rest of the sprite record to RAM_SPRVAL, in order
• After this, the value that was written to NINTH_WRITE has been right shifted by four. If you don't want to keep it, write a zero to NINTH_WRITE.

Each write to a byte in RAM_SPRVAL will update the corresponding extra bit from NINTH_WRITE.

New registers:

• SPR_PALROT_MASK (0x28c2) - Which of the sprites' rotation bits should be used for palette selection instead?
  • bit 0: diagonal flip bit for 4 color sprites
  • bit 1: horizontal flip bit for 4 color sprites
  • bit 2: vertical flip bit for 4 color sprites
  • bit 3: diagonal flip bit for 16 color sprites
• SPR_PALBASE_4 (0x28d0) - The 4 color palettes start at color index 4*SPR_PALBASE_4 in the main sprite palette
• SPR_PALBASE_16 (0x28d1) - The 16 color palettes start at color index 4*SPR_PALBASE_16 in the main sprite palette

The color index into RAM_SPRVAL is calculated differently for 4/16/256 color sprites:

• 4 color sprites: (vhdeeeepcc) + 4*SPR_PALBASE_4
• 16 color sprites: (deeeepcccc) + 4*SPR_PALBASE_16
• 256 color sprites: (ppcccccccc) + 16*(eeee)

where

• vhd are the sprite's rotation bits (vertical/horizontal/diagonal flip), zeroed in this calculation unless the corresponding bit in SPR_PALROT_MASK is set
• eeee are the sprite's ninth bits
• p are the sprite's original palette selection bits (two for 256 color sprites, one for the other)
• the c bits are given by the sprite's pixels

A short demo video of the mod is available at https://youtu.be/jUV9lBzyr1g

Note: The RAMB16_S9_S36 primitive used to implement RAM_SPRVAL in the Gameduino code didn't seem to synthesize correctly for my FPGA when I tried it (the ninth bits were always zero). If you encounter this problem, you can enable the RAM_SPRVAL_INFERRED define to use an alternate implementation (which did take more block RAM when I synthesized it for my FPGA, though).

Version history

• v1: Initial version (in this directory)
• v2/
  • The Gameduino's COMM registers (0x2890 - 0x28bf) turned out to be implemented in the same memories as the 4 color palettes, and disappeared when they were removed. Put them back.
  • Based on v2 of mods/512_tiles, with further bug fixes