Sprite compression (Generation I)

This article is incomplete. Please feel free to add any missing information about the subject. It is missing: needs proper analysis. The official method may also have been in the source code.

Sprite compression is the means in Generation I for where pure graphics data is compressed into a Pokémon or Trainer sprite. The sprite is decompressed again using the sprite decompression routine. While the source compressor code is not in the ROMs; there are a few rules for a compressed sprite:

• The image to compress is usually a maximum of 56x56 pixels. Other Pokémon sprites can be 32x32, 40x40, 48x48, or 56x56 pixels.
• Each nybble of the starting offset to the compressed sprite is dimensions/8. Front sprites can be of various sizes; a 56x56 sprite is represented as 0x77, a 48x48 one as 0x66, a 40x40 one as 0x55, a 32x32 as 0x44.
• Backsprites are internally 32x32 pixels (0x44). Usually, these backsprites also have a little white space at the bottom (hidden by the battle menu).
• White space occupies smaller sprites to add up to 56x56.
• Unofficially, a .2bpp file can be converted into a compressed Pokémon sprite using the tool pkmncompress, and a .PNG into a compatible .2bpp file using rgbds. "Sprite import tools for RBY" can be downloaded within Evie's Google Sites backup here (based on the previous work of the GB Dev and Skeetendo communities).

If copying individual rows from a VRAM viewer, it may be necessary to rearrange the bytes; i.e. in this sample image for a 56x56 (0x44) sprite:

Unintended compressed sprites with dimensions of 0 are known for their memory overflows, such as Yellow MissingNo.'s sprite (treated as dimensions of 256, or (256/8)=32).

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

Split:

5100FF001D0000000100010206010502|1000FE00D805080019041D02CA15946B|0000A200ED100000B7400EF131DEF992|00000000550000004000802018040400
07000102060109000201010000000000|730D37086B145CA3CDF753EF2FD7B1CE|2FD0FD0211018C00C59EFB8E8E5FDF0E|04000400000448807C5040303040C020
00000000000000000000000000000000|66DD87FFEF5DAD7EBB7CDC3FB17F3DFE|570CEF00D500FF00E91741BE2CD7F30C|7000A04020C0C080C000E08040C0E0C0
00000000000000000000000000000000|F17F3DFADE7D3CFFEF7F26FF32FD3EFF|22DDAFF06FF105FB3BD542BF25DF37C8|60C000E0A060C020806000E060E000E0

Rearrange (left-most columns top-bottom; next set of columns to the right, repeat, and unsplit):

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

Processing through pkmncompress gives:

44BE9B70D04EA91B0AA9506666A2D45C1EC318D5043EF0EE324C70BEC28870D850F232206048970A852221250837210CA6C323B86182898288C3829C72970D0506250B06CC626D0C5BB082EC62B2FAC288A97552D210A3A62239AB9DEE9DFE42A0C72BA0FFFBA72791E8385B37FF777EE82BFFFFFE844C0A47DFFFF7742861A458C83FDFC3E17F77E0E8587EF84BEFFFFB7FE161F5E83AB3FFEBDF8E0FDE1C8DBFDFFFD8487FF8EAFBDFFD4D8A0C560BBEEC17FF628C957DD0C1505410A46AB6732770

(Can be applied with arbitrary sprites or ROM hacking)

Sample image loaded using arbitrary sprites (9ゥ (E6) with default palette); exported drawing of Torchic from Game Boy Camera