SPR Spritesheet

SPR (Unit Sprite) files contain the bitmap data for a sprite. Sp2 are similar format used for extra images used by MON shp types, but they do not have palette data or compressed regions.

Palette Data

The file starts with the palette data and contains 256 colors (512 bytes), 16 palettes of 16 colors each. Palette's use 2 bytes per color, 1 bit to indicate color semi-transparency, followed by 5 bits per channel (B,G,R). The first 8 palettes are for the unit, the second 8 palettes are for the portrait.

See this post by LameGuy64 about how psx handles transparency: https://www.psxdev.net/forum/viewtopic.php?t=953

 Semi Transparency Modes (see P00 GetTPage and semi-transparency)
   0x00: 0.5 back + 0x5 forward
   0x01: 1 back + 1 forward
   0x02: 1 back - 1 forward
   0x03: 1 back + 0.25 forward

 Example of Mode 0 if semi-transparency bit is on or off:
 -- If Semi-Transparent mode is off --
 Full-black without STP bit = Transparent (alpha = 0)
 Full-black with STP bit = Opaque black (alpha = 255)
 Non full-black without STP bit = Solid color (alpha = 255)
 Non full-black with STP bit = Solid color (alpha = 255)
 
 -- If Semi-Transparent is on --
 Full-black without STP bit = Transparent (alpha = 0)
 Full-black with STP bit = Semi-transparent black (alpha = 127)
 Non full-black without STP bit = Solid color (alpha = 255)
 Non full-black with STP bit = Semi-transparent color (alpha = 127)

OTHER.SPR is a special case and has 512 colors. 5 palettes each for chicken, frog, chest, and crystal, an all black palette between crystal and chests, and 11 mostly black palettes at the end that are normally unusable.

Pixel Data

Each set of 4 bits is the palette color index of the pixel. The total decompressed spritesheet is 256 pixels wide and 488 pixels tall.

The lower half of unit spritesheets has compressed regions (see Decompress Attack spritesheet). This starts at file offset 0x9200 (row 288). Non-unit sprites, such as OTHER and WEP do not have the compressed section. The portrait and body parts in the row are in the uncompressed portion of the spr.

The compression algorithm is: 0 - some half byte - how many 0s there are

if halfbyte = 0, 1 digit in number of 0s

if halfbyte = 7, 2 digits in number of 0s

if halfbyte = 8, 3 digits in number of 0s

if halfbyte != [0,7,8], number of 0s = halfbyte

Example: 08100 is the hex string for 256 0s (128 bytes of 0).

Note, when looked at in the rom in a hex editor, 08100 may appear as 08001 due to little-endian format.