Sega Ages 2500 Series Vol. 17: Phantasy Star generation:2/Notes

Compression

CUE found out the coding algorithm. It's a Lempel-Ziv related one, called Lempel-Ziv Renau.

Structure of files :

• Bytes $0000 to $000a : header
  • 2 bytes : signature, always "CM"
  • 4 bytes: decoded data length, low endian
  • 4 bytes: encoded data length, low endian

• From $000a : ENCODED_DATA

• Then FLAGS. Flags are read bit per bit :
  • if bit = 0, then put one byte from ENCODED_DATA and put into DECODED_STREAM
  • if bit = 1, then the corresponding half-word in ENCODED_DATA is of the form lllloooooooooooo, where l = length and o = offset. Copy then 'length + 3' bytes from DECODED_STREAM[current_position - offset + 1] to DECODED_STREAM[curent_position].

• Fill the file with $00, up to have a file size dividible by $800.

Files specifications

Graphics files (SGGG)

They are just 256 colors palette indexed bitmap files.

• bytes $0 to $3 : 'SGGG' (identifier)
• bytes $4 to $7 : always 1, usage unknown
• bytes $8 to $9 : width (Little Endian)
• bytes $a to $b : height (LE)
• bytes $c to $f : unknown

• bytes $10 to $410 : palette. Each color is 32b RGBA. Alpha max seems to be $80 (and most colors are alpha max). Colors are stored in a weird way though :
  • colors $0 to $7
  • colors $10 to $17
  • colors $8 to $f
  • colors $18 to 1f
  • and so on...

• from bytes $410 : bitmap. The picture seems cut in 512×512 blocks in scan order, each of which described row per row, from top to bottom.

For example, a 640×448 bitmap is cut in 2 rects, one with pixels (0 - 511, 0 - 447) and the other (512 - 639, 0 - 447). No bitmap actually need more than 2 rects.