OBJ file: Difference between revisions
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OBJ files contain the [[sprites]] graphics (equivalent to the [[C ROM]]s on [[:Category:cartridge systems|cartridge systems]]). Data is compressed however using a simple RLE compression removing runs with a value of 0x00000000. Once decompressed the format is described here: [[sprite graphics format]]. | OBJ files contain the [[sprites]] graphics (equivalent to the [[C ROM]]s on [[:Category:cartridge systems|cartridge systems]]). Data is compressed however using a simple RLE compression removing runs with a value of 0x00000000. Once decompressed the format is described here: [[sprite graphics format]]. | ||
RLE compression | ==RLE compression== | ||
* First long word value is the compressed data length (/4-1). | * First long word value is the compressed data length (/4-1). | ||
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* When a zero value is found it is stored and the next value is checked. A fill of zero is created to match the value read unless it is zero. | * When a zero value is found it is stored and the next value is checked. A fill of zero is created to match the value read unless it is zero. | ||
===Example=== | |||
Example | |||
compressed: | compressed: | ||
Latest revision as of 02:32, 27 March 2018
OBJ files contain the sprites graphics (equivalent to the C ROMs on cartridge systems). Data is compressed however using a simple RLE compression removing runs with a value of 0x00000000. Once decompressed the format is described here: sprite graphics format.
RLE compression
- First long word value is the compressed data length (/4-1).
- The system works by checking single long words at a time.
- When a zero value is found it is stored and the next value is checked. A fill of zero is created to match the value read unless it is zero.
Example
compressed:
0x00000007, 0x11111111, 0x00000000, 0x00000003, 0x22222222, 0x00000000, 0x00000000, 0x33333333, 0x44444444
decompressed:
0x11111111, 0x00000000, 0x00000000, 0x00000000, 0x00000000, 0x22222222, 0x00000000, 0x33333333, 0x44444444
This system will not compress data very well unless there are large areas of zero data within the decompressed sprite data. This is due to a single zero value being inflated by 100% to let the decompressor know it is not a run. To my current knowledge only Art of Fighting uses the OBJ format.
For reference below is a 68000 code snippet from bios TOP-SP1 that handles decompresses.
C0BC5A 203C 00E0 0000 MOVE.L #0xE00000,D0
C0BC60 D0AD 7EF4 ADD.L (0x7EF4,A5),D0
C0BC64 2240 MOVEA.L D0,A1 <---- Where to decompress data
C0BC66 206D 7EF8 MOVEA.L (0x7EF8,A5),A0
C0BC6A 7000 MOVEQ #0x0,D0
C0BC6C 2E18 MOVE.L (A0)+,D7 <---- Get Length
C0BC6E 6738 BEQ.S *+0x3A [0xC0BCA8]
C0BC70 22D8 MOVE.L (A0)+,(A1)+ <---- Move single long word
C0BC72 6712 BEQ.S *+0x14 [0xC0BC86] <---- Branch if zero.
C0BC74 51CF FFFA DBF D7,*-0x4 [0xC0BC70] <---- check data end
C0BC78 2009 MOVE.L A1,D0
C0BC7A 0480 00E0 0000 SUBI.L #0xE00000,D0
C0BC80 2B40 7EF4 MOVE.L D0,(0x7EF4,A5)
C0BC84 4E75 RTS
C0BC86 2C18 MOVE.L (A0)+,D6 <---- Get next value
C0BC88 51CE 0014 DBF D6,*+0x12 [0xC0BC9E] <---- Decompress if non zero
C0BC8C 51CF FFE2 DBF D7,*-0x1C [0xC0BC70] <---- Check data end
C0BC90 2009 MOVE.L A1,D0
C0BC92 0480 00E0 0000 SUBI.L #0xE00000,D0
C0BC98 2B40 7EF4 MOVE.L D0,(0x7EF4,A5)
C0BC9C 4E75 RTS
C0BC9E 22C0 MOVE.L D0,(A1)+ <---- Recreate zero run
C0BCA0 51CE FFFC DBF D6,*-0x2 [0xC0BC9E] <---- and Loop until complete
C0BCA4 51CF FFCA DBF D7,*-0x34 [0xC0BC70] <---- Check data end
C0BCA8 2009 MOVE.L A1,D0
C0BCAA 0480 00E0 0000 SUBI.L #0xE00000,D0
C0BCB0 2B40 7EF4 MOVE.L D0,(0x7EF4,A5)
C0BCB4 4E75 RTS