SGA
- Extension: SGA
- Samples: http://samples.mplayerhq.hu/game-formats/segacd/sga/
SGA is a multimedia file format commonly used for games on the Sega CD console system. Early versions of the format store uncompressed video frames. Newer versions use a simple LZ compression scheme. The final version of the format, used on Sega 32X/CD games use a yet to be determined compression method.
File Format
All multi-byte numbers in a SGA file are big-endian.
Due to using CD storage each chunk is stored using 2048 byte sectors. The first sector in the file contains 2048 bytes of data, each subsequent sectors contain a 2 byte header specifying how much of the chunk is left followed by 2046 bytes of data. One item to be aware of is if the value of the header is zero then to skip the next 2046 bytes and check the next header. (I'm presuming this has something to do with padding the CD for faster loading?)
An SGA file does not contain a global header, but has headers for each chunk. Each chunk can contain video or audio. Chunks are word aligned. Chunk header is 12-bytes
byte 0 chunk type byte 1 Stream bytes 2-3 payload length bytes 4-7 time indicator non 32x byte 8 column start (can be ignored since we are not drawing on a 320x224 screen) byte 9 row start (again can be ignored) 32x byte 8 palette start (usually 1) byte 9 palette update size (0 if use existing palette) byte 10 column size byte 11 row size bytes 12.. chunk payload
Bytes 4-5 are presumed to be part of the chunk preamble since the payload length specifies the length of the chunk.
When reading an audio chunk, bytes 8 and 9 represent the frequency counter value of the audio (For the PCM chip on the Sega CD). Bytes 11 and 12 are unknown (Usually 0x01 and 0x00 in Sewer Shark).
Chunk Types
Some observed chunk types are:
- 0x81: compressed video (Night Trap 32x uses this)
- 0xC1: uncompressed video (used in Night Trap and Make My Video: INXS but with 3 palettes)
- 0xC6: compressed video
- 0xC7: compressed video
- 0xC8: compressed video (used in Sewer Shark)
- 0xCD: video, perhaps followed by audio
- 0xCB: video, perhaps in files that are video only
- 0xA1: audio, sign/magnitude 8-bit PCM
- 0xE7: compressed video (Used in Make My Video: INXS)
- 0xE8: compressed video (Used in Ground Zero Texas)
Uncompressed Video 0xC1
Genesis Video uses 4-bit(16 color) pixels and upto 4 palettes
Video is made up of tiles. (column * 4 * row * 8) (each byte has 2 pixels) Then palette data follows (if there is only 24 bytes then only one palette is used, otherwise there is 4 palettes) A palette map follows for videos that use 4 palettes.
Palette Data
Palettes are stored in an unusual format. As the genesis normally uses either RGB or BGR stored in nibbles (even though only the top 3 bits are used).
bitmap={1,2,4}
For bit=0 to 2
for color=0 to 15
red[color]+=Top Most Bit of Data *bitmap[bit]
next
next
Repeat for green and blue.
Palette Map
Reading 2 bits for each tile, determines which of the 4 palettes to use.
For Row=0 to RowMax
For Col=0 to ColMax
PalMap[Row*ColMax+Col]=Top 2 Bits of data
Next
Next
When Drawing the Tile you would select the palette based upon Map.
Compressed Video 0xC6, 0xC7, 0xC8
Chunks in this format consist of several 34 byte LZSS blocks. Each block contains a one word (2 bytes) tag of compression flags followed by 16 words of data.
The tag is read in bits, starting with the most significant (left most) bit. For each bit set to 0, there is an uncompressed word literal. For each bit set to 1, the following word is a displacement/length reference in the following format:
LLLD DDDD DDDD DDDD L = Amount of words to copy (amount of bytes to copy * 2) D = Displacement
This may be calculated as:
for count = 0 to top 3 bits of LZ word * 2 data[current + count] = data[current + count - last 13 bits of LZ word] next
Note that the displacement, unlike the copy amount, is based on bytes, not words. Also, the displacement does not have to be word aligned.
After the entire tag is read, the next flag block is read, and the process continues. The sequence ends when an reference word's top three bits are all zeros.