SCHG:Sonic the Hedgehog 2 (Master System)/Art

Palettes

Palettes are stored in bank 9, address $811F (absolute $2411F) in the standard SMS format. Individual palettes are accessed using an index value which is multiplied by 16 and added to $811F to calculate an absolute address.

Index	Address	Description
$00	$811F	Unknown (4 shades of grey).
$01	$812F	Unknown.
$02	$813F	Unknown
$03	$814F	Title Card Background
$04	$815F	GHZ Sprite
$05	$816F	SHZ 1 & 3 Sprite
$06	$817F	SHZ 2 Sprite
$07	$818F	ALZ 1 & 3 Sprite
$08	$819F	ALZ 2 Sprite
$09	$81AF	UGZ Sprite
$0A	$81BF	GMZ Sprite
$0B	$81CF	SEZ Sprite
$0C	$81DF	CEZ 1 & 2 Sprite
$0D	$81EF	CEZ 3 Sprite
$0E	$81FF	UGZ Background
$0F	$820F	SHZ 1 & 3 Background
$10	$821F	SHZ 2 Background
$11	$822F	ALZ 1 & 3 Background
$12	$823F	ALZ 2 Background
$13	$824F	GHZ Background
$14	$825F	GMZ Background
$15	$826F	SEZ Background
$16	$827F	CEZ 1 & 2 Background
$17	$828F	CEZ 3 Background
$18	$829F	Intro Sequence Sprite
$19	$82AF	Title Screen Sprite
$1A	$82BF	Intro Sequence Background
$1B	$82CF	Title Screen Background
$1C	$82DF	Unknown.
$1D	$82EF	UGZ Title Card Sprite
$1E	$82FF	SHZ Title Card Sprite
$1F	$830F	ALZ Title Card Sprite
$20	$831F	GHZ Title Card Sprite
$21	$832F	GMZ Title Card Sprite
$22	$833F	SEZ Title Card Sprite
$23	$834F	CEZ Title Card Sprite
$24	$835F	Unknown.
$25	$836F	Ending Sequence Sprite
$26	$837F	Unknown.
$27	$838F	8 background palettes used by the ending sequence.

Uncompressed Art

Uncompressed art is used for sprites that are loaded on-the-fly throughout the level (i.e. need to be loaded quickly). This includes Sonic's animations, rings, monitors, chaos emeralds, etc. Data is stored using the standard SMS planar format (see documentation on SMS Power!).

Compression

All level tiles and badnik art is compressed. The compression format is based on a simple principle: if the value is zero it can be omitted. Each omission is recorded in a flag bitfield at the end of the compressed data stream. The file structure is as follows:

Offset	Size	Description
$00	Word	The value $0001 stored in little-endian format.
$02	Word	The number of tiles in the compressed data.
$04	Word	Pointer the compression bitfields (relative to start of file).
$06	Variable	The compressed tile data.
Variable	ceil(tile_count * 2 / 8) bytes	Compression flags.

Compression Flags

Four types of storage are employed in the compressed stream: 0 = tile is blank (32 zero bytes) and no data is stored (flags only), 1 = tile stored uncompressed, 2 = tile is compressed, 3 = tile is XOR encoded and then compressed. This means that 2 bits are required to store the compression type for each tile; thus each flag byte can store compression type for 4 tiles.

Flag bytes are read from the stream as required. The bytes are then rotated right twice to extract the compression type for a tile. Therefore, compression flag bytes will look like this:

[44332211][--776655]

Where:

• 11 - Type for first tile;
• 22 - Type for second tile;
• 33 - Type for third tile;

Compression Types

Type 0

Blank Tile. To decompress, simply write 32 zero bytes.

Type 1

No compression. Straight copy of 32 bytes from the compressed data stream.

Type 2

Compressed data. A variable number of compressed bytes are preceded by 4 flag bytes (32 bits - one for each byte of uncompressed tile data). The decompression algorithm works as follows:

For each flag byte

Loop 8 times

Rotate right by one place

If bit rotated off was zero write a zero to the output

Else read one byte from the compressed data and copy to the output.

For example, given a flag byte of '01100101' and an input of 'E0 AB 43 A3 01', the decompressed output would be: 'E0 00 AB 00 00 43 A3 00'.

Type 3

XOR encoded, compressed data. To decompress follow the instructions for type 2 then decode according to the following algorithm (extracted from the Sonic 2 SMS Disassembly): <z80> XORDecode: ld ix, $D300 ;decode decompressed data at $D300 ld b, $07 -: ld a, (ix+0) ;xor byte at (ix+0) with byte at (ix+2)... xor (ix+2) ld (ix+2), a ;...and store the result at (ix+2)

ld a, (ix+1) ;xor byte at (ix+1) with byte at (ix+3)... xor (ix+3) ld (ix+3), a ;...and store the result at (ix+3)

ld a, (ix+16) ;xor byte at (ix+16) with byte at (ix+18)... xor (ix+18) ld (ix+18), a ;...and store result at (ix+18)

ld a, (ix+17) ;xor byte at (ix+17) with byte at (ix+19)... xor (ix+19) ld (ix+19), a ;...and store result at (ix+19)

inc ix inc ix ;ix += 2 djnz - ret </z80>