6.6 KiB
The Image Format
Images are split up into two segments on the tapes: tile data and CHR image data. The CHR data is the simplest to explain. It is exactly the same as your standard NES CHR. There are no headers or extra metadata. (i think the firmware counts tiles and stores that somewhere for the engine to use)
Images can be either nametable data or sprite data. Both follow the same data format. The only difference in decoding is nametable data expects the first four CHR tiles to be solid colors (one for each palette color), while the sprites do not.
Multi-byte values are little endian.
type DataHeader struct {
// Offset from the start of the ImageCount byte
PaletteOffset uint16
// Unused I think?
ArgB uint16
// Number of images in this segment
ImageCount uint8
}
type ImageHeader struct {
// in tiles
Width uint8
Height uint8
// in bytes
AttrLength uint16
// in pixels
X uint8
Y uint8
}
Each file starts with a DataHeader and is followed by one or more
ImageHeaders. DataHeader.ImageCount containts the number of images (and
headers) are contained in this segment. After the last ImageHeader is tile and
attribute data for every image in sequence. The length of data for each image
is Width * Height + AttrLength. Immediately following the data for the last
image is the palette data. This data is 16 bytes (either all of the BG
palettes, or all of the sprite palettes).
Full Backgrounds
This image is a full background. The palette offset is $0407 + 4 = $040B and
there is a single image in this segment ($01 at offset $04).
The image header starts at offset $05: $20 $1E $40 $00 $00 $00. The tile
data is immediately following. Attribute data starts at $03CA and is 64
bytes long ($40 at offset $07).
This image is 32x30 tiles in size starting at an X/Y coordinate of (0, 0). there are a few padding bytes after the palette data (27 bytes in this case). This padding doesn't seem to be required.
00000000 07 04 2b 00 01 20 1e 40 00 00 00 02 02 02 02 02 |..+.. .@........|
00000010 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 |................|
...
skipping to attribute and palette data
...
000003b0 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 |................|
000003c0 02 02 02 02 02 02 02 02 02 02 02 00 00 00 00 00 |................|
000003d0 00 00 00 00 00 00 00 00 40 50 10 04 05 01 00 00 |........@P......|
000003e0 00 05 01 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000003f0 00 00 00 00 00 00 00 00 40 10 00 00 00 00 00 00 |........@.......|
00000400 04 01 00 00 00 00 00 00 00 00 00 0f 12 20 16 0f |............. ..|
00000410 27 20 16 0f 27 20 12 0f 28 20 12 00 00 00 00 00 |' ..' ..( ......|
00000420 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000430 00 00 00 00 00 00 |......|
Partial Backgrounds & Metasprites
A partial background is laid out exactly like a full background. The only
difference is that the X/Y coordinates will most likely not be (0, 0) and
Width * Height != 960. The attribute data, however, will still be 64 bytes
long.
The only difference with sprite data is that there is one byte of attribute data for each tile. Sprites with 6 tiles will have 6 bytes of attribute data.
An examle of a segment with three metasprites follows.
Complete data
00000000 79 00 2b 00 03 03 05 0f 00 40 38 05 06 1e 00 78 |y.+......@8....x|
00000010 28 03 02 06 00 60 40 00 01 02 03 04 05 06 07 08 |(....`@.........|
00000020 09 0a 0b 0c 0d 0e 00 02 02 02 02 03 03 03 03 02 |................|
00000030 03 02 00 00 02 0c 0f 10 11 12 00 13 14 15 16 17 |................|
00000040 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27 |........ !"#$%&'|
00000050 28 29 2a 00 00 00 00 00 00 00 00 00 00 00 00 00 |()*.............|
00000060 00 00 01 00 00 00 00 01 01 01 01 01 01 01 01 01 |................|
00000070 00 2b 2c 2d 2e 2f 30 00 00 00 00 00 00 3d 01 17 |.+,-./0......=..|
00000080 37 3d 01 26 37 3d 01 20 37 3d 01 20 02 00 00 00 |7=.&7=. 7=. ....|
00000090 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000000a0 00 00 00 00 00 00 00 00 |........|
Segmented & Labeled
Main header: Palettes at $007D ($0079 + 4), 3 images.
00000000 79 00 2b 00 03 .. .. .. .. .. .. .. .. .. .. .. |y.+.............|
Image #1 Header: 3x5 tiles, 15 attribute bytes, at coord (64, 48)
00000000 .. .. .. .. .. 03 05 0f 00 40 38 .. .. .. .. .. |.........@8.....|
Image #2 Header: 5x6 tiles, 30 attribute bytes, at coord (120, 40)
00000000 .. .. .. .. .. .. .. .. .. .. .. 05 06 1e 00 78 |...............x|
00000010 28 .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. |(...............|
Image #3 Header: 3x2 tiles, 6 attribute bytes, at coord (96, 64)
00000010 .. 03 02 06 00 60 40 .. .. .. .. .. .. .. .. .. |.....`..........|
Image #1 Tile Data
00000010 .. .. .. .. .. .. .. 00 01 02 03 04 05 06 07 08 |......@.........|
00000020 09 0a 0b 0c 0d 0e .. .. .. .. .. .. .. .. .. .. |................|
Image #1 Attribute Data
00000020 .. .. .. .. .. .. 00 02 02 02 02 03 03 03 03 02 |................|
00000030 03 02 00 00 02 .. .. .. .. .. .. .. .. .. .. .. |................|
Image #2 Tile Data
00000030 .. .. .. .. .. 0c 0f 10 11 12 00 13 14 15 16 17 |................|
00000040 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27 |........ !"#$%&'|
00000050 28 29 2a .. .. .. .. .. .. .. .. .. .. .. .. .. |()*.............|
Image #2 Attribute Data
00000050 .. .. .. 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000060 00 00 01 00 00 00 00 01 01 01 01 01 01 01 01 01 |................|
00000070 00 .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. |................|
Image #3 Tile Data
00000070 .. 2b 2c 2d 2e 2f 30 .. .. .. .. .. .. .. .. .. |.+,-./0.........|
Image #3 Attribute Data
00000070 .. .. .. .. .. .. .. 00 00 00 00 00 00 .. .. .. |................|
Palette Data
00000070 .. .. .. .. .. .. .. .. .. .. .. .. .. 3d 01 17 |.............=..|
00000080 37 3d 01 26 37 3d 01 20 37 3d 01 20 02 .. .. .. |7=.&7=. 7=. ....|
Padding
00000080 .. .. .. .. .. .. .. .. .. .. .. .. .. 00 00 00 |................|
00000090 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000000a0 00 00 00 00 00 00 00 00 |........|