[docs] Add images.md, move instructions.md, add some comments
- Added some clarifying comments to cmd/extract-imgs.go. - Moved instructions.md into a docs/ folder at the root of the repo. - Created images.md that describes the image format on the tapes.
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@ -219,8 +219,8 @@ func BytesToInt(raw []byte) int {
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}
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type DataHeader struct {
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PaletteOffset uint16
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ArgB uint16
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PaletteOffset uint16 // offset from the first image header
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ArgB uint16 // ?? unused i think?
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ImageCount uint8
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}
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@ -233,8 +233,11 @@ func (h DataHeader) String() string {
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}
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type ImageHeader struct {
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// in tiles
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Width uint8
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Height uint8
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// in bytes
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AttrLength uint16
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// in pixels
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@ -0,0 +1,147 @@
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# The Image Format
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Images are split up into two segments on the tapes: tile data and CHR image
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data. The CHR data is the simplest to explain. It is exactly the same as your
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standard NES CHR. There are no headers or extra metadata. (i think the
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firmware counts tiles and stores that somewhere for the engine to use)
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Images can be either nametable data or sprite data. Both follow the same data
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format. The only difference in decoding is nametable data expects the first
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four CHR tiles to be solid colors (one for each palette color), while the
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sprites do not.
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Multi-byte values are little endian.
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type DataHeader struct {
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// Offset from the start of the ImageCount byte
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PaletteOffset uint16
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// Unused I think?
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ArgB uint16
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// Number of images in this segment
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ImageCount uint8
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}
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type ImageHeader struct {
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// in tiles
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Width uint8
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Height uint8
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// in bytes
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AttrLength uint16
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// in pixels
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X uint8
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Y uint8
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}
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Each file starts with a `DataHeader` and is followed by one or more
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`ImageHeader`s. `DataHeader.ImageCount` containts the number of images (and
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headers) are contained in this segment. After the last `ImageHeader` is tile and
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attribute data for every image in sequence. The length of data for each image
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is `Width * Height + AttrLength`. Immediately following the data for the last
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image is the palette data. This data is 16 bytes (either all of the BG
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palettes, or all of the sprite palettes).
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## Full Backgrounds
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This image is a full background. The palette offset is `$0407 + 4 = $040B` and
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there is a single image in this segment (`$01` at offset `$04`).
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The image header starts at offset `$05`: `$20 $1E $40 $00 $00 $00`. The tile
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data is immediately following. Attribute data starts at `$03CA` and is 64
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bytes long (`$40` at offset `$07`).
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This image is 32x30 tiles in size starting at an X/Y coordinate of (0, 0).
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there are a few padding bytes after the palette data (27 bytes in this case).
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This padding doesn't seem to be required.
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00000000 07 04 2b 00 01 20 1e 40 00 00 00 02 02 02 02 02 |..+.. .@........|
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00000010 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 |................|
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...
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skipping to attribute and palette data
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...
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000003b0 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 |................|
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000003c0 02 02 02 02 02 02 02 02 02 02 02 00 00 00 00 00 |................|
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000003d0 00 00 00 00 00 00 00 00 40 50 10 04 05 01 00 00 |........@P......|
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000003e0 00 05 01 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000003f0 00 00 00 00 00 00 00 00 40 10 00 00 00 00 00 00 |........@.......|
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00000400 04 01 00 00 00 00 00 00 00 00 00 0f 12 20 16 0f |............. ..|
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00000410 27 20 16 0f 27 20 12 0f 28 20 12 00 00 00 00 00 |' ..' ..( ......|
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00000420 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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00000430 00 00 00 00 00 00 |......|
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## Partial Backgrounds & Metasprites
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A partial background is laid out exactly like a full background. The only
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difference is that the X/Y coordinates will most likely not be (0, 0) and
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`Width * Height != 960`. The attribute data, however, will still be 64 bytes
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long.
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The only difference with sprite data is that there is one byte of attribute
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data for each tile. Sprites with 6 tiles will have 6 bytes of attribute data.
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An examle of a segment with three metasprites follows.
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### Complete data
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00000000 79 00 2b 00 03 03 05 0f 00 40 38 05 06 1e 00 78 |y.+......@8....x|
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00000010 28 03 02 06 00 60 40 00 01 02 03 04 05 06 07 08 |(....`@.........|
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00000020 09 0a 0b 0c 0d 0e 00 02 02 02 02 03 03 03 03 02 |................|
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00000030 03 02 00 00 02 0c 0f 10 11 12 00 13 14 15 16 17 |................|
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00000040 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27 |........ !"#$%&'|
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00000050 28 29 2a 00 00 00 00 00 00 00 00 00 00 00 00 00 |()*.............|
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00000060 00 00 01 00 00 00 00 01 01 01 01 01 01 01 01 01 |................|
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00000070 00 2b 2c 2d 2e 2f 30 00 00 00 00 00 00 3d 01 17 |.+,-./0......=..|
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00000080 37 3d 01 26 37 3d 01 20 37 3d 01 20 02 00 00 00 |7=.&7=. 7=. ....|
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00000090 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000000a0 00 00 00 00 00 00 00 00 |........|
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### Segmented & Labeled
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Main header: Palettes at $007D ($0079 + 4), 3 images.
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00000000 79 00 2b 00 03 .. .. .. .. .. .. .. .. .. .. .. |y.+.............|
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Image #1 Header: 3x5 tiles, 15 attribute bytes, at coord (64, 48)
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00000000 .. .. .. .. .. 03 05 0f 00 40 38 .. .. .. .. .. |.........@8.....|
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Image #2 Header: 5x6 tiles, 30 attribute bytes, at coord (120, 40)
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00000000 .. .. .. .. .. .. .. .. .. .. .. 05 06 1e 00 78 |...............x|
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00000010 28 .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. |(...............|
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Image #3 Header: 3x2 tiles, 6 attribute bytes, at coord (96, 64)
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00000010 .. 03 02 06 00 60 40 .. .. .. .. .. .. .. .. .. |.....`..........|
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Image #1 Tile Data
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00000010 .. .. .. .. .. .. .. 00 01 02 03 04 05 06 07 08 |......@.........|
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00000020 09 0a 0b 0c 0d 0e .. .. .. .. .. .. .. .. .. .. |................|
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Image #1 Attribute Data
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00000020 .. .. .. .. .. .. 00 02 02 02 02 03 03 03 03 02 |................|
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00000030 03 02 00 00 02 .. .. .. .. .. .. .. .. .. .. .. |................|
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Image #2 Tile Data
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00000030 .. .. .. .. .. 0c 0f 10 11 12 00 13 14 15 16 17 |................|
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00000040 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27 |........ !"#$%&'|
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00000050 28 29 2a .. .. .. .. .. .. .. .. .. .. .. .. .. |()*.............|
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Image #2 Attribute Data
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00000050 .. .. .. 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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00000060 00 00 01 00 00 00 00 01 01 01 01 01 01 01 01 01 |................|
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00000070 00 .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. |................|
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Image #3 Tile Data
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00000070 .. 2b 2c 2d 2e 2f 30 .. .. .. .. .. .. .. .. .. |.+,-./0.........|
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Image #3 Attribute Data
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00000070 .. .. .. .. .. .. .. 00 00 00 00 00 00 .. .. .. |................|
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Palette Data
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00000070 .. .. .. .. .. .. .. .. .. .. .. .. .. 3d 01 17 |.............=..|
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00000080 37 3d 01 26 37 3d 01 20 37 3d 01 20 02 .. .. .. |7=.&7=. 7=. ....|
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Padding
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00000080 .. .. .. .. .. .. .. .. .. .. .. .. .. 00 00 00 |................|
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00000090 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000000a0 00 00 00 00 00 00 00 00 |........|
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