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rgbgfx

Game Boy graphics converter

rgbgfx [-CmhOuVwXYZ] [-v [-v ...]] [-a attrmap | -A] [-b base_ids] [-c pal_spec] [--color when] [-d depth] [-i input_tiles] [-L slice] [-l base_pal] [-N nb_tiles] [-n nb_pals] [-o out_file] [-p pal_file | -P] [-q pal_map | -Q] [-r width] [-s nb_colors] [-t tilemap | -T] [-W warning] [-x quantity] file

The rgbgfx program converts PNG images into data suitable for display on the Game Boy and Game Boy Color, or vice-versa.

The main function of rgbgfx is to divide the input PNG into 8×8 pixel , convert each of those squares into 1bpp or 2bpp tile data, and save all of the tile data in a file. It also has options to generate a tile map, attribute map, and/or palette set as well; more on that and how the conversion process can be tweaked below.

rgbgfx accepts the usual short and long options, such as -V and --version. Options later in the command line override those set earlier, except for when duplicate options are considered an error. Options can be abbreviated as long as the abbreviation is unambiguous: --verb is --verbose, but --ver is invalid because it could also be --version.

Unless otherwise noted, passing ‘-’ (a single dash) as a file name makes rgbgfx use standard input (for input files) or standard output (for output files). To suppress this behavior, and open a file in the current directory actually called ‘-’, pass ‘./-’ instead. Using standard input or output for more than one file in a single command may produce unexpected results.

rgbgfx accepts decimal, hexadecimal, octal, and binary for numeric option arguments. Decimal numbers are written as usual; hexadecimal numbers must be prefixed with either ‘$’ or ‘0x’; octal numbers must be prefixed with either ‘&’ or ‘0o’; and binary numbers must be prefixed with either ‘%’ or ‘0b’. (The prefixes ‘$’ and ‘&’ will likely need escaping or quoting to avoid being interpreted by the shell.) Leading zeros (after the base prefix, if any) are accepted, and letters are not case-sensitive. For example, all of these are equivalent: ‘42’, ‘042’, ‘0x2A’, ‘0X2A’, ‘0x2a’, ‘&52’, ‘0o52’, ‘0O052’, ‘0b00101010’, ‘0B101010’.

The following options are accepted:

attrmap, --attr-map attrmap
Generate an attribute map, which is a file containing tile “attributes”. For each square of the input image, its corresponding attribute map byte contains the mirroring bits (if -m was specified), the bank bit (see -N), and the palette index. See Pan Docs for the individual bytes' format. The output is written just like the tile map (see -t), follows the same order (-Z), and has the same size.
, --auto-attr-map
Same as -a base_path.attrmap (see Automatic output paths).
color, --background-color color
Set a background color to be omitted from output. Colors are accepted in ‘#rgb’ or ‘#rrggbb’ format, or as ‘transparent’. Input tiles which are entirely the specified background color are ignored and will not be output in tile data file. The tilemap, atrribute map, or palette map files use placeholder values where background tiles were. If a background color is specified, it cannot be used within tiles which are not ignored.
base_ids, --base-tiles base_ids
Set the base IDs for tile map output. base_ids should be one or two numbers between 0 and 255, separated by a comma; they are for bank 0 and bank 1 respectively. Both default to 0.
, --color-curve
Modifies the color palettes (whether they are generated from the input image or taken from an input palette specification) with a color curve mimicking the Game Boy Color's screen. This adjusts the RGB color values so that the colors, when displayed on Game Boy Color hardware (or an emulator with an accurate display filter), will look like the original colors as displayed on a backlit computer screen. Note that GBC displays can look very different depending on the ambient light and their exact hardware model, so this color curve is only a "best effort".
pal_spec, --colors pal_spec
Use the specified color palettes instead of having rgbgfx automatically determine some. pal_spec can be one of the following:
If pal_spec begins with a hash character ‘#’, it is treated as an inline palette specification. It should contain a comma-separated list of hexadecimal colors, each beginning with a hash. Colors are accepted in ‘#rgb’ or ‘#rrggbb’ format. To leave one or more gaps in the palette, ‘#none’ can be used instead of any color. Palettes must be separated by a colon or semicolon (the latter may require quoting to avoid special handling by the shell), and spaces are allowed around colons, semicolons and commas; trailing commas and semicolons are allowed. See EXAMPLES for an example of an inline palette specification.
If pal_spec is the case-insensitive word embedded, then the first four colors of the input PNG's embedded palette are used. It is an error if the PNG is not indexed, or if colors other than these 4 are used. (This is different from the default behavior of indexed PNGs, as then unused entries in the embedded palette are ignored, whereas they are not with -c embedded).
If pal_spec starts with case-insensitive dmg=, then the following two-digit hexadecimal number specifies four grayscale DMG color indexes. The number functions like the DMG's $FF47 register (see Pan Docs for more information): the low two bits 0-1 specify which gray shade goes in color index 0, the next two bits 2-3 specify which gray shade goes in color index 1, and so on. Gray shade 0 is the lightest (white), 3 is the darkest (black). If pal_spec is the case-insensitive word dmg, then it acts like dmg=E4, i.e. the darkest gray will end up in color index 0, and so on. The same gray shade cannot go in two color indexes. To specify a DMG palette, the input PNG must have all its colors in shades of gray, without any transparent colors.
If pal_spec is the case-insensitive word auto, then a palette is automatically generated using the procedure described in PALETTE GENERATION. This is the default behavior if -c was not specified.
Otherwise, pal_spec is assumed to be an external palette specification. The expected format is ‘format:path’, where path is a path to a file (‘-’ is not treated specially), which will be processed according to the format. See PALETTE SPECIFICATION FORMATS for a list of formats and their descriptions.
when
Specify when to highlight warning and error messages with color: ‘always’, ‘never’, or ‘auto’. ‘auto’ determines whether to use colors based on the ‘NO_COLOR’ or ‘FORCE_COLOR’ environment variables, or whether the output is to a TTY.
depth, --depth depth
Set the bit depth of the output tile data, in bits per pixel (bpp), either 1 or 2 (the default). This changes how tile data is output, and the maximum number of colors per palette (2 and 4 respectively).
, --help
Print help text for the program and exit.
input_tiles, --input-tileset input_tiles
Use the specified input tiles in addition to having rgbgfx automatically determine some. The input tiles will always be first in the -o image output, and will always get the first IDs in the -t tilemap output. input_tiles must contain 1bpp or 2bpp tile data (whichever matches the -d option used here), as could be previously generated with the -o option.

If the -o option is also specified, then the input tiles will be assigned the first tile IDs, and any tiles from the input image that are not in the input tileset will be assigned subsequent IDs. But if the -o option is specified, then the tile map can use tiles from the input tileset. Using -o with -i is useful if you want to precisely control the tile IDs of its tile map. Using -i alone is more useful if you want several images to use a subset of shared tiles.

If the image will use more than one color palette, it is advised to generate the palette set along with the input tile data, and pass -c gbc:input_palette along with -i input_tiles. This is because rgbgfx might not generate the same palette set for this image as it did for its input tileset.

See EXAMPLES for examples of how to use this option.

This option is ignored in REVERSE MODE.

slice, --slice slice
Only process a given rectangle of the image. This is useful for example if the input image is a sheet of some sort, and you want to convert each cel individually. The default is to process the whole image as-is.

slice must be formatted as ‘X,Y:W,H’: two comma-separated number pairs, separated by a colon. Whitespace is allowed around all punctuation. The first number pair specifies the X and Y coordinates of the top-left pixel that will be processed (anything above it or to its left will be ignored). The second number pair specifies how many tiles to process horizontally and vertically, respectively.

-L , no padding is inserted.

base_pal, --base-palette base_pal
Set the base ID for attribute map and palette map output. base_pal should be a number between 0 and 255. It defaults to 0.
, --mirror-tiles
Deduplicate tiles that are horizontally and/or vertically symmetrical mirror images of each other. Only one of each unique tile will be saved in the tile data file, with mirror images counting as duplicates. Useful with a tile map and attribute map together (see -a and -t) to keep track of the duplicated tiles and the dimension(s) mirrored. Implies -u. Equivalent to -XY.
nb_tiles, --nb-tiles nb_tiles
Set a maximum number of tiles that can be placed in each VRAM bank. nb_tiles should be one or two numbers between 0 and 256, separated by a comma; if the latter is omitted, it defaults to 0. Setting either number to 0 prevents any tiles from being output in that bank.

If more tiles are generated than can fit in the two banks combined, rgbgfx will abort. If -N is not specified, no limit will be set on the amount of tiles placed in bank 0, and tiles will not be placed in bank 1.

nb_pals, --nb-palettes nb_pals
Abort if more than nb_pals palettes are generated. This may not be more than 256.

Note that attribute map output only has 3 bits for the palette ID, so a limit higher than 8 may yield incomplete data unless relying on a palette map (see -q).

, --group-outputs
Sets the ‘base path’ to be the output tile data path from -o instead of the input image path (see Automatic output paths).
out_file, --output out_file
Output the tile data in native 2bpp format or in 1bpp (depending on -d) to this file.
pal_file, --palette pal_file
Output the image's palette set to this file.
, --auto-palette
Same as -p base_path.pal (see Automatic output paths).
pal_map, --palette-map pal_map
Output the image's palette map to this file. This is useful if the input image contains more than 8 palettes, as the attribute map only contains the lower 3 bits of the palette indices.
, --auto-palette-map
Same as -q base_path.palmap (see Automatic output paths).
width, --reverse width
Switches rgbgfx into “” mode. In this mode, instead of converting a PNG image into Game Boy data, rgbgfx will attempt to reverse the process, and render Game Boy data into an image. See REVERSE MODE below for details.

width is the width of the image to generate, in tiles. -r -0 chooses a width to make the image as square as possible. This is useful if you do not know the original width.

nb_colors, --palette-size nb_colors
Specify how many colors each palette contains, including the transparent one if any. nb_colors cannot be more than ‘1 << depth’ (see -d).
tilemap, --tilemap tilemap
Generate a file of tile indices. For each square of the input image, its corresponding tile map byte contains the index of the associated tile in the tile data file. The IDs wrap around from 255 back to 0, and do not include the bank bit; use -a for that. Useful in combination with -u and/or -m to keep track of duplicate tiles.
, --auto-tilemap
Same as -t base_path.tilemap (see Automatic output paths).
, --unique-tiles
Deduplicate identical tiles. Only one of each unique tile will be saved in the tile data file. Useful with a tile map (see -t) to keep track of the duplicated tiles.

Note that if this option is enabled, no guarantee is made on the order in which tiles are output; while it be consistent across identical runs of a given rgbgfx release, the same is not true for different releases.

, --version
Print the version of the program and exit.
, --verbose
Be verbose. The verbosity level is increased by one each time the flag is specified, with each level including the previous:
  1. Print the rgbgfx configuration before taking actions.
  2. Print a notice before significant actions.
  3. Print some of the actions' intermediate results.
  4. Print some internal debug information.
  5. Print detailed internal information.
The verbosity level does not go past 6.

Note that verbose output is only intended to be consumed by humans, and may change without notice between RGBDS releases; relying on those for scripts is not advised.

warning, --warning warning
Set warning flag warning. A warning message will be printed if warning is an unknown warning flag. See the DIAGNOSTICS section for a list of warnings.
Disable all warning output, even when turned into errors.
, --mirror-x
Deduplicate tiles that are horizontally symmetrical mirror images of each other across the X axis. Implies -u.
quantity, --trim-end quantity
Do not output the last quantity tiles to the tile data file; no other output is affected. This is useful for trimming “filler” / blank squares at the end of an image. If fewer than quantity tiles would have been emitted, the file will be empty.

Note that this is done deduplication if -u was enabled, so you probably don't want to use this option in combination with -u. Note also that the tiles that don't get output will not count towards -N's limit.

, --mirror-y
Deduplicate tiles that are vertically symmetrical mirror images of each other across the Y axis. Implies -u.
, --columns
Read squares from the PNG in column-major order (column by column), instead of the default row-major order (line by line). This primarily affects tile map and attribute map output, although it may also change generated tile data and palettes.
@at_file
Read more options and arguments from a file, as if its contents were given on the command line. Arguments are separated by whitespace or newlines. Lines starting with a hash sign (‘#’) are considered comments and ignored.

No shell processing is performed, such as wildcard or variable expansion. There is no support for escaping or quoting whitespace to be included in arguments. The standard ‘--’ to stop option processing also disables at-file processing. Note that while ‘--’ can be used an at-file, it only disables option processing within that at-file, and processing continues in the parent scope.

See At-files below for an explanation of how this can be useful.

In a given project, many images are to be converted with different flags. The traditional way of solving this problem has been to specify the different flags for each image in the Makefile or build script; this can be inconvenient, as it centralizes all those flags away from the images they concern.

To avoid these drawbacks, you can use “at-files”: any command-line argument that begins with an at sign (‘@’) is interpreted as one, as documented above. At-files can be stored right next to the corresponding image, for example:

$ rgbgfx -o image.2bpp -t image.tilemap @image.flags image.png

This will read additional flags from the file ‘image.flags’, which could contain, for example, ‘-b 128’ to specify a base offset for the image's tiles. The above command could be generated from the following make(1) rule:

%.2bpp %.tilemap: %.flags %.png
        rgbgfx -o $*.2bpp -t $*.tilemap @$*.flags $*.png

The following formats are supported:

Adobe Photoshop color table.
Adobe Photoshop color swatch.
A GBC palette memory dump, as emitted by rgbgfx -p. Useful to force several images to share the same palette.
GIMP palette.
Plaintext lines of hexadecimal colors in ‘rrggbb’ format.
An image of square color swatches, with each row defining the colors for one palette. Color swatches can be any square size.
Paint Shop Pro palette.

If you wish for another format to be supported, please open an issue (see BUGS below) or contact us, and supply a few sample files.

rgbgfx must generate palettes from the colors in the input image, unless -c was used; in that case, the provided palettes will be used. , for example because you want to use palette swaps, please use -c to specify the palette explicitly.

First, if the image contains transparent pixel, color #0 of all palettes will be allocated to it. This is done -c; then the specification only covers color #1 onwards. (If you do not want this, ask your image editor to remove the alpha channel.)

After generating palettes, rgbgfx sorts colors within those palettes using the following rules:

  • If the PNG file internally contains a palette (often dubbed an “indexed” PNG), then colors in each output palette will be sorted according to their order in the PNG's palette. Any unused entries will be ignored, and only the first entry is considered if there are any duplicates. (If you want a given color to appear more than once, or an unused color to appear at all, you should specify the palettes explicitly instead using -c; -c embedded may be appropriate.)
  • Otherwise, if the PNG only contains shades of gray, they will be categorized into as many “bins” as there are colors per palette, and the palette is set to these bins. The darkest gray will end up in bin #0, and so on; note that this is the opposite of the RGB method below. This is equivalent to having specified a DMG palette of -c dmg=E4. If two distinct grays end up in the same bin, the RGB method is used instead.

    Be careful that rgbgfx is picky about what it considers “grays”: the red, green, and blue components of each color must all be the same.

  • If none of the above apply, colors are sorted from lightest (first) to darkest (last). The definition of luminance that rgbgfx uses is “ 2126×red+7152×green+722×blue ”.

Note that the “indexed” behavior depends on an internal detail of how the PNG is saved, specifically its ‘PLTE’ chunk. Since few image editors (such as GIMP) expose that detail, this behavior is only kept for compatibility and should be considered deprecated.

It turns out that palette generation is an NP-complete problem known as "pagination", so rgbgfx does not attempt to find the optimal solution, but instead uses an "overload-and-remove" heuristic to find a good one in a reasonable amount of time. (There are no guarantees about how this algorithm will generate palettes, apart from the constraints documented above.) It is possible to compute the optimal solution externally (using a solver, for example), and then provide it to rgbgfx via -c.

All files output by rgbgfx are binary files, and designed to follow the Game Boy and Game Boy Color's native formats. What follows is succinct descriptions of those formats, including rgbgfx-specific details. For more complete, beginner-friendly descriptions of the native formats with illustrations, please check out Pan Docs.

Tile data is output like a binary dump of VRAM, with no padding between tiles. Each tile is 16 bytes, 2 per row of 8 pixels; the bits of color IDs are split into each byte (or “bitplane”). The leftmost pixel's color ID is stored in the two bytes' most significant bits, and the rightmost pixel's color ID in their least significant bits.

When the bit depth (-d) is set to 1, the most significant bitplane (second byte) of each row, being all zeros, is simply not output.

Palette data is output like a dump of palette memory. Each color is written as GBC-native little-endian RGB555, with the unused bit 15 set to 0. There is no padding between colors, nor between palettes; however, empty colors in the palettes are output as 0xFFFF. For example, if 5 palettes are generated with -s 4, the palette data file will be 2×4×5=40 bytes long, even if some palettes contain less than 3 colors. Note that -n only caps how many palettes are generated (and thus this file's size), but fewer may be generated still.

A tile map is an array of tile IDs, with one byte per tile ID. The first byte always corresponds to the ID of the tile in top-left corner of the input image; the second byte is either the ID of the tile to its right (by default), or below it (with -Z); and so on, continuing in the same direction. Rows / columns (respectively) are stored consecutively, with no padding.

Attribute maps mirror the format of tile maps, like on the GBC, especially the order in which bytes are output. The contents of individual bytes follows the GBC's native format:

Bit 7 BG-to-OAM Priority Set to 0
Bit 6 Vertical Flip 0=Normal, 1=Mirror vertically
Bit 5 Horizontal Flip 0=Normal, 1=Mirror horizontally
Bit 4 Not used Set to 0
Bit 3 Tile VRAM Bank number 0=Bank 0, 1=Bank 1
Bit 2–0 Background Palette number BGP0-7

Note that if more than 8 palettes are used, only the lowest 3 bits of the palette ID are output.

For convenience, rgbgfx provides shortcuts to generate all files in the same directory. This is done by using the uppercase version of a flag (for example, -A instead of -a). The base_path is the input image path (or the output tile data path from -o, if -O was given) with its extension, if any, removed.

For example, these two commands are equivalent:

$ rgbgfx img/player.png -o build/player.2bpp -P
$ rgbgfx img/player.png -o build/player.2bpp -p img/player.pal

And so are these two:

$ rgbgfx img/player.png -o build/player.2bpp -O -P
$ rgbgfx img/player.png -o build/player.2bpp -p build/player.pal

rgbgfx can produce a PNG image from valid data. This may be useful for ripping graphics, recovering lost source images, etc. An important caveat on that last one, though: the conversion process is both ways, so the “reversed” image won't be perfectly identical to the original—but it should be close to a Game Boy's output. (Keep in mind that many of consoles output different colors, so there is no true reference rendering.)

When using reverse mode, make sure to pass the same flags that were given when generating the data, especially -C, -d, -N, -s, -x, and -Z. “At-files” may help with this. rgbgfx will warn about any inconsistencies it detects.

Files that are normally outputs (-a, -p, -t) become inputs, and file will be written to instead of read from, and thus needs not exist beforehand. Any of these inputs not passed is assumed to be some default:

palettes Unspecified palette data makes rgbgfx assume DMG (monochrome Game Boy) mode: a single palette of 4 grays. It is possible to pass palettes using -c instead of -p.
tile data Tile data must be provided, as there is no reasonable assumption to fall back on.
tile map A missing tile map makes rgbgfx assume that tiles were not deduplicated, and should be laid out in the order they are stored.
attribute map Without an attribute map, rgbgfx assumes that no tiles were mirrored.

Warnings are diagnostic messages that indicate possibly erroneous behavior that does not necessarily compromise the conversion process. The following options alter the way warnings are processed.

Make all warnings into errors. This can be negated as -Wno-error to prevent turning all warnings into errors.
Make the specified warning or meta warning into an error. A warning's name is appended (example: -Werror=obsolete), and this warning is implicitly enabled and turned into an error. This can be negated as -Wno-error= to prevent turning a specified warning into an error, even if -Werror is in effect.

The following warnings are “meta” warnings, that enable a collection of other warnings. If a specific warning is toggled via a meta flag and a specific one, the more specific one takes priority. The position on the command-line acts as a tie breaker, the last one taking effect.

This enables warnings that are likely to indicate an error or undesired behavior, and that can easily be fixed.
Enables literally every warning.

The following warnings are actual warning flags; with each description, the corresponding warning flag is included. Note that each of these flags also has a negation (for example, -Wobsolete enables the warning that -Wno-obsolete disables; and -Wall enables every warning that -Wno-all disables). Only the non-default flag is listed here. Ignoring the “no-” prefix, entries are listed alphabetically.

Warn when a generated palette is sorted according to the input PNG's embedded palette but -c embedded was not provided. This warning is enabled by -Weverything.
Warn when obsolete features are encountered, which have been deprecated and may later be removed.
Warn when -x trims a nonempty tile. An "empty" tile uses entirely color 0 of its palette. This warning is enabled by -Wall.

The following will only validate the ‘tileset.png’ image (check its size, that all tiles have a suitable amount of colors, etc.), but output nothing:

$ rgbgfx src/res/maps/overworld/tileset.png

The following will convert the ‘tileset.png’ image using the two given palettes (and only those), and store the generated 2bpp tile data in ‘tileset.2bpp’, and the attribute map in ‘tileset.attrmap’.

$ rgbgfx -c '#ffffff,#8d05de, #dc7905,#000000; #fff,#8d05de, #7e0000 , #000' -A -o tileset.2bpp tileset.png

The following will deduplicate the tiles in the ‘title_screen.png’ image, keeping only one of each unique tile, and store the generated 2bpp tile data in ‘title_screen.2bpp’, and the tile map in ‘title_screen.tilemap’.

$ rgbgfx -u title_screen.png -o title_screen.2bpp -t title_screen.tilemap

The following will convert the given inline palette specification to a palette set, and store the palette set in ‘colors.pal’, without needing an input image.

$ rgbgfx -c '#fff,#ff0,#f80,#000' -p colors.pal

The following will convert two level images using the same tileset, and error out if any of them contain tiles not in the tileset.

$ rgbgfx tileset.png -o tileset.2bpp -O -P
$ rgbgfx level1.png -i tileset.2bpp -c gbc:tileset.pal -t level1.tilemap -a level1.attrmap
$ rgbgfx level2.png -i tileset.2bpp -c gbc:tileset.pal -t level2.tilemap -a level2.attrmap

Please report bugs or mistakes in this documentation on GitHub.

rgbasm(1), rgblink(1), rgbfix(1), rgbds(7)

The Game Boy hardware reference Pan Docs, particularly the section about graphics.

rgbgfx was originally written by stag019 as a program to be packaged in RGBDS. It was later rewritten by ISSOtm, and is now maintained by a number of contributors at https://github.com/gbdev/rgbds.