RGB2PCT(1) GDAL RGB2PCT(1)

NAME

rgb2pct - Convert a 24bit RGB image to 8bit paletted.

SYNOPSIS

rgb2pct [--help] [--help-general]
[-n colors | -pct palette_file] [-of format] <source_file> <dest_file>

DESCRIPTION

This utility will compute an optimal pseudo-color table for a given
RGB image using a median cut algorithm on a downsampled RGB
histogram. Then it converts the image into a pseudo-colored image
using the color table. This conversion utilizes Floyd-Steinberg
dithering (error diffusion) to maximize output image visual quality.

NOTE:
rgb2pct is a Python utility, and is only available if GDAL Python
bindings are available.

--help Show this help message and exit

--help-general
Gives a brief usage message for the generic GDAL commandline
options and exit.

-n <color>
Select the number of colors in the generated color table.
Defaults to 256. Must be between 2 and 256.

-pct <palette_file>
Extract the color table from <palette_file> instead of
computing it. Can be used to have a consistent color table
for multiple files. The <palette_file> must be either a
raster file in a GDAL supported format with a palette or a
color file in a supported format (txt, qml, qlr).

-of <format>
Select the output format. Starting with GDAL 2.3, if not
specified, the format is guessed from the extension
(previously was GTiff). Use the short format name. Only output
formats supporting pseudo-color tables should be used.

<source_file>
The input RGB file.

<dest_file>
The output pseudo-colored file that will be created.

EXAMPLE

If it is desired to hand create the palette, likely the simplest text
format is the GDAL VRT format. In the following example a VRT was
created in a text editor with a small 4 color palette with the RGBA
colors 238/238/238/255, 237/237/237/255, 236/236/236/255 and
229/229/229/255.

rgb2pct -pct palette.vrt rgb.tif pseudo-colored.tif
more < palette.vrt
<VRTDataset rasterXSize="226" rasterYSize="271">
<VRTRasterBand dataType="Byte" band="1">
<ColorInterp>Palette</ColorInterp>
<ColorTable>
<Entry c1="238" c2="238" c3="238" c4="255"/>
<Entry c1="237" c2="237" c3="237" c4="255"/>
<Entry c1="236" c2="236" c3="236" c4="255"/>
<Entry c1="229" c2="229" c3="229" c4="255"/>
</ColorTable>
</VRTRasterBand>
</VRTDataset>

AUTHOR

Frank Warmerdam <warmerdam@pobox.com>

COPYRIGHT

1998-2025

January 8, 2025 RGB2PCT(1)