NAME

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

SYNOPSIS

rgb2pct.py [-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.

-n colors:

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 a raster file in a GDAL supported format with a palette.

-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.

NOTE: rgb2pct.py is a Python script, and will only work if GDAL was built with Python support.

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.py -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