.. _rgb2pct:

================================================================================
rgb2pct
================================================================================

.. only:: html

    Convert a 24bit RGB image to 8bit paletted.

.. Index:: rgb2pct

Synopsis
--------

.. code-block::

    rgb2pct [--help] [--help-general] [--creation-option OPTION]
               [-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.

.. tip:: Equivalent in new "gdal" command line interface:

    See :ref:`gdal_raster_rgb_to_palette`.

.. program:: rgb2pct

.. include:: options/help_and_help_general.rst

.. option:: -n <color>

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

.. option:: --creation-option OPTION

    Optional creation parameters for the GeoTIFF driver,
    for example "COMPRESS=LZW". Can be specified multiple times.

.. option:: -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).

.. option:: -of <format>

    Select the output format. If not specified, the format is guessed from the
    extension. Use the short format name. Only output formats
    supporting pseudo-color tables should be used.

.. option:: <source_file>

    The input RGB file.

.. option:: <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.

.. code-block:: console

    $ 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>
