.TH "rgb2pct" 1 "Fri Mar 22 2019" "GDAL" \" -*- nroff -*-
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.SH NAME
rgb2pct \- Convert a 24bit RGB image to 8bit paletted\&.
.SH "SYNOPSIS"
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rgb2pct.py [-n colors | -pct palette_file] [-of format] source_file dest_file
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.SH "DESCRIPTION"
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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\&.
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.IP "\fB\fB-n\fP \fIcolors\fP:\fP" 1c
Select the number of colors in the generated color table\&. Defaults to 256\&. Must be between 2 and 256\&.  
.IP "\fB\fB-pct\fP \fIpalette_file\fP:\fP" 1c
Extract the color table from \fIpalette_file\fP instead of computing it\&. Can be used to have a consistent color table for multiple files\&. The \fIpalette_file\fP must be a raster file in a GDAL supported format with a palette\&. 
.IP "\fB\fB-of\fP \fIformat\fP:\fP" 1c
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\&.  
.IP "\fB\fIsource_file\fP:\fP" 1c
The input RGB file\&.  
.IP "\fB\fIdest_file\fP:\fP" 1c
The output pseudo-colored file that will be created\&. 
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NOTE: rgb2pct\&.py is a Python script, and will only work if GDAL was built with Python support\&.
.SH "EXAMPLE"
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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\&.
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% 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>
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.SH "AUTHOR"
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Frank Warmerdam warmerdam@pobox.com