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#### 37.8.2 Color Quantization

In order to display a RGB image on a color-mapped device of limited depth, the number of colors in the original image will have to be reduced. Color quantization is one way of doing this.

Two color quantization algorithms are available in the Forms Library. One uses Heckbert’s median cut algorithm followed by Floyd-Steinberg dithering after which the pixels are mapped to the colors selected. The code implementing this is from the Independent JPEG Group’s two pass quantizer (`jquant2.c` in the IJG’s distribution), which under copyright (c) 1991-1996 by Thomas G. Lane and the IJG.

Another method is based on the Octree quantization algorithm with no dithering and is implemented by Steve Lamont (`spl@ucsd.edu`) and is under vopyright (c) 1998 by Steve Lamont and the National Center for Microscopy and Imaging Research. This quantization library is available from ftp://ncmir.ucsd.edu/pub/quantize/libquantize.html. The quantizer based on this library is not compiled into the image support. The source code for using this quantizer is in `image` subdirectory.

By default, the median cut algorithm is used. You can switch to the octree based algorithm using the following call

    void flimage_select_octree_quantizer(void);

To switch back to the median cut quantizer use

    void flimage_select_mediancut_quantizer(void);

The median-cut quantizer tends to give better images because of the dithering step. However, in this particular implementation, the number of quantized colors is limited to 256. There is no such limit with the octree quantizer implementation.