Image Processing Toolbox User's Guide

Reducing Colors in an Indexed Image

Use imapprox when you need to reduce the number of colors in an indexed image. imapprox is based on rgb2ind and uses the same approximation methods. Essentially, imapprox first calls ind2rgb to convert the image to RGB format, and then calls rgb2ind to return a new indexed image with fewer colors.

For example, these commands create a version of the trees image with 64 colors, rather than the original 128.

• load trees
  [Y,newmap] = imapprox(X,map,64);
  imshow(Y, newmap);
  

The quality of the resulting image depends on the approximation method you use, the range of colors in the input image, and whether or not you use dithering. Note that different methods work better for different images. See Dithering for a description of dithering and how to enable or disable it.

Color Approximation

Dithering

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