imapprox (Image Processing Toolbox User's Guide)

Image Processing Toolbox User's Guide

Approximate indexed image by one with fewer colors

Syntax

[Y,newmap] = imapprox(X,map,n)
[Y,newmap] = imapprox(X,map,tol)
Y = imapprox(X,map,newmap)
[...] = imapprox(...,dither_option)

Description

[Y,newmap] = imapprox(X,map,n) approximates the colors in the indexed image X and associated colormap map by using minimum variance quantization. imapprox returns indexed image Y with colormap newmap, which has at most n colors.

[Y,newmap] = imapprox(X,map,tol) approximates the colors in X and map through uniform quantization. newmap contains at most (floor(1/tol)+1)^3 colors. tol must be between 0 and 1.0.

Y = imapprox(X,map,newmap) approximates the colors in map by using colormap mapping to find the colors in newmap that best match the colors in map.

Y = imapprox(...,dither_option) enables or disables dithering. dither_option is a string that can have one of these values. The default value is enclosed in braces ({}).

Value
Description

{'dither'}
Dithers, if necessary, to achieve better color resolution at the expense of spatial resolution.

'nodither'
Maps each color in the original image to the closest color in the new map. No dithering is performed.

Value	Description
`{'dither'}`	Dithers, if necessary, to achieve better color resolution at the expense of spatial resolution.
`'nodither'`	Maps each color in the original image to the closest color in the new map. No dithering is performed.

Class Support

The input image X can be of class uint8, uint16, or double. The output image Y is of class uint8 if the length of newmap is less than or equal to 256. If the length of newmap is greater than 256, Y is of class double.

Algorithm

imapprox uses rgb2ind to create a new colormap that uses fewer colors.

Example

Approximate the indexed image trees.tif by another indexed image containing only 16 colors.

[X, map] = imread('trees.tif');
[Y, newmap] = imapprox(X, map, 16);
imshow(Y, newmap)

See Also

cmunique, dither, rgb2ind

imagemodel imattributes