cmunique

Eliminate duplicate colors in colormap; convert grayscale or truecolor image to indexed image

Syntax

[Y,newmap] = cmunique(X,map)

[Y,newmap] = cmunique(RGB)

[Y,newmap] = cmunique(I)

Description

[Y,newmap] = cmunique(X,map) removes duplicate rows from the colormap map to produce a new colormap, newmap. The function also adjusts the indices in intensity image X to maintain correspondence between the indices and the colormap, and returns the result in Y. The image Y and associated colormap newmap produce the same image as X and map but with the smallest possible colormap.

[Y,newmap] = cmunique(RGB) converts the truecolor image RGB to the indexed image Y and its associated colormap, newmap. The returned colormap is the smallest possible colormap for the image, containing one entry for each unique color in RGB.

Note

newmap might be very large, because the number of entries can be as many as the number of pixels in RGB.

[Y,newmap] = cmunique(I) converts the grayscale image I to an indexed image Y and its associated colormap, newmap. The returned colormap is the smallest possible colormap for the image, containing one entry for each unique intensity level in I.

Examples

Eliminate Duplicate Entries in Colormap

Use the magic function to define X as a 4-by-4 array that uses every value in the range between 1 and 16.

X = magic(4);

Use the gray function to create an eight-entry colormap. Then, concatenate the two eight-entry colormaps to create a colormap with 16 entries, map. In map, entries 9 through 16 are duplicates of entries 1 through 8.

map = [gray(8); gray(8)];
size(map)

ans = 1×2

    16     3

Use cmunique to eliminate duplicate entries in the colormap.

[Y, newmap] = cmunique(X, map);
size(newmap)

ans = 1×2

     8     3

cmunique adjusts the values in the original image X so that Y and newmap produce the same image as X and map.

figure
image(X)
colormap(map)
title('X and map')

figure
image(Y)
colormap(newmap)
title('Y and newmap')

Input Arguments

X — Indexed image with duplicate colors

m-by-n matrix of integers

Indexed image with duplicate colors, specified as an m-by-n matrix of integers.

Data Types: double | uint8 | uint16

map — Colormap with duplicate colors

c₁-by-3 matrix

Colormap with duplicate colors associated with indexed image X, specified as a c₁-by-3 matrix with values in the range [0, 1]. Each row of map is a three-element RGB triplet that specifies the red, green, and blue components of a single color of the colormap.

Data Types: double

RGB — RGB image

m-by-n-by-3 array of nonnegative numbers

RGB image, specified as an m-by-n-by-3 array of nonnegative numbers.

Data Types: double | uint8 | uint16

I — Grayscale image

m-by-n numeric matrix

Grayscale image, specified as an m-by-n numeric matrix.

Data Types: double | uint8 | uint16

Output Arguments

Y — Indexed image with unique colors

m-by-n matrix of integers

Indexed image with unique colors, returned as an m-by-n matrix of integers. If the length of newmap is less than or equal to 256, then the output image is of class uint8. Otherwise, the output image is of class double.

Data Types: double | uint8

newmap — Colormap with unique colors

c₂-by-3 matrix

Colormap with unique colors associated with the output indexed image Y, returned as a c₂-by-3 matrix with values in the range [0, 1]. Each row of newmap is a three-element RGB triplet that specifies the red, green, and blue components of a single color of the colormap.

Data Types: double

Version History

Introduced before R2006a