label2rgb

Convert label matrix into RGB image

Syntax

RGB = label2rgb(L)

RGB = label2rgb(L,cmap)

RGB = label2rgb(L,cmap,zerocolor)

RGB = label2rgb(L,cmap,zerocolor,order)

Description

RGB = label2rgb(L) converts a label matrix, L, such as those returned by labelmatrix, bwlabel, bwlabeln, or watershed, into an RGB color image for the purpose of visualizing the labeled regions. The label2rgb function determines the color to assign to each object based on the number of objects in the label matrix. The label2rgb function picks colors from the entire range of the color map.

RGB = label2rgb(L,cmap) specifies the colormap cmap to be used in the RGB image.

RGB = label2rgb(L,cmap,zerocolor) specifies the RGB color of the background elements (pixels labeled 0).

RGB = label2rgb(L,cmap,zerocolor,order) controls how label2rgb assigns colors to regions in the label matrix.

Examples

Use Color to Highlight Elements in Label Matrix

Read an image and display it.

I = imread('rice.png'); 
imshow(I)

Create a label matrix from the image.

BW = imbinarize(I); 
CC = bwconncomp(BW);
L = labelmatrix(CC);

Convert the label matrix into RGB image, using default settings.

RGB = label2rgb(L);
figure
imshow(RGB)

Convert the label matrix into an RGB image, specifying optional parameters. This example uses the 'spring' colormap, sets background pixels to the color cyan, and randomizes how colors are assigned to the labels.

RGB2 = label2rgb(L,'spring','c','shuffle'); 
figure
imshow(RGB2)

Input Arguments

L — Label matrix

m-by-n matrix of nonnegative integers

Label matrix of contiguous regions, specified as an m-by-n matrix of nonnegative integers. Pixels labeled 0 are the background. Pixels labeled 1 make up one object; pixels labeled 2 make up a second object; and so on.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | logical

cmap — Color map

'jet' (default) | c-by-3 matrix | colormap function | handle

Color map, specified as one of the following.

Value	Description
c-by-3 matrix of data type double	Color map matrix specifying c colors, each as an RGB triple. c must be greater than or equal to the number of labels, numlabels, in label matrix L. You can compute the number of labels as numlabels = max(L(:)). If c is greater than numlabels, then label2rgb creates the RGB image using only the first numlabels rows in the matrix.
colormap function	Name of a MATLAB® colormap function, such as 'jet' or 'gray'. See colormap for a list of supported colormaps.
colormap handle	Handle of a colormap function, such as @jet or @gray.

zerocolor — Color map

[1 1 1] (white) (default) | 3-element vector | 'b' | 'c' | 'g'

Label matrix of contiguous regions, specified as a 3-element vector representing an RGB triple, or one of the following color abbreviations.

Value	Color
'b'	Blue
'c'	Cyan
'g'	Green
'k'	Black
'm'	Magenta
'r'	Red
'w'	White
'y'	Yellow

order — Color order

'noshuffle' (default) | 'shuffle'

Color order, specified as 'noshuffle' or 'shuffle'. The 'noshuffle' order arranges colormap colors to label matrix regions in numerical order. The 'shuffle' order assigns colormap colors pseudorandomly.

Output Arguments

RGB — RGB image

m-by-n-by-3 numeric matrix

RGB image, returned as an m-by-n-by-3 numeric matrix.

Data Types: uint8

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• label2rgb supports the generation of C code (requires MATLAB Coder™). For more information, see Code Generation for Image Processing.

• When generating code, for best results when using the standard syntax RGB = label2rgb(L,cmap,zerocolor,order):

  • Submit at least two input arguments: the label matrix, L, and the colormap matrix, cmap.

  • cmap must be a c-by-3 matrix of data type double. You cannot specify the name of a MATLAB colormap function or a function handle of a colormap function.

  • If you set the background color zerocolor to the same color as one of the regions, then label2rgb will not issue a warning.

  • If you supply a value for order, then it must be 'noshuffle'.