labelmatrix

Create label matrix from bwconncomp structure

Syntax

L = labelmatrix(CC)

Description

L = labelmatrix(CC) creates a label matrix, L, from the connected components structure CC returned by bwconncomp.

labelmatrix is more memory efficient than bwlabel and bwlabeln because it returns its label matrix in the smallest numeric class necessary for the number of objects.

Examples

Find and Display Connected Components

Read a binary image into the workspace.

BW = imread('text.png');

Calculate the connected components using bwconncomp.

CC = bwconncomp(BW);

Create a label matrix using labelmatrix.

L = labelmatrix(CC);

For comparison, create a second label matrix using bwlabel.

L2 = bwlabel(BW);

View both label matrices in the workspace. Note that labelmatrix uses the smallest numeric class necessary for the number of objects and therefore is more memory efficient than bwlabel .

whos L L2

  Name        Size              Bytes  Class     Attributes

  L         256x256             65536  uint8               
  L2        256x256            524288  double              

Display the label matrix as an RGB image using label2rgb. To make it easier to differentiate the different connected components, shuffle the color order of the labels.

imshow(label2rgb(L,'jet','w','shuffle'));

Input Arguments

CC — Connected components

struct

Connected components, specified as a structure with four fields.

Field	Description
Connectivity	Connectivity of the connected components (objects)
ImageSize	Size of the binary image
NumObjects	Number of connected components (objects) in the binary image.
PixelIdxList	1-by-NumObjects cell array where the k-th element in the cell array is a vector containing the linear indices of the pixels in the k-th object.

Output Arguments

L — Label matrix

matrix of nonnegative integers

Label matrix of contiguous regions, returned as matrix of nonnegative integers. The pixels labeled 0 are the background. The pixels labeled 1 make up one object; the pixels labeled 2 make up a second object; and so on.

The size of L is CC.ImageSize. The class of L depends on CC.NumObjects, as shown in the table.

Class	Range
'uint8'	CC.NumObjects $$\le$$ 255
'uint16'	256 $$\le$$ CC.NumObjects $$\le$$ 65535
'uint32'	65536 $$\le$$ CC.NumObjects $$\le 2^{32}-1$$
'double'	CC.NumObjects $$\ge 2^{32}$$

Data Types: double | uint8 | uint16 | uint32