label2idx

R2026b

Convert label matrix to cell array of linear indices

Syntax

pixelIndexList = label2idx(L)

Description

pixelIndexList = label2idx(L) converts the regions described by the label matrix L into linear indices pixelIndexList.

Examples

Calculate Pixel Index List for Small Label Matrix

Create a small sample matrix containing three regions.

BW = logical([1 1 1 0 0 0 0 0
              1 1 1 0 1 1 0 0
              1 1 1 0 1 1 0 0
              1 1 1 0 0 0 0 0
              1 1 1 0 0 0 1 0
              1 1 1 0 0 0 1 0
              1 1 1 0 0 1 1 0
              1 1 1 0 0 0 0 0]);

Create a label matrix from this sample image.

L = bwlabel(BW)

L = 8×8

     1     1     1     0     0     0     0     0
     1     1     1     0     2     2     0     0
     1     1     1     0     2     2     0     0
     1     1     1     0     0     0     0     0
     1     1     1     0     0     0     3     0
     1     1     1     0     0     0     3     0
     1     1     1     0     0     3     3     0
     1     1     1     0     0     0     0     0

Get a linear index list of all the pixels in each region. The function returns a cell array with an element for each region it finds in the label matrix.

pixelIndexList = label2idx(L)

pixelIndexList = 1×3 cell array
    {24×1 double}    {4×1 double}    {4×1 double}

Examine one of the pixel index lists returned. For example, look at the second cell in the returned cell array. It contains the linear indices for all the pixels in the region labeled "2". The upper left corner of the region is pixel BW(2,5), which is the 34th pixel in linear indexing.

pixelIndexList{2}

ans = 4×1

    34
    35
    42
    43

Input Arguments

L — Label matrix

numeric array

Label matrix, specified as a numeric array of any dimension.

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

Output Arguments

pixelIndexList — Linear indices of pixels in regions

1-by-n cell array

Linear indices of pixels in regions, returned as a 1-by-n cell array. Each element of the output, pixelIndexList{n}, is a vector that contains all the linear indices in L where L is equal to n.

Extended Capabilities

C/C++ Code Generation

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

label2idx supports the generation of C and C++ code (requires MATLAB^® Coder™). For more information, see Code Generation for Image Processing.

GPU Code Generation
Generate CUDA® code for NVIDIA® GPUs using GPU Coder™.

Thread-Based Environment

Run code in the background using MATLAB® backgroundPool or accelerate code with Parallel Computing Toolbox™ ThreadPool.

The label2idx function fully supports thread-based environments. For more information, see Run MATLAB Functions in Thread-Based Environment.

Version History

Introduced in R2016a

R2022b: Support for thread-based environments

label2idx now supports thread-based environments.