superpixels

2-D superpixel oversegmentation of images

Syntax

[L,numLabels] = superpixels(A,N)

[L,numLabels] = superpixels(A,N,Name=Value)

Description

[L,numLabels] = superpixels(A,N) computes superpixels of the 2-D grayscale or RGB image A. N specifies the number of superpixels you want to create. The function returns the label matrix L and the actual number of superpixels that were computed, numLabels.

The superpixels function uses the simple linear iterative clustering (SLIC) algorithm [1]. This algorithm groups pixels into regions with similar values. Using these regions in image processing operations, such as segmentation, can reduce the complexity of these operations.

[L,numLabels] = superpixels(A,N,Name=Value) computes superpixels of image A using name-value arguments used to control aspects of the segmentation.

Examples

Compute Superpixels of Input RGB Image

Read image into the workspace.

A = imread('kobi.png');

Calculate superpixels of the image.

[L,N] = superpixels(A,500);

Display the superpixel boundaries overlaid on the original image.

figure
BW = boundarymask(L);
imshow(imoverlay(A,BW,'cyan'),'InitialMagnification',67)

Set the color of each pixel in the output image to the mean RGB color of the superpixel region.

outputImage = zeros(size(A),'like',A);
idx = label2idx(L);
numRows = size(A,1);
numCols = size(A,2);
for labelVal = 1:N
    redIdx = idx{labelVal};
    greenIdx = idx{labelVal}+numRows*numCols;
    blueIdx = idx{labelVal}+2*numRows*numCols;
    outputImage(redIdx) = mean(A(redIdx));
    outputImage(greenIdx) = mean(A(greenIdx));
    outputImage(blueIdx) = mean(A(blueIdx));
end    

figure
imshow(outputImage,'InitialMagnification',67)

Input Arguments

A — Image to segment

2-D grayscale image | 2-D truecolor image

Image to segment, specified as a 2-D grayscale image or 2-D truecolor image. For int16 data, A must be a grayscale image. When the argument isInputLab is true, then the input image must be data type single or double.

Data Types: single | double | int16 | uint8 | uint16

N — Desired number of superpixels

positive integer

Desired number of superpixels, specified as a positive integer.

Name-Value Arguments

Specify optional pairs of arguments as Name1=Value1,...,NameN=ValueN, where Name is the argument name and Value is the corresponding value. Name-value arguments must appear after other arguments, but the order of the pairs does not matter.

Example: B = superpixels(A,100,NumIterations=20); performs twenty iterations.

Before R2021a, use commas to separate each name and value, and enclose Name in quotes.

Example: B = superpixels(A,100,"NumIterations",20);

Compactness — Shape of superpixels

10 (default) | positive number

Shape of superpixels, specified as a positive number. The compactness parameter of the SLIC algorithm controls the shape of superpixels. A higher value makes superpixels more regularly shaped, that is, a square. A lower value makes superpixels adhere to boundaries better, making them irregularly shaped. Typical values for compactness are in the range [1, 20].

IsInputLab — Input image data is in L*a*b* color space

false or 0 (default) | true or 1

Input image data is in the L*a*b* color space, specified as a numeric or logical 0 (false) or 1 (true).

Method — Algorithm used to compute superpixels

"slic0" (default) | "slic"

Algorithm used to compute superpixels, specified as one of the following values. The superpixels function uses two variations of the simple linear iterative clustering (SLIC) algorithm.

Value	Meaning
"slic0"	superpixels uses the SLIC0 algorithm to refine Compactness adaptively after the first iteration. This is the default.
"slic"	Compactness is constant during clustering.

Data Types: char | string

NumIterations — Number of iterations

10 (default) | positive integer

Number of iterations used in the clustering phase of the algorithm, specified as a positive integer. For most problems, it is not necessary to adjust this argument.

Output Arguments

L — Label matrix

array of positive integers

Label matrix, returned as an array of positive integers. The value 1 indicates the first region, 2 the second region, and so on for each superpixel region in the image.

Data Types: double

numLabels — Number of superpixels computed

positive integer

Number of superpixels computed, returned as a positive integer.

Data Types: double

References

[1] Radhakrishna Achanta, Appu Shaji, Kevin Smith, Aurelien Lucchi, Pascal Fua, and Sabine Susstrunk, SLIC Superpixels Compared to State-of-the-art Superpixel Methods. IEEE Transactions on Pattern Analysis and Machine Intelligence, Volume 34, Issue 11, pp. 2274-2282, May 2012

Extended Capabilities

C/C++ Code Generation

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

Usage notes and limitations:

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

• All string and character vector inputs must be compile-time constants.

• The value of IsInputLab (true or false) must be a compile-time constant.

GPU Code Generation

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

Usage notes and limitations:

• All string and character vector inputs must be compile-time constants.

• The value of IsInputLab (true or false) must be a compile-time constant.

Thread-Based Environment

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

This 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

superpixels now supports thread-based environments.