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 L, a label matrix of type double, and NumLabels, the actual number of superpixels that were computed.

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 pair 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 parameter isInputLab is true, 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.

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

Name-Value Pair Arguments

Specify optional comma-separated pairs of Name,Value arguments. Name is the argument name and Value is the corresponding value. Name must appear inside quotes. You can specify several name and value pair arguments in any order as Name1,Value1,...,NameN,ValueN.

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

'Compactness' — Shape of superpixels

10 (default) | numeric scalar

Shape of superpixels, specified as a numeric scalar. 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. The allowed range is (0 Inf). Typical values for compactness are in the range [1,20].

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

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

false (default) | true

Input image data is in the L*a*b* color space, specified as true or false.

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

'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 parameter.

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

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 code (requires MATLAB^® Coder™). For more information, see Code Generation for Image Processing.

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

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