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superpixels

2-D superpixel oversegmentation of images

Syntax

[L,NumLabels] = superpixels(A,N)
[L,NumLabels] = superpixels(___,Name,Value,...)

Description

example

[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(___,Name,Value,...) computes superpixels of image A using with Name-Value pairs used to control aspects of the segmentation.

Examples

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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

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Input image, specified as a real, nonsparse matrix. For int16 data, A must be a 2-D grayscale image. For all other data types, A can be a 2-D grayscale or 2-D RGB image. When the parameter isInputLab is true, the input image must be single or double.

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

Desired number of superpixels, specified as a numeric scalar.

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 single 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);

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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

Input image data is in the L*a*b* colorspace, specified as the logical scalar true or false.

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

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

Number of iterations used in the clustering phase of the algorithm, specified as a numeric scalar. 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

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Label matrix, returned as a numeric array of type double. The values are positive integers, where 1 indicates the first region, 2 the second region, and so on for each superpixel region in the image.

Number of superpixels computed, returned as a numeric scalar of type 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

Introduced in R2016a

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