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function nCutValue = calculateNcut (anAffinityMat , clusterIdx)
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%Calculation of the NCut metric
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% Usage nCutValue = calculateNcut (anAffinityMat , clusterIdx), where:
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% Inputs
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% - anAffinityMat is a rectangular, symmetrical affinity matrix
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% representation of an image
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% - clusterIdx is a vector storing the cluster Id of each node
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% Output
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% - nCutValue is a vector storing the cluster Id of each node
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% Gets the unique cluster IDs
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clusterIds = unique(clusterIdx);
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if size(clusterIds, 1) ~= 2
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error('Too many different clusters! Number of clusters must be two');
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end
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% Finds the indices of the samples of each cluster
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clusterOneIndices = (clusterIdx == clusterIds(1));
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clusterTwoIndices = (clusterIdx == clusterIds(2));
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% Calculates the N-Cut metric
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nCutValue = 2 - ...
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(sum(sum(anAffinityMat(clusterOneIndices, clusterOneIndices))) / ...
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sum(sum(anAffinityMat(clusterOneIndices, :))) + ...
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sum(sum(anAffinityMat(clusterTwoIndices, clusterTwoIndices))) / ...
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sum(sum(anAffinityMat(clusterTwoIndices, :))));
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end
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