Efficiency Optimization in Correlation Problem

Iñigo Belío

2019 4 30

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I developed a part of my program that I need to optimize as much as possible. I will give a bit of context first:

The starting point is an image (img) lets say 448x448. This image contains several light spots and I want to find the centroid position of each one.

Each spot lies on a subaperture of the image of lets say of 32x32 pixels. So we can divide the image (img), into multiple sub-images (subI) of size 32x32.

The centroid of the light spot at each subaperture is obtained by calculating the correlation of a model light spot (template) to every single position possible withing the sub-image.

Here is the code I've made so far:

[m,n] = size(img);
centroids.X = zeros(m/gridSize,n/gridSize);
centroids.Y = zeros(m/gridSize,n/gridSize);
template = fspecial('gaussian', [7 7], 3); %Spot light model
template = template/max(template(:)); %Normalize to maximum
comparations = (gridSize-size(template,1) + 1); %Number of times to move the template around
    %First two loops are two run over all the subapertures positions
    for k = 0:size(centroids.X,1)-1
        for l = 0:size(centroids.X,2)-1
            %Now we get the sub images corresponding to each subaperture
            %of the image where we want to find the centroid
            lim = [k*gridSize+1,l*gridSize+1];
            subI = imcrop(img,[lim(2),lim(1),gridSize-1,gridSize-1]);
            correlations = zeros(comparations,comparations);
            %In this two loops we compute the correlation coefficient
            %between the template and each possible position in subI
            for m = 1:comparations
                for n = 1:comparations
                    correlations(m,n) = corr2(template,subI(m:m+7-1,n:n+7-1));
                end
            end
          [~,maxCorr] = max(abs(correlations(:)));%We get the maximum value
          [row,col] = ind2sub(size(correlation),maxCorr);%Position of the maximum
          %Position of the maximum in the full image
          rowAbs = row + gridSize*k + 3;
          colAbs = col + gridSize*l + 3;
          %Store the position as the centroid for the subaperture
          centroids.X(k+1,l+1) = colAbs;
          centroids.Y(k+1,l+1) = rowAbs;
        end
    end