maxIntensity of image/matrix and ratio

Question

zozo 2012-2-1

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https://ww2.mathworks.cn/matlabcentral/answers/27745-maxintensity-of-image-matrix-and-ratio

I have a matrix H(360x180). My final goal is to find the ratio of second max peak to max peak in the matrix.

I have done the following:

peaks=imregionalmax(H);
[label n]=bwlabel(peaks);
peaks = imdilate(peaks,strel('disk',1));
G=regionprops(label,'PixelIdxList');

I have G < 15x1 struct > How can I find the max intesity from each field in G, and take the ratio of second max intesity to max intensity from the set of all maximums/peaks?

Please help.

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Answer 1

Image Analyst 2012-2-1

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You need to put into a loop from 1 to n and get the max value for each region, something like (untested)

for blob = 1 : n
  % Get pixel intensities of this particular blob.
  thisBlob = H(G(blob).PixelIdxList);
  % Find the max intensity out of those pixel intensities.
  maxForThisBlob(blob) = max(thisBlob(:));
end
% Sort them in descending order.
sortedMaxes = sort(maxForThisBlob, 'descend');
% Get ratios of all peaks heights to the first peak height.
ratios = sortedMaxes / sortedMaxes(1);

That's just off the top of my head. If it doesn't work and you can't figure it out, write back.

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zozo 2012-2-2

thank you @image analyst and @walter.

I have 1 more query.

In matrix H, I have the values in dB scale with maximum (main lobe) at 0dB. I need to find the 3dB beamwidth of mainlobe(in degrees across elevation and azimuth).

How can I do it? any function or short syntax shall be helpful.

zozo 2012-2-2

@image analyst: got it Sir.

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Answer 2

Walter Roberson 2012-2-1

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firstmax = arrayfun( @(S) max(H(S.PixelIdxList)), G);

Second max depends on how you want to handle the possibility of duplicate values. If two locations have the same intensity, are those the first and second max? Or should the second max be the next unique value down?

almostlast = @(V) V(end-1);
secondmax = arrayfun( @(S) almostlast(unique(H(S.PixelIdList))), G);

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Walter Roberson 2012-2-2

G = regionprops(label,'PixelIdxList','Centroid','MaxIntensity');

numblobs = length(G);

sortedMaxes = sort([G.MaxIntensity], 'descend');

ratio = double(sortedMaxes(2)) ./ double(sortedMaxes(1));

Hc = zeros(size(H));

cents = zeros(numblobs,3);

for K = 1 : numblobs

cents(K,:) = [G(K).Centroid, G(K).MaxIntensity];

thesepixels = G(K).PixelIDList;

Hc(thesepixels) = H(thesepixels);

end

imagesc(Hc);

for K = 1 : numblobs

text(cents(K,1), cents(K,2), num2str(cents(K,3)))

end

But perhaps I did not understand what you meant about plotting the blobs on your imagesc() plot. If you have an existing image and want to draw a border around each blobs, then matters get a bit more complicated in order to trace the boundary... and I think it is about time to head home for supper.

zozo 2012-2-2