Speeding up matrix comparison within a loop (logical indexing???)

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Andriy Boubriak 2015-1-27

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https://ww2.mathworks.cn/matlabcentral/answers/172538-speeding-up-matrix-comparison-within-a-loop-logical-indexing

评论： Andriy Boubriak 2015-1-28

At the moment as part of a larger programme I have the following code, however it takes a 10s of minutes to run. I've seen people on these forums reference something called "logical indexing" that removes the need for the loop however googling logical indexing I couldn't figure out quite what it is or how it works.

for i=2:12
  sizeOfIm=size(squeeze(imlab50(i,:,:,:)));
  outputIm=zeros(sizeOfIm(1),sizeOfIm(2),sizeOfIm(3));
  outputIm(:,:,:)=squeeze(imlab50(i,:,:,:));
  outputIm=uint8(outputIm);
  for loopy = 1 : sizeOfIm(1)
      for loopx = 1:sizeOfIm(2)
          if (squeeze((backGrndColour(i,:)).'-mindif(:))>squeeze(imlab50(i,loopy,loopx,:))) 
                if (squeeze(backGrndColour(i,:)).'-maxdif(:) <squeeze(imlab50(i,loopy,loopx,:)))
                    outputIm(loopy,loopx,:)=0;
                end
            end 
        end
    end

In short how do I do the following comparison for every value in the matrices, without having three serperate nested loops that vary i, loopy, and loopx?

 if (squeeze((backGrndColour(i,:)).'-mindif(:))>squeeze(imlab50(i,loopy,loopx,:)))

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Answer 1

Sara 2015-1-27

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在 MATLAB Online 中打开

Without having any data to test it, I'd say you could do:

for i=2:12
      x = squeeze(imlab50(i,:,:,:));
      outputIm=uint8(x);
      [j,k] = find(squeeze((backGrndColour(i,:)).'-mindif(:)) > x);
      outputIm(j,k,:) = 0;
end

In the find, check if it should be x or outputIm.

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Andriy Boubriak 2015-1-27

I got the followng error

"Error using > Number of array dimensions must match for binary array op.

Error in locator (line 32) [j,k] = find(squeeze((backGrndColour(i,:)).'-mindif(:)) > x);"

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Answer 2

Image Analyst 2015-1-27

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https://ww2.mathworks.cn/matlabcentral/answers/172538-speeding-up-matrix-comparison-within-a-loop-logical-indexing#answer_166130

在 MATLAB Online 中打开

What is imlab50? Is it a uint8 video?

And this code:

outputIm=zeros(sizeOfIm(1),sizeOfIm(2),sizeOfIm(3));
outputIm(:,:,:)=squeeze(imlab50(i,:,:,:));
outputIm=uint8(outputIm);

can most likely be replaced with this to save some time possibly.

outputIm = uint8(squeeze(imlab50(i,:,:,:)));

Another thing - you made a very common error, getting x and y wrong. You're calling the first index x and the second one y - that's wrong. The first index is the row index and thus should be y not x, and the second index is the column and thus should be called x, not y.