Summing specific pixel values

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Jason 2014-10-22

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https://ww2.mathworks.cn/matlabcentral/answers/159637-summing-specific-pixel-values

评论： Bjorn Gustavsson 2014-10-22

Hi. I am wanting to find the sum of all the positons xi, yi of an image IM. I thought the following would do it - but it doesn't.

sum(im (xi,yi));

Also, If I want to take this a step further and find the sum of all pixels at locations xi, yi but this time include the surrounding 8 pixels (so its a 3x3 centred on xi,yi), how do I do that?

thanks

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Answer 1

Image Analyst 2014-10-22

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You can use a for loop:

m=magic(5) % Whatever...
xi=[2,3,4]
yi = [2,3,4]
theSum = 0;
the8Sum = 0;
for k = 1 : length(xi)
  row = yi(k);
  col = xi(k);
  theSum = theSum + m(row, col);
  the8Sum =the8Sum + ...
    m(row-1, col-1) + ...
    m(row-1, col) + ...
    m(row-1, col+1) + ...
    m(row, col-1) + ...
    m(row, col) + ...
    m(row, col+1) + ...
    m(row+1, col-1) + ...
    m(row+1, col) + ...
    m(row+1, col+1);
end
theSum
the8Sum

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Jason 2014-10-22

Thanks, I will need to work out boundary issues.

Bjorn Gustavsson 2014-10-22

在 MATLAB Online 中打开

I suggest you do something like this instead:

ind1D = sub2ind(size(im),xi,yi);
sumPixxiyi = sum(im(ind1D));

Then for the nearest Neighbour you'd have to make arrays for them too. In my opinion such a solution is so much easier to maintain since it will be so many fewer lines of code that potential speed losses might be preferable.

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