sum over a matrix with condition in matlab

assume I have a 10x10 matrix M

M=[64 36 50 87 22 45 37 23 68 88;
   33 23 87 49 54 25 35 98 78 52;
   12 54 76 43 24 87 54 98 45 34; 
   77 87 23 45 34 65 23 76 12 76; 
   12 34 55 44 76 98 93 23 54 67; 
   22 55 78 90 88 56 34 23 12 76;
   99 23 67 89 34 23 12 87 45 23; 
   22 54 76 89 65 23 45 12 93 12;
   44 56 23 88 67 14 15 67 34 12;
   11 44 77 99 34 23 78 34 12 79];

• I want to first find out the local maximum in the matrix
• and then accordi开发者_运维百科ng to the maximum position do a sum over a 3x3 region over M

For the first step, the code I used is local_max=imregionalmax(M). to find out the local maximum position, but how can I go further to use this coordination to sum over a 3x3 matrix over M?

Thanks for the help.

You can calculate the sum for the whole matrix and then only keep the values that you're interested in. This should work:

local_max=imregionalmax(M)
sums = imfilter(M, ones(3));
local_max_sums = sums(local_max);

And if what you want is a matrix with non-zero entries where the local maxima are located:

local_max_sums = sums .* local_max;

You seem to be looking for the matrix subset functionality of Matlab.

Basically, for

M = [ 1 2 3 4 5 6;
      4 5 6 7 8 9;
      7 8 9 0 1 2;
      0 1 2 3 4 5;
      3 4 5 6 7 8;
      6 7 8 9 0 1];

If you have a max at (3,3), you can use M(2:4, 2:4) to get

N = [ 5 6 7;
      8 9 0;
      1 2 3];

Summing that matrix is all that remains - as simple as

total = sum(N(:));

This is kind of brute force for Matlab, but I think it works.

bw = imregionalmax(M);
[x,y] = find(bw);

s = [];
for i = 1:length(x)
    startX = x(i)-2;
    if(startX < 1)
        startX = 1;
    end

    endX = x(i)+2;
    if endX > 10
        endX = 10;
    end

    startY = y(i)-2;
    if startY < 1
        startY = 1;
    end

    endY = y(i)+2;
    if endY > 10
        endY = 10;
    end

    s(i) = sum2(M(startX:endX, startY:endY));
end