Summing specific pixel values

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

Answer 1

[Image Analyst]

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

[Jason]

Thanks, I will need to work out boundary issues.

[Bjorn Gustavsson]

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.