# Correctly sum image values

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Andy on 1 Nov 2013
Commented: Image Analyst on 4 Nov 2013
I have a standard rob image
I want to access values within this image so that i can calculate the average for different regions. The coordinates for these rejoins are held in variables xx and yy. I have tried averaging the red, green, blue values from the image as below but instead of the sum returning a single value it returns an array of doubles. xx and yy are of size 1x1028 which is the size of the average i get back.
avRed = sum(im(xx,yy,1))/el;
avGre = sum(im(xx,yy,2))/el;
avBlu = sum(im(xx,yy,3))/el;
I want to sum up all the image values at points xx and yy, and then divide them by the number of elements el, and have the average colour value returned as a single value between 0-255 and not an array. Could anyone advise me on where i have gone wrong.

sixwwwwww on 1 Nov 2013
Edited: sixwwwwww on 1 Nov 2013
Dear Andy, try this:
avRed = sum(sum(im(xx,yy,1)))/el;
avGre = sum(sum(im(xx,yy,2)))/el;
avBlu = sum(sum(im(xx,yy,3)))/el;
Since in your case you have a matrix so you have to sum twice because in first summation MATLAB calculates the sum of each column individually and stores them in a row vector and in the second summation it sum this row vector which is your desired summation.
I hope it helps. Good luck!
Image Analyst on 4 Nov 2013
Flipping up/down is not going to make it magically work. It's a completely different concept. Evidently I didn't explain it well enough because you don't understand how im(xx,yy,1) gives you 4 pixel values instead of 2 pixel values.

Azzi Abdelmalek on 1 Nov 2013
Edited: Azzi Abdelmalek on 1 Nov 2013
n=size(im);
m=ones(size(xx));
avRed = sum(im(sub2ind(n,xx,yy,m)))/el;
avGre = sum(im(sub2ind(n,xx,yy,2*m)))/el;
avBlu = sum(im(sub2ind(n,xx,yy,3*m)))/el;