# Correctly sum image values

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I have a standard rob image

im = imread('myIm.jpg');

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.

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### Accepted Answer

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!

##### 5 Comments

Image Analyst
on 4 Nov 2013

### More Answers (1)

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;

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