MATLAB: How to create 2D Gray image from 3D colored image

2d3darraysimagematrix

I've been trying to create a 2D matrix using the Average of RGB values from each pixel in a 3D matrix.
for i = 1:m
    for j = 1:n
        
            gray(i,j) = double((A(i,j,1) + A(i,j,2) + A(i,j,3))/3);
            %%gray(i,j) = round(gray(i,j)/255);
        
    end
end
imshow(gray, []);
This only shows a red image with some greens in it. It is possible that it still is trying to print using RGB values maybe?
IF I uncomment this: "gray(i,j) = round(gray(i,j)/255);" , I get a black image.
All I need is a simple direction. What am I doing wrong? The image should be grayscale form the averages of RGB values.

Best Answer

When you add data of non-double type you'll run into all sorts of wrap-around problems. When I work with images I typically cast the data to double ASAP - that is instantly after reading the image-file:
A = imread('peppers.png');
A = double(A);
Then I don't have to worry about issues like overflow and wraparound.
subplot(2,2,1)
imagesc(A/max(A(:)))
disp([min(A(:)),max(A(:))])
subplot(2,2,2)
imagesc(A/max(A(:)))
Then a couple of unrelated advices:
When you have to loop, try to pre-allocate your variables, otherwise matlab will have to re-allocate space on every step, that becomes slow. If you cannot figure-out how big matrices you have but can run the loop in any direction you might run it from the largest indices and down towards 1: 
for i = m:-1:1
  for j = n:-1:1
    gray(i,j) = (A(i,j,1) + A(i,j,2) + A(i,j,3))/3;
  end
end
That way you'll allocate gray to its full size first time around the loops.
You very often do not need to loop. matlab is short for MATrix LABoratory after all. You can do something like this:
gray = (A(:,:,1)+A(:,:,2)+A(:,:,3))/3;
...or even better there are loads of functions doing what you need or 60-90 % of what you need. You could do:
gray = mean(A,3);
HTH
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