MATLAB: Index must be a positive integer or logical.

Question

how to correct this error "Attempted to access thinned(0,2); index must be a positive integer or logical."

 img = imread('5_1_4.jpg'); figure, imshow(img)
binary_image= im2bw(img,graythresh(img));
thinned=im2double(bwmorph(binary_image,'thin',Inf));
for x=1:size(thinned, 2)
    for y=1:size(thinned,1)
        if (thinned(x, y) == 1)  
CN=(abs(thinned(x+1,y)-thinned(x+1,y-1))+ abs(thinned(x+1,y-1)-thinned(x,y-1))+ abs(thinned(x,y-1)-thinned(x-1,y-1))+ abs(thinned(x-1,y-1)-thinned(x-1,y))+ abs(thinned(x-1,y)-thinned(x-1,y+1))+ abs(thinned(x-1,y+1)-thinned(x,y+1))+ abs(thinned(x,y+1)-thinned(x+1,y+1))+ abs(thinned(x+1,y+1)-thinned(x+1,y)));         
CN=CN/2;           
        end  
    end
end 
end

Answer 1

You access the indices x-1, y-1 and x+1, y+1. Then replace:

for x=1:size(thinned, 2)
  for y=1:size(thinned,1)

by:

for x = 2:size(thinned, 2) - 1   % [EDITED: x is 2nd dim, y is 1st dim]
  for y = 2:size(thinned, 1) - 1

Currently you overwrite CN in each iteration. Is this wanted?

I think, that a proper indentation and sorting of the indices makes it much easier to debug:

% [EDITED, x and y swapped]
CN = (abs(thinned(y+1, x-1) - thinned(y,   x-1)) + ... 
      abs(thinned(y+1, x)   - thinned(y+1, x-1)) + ...
      abs(thinned(y+1, x+1) - thinned(y+1, x)))  + ...
      abs(thinned(y,   x-1) - thinned(y-1, x-1)) + ...
      abs(thinned(y,   x+1) - thinned(y+1, x+1)) + ...
      abs(thinned(y-1, x-1) - thinned(y-1, x))   + ...
      abs(thinned(y-1, x)   - thinned(y-1, x+1)) + ...
      abs(thinned(y-1, x+1) - thinned(y,   x+1));
CN = CN/2;