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What's wrong with the simple call
max(A(:)) or min(A(:))
? Since Matlab stores A in column format anyway, I don't think there is much computational work required to transform the array into a column vector.
Refers PRESS to the well-known leave-one-out-crossvalidation? If yes, it would be good to indicate so in the description.
No, it is not an alternativ since edge can handle grayscale images, too. However, I like the good documentation, the comments in the code and the H1 line.
refer to the function conv2 to be much more efficient in calculating an averaging filter.