Weird Calculation difference using uint16 and double for an image

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Jason
Jason 2019-10-30
评论： Steven Lord 2019-10-30
Hello, I have a function that calculates the "contrast" of a greyscale image
function FM=CalculateBrenner(Image)
[M N] = size(Image);
DH = Image;
DV = Image;
DH(1:M-2,:) = diff(Image,2,1);
DV(:,1:N-2) = diff(Image,2,2); % second order difference between
% columns, i.e., along x-direction.
FM = max(DH, DV);
FM = FM.^2;
FM = mean2(FM);
end
when my image is a uint16, I get
FM= 100.39
max=438
min=22
But when my image is cast as a double
I get:
FM = 204.14
max=438.00
min= 22.00
So why is the calculation FM different for both cases, yet the values its using are the same (indicated by the max & min)

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采纳的回答

Steven Lord
Steven Lord 2019-10-30
d = [1 2 1];
u = uint16(d);
dd = diff(d)
du = diff(u)
The smallest value you can store in an unsigned 16-bit integer is 0. In double 1 - 2 is -1 but in uint16 it is 0.
  2 个评论
Jason
Jason 2019-10-30
编辑：Jason 2019-10-30
OK, so I should use a double as it doesn't ignore negative values.
Steven Lord
Steven Lord 2019-10-30
You could use imabsdiff if you have Image Processing Toolbox, or just take the max of A-B and B-A (one will be zero, one may not be) for appopriate pieces A and B of your Image variable.
d = [1 2 1];
u = uint16(d);
max(u([3 2])-u([2 1]), u([2 3])-u([1 2]))
I'll leave that last line for you to generalize.

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