Comparing images

I want to compare images by taking a difference of their images but those values are just limited to 0 and 255. if i subtract 0-255 it gives me a 0 and if its 255-0 the answer is 255. how do i make them subtract like normal numbers so i can calculate the difference and get negative values too?

Answer 1

[Image Analyst]

Cast them to double before you add or subtract uint8 or uint16 images.

diffImage = double(uint8Image1) - double(uint8Image2);

Answer 2

[Wayne King]

You could do

    diffim = double(X)-double(Y);

Answer 3

[Alex Taylor]

The results you are getting indicate that you are working with 8-bit unsigned integer data.

a = uint8(10*rand(3,3))
b = uint8(10*rand(3,3))
class(a)

If you want to do signed arithmetic on your data, you will need to work in a signed datatype. In the following example, I'll cast my matrices to double precision floating point.

   c = double(a)-double(b);

If you want to work with integer datatypes, you'll need a signed integer datatype big enough to represent the dynamic range of your data:

c = int16(c)

Hope that helps,

Alex.