MATLAB: Why is the differnce in subtracting two images and subtracting two matrices

Question

Hi, when you subtract two images we get another image which will be having no negative elements in it,ie intensity level below zero is assigned to Zero value. But when we subtract another two matrices,for example a=[1,2,4;4,6,8;6,8,9] and b=[4,6,9;4,1,3;4,2,7] we get another matrix [-3,-4,-5;0,5,4;2,6,2]. but this final matrix has some negative elements. my doubt is that,even if two images are saved in two variables(ie equivalent to two matrices) their subtraction always gives another matrix having no negative elements. but i want to get negative elements after subtracting them how to achieve this in MATLAB ? i need to sum all those -ve elements. plz help me…

Answer 1

Hi Pramod,

I think this is related due the fact that an image is hold as an uint8 matrix and those have no sign. What you can do to get the negative values is to cast it,e.g. to int16 before:

difference = int16(A) - int16(B);

Friedrich