How to subtract single element of matrix from any number

i am having a matrix 'devi1' and graylevel image 'b'

a variable av2 is having value 79

my code is

b(1,1)=abs(a(1,1)-av2);

currently a(1,1)=0

then b(1,1) should be 78

but it is caming '0'

please help me !

Answer 1

[Walter Roberson]

abs(double(a(1,1)) - av2)

Your array is uint8. When you subtract a larger value from a uint8 value, then the result does not become negative: the result is uint8 and so the result becomes 0. You need to convert to a signed representation to avoid the problem.