value of Z in in the program doesn't take above 255?? program used for edge detection('salt & pepper nois'e)

value of Z cannot take any value above 255 why?

PROGRAM

for i = 1:imageHeight - windowHeight + 1
  for j = 1:imageWidth - windowWidth + 1
      window = I(i:i + windowHeight - 1, j:j + windowWidth - 1);
      d=window(2,2);
      if(window(2,2)~=0 && window(2,2)~=255)
          a=0;
          for m=1:3
             for p=1:3
                if(m~=2 || p~=2)
                  if(window(m,p)~=0 && window(m,p)~=255) 

                          a=a+1
                      else
                     end
                  else
                  end
               end
            end
            z=a*d

[EDITED, Jan, please format your code properly to make it readable]

Answer 1

[Wayne King]

The program is most likely designed to work on uint8 data - that data is in the range [0, 255] for 2^8 data values.

[Jan]

I did reload this thread before I've answered, but I did not see your almost equivalent answer. Strange.

Answer 2

[Jan]

There is no varaible called "Z" in your code. I guess you mean the lower case "z".

window = I(...)
d = window(2, 2);
z = a * d;

When "I" is an UINT8 array, and a is a DOUBLE, the multiplication replies the type with the highest restrictions. Therefore the class of z is UINT8 also, which saturates at 255. Test this:

class(z)

You can try:

z = a * double(d);