OK, so I'm trying to walk the line between helping you and doing your homework... so, how about this? I'll step you through solving a very similar problem, but in 1 dimension. I'll let you figure out the 2-D version.
Problem: given a vector, such as x = [3,5,0,0,7,6,3,5,2,0], find the difference between the consecutive pairs. That is, [2,0,-1,2,-2].
Solution: The first element of the solution is x(2)-x(1); the 2nd element is x(4)-x(3); the 3rd is x(6)-x(5); the 4th is x(8)-x(7)... OK, so I see a pattern. solution(k) = x(2*k)-x(2*k-1). Hence, brute-force approach would be
y = zeros(1,5);
for k=1:5
y(k) = x(2*k) - x(2*k-1);
end
But a more "MATLAB" way to look at this would be to see that we're taking the even-numbered elements minus the odd-numbered ones. To get the even-numbered elements: x(2:2:end) This will return the vector [5,0,6,5,0]. Similarly, the odd-numbered elements are x(1:2:end), which gives [3,0,7,3,2]. So now we have two vectors of 5 points. Take the difference and we're done:
y = x(2:2:end) - x(1:2:end)
Now change difference into average, and extend to 2-D arrays, and your homework is done and dusted. I hope!
A final word of warning: make sure you're doing everything with doubles, not uint8. Images are often in uint8, but that will cause headaches when you try to do calculations (like add together and divide by 4, for example). If necessary: x = double(x).
