It's defined to work on a 2x2 grid called a quad. If you're the top left pixel, then dFdX(v) = v(x+1,y) - v. If you're the top right pixel, then it's dFdX(v) =

It's defined to work on a 2x2 grid called a quad. If you're the top left pixel, then dFdX(v) = v(x+1,y) - v. If you're the top right pixel, then it's dFdX(v) = v(x-1,y) - v