What I'm doing is calculating for the entire 2x2 quad: abs(p(1,0) - p(0,0)) and abs(p(0,1) - p(0,0)) since I thought the rate of change is an absolute value that should be the same for all x and y in the 2x2 quad. Is that correct ?
Edit: I assume the part I'm missing is only for non-uniform textures ? (e.g. 128x64 instead of 128x128)
If I understand correct I also need to multiply the derivatives of X and Y both by the texWidth and both by texHeight and then do the max(dot(xx,xx), dot(yy, yy)) right ?
I think I've got it working now