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This is a mathematical problem in essence. Don't worry about it too much. It's a little old, and you don't care about it now. Ha ha ha
??????? The code is what they say. I just explain how this code is derived
Because of mathematics,u,v is un-normalize, u=u(x,y),v=v(x,y). (x,y)->(u1,v1), (x+1, y)->(u2,v2), ddx(u1v1) = (u2-u1, v2-v1), set (u2-u1,v2-v1) is...