I have been looking over this per pixel lighting presentation again and i just cant figure out when they talk about solving dudx, dudy, etc how they are coming up with the solution. Under the slide that says “Generating the Partial Derivatives” they show some math that gets dudx from the plane equation Ax + Bu + Cv + D = 0. This is what they do:

Assume: Cv and D = 0

Ax + Bu = 0

Ax = -Bu

x = -Bu/A

dudx = -Bu/A

Ok where was a derivative done? I see how the Cv and D were set as constants to they would go away since we are doing a partial derivative. But they just solve for x and then say that dudx is the same as what we got for x. What gives? Could someone explain this? Its just annoying to me that i dont completly understand it, i dont like just using code im not sure about.

-SirKnight