I've been playing with hl2 sdk and ran across some weird looking dudv map things. I'm familiar with normal mapping but don't really see how this one is used. Intuitively i understand it means change in horizontal (derivative of u coord) along with change in vertical (derivative of v coord).

I have no idea what im talking about, so take this as pure conjecture. Are the dudv's stored in smaller datatypes than uv's would be? that might explain a reason.