This is a really bad idea. Most stream ciphers, certainly RC4 and anything in a Counter Mode construction are ultimately a PRNG XORed onto the plaintext. This is fine for encryption -- the information theory is pretty easy to understand -- but is no basis for a hash function.

The goals of a hash function and a cipher are different. As it turns out, you can make a block cipher into a hash function, but even then not any block cipher makes for a good hash function.

Think of it this way -- a cipher is like a game of cards. It relies upon the fact that you can't see what's in your opponent's hand, and what makes it hard or intractable is the uncertainty. But a hash function is like a game of chess. Everyone knows everything, and the goal of the hash function is to make it so complex that you can't go backwards and that there's no easy way to predict forwards.

Can you make a hash out of a stream cipher?
I'm trying to get my head around heightmap terrain generation. If I have a plane say 64 x 64 verts will I need to create a 64px by 64px greyscale heightmap in order to displace the geometry of the plane correctly?
