Regardless of the complexity of the protection scheme, there is a simple way to thwart keygens.  Just have two keyspaces, A and B, where B is a subset of A (and where A is significantly larger than B so a key in A has a very small probability of being in B).  All keys distributed with licensed copies of the program are from the smaller set B.  The program will accept any key from the larger set A, while the algorithm for checking to see if the key belongs in B is kept a secret.

Although the program will run with invalid keys, any key that is sent back to the company can be easily checked and determined if it was generated by an unauthorized keygen (and the installation using it locked out of updates and such, or possibly even the user of the unauthorized installation finding themselves in legal hot water if the company chooses to proceed with legal action)

Of course, checking serial numbers here to make sure that people are only using licensed software for their renders is simply infeasible.