Simple improvements to code

[DavidWC]

I'd like to give a couple of suggestions to improvements to the code. In particular, the use of strings to identify the different variations of the transforms is error-prone, inefficient, and hard to use. These would be much better handled as enumeration types.

I would also recommend liberal use of "const" and "restrict" for your pointers to the data. These will improve the quality of the code by reducing the risk of errors and improving the clarity to the reader, and can give significant speed benefits.