How to recreate the math behind photoshop curves

Question

Basically, what I want to do is understand how to calculate the values along a \'curve\' as represented here in the photoshop curves box:

Answer 1

Here's a link to a vbscript that appears to implement the cubic-spline curves photoshop uses.

http://www.planet-source-code.com/vb/scripts/ShowCode.asp?txtCodeId=68577&lngWId=-1