Hi Brendan, glad to see you're still using the libs.. Below is a fairly naive implementation, which is simply extruding the patch with a fixed offset vector. It's "naive" in the sense, that depending on the amount of curvature in your patch you might get quite drastic differences in thickness. The image shows that effect a little bit... To do this more properly, the patch should be extruded by a fixed distance along the surface normal of each point, which actually isn't that difficult to achieve based on the code below... In the example below, simply replace the line defining point Q from this:

That's great - much appreciated.