Bear in mind that this is the first alpha version of this, and it will likely be wrapped in a script, so I am just figuring out how things should be implemented, and how heavy the processing is, and how it can be improved.

No error handling is present, so expression errors are likely caused by source layers not being selected or something similar, and should go away when these are selected.

The controls layers is currently simple shape layers that are sampled inside the rectangle. I will fix this eventually, but you should try and keep the bezier shape within the rectangle to prevent unexpected results. ;)