With the technique that I intend to use I can have up to 6 clipping lines.
Sounds good, although more lines, and a circular mask would be really useful.
You can also let maks be extended or to accept grayscale arrays/images for raster alpha channel masks
Now that would be REALLY cool! With that you could have a simple way, with the six lines, and an advanced way, with infinite possibilities. That might just be the best way to take care of users of all levels...