Types of masks
You can draw four types of masks:
Rectangular
A rectangular mask can also be square. This type of mask is previewed and rendered faster than any other kind of drawn mask.
Elliptical
An elliptical mask can also be circular.
Bezier
Use the pen tool to draw a freeform Bezier path as a mask. You can create any shape of Bezier mask by using the pen tool. (See Drawing a Bezier mask with the pen tool.)
RotoBezier
Select the RotoBezier option in the Tools palette after selecting the pen tool to draw a freeform RotoBezier path as a mask. The main difference between RotoBezier and Bezier is that the tangent handles are calculated automatically with RotoBezier masks.
After you first create a mask, you can resize it or rotate it as a whole or you can change parts of it. (See Scaling and rotating selected masks or vertices and Changing the shape of a mask).
