Camera Mask

because cameras in our scenes are already set, we can not import a camera which I hav set a camera mask any more
however, an easy way to put a blask mask on the screen is
1. create a polygon plane with 3x3 division
2. change the move and rotate setting to object then parent this plane to the shot camera
3. the attributes of this plane will change because of the on the camera then change the attributes Tx Ty Tz Rx Ry Rz back to 0 (do not use freeze).
4. type 90 or -90 on the Ry (or Rx or Rz....I forgot), this action will make the 90 degree to the camera.
5. now you can delete the middle face in the plane then move edges to match the camera's resolution gate.