Place the clip you want to reveal in track one. Then place the clip that you are going to roto scope…lets say it is a clip of a diesel truck driving by above the clip in track 1. Select the clip in track 2 by double clicking on it and go up to Effects, video filters, matte and then choose Eight Point Garbage Matte. In the Viewer click on the Filters tab. Click on the plus symbol in a circle button. Then in the Canvas click and hold down the left mouse button. Move the mouse to where you want the point to be. Move all eight points to where you want them. (I typically do this mask where I see all of the sides of the truck I need…if the truck is so tight that all you need to do is have just a line aka two points move then use the “Four Point Garbage matte.”) Then click on the diamond in the circle button for each point. That creates a keyframe. Then move to where the truck almost leaves frame and then move the necessary points around. FCP will automatically create new keyframes. That is how to do it in a nutshell. Hope this helps and best of luck.
Stephen Smith
Utah Video Productions
Check out my Motion Training DVD
Check out my Motion Tutorials