I don't know if "crop" is what I want but here's what I'm trying to do...
I have an image on the lowe track. (It is the video source.
I picket gate closes over the image so you can see through the gate. the gate is rounded at the top. I want to lose all of the video that falls ouside the gate. I can easily crop the straigh sides but since the top is rounded I don't know how to crop the corners. I can get the pixel chooser to identify what I want to keep/take away but I don't know what filter would do that. Is there an opacity filter with a pixel chooser?
Regarding the gate swinging shut. I'm using a page turn. It will do but it looks more like a page then a solid gate.
You should be able to drop the gate image into the mask track to generate transparency.
If you like, you can send me the RED project via yousendit ftp and I'll set it up and send it back to you. You can send that to me at email@example.com. And just a frame or two of the media would be enough to set the project up for you.