Letterbox Mask

Click here to see what the final product might look like.

(1) Media Generators>Color Gradient>Linear Black to White

(a) Add two more points to the gradient (+). Make the two outer points black and the two inner points white.

(2) Add a luminance mask to the gradient clip. Video FX> Mask Generators. Check the Invert box.

(3) Change the compositing mode to Multiply (mask)

(4) Insert a new video track. Drag an animated background that you have rendered onto this track. Press the "Make Compositing Child" button on this second video track. (Notice how track two below is indented?)

(5) Finally insert a new video track and add your main video.

 

Here's how your tracks will look like in the end.