I have a sequence with HD 16:9 DSLR footage. I am overlaying a 2.35 widescreen crop over it all. However, for some of the clips I want to add some push-in motion, but when I apply that the black bars shrink or grow with the clip's borders. Is there a way to apply motion while maintaining the black 2.35 bars?
The widescreen mask isn't really a mask in the sense that we perceive it, it's an alpha channel exposing the base layer. I figured out a way of doing what you need.
Apply the widescreen mask then make a freeze frame.
Export the freeze frame as a sill image.
Open the still in Photoshop and you will see the alpha top and bottom.
Precisely crop the image part of the frame and delete it.
Invert the selection and fill with black.
You will now have a frame with black bars top and bottom and with transparency between. Save it with a descriptive name and as a PNG.
In FCP remove the previously applied widescreen mask
Import the Photoshop mask into FCP and place it on track 2 above the clip on track 1. It looks exactly the same as the FCP generated mask but really is a mask and so enables you to re-frame the clip on track 1 independently. If it comes in as a black frame go to the pull down menu > Format, and make sure that the alpha is set to straight.