It sounds like what you might be doing wrong is not using the Sony Mask Generator on the mask track. Your tracks should be laid out top to bottom as: Mask track (top), background A (middle), background B (bottom).
Then follow these steps:
- Change the Compositing Mode of the Mask track to Multiply (Mask). This will make background A show through the white part of the mask.
- Add a Sony Mask Generator as an FX on the Mask track and make sure it’s set to Luminance (the default)
- Press the Make Compositing Child button on the background A track to make it a child of the Mask track.
Background track B should now show through where there is black on the mask track. Most people forget about adding the Mask Generator. This is the crucial step.
~jr
http://www.johnrofrano.com
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