If you have a seperate matte (black and white) you place it above the video track and add the "track matte key" filter to the video track. If you need to adjust the video inside the matte (ie. scale it, slide it, etc) you used to be able to put a transforme filter ABOVE the trackmatte and you could move the video around inside the matte without affecting the matte scale or position. If the tranforme was below the trackmatte it affected both. This behaviore no longer works as of 2020. Now the track matte doesn't respect the stack order of the filters.
A work arond is to use setmatte BUT setmatte has an issue where it won't use and essental graphic as a matte either as a luma matte or using its alpa. There is a work around for this too by nesting the essential graphic. But these behaviors look like they are revealing a deeper problem.
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