In order to edit alpha channels in FCPX, please follow the steps below:
1. Cut the luminance matte off the tail end of the clip (the black and white outline)
2. Drop the second half into the layer above the first half. They should be the exact same length.
3. Go to effects / keying and select Luma Keyer. Drop it onto the luma matte (black and white outline)
4. If the desired effect is not achieved, go the effects panel and check the "Invert" dialog box.
At this time Bigstock does not provide video editing support. However we are offering this information as a courtesy. If you have any further post productions inquiries, we recommend contacting your software provider.