Compositing
Compositing is the process of mixing video tracks to create a single layered output.
You can use compositing in conjunction with masks to cover portions of video or to limit the effects of a filter. Masks are frequently based on dark versus light areas, specific colors, or an alpha channel.
Click the Parent Composite Mode or Compositing Mode button 
in the track header and choose a mode from the menu to determine how the transparency in a video track is generated. Since lower tracks show through higher tracks, it is the compositing mode of the higher track that determines how much of the lower track shows though. The compositing mode of the lowest video track adjusts its transparency against the background.
To change the opacity/transparency of all events on the track, drag the Level slider in the track header or use a composite-level envelope adjust the opacity/transparency over time. For more information, see “Video Track Controls“ and “Video Track Automation.”
To change the opacity/transparency of a single event, you can use an event envelope. For more information, see “Event Envelopes.
Creating a mask
- If you have not already done so, place and position the image or video you want to use as a mask on the top-level track in your project. For more information, see “Adding media files to your project.”
- Place video files on the tracks below the mask.
- Ensure your mask is a compositing parent the tracks below are child tracks.
- Click the Compositing Mode button on the mask track and choose Multiply (Mask) from the menu.If your mask image is grayscale, you’re done. If not, continue with Step 5.
- Add the Mask Generator plug-in as a track effect on the mask track and adjust the controls as needed to set transparency in the mask track.
You can also use the Event Pan/Crop dialog to create event-level Bézier masks. For more information, see “Panning and Cropping Video Events.”
Solid-color mask
Gradient mas
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