Finalize the key
Quite often your foreground image will have objects that you don’t want to appear in the composited output. These could be production items like microphone booms and light stands or the edges of the chroma key backdrop. Additionally, you might need to resize or reposition the foreground object to better fit with the background. As a final step, you can use the color corrector to adjust the foreground so that it matches the look of the background.
Final Cut Pro includes effects that you can use for these purposes.
Use a mask effect to remove unwanted parts of the foreground image
In the Browser, select the part of the foreground clip (the chroma key clip with the color you want to remove) you want to key over the background, and choose Edit > Connect to Primary Storyline (or press Q).
For details about connecting clips in this way, see Connect clips to add cutaway shots, titles, and synchronized sound effects.
Select the foreground clip.
In the above example, the light stands need to be removed.
Click the Effects button in the toolbar (or press Command-5).
In the Effects Browser, double-click a mask effect to apply it to the selected foreground clip.
Tip: Type “mask” in the Effects Browser’s search field to quickly find the various mask effects. For more information, see Masking overview.
Adjust the edges of the mask to crop out the objects you want to remove, in this case, the light stands.
When adjusting the mask, be sure to leave shadows and other details that can help make the key more realistic later.
Apply the appropriate keyer effect to the foreground clip.
The foreground is composited over the background.
Use Transform to position the foreground image
The following steps assume you have already configured a key but need to reposition the foreground image.
Select the foreground clip in the Timeline.
In the lower-left corner of the Viewer, choose Transform from the pop-up menu and move the image to the position you want. In the above example, you might want to move the woman to the left.
You can also resize the image.
Click Done in the Viewer when you are finished repositioning the foreground image.