Creating an Alpha Channel from a Video Tape

Posted on by Larry

[ This article was first published in the August, 2007, issue of
Larry’s Final Cut Pro Newsletter. Click here to subscribe. ]

Charlie Wilson writes:

I am in the process of converting my daily editing from Avid to Final Cut at CBS and I have run into something I can’t figure out. I have animation for Face The Nation that uses a foreground video with matching b&W matt that I sync together. In avid I use MATT KEY and place the video that is in the cut box on V1, that animation on V2 and the Alpha on V3 and apply the MATT KEY effect to V3. I know that FCP is really set up for transparency keying and I love that method, but this animation does not exist that way.

Can I:

  • Create a file that has the matt embedded and use that as a transparency or
  • Do it in a similar manner to avid. I have tried every key effect with inverse and feathering the settings, and color effect but nothing will let the video on V1 come through. I am stumped on this one.
  • To clarify, I have animation sent from an agency in NY that makes a frame with floating letters of Face the Nation with a black hole in the middle, and when the CBS Eyeball comes up from the lower right it becomes full screen with the Face ID, then wipes to a lower 3rd for a beauty shot. Then immediately following that is another piece of video with a sync X that is just a black hole with white where the animation is with a white CBS Eyeball as it comes up from lower right. I have attached a section of the clip foreground and matching Alpha Key.

    Larry replies: Piece of cake. I learned this trick a while ago from Ed Kaufholz over at ESPN.

    This works with any black and white matte source and any version of Final Cut since about 4.0. Traditionally, these mattes are recorded on tape for use in a keyer, but you can use this with any gray-scale image; even TIFF stills from a clip-art catalog.

    1) Stack your images in the Timeline such that the background image is on V1, the image that generates the matte is on V2, and the image that goes inside the matte is on V3.

    2) Select the matte clip on V2 and apply Effects > Video filters > Matte > Extract. This generates the alpha channel, but you won’t see any changes to your image, yet.

    3) Select the foreground video on V3 and apply Modify > Composite Modes > Travel Matte – Alpha.

    Poof! Your foreground image is magically composited into the background based upon the gray-scale values in the V2 clip.

    If you need to tweak the key, adjust the Threshold and Tolerance settings in the Filters tab.

    Very, very cool!

    UPDATE – Aug. 22, 2007

    Tom Wolsky, Apple-Certified FCP Guru, writes:

    Why do you have to do the extract filter? If it’s a black and white matte, can’t you just use travel matte – luma, or does the matte need to be inverted?

    Larry replies: For some reason, I’ve always gotten better results with the Extract filter, but Travel Matte – Luma will also work.

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