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Effects question: removing a background
  • Let's say I have a shot with an actor in a room. The camera doesn't move, so the background is completely static, and doesn't change from frame to frame beyond the usual grain. I'm figuring that, if I had a frame of just the background, I ought to be able to use it to filter out the background and isolate just the actor, as though it were a greenscreen.

    I'm having some trouble doing this in Premiere Pro. Any suggestions?

  • 5 Replies sorted by
  • @Brian_Siano You're probably going to have to rotoscope (if I correctly understand what you are trying to do).

  • Yes, you need to roto out the actor by animating masks, the frame of the background serves as the plate for the parts of the image that are covered by actor, if you had not have those, you would need to reconstruct those areas (*in case that you need any of the background parts at all). You would have much more success in After Effects or Mocha for the roto job, Premiere recently added masks, but still rudimentary (although very useful in many situations) comparing to other platforms dedicated for compositing.

    But to come back to the title of the topic, if I understood it correctly, if you wanted to 'automatically' remove background (create transparent alpha regions in a layer) similar to the keying process, nice idea, but I'm not aware of that technology.

  • You are referring to a difference key, which should do what you want in theory but usually does not work that well. Try it and it may get you some of the way.

  • huh, to much variables for difference key in video, temporal noise, changes of light due to any movement of the talent, might even be corrected and fiddled for one frame, but as far as I know, not a reliable solution as an overall tool, maybe in some areas but to slap it on an entire clip, hardly.

  • Difference keying has been around since the 1990's. I have used it on many effects productions. It is important to setup and shoot for a difference key so the background clean plate and the final take are an exact match. A difference of even one pixel will make if extremely hard to get an acceptable result. The reason it's not used much is because it's easier to get a clean key with a chroma key.