Steps for seamless blending of a blue-screen video onto a background
Sometimes you’ll want to have a full-screen scene that includes Quicktime video in a CD ROM production. Specifically, you’ll shoot your subject on a blue screen and want to key out the background and integrate the subject into the scene. If you’ve tried to composite 24 bit or even 8 bit cinepaked movies onto a background--I’ll use the example of a dithered 8 bit background-- you know it won’t match the scene. The video, when compressed, plays as a a blurry rectangle, standing out sharply against the nicely dithered environment graphic. One can clearly see where the square movie borders lie, and thus the scene displays its production and doesn’t offer the user a “seamless experience.” What I’d like to do is note some solutions to making the integration of video and graphics in a given scene.
I know of three ways to get around this problem: The first is obvious, yet sometimes difficult to create. Design your interface with a tool, screen, or a special integrated area for the Quicktime movie to play. Another way to subtly composite movies is to design the screen so that the movie’s borders are integrated into the scene--putting a movie of a figure, for example, in a doorway clearly will help mask where the movie lies, since the movie can be made to its rectangular dimensions and only the background, the door, would be affected by compression. Finally, you can truly integrate the video into the background by using tools such as Adobe After Effects, Convert to Movie, and Movie Analyzer.
I’ve used this technique for host introductions for two commercial titles so far: Nile: Passage to Egypt (1995, The Discovery Channel) and SkyTrip America (1996, The Discovery Channel). There are some drawbacks to this technique. You must use a low frame rate and you must have a pretty small movie on the background to keep the data rate low. It is possible to get the data rate to 150k/sec with this technique if necessary.
Blue screen a movie onto an 8 bit dithered pict file
1. Take the blue screen footage (24 bit) into After Effects. Remove the blue with a color key effect in one composition, using the background as the bottom layer in a second composition. Crop the movie close to the action--bring the composite into a new composition in which the movie just fits. Make the movie with animation compression, millions of colors, 8 fps-10 fps (you’ll have to go through a test process and use Movie Analyzer to see how low a frame rate you must have. This depends primarily on the data rate limitations of your delivery platform.)
2. After the movie has processed, go to the latest version of Convert to Movie and open up movie #1. Process (or, I prefer to say, “cook”) with settings: Graphics compression, same frame rate, 999 key frames, dither on. This process is to make the blue screened live action part of the scene nicely dithered to match the dithered background.
3. Back to After Effects. Import processed movie#2. Here’s where we’ll do the magic. Make a composition with movie #1’s key effect composition where you removed the blue in the first place), movie#2 (the compressed file), and the 8 bit background pict. The 8 bit dithered background pict should be the background, then movie #2, then on top, place the blue screened layer from movie #1. Turn the video off from this layer--it’s not needed since this layer is used only as a moving matte. On the movie #2 layer, do an effect: extract matte from layer movie #1. This will allow the dithered subject to show through, yet hide the compressed, dithered background that is effected by “jingly pixels” and let the stationary, original background show through in place of the compresssed background. The only part of movie#2 that will show through is the predithered live video figure. Play with this a little, you may want to change the matte to fit closer or looser around the compressed figure. Make this into a new movie #3, graphics compression, same frame rate. Note that you need one second pause at the beginning of the movie or else use Movieplayer to add a second of non-motion at the start of the movie to avoid a video hang upon movie startup.
4. Import movie #3 in Convert to Movie, cook at graphics compression, 999 key frames, no dither, single fork, no duplicates. This is your final movie, platform independent.
I recommend using a small section of movie #1 to use as a test movie--about 10 seconds. Run all four steps on the test and then use Movie Analyzer to see if you’re in the ball park in terms of data rate.
