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I'm trying to map some Quicktime files onto the green and blue screens
Yeah, a tga sequence with alpha usually works best, thought I have had success with quicktime movies compressed with the Animation codec.
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Also, I wanted to add some people (green screen and keyed with alfa) onto the middle stage. Can I place them there without having to map onto a grid/wall? Can they stand alone away from the wall right up to the railing or do I have to map them onto a solid grid/wall?
I've had best results making the background behind an image of the person black after I create the alpha channel, and make the alpha channel 1 or 2 pixels smaller than the object. This helps to eliminate the fringe that sometimes results from some programs like Photoshop.
You can create a Layer in the choreography in A:M to map the image or image sequence onto. First import your image or sequence, Right-click on the chor and choose New > Layer and choose your image. You can move a Layer anywhere you want in the chor.