Key / Alpha Channel
From CasparCG Wiki
CasparCG Server automatically generates a key signal on the the key output of your DeckLink or Bluefish card. If your video files have an embedded alpha channel, that will be used. Everything that cover the stage background in your Flash template graphics (even FLV video or PNG sequences) will become your key signal. When you play both video (one layer) and several Flash layers at once, CasparCG Server handles the compositing and outputs the combined fill and key signals separately.
Content played by the Flash Producer is always pre-multiplied against black (no matter what the Stage background color is set to.) When composited on top of other content (either Flash content in FTs or pre-rendered videos played by the FFmpeg Producer, Flash's Stage will become transparent and reveal underlying layers.
The alpha channel in content played by the FFmpeg Producer can be either straight or premultiplied, but we recommend using premultiplied alphas so you can play content via the Flash Producer on top and have them correctly composited.
Resolving Key Output with DeckLink Cards
If you don't get any alpha channel / key output from DeckLink cards using CasparCG 1.8, please launch the DeckLink Live-key application just before starting CasparCG (Decklink) server and you'll get receive both signals; Fill channel on SDI Out A and key channel on SDI out B.
