Applying Chroma Key to Video

chroma key filterChroma key is one of the principal ways the movie and television industry add special effects. Think of a sci-fi thriller with an impossibly large spaceship hovering over New York city or a weatherman who appears in front of a computer generated map. Chroma key works by automatically selecting everything close to a certain color and turning it transparent. Often Chroma key is used to erase the background behind actors or special effects elements, so they can be seamlessly placed onto a different background. But first, they must be filmed against a solid color background. KoolMoves makes it easy for you do chroma key. You just import a video clip, and apply the right filter (Effects > Flash 10 Filters> ChromaKey). This filter has sliders that let you adjust the settings as you like. This is discussed at board.flashkit.com/board/showthread.php?t=822703.

Here are a few tricks you will need to get the results you want. If your goal is to avoid making things appear ghostly or full of holes, the background color should be very different from that of the props, clothing, or skin colors. Super saturated blue or green are often the best choices. For that reason, this technique is sometimes called "blue screen" or "green screen". A different background color, like super saturated magenta, is needed in some situations, such as a scene in which a green Godzilla arm wrestles a blue Smurf. To chroma key with a nice clean edge, one more trick is needed -- keeping the proper distance from the background. If things are filmed to close to the background they tend to pick up or reflect its color.

Alternatively, you can code chroma key using action script. When you import a video file using the gui and the export version is set to Flash 9 or higher, the object that is created on stage is an instance of the GUI_VideoObject class. An object of that class has a method setChromaKey. The documentation on the parameters for the method is in the Koolmoves documentation for the GUI_VideoObject class and there is an example on the KoolMoves support page. Basically, create an AS3 frame action like flv1.setChromaKey(... where flv1 is the name of the video object. The only chroma key argument that is necessary in the function call is the key color -- setChromaKey(keyColor:uint = 0x00ff00, chromaTolerance:int = 60, lumaTolerance:int = 255, preBlur:Boolean = true, postBlur:Boolean = false, invertMask:Boolean = false, preserveLuma:Boolean = false) KoolMoves also lets you control this filter with action script, so you can have the chroma key settings change over time or be set interactively by your audience.

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