The chromakey effect effectively removes all pixels of a particular color from its
composited media, making it transparent to any underlying media.
Refactored from the excellent chromakey example freely available on
Mario Klingemann's blog.
composites: valid video or image media id
defringe: number between 0 and 300
defringeradius: number between 0 and 1000
editable: comma delimited list of property names
forecolor: six digit hex value
id: single word string identifier, starting with a letter
keygamma: number between 1 and 400
keyin: number between 0 and 256
keyout: number between 0 and 256
length: floating point number (frames)
preblur: number between 0 and 100
postblur: number between 0 and 160
symbol: a valid SWF URL, followed by an ampersand and class name
type: the string 'effect'