See Also: -HCKey
This mode overrides the transparency of the lower layer composite with transparency that is proportional to the difference between the color of the upper layer and the color of the lower layer composite.
Where the colors match exactly, the lower layer composite transparency is 100% (it is transparent.) Where the colors are different, the lower layer composite transparency is lower (more opaque.) Where the layer factor is zero, the colors must match exactly to produce transparency. Where the layer factor is one hundred, the colors don't have to match at all to create full transparency.
This is what you would use for a traditional "green screen" or "blue screen" color keying operation.
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