Ensemble Designs 7420 HD/SD Logo Inserter - Manual (Page 24)

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Page 24
Avenue Logo HD Application
At this point, the background has a perfectly cut hole into which the foreground video can be added.
And when that is done, the result will look like this:
In order for additive keying to work, the foreground material (the RGB of the graphic in this case)
must be perfectly black EXCEPT where the foreground is intended to appear in the final output. This
is because the Alpha signal is going to be applied only to the background material, and not to the
foreground.
If we use the Alpha to control both the foreground and the background, the results will not be
as good. Since both the Alpha and the foreground (RGB) have anti-aliasing applied to them, the
foreground would have too much anti-aliasing applied if we multiply it by the Alpha.
This example uses a logo that casts a shadow because it so clearly demonstrates how this technique
works. The linear performance of the keyer is also essential to the appearance of the edges of the text.
Here is a close up: