Ensemble Designs 7420 HD/SD Logo Inserter - Manual (Page 35)

We will darken the background in proportion to how white the alpha is. This means that
the alpha signal itself never contributes to the image the viewer sees -- it only controls
how much of the background or foreground is displayed. It will then look like this:
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 only going to be applied to the
background material -- not the foreground.
7420-35
Model 7420 HD/SD Logo Inserter