Ensemble Designs 7420 HD/SD Logo Inserter - Manual (Page 36)

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.
In this sample I chose to have the logo cast 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 closeup:
One of the reasons that the additive keying technique is so powerful is that television
itself relies upon “additive” color. The television signal naturally starts with a black raster
and we “add” color and light to it. The primary colors in television are Red, Green, and
Blue because in an additive system, they combine to produce White. This is in contrast to
printing which is a subtractive process. Color printing begins with white paper -- the ink
that is applied “subtracts” color from the original white background. And we can see this
because the primary colors in printing are Cyan, Magenta, and Yellow.
7420-36
Model 7420 HD/SD Logo Inserter