Keying

Mixing to a caption doesn’t work

First let’s clarify why we need to key at all. Suppose you have a white caption on a black background. You want to superimpose this over a general view. If we try just mixing the two together, the background picture dims and the caption doesn’t fully come through.

Adding a caption to a background picture

The next logical thing to do is to add a caption to the background picture. That would be fine if all of the background picture was dark, but if you added a white caption to a bright background, you would quickly end up with an illegal signal that is way too bright.

Cutting a hole

The way we get around the problem is to cut a hole in the background picture first, then fill it with the video from the caption generator. That’s fine, If you have white text on a black background. It would be easy to use the white signal to cut the hole, but what if you want to use black text?

The solution is for the caption generator to send two signals. One, the video fill, is the picture as the caption will appear on screen. The second, the video key signal, tells the vision mixer the exact shape of the caption.

Shadows

One benefit of this key (or ‘matte’) signal is that we can use it to create shadows of the text. If the key signal is wider than the text, we get a border around the text on screen. We use the same idea to create drop shadows.

Soft shadows

We can also use the key signal to generate soft edges. If, instead of sending a solid key signal, we send a mid level, or grey key signal, the vision mixer will interpret that as a half mix of that part of the caption. We can use this technique to create solid captions over soft, translucent backgrounds.

Self keying

Occasionally we don’t want to use the key signal. For example, suppose you have a full frame logo going into your DVE. If you want to place that on a background picture, you wouldn’t want the solid box from the DVE. Instead you would want to keep the bright parts of the picture, i.e. key off the video coming out of the DVE, not from the DVE key signal.