Keying Graphics in GenCG

Sometimes we get asked how a graphic key (superimposition or layer) works and how a TV graphic can be keyed (superimposed or layered) on top of live video.

Video and graphics you see on your TV screen are made up of red, green, and blue (RGB) colors that combine to make all visible colors. Shown is the fill video which you see on TV.

In still graphics, a key is referred to as an alpha.  A TV graphics system cuts a key hole for the fill video to fit into. The key hole is black and white, not visible on TV, and behind the fill. This allows the fill video to have transparency over the background video. Targa (.tga) files made in software like Adobe Photoshop™ make these alpha or key layers.

Here is a key for the above fill:

Here is a background with no fill or key:

Here is the final output with the foreground fill from picture #1 and the invisible key in picture #2 combined with the background from picture #3.  Notice the transparency of the fill. You can see through it to the background. This is made possible by the alpha, or key.

This is a very quick overview of the concept of keying graphics over video. Until next time!


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