SI FAQ: Shaped/Unshaped

About Shaped/Unshaped Key and Fill

Shaped/Unshaped Key and Fill DekoCast carries transparency information in addition to the red, green, blue color infomation for all its elements: the DekoCast output can be configured to use two video spigots - one carrying a color (or "fill") signal and the other a black-and-white transparency (or "key") signal; image files can use a 4th channel in addition to the usual three red green and blue channels; the Clip Playback object can accept two clip file names, one carrying compressed color frame and the other carrying compressed black-and-white transparency information.

Shaped and Unshaped - defined
Downstream Keying - DekoCast signals vs traditional CG's.
Keying Problems - if the downstream keyer is not set up correctly
DekoCast Elements - dealing with unshaped images and clips

Shaped and Unshaped
By convention, black in the transparency channel means completely transparant, white means completely opaque, and gray indicates something proportionately in between. This convention is not standard across the universe - some applications assume it's the other way around.

Also by convention, DekoCast assumes that the color channels are black in the areas where the image is transparent. In general, the color value of each red, green, blue component should be less than the transparency component at every point in the image. This is called a Shaped (or sometimes Premultiplied) image:

This is generally not the convention used by traditional character generators, which may output

or even

This is referred to as an Unshaped image.

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Downstream Keying
With an unshaped image, the shaping is done by the downstream keyer, which uses the key to shape the foreground color, and the inverse of the key to shape the background image:

Since DekoCast fill is already shaped, it must be combined with other elements with a downstream key process which does not shape the DekoCast fill, known as an Additive Mix:

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Keying Problems
If you run a shaped signal through a downstream keyer expecting an unshaped signal, the fill essentially is shaped twice, resulting in black outlines around shooth edges. As elements fade on and off, you'll also see the result dip to a dark color in the middle of the fade.
Double Shaped

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DekoCast Elements
The Cel Animation and Clip Playback objects expect their image and clip files to be shaped. A Non-additive downstream keyer may hide problems when the transparent pieces are around the edges of the DekoCast graphic, but you will see problems where DekoCast objects overlap, since DekoCast only does Additive Mixes internally.

The Cel Animation object can shape image files as they are being read in. In the Cel Animation object editor, change the Appearance chooser from "Normal" to "Shape". Changing this setting does not affect the image(s) currently being displayed; press the Update button to force them to be read and processed again.

The Clip Playback object has no way to handle unshaped clips in DekoCast 3.x. It will be necessary to go back to the artist who created the clip and re-create it shaped.

last modified 8/26/2005

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