Grass Valley Switcher Products - Manual (Page 135)

Switcher Products — Protocols Manual 135
Introduction
When distribution paths were predominately analog, this approach did not
work well. Black level errors and noise could corrupt the background por-
tions of the final composite, so the video output by one device was divided
by its own key signal (“unshaped”) so that the receiving device could
reapply that key signal, eliminating the opportunity for signal errors to
corrupt the background.
The still store accepts either “shaped” or “unshaped” video for video + key
pairs. When a still store output is selected on a keyer, the still store notifies
the keyer which mode to operate in. If the mode is set incorrectly, the com-
posite will not look right.
Part of the metadata identifies if the video is shaped or if it needs to be
shaped prior to compositing (unshaped). The application generating the
image file is responsible for setting this flag correctly.
In general, the output of a graphics system that produces red, green, blue
and alpha channels will be producing shaped video. A shaped video signal
is black everywhere outside the alpha area. Inside the alpha area, it should
reduce in amplitude as the image becomes more transparent.