Appendix B: Using Shaped And Unshaped Keying
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Deko1000 & Deko550 Technical Reference And Service Guide
Unshaped
“Unshaped” refers to a video and key pair that does not have the key “pre-applied” to the video. In
this case, all transparency is represented by the key signal. The graphics in Figures B-4 and B-5
show the same example in the unshaped environment.
Figure B-4: Unshaped Video Signal
Figure B-5: Key Signal
Once this graphic is keyed, the background image shows through the semi-transparent border just
as in the shaped case. Refer to Figure B-6.
Figure B-6: Keyed Result
Mathematical Equations
A keyer or switcher that combines two keyed sources must take into account their formats
(shaped or unshaped). Two distinct mathematical functions are used depending on the format of
the keyed inputs. For the mathematically inclined, the equations are provided below.
Unshaped Keying uses the following equations for mixing two Video + Key sources:
K
out
= K
foreground
+ K
background
* (1 – K
foreground
)
V
out
= V
foreground
* K
X
+ V
background
* (1 – K
X
)
where K
X
= (K
foreground
- K
foreground
*K
background
) / K
out
Shaped Keying uses the following equations for mixing two Video + Key sources.:
K
out
= K
foreground
+ K
background
* (1 – K
foreground
)
V
out
= V
foreground
+ V
background
* (1 – K
foreground
)
While the Key equation is the same in both cases, it is clear that the Video equation is much
simpler for Shaped Keying than for Unshaped. Having the Key pre-applied to the Video greatly
reduces the computations required produce the combined result. It is for this reason that shaped
keying has become standard in the world of graphics and multi-layer composting.
