Grass Valley Zodiak v.6.0 Mar 15 2006 - Manual (Page 407)

Zodiak — User Manual 407
Chroma Key Operating Notes
2. Adjust Hue, Selectivity, Chroma, and Luma primary suppression to
eliminate the backing color.
• Hue can be set accurately with Auto Setup. Hue should center on the
primary color of the backing area of the foreground scene.
Depending on where
Luma and Chroma primary suppression are set,
adjusting Hue may not make any noticeable change on the scene.
Chroma suppression should be preset to 100% and Luma set to 0%.
Hue can then be tuned to remove the backing color.
• Selectivity may need to be increased if there are colors in the fore-
ground image that are being suppressed. Selectivity should be set as
low as possible without including colors that should not be sup-
pressed.
For example, when keying on green, a greenish yellow shirt might
be affected by the suppression. If so, adjust the selectivity high
enough to reject that color. Too high a selectivity is one of the classic
causes of a noisy key. If the foreground subject is stationary, con-
sider using a force mask instead of increasing selectivity.
• Chroma suppression can be set accurately with Auto Setup. To
adjust, increase Chroma suppression and observe the backing color
dot on the vector scope move toward the center. You want to center
it exactly, so no chroma exists in the backing area. 100% chroma
suppression is the correct setting for all chroma keys. At this point,
you will probably see a line through the center of the vector scope.
With increased selectivity, this line will become an arc.
• Luma suppression adjustments may be necessary if shading is
visible in the backing area with FG Only selected, or if the shading
adversely affects the background image. Primary Luma suppres-
sion is hardly ever desired when Reshape is on. To adjust, increase
Luma suppression and observe the backing color move toward
black. You want to make the backing color just black. Increasing
this control too much will make the chroma key hard and noisy.
When not enough, highlights will be added to the background.
Note that incomplete luminance suppression is not necessarily bad.
The highlights added to the background will match the shading on
the backing wall, adding natural shadows and perhaps eliminating
the need to add artificial shadows.
• All the above adjustments may need to be revisited later.
3. Another potential artifact of chroma keying is a tinting of the overall
foreground subject due to lighting splash from the backing color or lens
flare.
Flare Suppression adds a small amount of color to the entire
foreground image to cancel the splash or flare. Typically less than 2%
of the backing color is needed to neutralize the flare.