GRASS VALLEY KARRERA, Руководство пользователя

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58 KARRERA — User Manual
Section 3 — Video Mix/Effects
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 selec-
tivity 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 sta-
tionary, consider using a force mask instead of increasing selec-
tivity.
• 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 sup-
pression 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 high-
lights 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.