GRASS VALLEY Kayak HD 100C, User Manual

Page: 339 / 558

Kayak — User Manual 339
Chroma Key Operating Notes
•
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 foreground
image that are being suppressed. Selectivity should be set as low as pos-
sible without including colors that should not be suppressed. 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, consider 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 suppression is hardly ever
desired when FGD Fade 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.