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Video scopes make these problems
even clearer. Since all major video
editing software has video scopes
built-in, we can easily check levels on
set before beginning production for the
day. There are two relevant scopes:
Waveform Monitor, which
measures gray-scale (light and
Vectorscope, which measures
We use the two scopes in tandem to
evaluate an image, because, for a key
to work, we need to be able to isolate
a SPECIFIC background color from
all other colors in the image. Since a
“color” consists of three values: hue,
saturation and gray-scale these two
scopes help us make sure we have a
clearly isolated color.
A better way to work
Compare that fi rst image with the one
Perfectly smooth background
• Evenly lit background
• And note that the actor is lit
dramatically, while the background
is lit evenly.
This will look magnifi cent when the
key is pulled, even down to individual
strands of hair.
Here’s the same shot, this time using
the Vectorscope. Look how the colors
are desaturated (close to the center)
and range in value from close to gray
to kinda-sorta-leaning-toward green.
Maybe. There’s no clear-cut color to grab. This
causes edges to tear, strands of hair
will be lost, and, in general, the fi nal
key will look awful.
KEY RULE: The more of the actor’s body you need to
see, the better your background and lighting need to be...
or, plan to spend a LOT of money in post cleaning this up.
Here’s the Waveform Monitor for that
second image. Look how even and
tightly controlled the background
is - almost exactly at 45% and dead
smooth. This is a near-perfect example
of how to light a background.
• Here’s that image displayed on the
Waveform Monitor. See how the green
values range from about 12% up to
50%? In fact, some of the green is
darker than the performer. This huge
difference in value will cause major
problems in editing, because we won’t
be able to isolate the talent from the
background; all the colors are blending
together. Here’s the same image on the
Vectorscope. Look at how clearly
defi ned that green is, and the color
separation between the background
and the talent. This is exactly what you
should expect to see when monitoring
your effects on set.
Summary I know that production never has enough time, but when it comes to
chroma-keys a few extra minutes properly setting and lighting the studio for a
chroma-key shot can save bundles of time and dollars later in post.
Whether you are a studio-owner or a producer, keep these rules in mind
when considering a green-screen shoot. Pick facilities that can make your life
easier and less stressful by giving you the room, surface and lights to make
your keys look great.
And don’t be afraid to record a quick test clip and load it into the editing
software of your choice. Check the shot on the scopes. It is far easier to fi x
lighting and set issues in production, than work around them in post.
TV-BAY MAGAZINE: ISSUE 92 AUGUST 2014 | 41