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Keying
with Pinnacle?
How does the Chroma
key tool work?
NOTE: The following information is also available from within the
Studio software. It can be accessed by going to the Help menu and
selecting Help Topics. Once in the help window, either browse through
the displayed topics or choose the Search tab and type in the desired
topic. It will display a list of topics based on the keyword(s) to
choose from.
Chroma key is a
widely-used technique that allows foreground objects to appear in a
video scene even though they were not present – and often could not have
been present – when the scene was shot. When an action star tumbles into
a volcano, or battles a giant cockroach, or saves the crew with a daring
space-walk, the chances are that chroma
key or a related
technology was involved in the scene.
Chroma key
effects are often called “blue-screen” or “green-screen” effects because
the foreground action is shot in front of a uniform blue or green
background. The background is then electronically removed, leaving only
the foreground action to be superimposed on the actual background of the
final scene, which has been separately prepared.
Blue and green are the generally-preferred colors for chroma
key use because
their removal from an image will not affect human skin tones, but in
principle any hue can be used with Studio’s chroma
key tool.
As with picture-in-picture and while editing in "timeline view", the
first step in using chroma
key is to drag
some video clips onto the Timeline. Drop the clips you want for
background video onto the video track. The foreground clip, which should
have a uniform, highly-saturated background like the center clip in the
illustration above, goes on the overlay track below the main clip.
With the clips in place, select the foreground clip and open the
Picture-in-picture and Chroma
key (PIP/CK)
tool. It is the sixth tool in the Movie Window’s video toolbox. Select
the Chroma key
tab to display the controls you will need.
Chroma-key tool
controls
The chroma key
tool constructs a “mask,” shown in the
Key channel
graphic on the left side of the tool, where the transparent part of the
frame is drawn in black, and the opaque part – the part you will see in
the final video – is drawn in white. Most of the remaining controls are
used to define exactly which areas of the frame will be included in the
transparent part of the mask by setting the “key color” and related
properties.
Transparency: Use this slider if you want the underlying video to show
through the normally opaque overlay. Moving the slider to the right
makes the overlay, with its border and shadow, increasingly transparent.
Presets: The tool provides two presets, called “Green screen key” and
“Blue screen key.”
These provide good starting points for setting up the tool if you are
using one of the standard chroma
key colors.
Key
color: Use the
color swatch or
eye dropper buttons to select the
color that will
be removed from the video frame leaving only the desired foreground.
Rather than an actual
color, you are really selecting only a hue, without regard to the
other properties – saturation and intensity – that in combination with
hue make a complete
color specification. The chosen hue is shown by the position of the
highlighted region on the circumference of the
color circle
display.
The color circle
on the Chroma key
tool highlights a range of hues (around the circumference) and
color saturation
values (along the radius). Any pixel in the overlay frame whose hue and
saturation fall within the highlighted region will be treated as
transparent.
Color tolerance:
This slider controls the width of the range of hues that will be
recognized as belonging to the “key
color." Moving
the slider to the right increases the angle of the arc covered by the
highlighted region on the
color circle.
Saturation minimum: Saturation is the amount of hue in a
color. A pixel
with zero saturation (corresponding to the center of the
color circle) has
no hue: it falls on the “gray scale, " whose extremes are white and
black. Chroma key
works most effectively when the background is highly and uniformly
saturated, allowing a high setting of this slider. In the real world,
vagaries of lighting and apparatus often result in a background that
falls short of the ideal. Moving the slider left allows a wider range of
saturation values to be matched, indicated by a highlighted region that
extends farther towards the center of the
color circle.
Softness: This slider controls the density of the underlying video. When
it is positioned all the way to the left, the main video is entirely
black. As you move the slider to the right, the main video is brought up
to full density.
Spill suppression: Adjusting this slider may help suppress video noise
or fringing along the edges of the foreground object.
Enable chroma keying: This checkbox allows you to turn the chroma
key effect on and
off.
Apply to new clips: This option is handy when you want to set up the
same chroma key
settings for a number of different clips. As long the option is checked,
chroma key will
automatically be applied to each new clip that you drag onto the overlay
track, using the same settings that were displayed the last time the
tool was open.
The chroma key
effect interface
If you prefer to enter your chroma
key parameter
settings numerically rather than graphically, you can turn to an
alternative interface provided by the Video effects tool. You can also
combine the two methods, using the chroma
key tool’s
graphical interface to specify the initial settings, then fine tuning
them with the numerical effect parameters.
The Chroma key
effect is found in the Overlay Effects group. The available parameter
settings are almost identical to those offered by the chroma
key tool, but
provides one more option, Invert
Key. When this
option is activated, the normally opaque parts of the
key are treated
as transparent, and the transparent parts as opaque, so that the
underlying video shows through only where the foreground object in the
overlay video is not.
Chroma key tips
No matter how good your software may be, successful use of chroma
key depends on
carefully setting up your shot, and may require experimentation to get
the details just right. Here are some tips to get you started:
Light the backdrop as evenly as possible: Very often, background
coloring that looks flat to the naked eye will prove on playback to have
areas that are too dark or too washed out to work well for chroma
keying, which favors even, saturated
color. Use
multiple lights on the backdrop to ensure that it is well-lit across its
whole area and without hotspots. Diffuse sunlight, as produced by a
light overcast sky, can work well when shooting out of doors is an
option.
Note: A professional background cloth for chroma
key work is
available as an inexpensive purchase at the Pinnacle web-site.
Don’t let the subject shadow the screen: Arrange your subject and
foreground lighting so that no shadows fall across the backdrop. The
subject should be not less than one meter (three feet) in front of the
backdrop.
Choose foreground colors carefully: Don’t have your subject wear green
if you are shooting on a green screen, or blue for a blue screen; those
areas will be removed if they are taken to match the
key
color. You have
to be especially careful about this when working with less even
backdrops that require you to set a wider
color tolerance
in the chroma keyer.
Make a smooth profile: Chroma keyers do better with a smooth edge than a
jagged or complex one, so try to have your subject present a smooth
profile to the camera. Hair is particularly tricky, and should be
slicked down if possible. If the subject can wear a hat, so much the
better.
Use tight framing: The wider your frame, the larger your background
needs to be, and the more difficult it is to control your shot. One way
to keep things simple is to shoot your subject from the waist up rather
than in full view.
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