Visual Effects
Now that I have explained how to generate mattes in situations where it must
be done manually, this section should shed some light on methods of "automated"
matte generation, which is know as keying.
The most common form of keying can be found in the blue screened
weather broadcast yet this same technique is used with many different variations
depending upon the situation. The color shot in the background, for instance, can
easily be changed to suit a different application. Blue is used often because it
has the quality of being the least often occuring color in human skin. This makes
it the perfect choice when working with sbjects who are showing flesh tones. Green
screens are often used in similar situations but have the benefit of allowing the
director the freedom to cast persons with blue eyes and put actors in blue clothing,
both of which would produce most undesireable effects in a blue screen situation.
Other colors used for varing reasons include red and sodium yellow. Because most
(but not all) computer systems that generate these mattes work in an RGB color-space
it is conveinent to use colors which are close to the extremes of each of the red,
green, and blue componets of that color model. When using a colored screen it is
important to keep the reflected light from spilling onto the supjects. Good tools
will try to do what is called spill suppression which will remove the offending tint
from the entirety of the plate.
Sodium lights have the characteristic
of being confined to a very small portion of the visable light spectrum and as
a result were used by some creative people at Disney in a method of keying which
combines the innovations of the digital world with the handpainted glass filters
of yesteryear. Since their sodium lights only emitted a tiny band of visable light
filters which blocked that portion of the electromagnetic spectrum were used to
keep a portion of the film negative un-exposed until later. This technique saw
extensive use in films like Mary Popins when combining live action and animated
charaters in the same scenes.
Keying is not however limited to removal of specific colors. Keying can also be
done using luminance (brightness) values. For instance a person wearing a bright
blue, green, and red Hawaiian shirt and carrying a
sodium light could be shot against a black background which could easily be
distinguished digitally by its zero luminance. Similarly, a white backdrop could
be used in a different situation.
Perhaps the most interesting technique is called diffrence keying. A differnce
key is accomplished by shooting what is known as a clean plate. Clean plates have
many uses but in this scenario the clean plate is used as a digital refrence point
from which once again a black and white high-contrast matte, known as a hi-con,
is generated to show the differences
between two shots. The first shot, the clean plate, would be everything that is not
to be shown in the foreground. Then, while keeping the camera locked down (stationary)
or using motion control to mimic the first camera move, the seccond plate is shot.
When the software compares the two plates it is able to generate a matte for the
diffrence between the two. This makes the difference keyer a tool that is useful in
situations where the object or group of objects that you are removing from a
background cover too much physical space for them to be practiacally blue screened.
For an explanation of how paintbox style tools are used in visual effects,
check out the digital paint section.
Back to the Visual Effects area.
Feedback is always welcome drop a line to:
mrehrer@csua.berkeley.edu.
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