Graphics Programs Reference
In-Depth Information
Becauseyouhavetoshootespeciallyforkeyingandyoucan'tshootonlocation,youhave
to do a lot of extra work to make a shot like this work. Extracting a key is not an easy
process,andproblemsofholesinthematteandfineedgessuchashairsarealwaysaprob-
lem. Also, standing an actor in the middle of a green-painted studio means the actor will
have a strong green discoloration (called
spill
) that will have to be removed somehow—a
process called
spill suppression
or just
spill
for short.
But still, keying is very often the better method of working, and is used extensively in the
VFX (visual effects) industry.
Mostapplicationstrytocreateamagickeyingtoolthatgetsridofthescreenwithacouple
of clicks. However, this hardly ever works. Most likely, you have to create a whole big
composition just for extracting the matte. Nuke's tree-based approach makes it easy to
combine keys, mattes, and color correction together to reach a better overall matte and
corrected (
spill suppressed
) foreground.
Basic Keying Terms
Therearefourbasictypesofkeyingtechniques.Withoutgoingintoacompleteexplanation
of the theory of keying, here are the four techniques:
•
Luma-keying uses the black and white (called
luminance
in video terms) values of
the image to extract a matte.
•
Chroma-keying uses the hue and saturation (called
chrominance
in video terms)
values of the image to extract a matte.
•
Different-keying comprises the difference between an image with a foreground ele-
ment and another image with that foreground element taken away (also called a
clean plate
).
•
Colordifference-keyingutilizesthedifferenceincolorbetweenthethreecolorchan-
nels to extract a matte. Spill suppression is a side effect of this technique.
Introducing Nuke's Keying Nodes
Nuke has many keying nodes, but I only have room to cover a few of them in detail. Here
is a a rundown of the options in Nuke's Keying toolbox:
•
Difference:
This undocumented node is rarely used. It is a simple difference key
node that creates a matte for the difference between two images. It has very little
Figure 7.1. The Difference node's Properties panel.
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