Visual Effects
Roto, for short, is the painstaking process of generating mattes for plates that are
to be composited. These black and white shapes are generated as pieces of digital
footage that are either stored separately or in the alpha channel of a file format
like Quicktime or sequences of pict files. Roto is a general term that covers many
techniques. Mattes may be generated with digital paint or, in most professional tools,
through the use of animatable splines.
Splines are paths drawn in one of two fashions. Bezier splines are more common and will
serve as a good introduction as they are found in many desktop tools that may not even
involve video. These tools include, but are not limited to, Adobe Illustrator and Macromedia's
Freehand. The other spline type is refered to as natural or B-splines. These are
not very common outside of high end tools.
However, B-splines are somewhat similar to the NURBS (nonuniform rational
B-splines)
that are used in many 3D modeling packages.
The main difference between the
two types of splining has to do with the way the shapes are drawn and later animated.
Bezier splines assume that the simplest way to connect three points is a triangle,
which with some more manipulation can be changed to show rounded edges while a natural
spline assumes that each of its points will serve as a tangent to the shape it describes.
This difference is best illustrated by the graphics on the left. Because the two types of
have significant
differences, the vast majority of artists are fiercely loyal to one or the other. Their
strengths, however, lie in different areas and the best tools will offer the ability to
work with either or both types of splines on the same plate. For example, if one were to
attempt to roto a walking person carrying a brief case with either natural or Bezier splines,
their individual weeknesses would cause hours of unnecessary pain. The difficulties are
due to the inefficient method of defining "square" corners with natural splines and the
unnatural curved path motion of Bezier splines caused by overly maleable "split" handles.
The best solution would be to quickly define a rectangular path for the brief case using
Beziers and then use the organic curves of
natural spline to follow the motion of the walking figure.
Tools which use spline-based matte generation include Adobe After Effects, Puffin Desgins'
Commotion, Avid's Matador & Elastic Reality, Discreet's Flint/Flame/Inferno, Post Digital's
Roto and several others. In my experience, the best tool out there for cutting mattes
is Commotion. I will not hide my bias; I spent over a year with Puffin as the
fourth employee on staff but in that time and since have not seen a tool which comes close.
No tool compares to Commotion's multiple spline types, multiple
paths per plate, resolution independant realtime playback of splines and footage, and
camera angle correct motion blurred mattes.
Commotion runs on a $2,000 Macintosh and soon will do the same on a NT box; this is in contrast
to half of the other products mentioned above which require that you go out and buy or
tie up an existing $40,000 - $500,000 workstation just so that someone can cut your mattes.
Rotoscoping has been aided a great deal by some slick digital tools but it will always be a
time consuming process when it is done right. Done wrong it can be a really time consuming
process, produce an unusable product, or both. The bottom line is that work of this
nature should be done as cheaply as possible to produce the best quality product that it
can. This means that its probally best to spend the cash that you would have spent on
one Flame session to outfit your own studio with a kick butt Macintosh Commotion station.
For a more in depth look at the features of each of these products, check out the
Tools section.
For an explanation of how these mattes could be generated in an automated fashion,
check out the keying section.
Back to the Visual Effects area.
Feedback is always welcome drop a line to:
mrehrer@csua.berkeley.edu.
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