Graphics Reference
In-Depth Information
2
Image Matting
Separating a foreground element of an image from its background for later com-
positing into a new scene is one of the most basic and common tasks in visual effects
production. This problem is typically called
matting
or
pulling amatte
when applied
insert weather maps behind meteorologists who are in fact standing in front of a
green screen. At its most difficult, an actor with curly or wispy hair filmed in a com-
plex real-world environment may need to be digitally removed from every frame of a
long sequence.
Imagematting is probably the oldest visual effects problem in filmmaking, and the
search for a reliable automaticmatting systemhas been ongoing since the early 1900s
[
393
]. In fact, the main goal of Lucasfilm's original Computer Division (part of which
later spun off to become Pixar) was to create a general-purpose image processing
computer that natively understoodmattes and facilitated complex compositing [
375
].
A major research milestone was a family of effective techniques for matting against a
blue background developed in the Hollywood effects industry throughout the 1960s
and 1970s. Such techniques have matured to the point that blue- and green-screen
matting is involved in almost every mass-market TV show or movie, even hospital
shows and period dramas.
On the other hand, putting an actor in front of a green screen to achieve an effect
isn't always practical or compelling, and situations abound in which the foreground
must be separated from the background in a natural image. For example, movie
credits are often inserted into real scenes so that actors and foreground objects
seem to pass in front of them, a combination of image matting, compositing, and
matchmoving. The computer vision and computer graphics communities have only
recently proposed methods for semi-automatic matting with complex foregrounds
and real-world backgrounds. This chapter focuses mainly on these kinds of algo-
rithms for still-image matting, which are still not a major part of the commercial
visual effects pipeline since effectively applying them to video is difficult. Unfortu-
nately, video matting today requires a large amount of human intervention. Entire
teams of rotoscoping artists at visual effects companies still require hours of tedious
work to produce the high-quality mattes used in modern movies.
1
The computer vision and graphics communities typically refer to the problem as matting, even
though the input is always digital video.
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