Graphics Reference
In-Depth Information
(a)
(b)
(c)
Figure 2.25.
Two images of the same scene taken (a) with a flash and (b) without a flash. (c) The
difference of the flash and no-flash images results in a mostly-foreground image.
subtracting the without-flash image from the flash image, a “flash-only” image is
created that contains mostly foreground pixels and that can be used to generate a
good trimap. An extension of Bayesian matting is then used to generate the matte. In
a sense, flash matting reverses Smith and Blinn's triangulation assumption: instead
of observing the same foreground in front of two different-color backgrounds, we
observe two different-color foregrounds in front of the same background. In practice,
it may be difficult to ensure that the foreground and background remain exactly the
same and that the flash doesn't create strong shadows.
If an estimate of the depth at each pixel is available in addition to a color image, it
forms a valuable prior on whether pixels should be classified as foreground or back-
ground, and can help disambiguate situations where the foreground and background
have nearly the same color. We'll discussmanymethods for directly acquiring or indi-
rectly estimating scene depth in Chapter
8
. In the context of this chapter, methods
for estimating depth to aid in creating digital mattes include using a color-filtered
aperture to slightly offset the color channels [
28
] or a time-of-flight sensor to directly
acquire depth [
580
].
2.11
INDUSTRY PERSPECTIVES
NickApostoloff, senior software engineer, Paul Lambert, compositing supervisor, and
Blake Sloan, software engineer fromDigital Domain in Venice, California, and David
Geoghegan, Flame operator fromLOOK Effects in Los Angeles, California, discuss the
process of image and video matting in feature filmmaking.
RJR: How does academic work in matting compare with how it's done in the visual
effects world?
Apostoloff: I came from an academic background — but academic matting is very
different than what you see in the film industry. It's primarily because in industry,
