Graphics Reference
In-Depth Information
FIGURE A.17
Various ways to render a drop shadow: black, transparent, and black with transparent edges.
transparent sections allow the camera to see objects behind the quadrilateral but not obscured by the
billboard image. An impostor usually implies that it is a temporary substitute for a three-dimensional
object, whereas billboards usually refer to independent graphical elements.
Billboards are useful when a three-dimensional graphical element is too complex to model and ren-
der for its perceptual effect. Billboards are suitable for objects that are to be viewed at a distance, that
are spherically or cylindrically perceptually symmetric, and that are just as effectively modeled in two
dimensions as in three dimensions. Common uses of billboarding include the representation of trees,
clouds, mountains, explosions, fire, and lens flares. See
Figure A.18
for an example. Billboard repre-
sentations can also be incorporated as one level of an object's level of detail representation.
FIGURE A.18
A scene containing several billboards of trees (Image courtesy of Ysheng Chen.)
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