Rendering Issues - Computer Animation: Algorithms and Techniques - page 402

FIGURE A.16
Circular drop shadows.
Globular shapes that may more closely indicate the shapes of the original objects can also be used as
shadows. The drop shadow, however it is produced, can be represented on the ground plane in several
different ways. It can be colored black and placed just above the ground plane. It can bemade into a darkly
colored transparent object so that any detail of the ground plane shows through. It can be colored darker in
the middle with more transparency toward the edges to simulate a shadow's penumbra 5 ( Figure A.17 ).
When placing a shadow over the ground plane, one must take care to keep it close enough to the
ground so that it does not appear as a separate object and at the same time does not conflict with the
ground geometry. To avoid the problems of using a separate geometric element to represent the drop
shadow, the user can incorporate the shadow directly into the texture map of the ground plane.
A.5 Billboarding and impostors
Billboarding and the use of impostors are a rendering shortcut that reduces the computational complex-
ity of a scene while maintaining its visual complexity (e.g., [ 4 ] [ 7 ]). These related techniques do this by
using a two-dimensional substitute (such as a planar projection) for a complex three-dimensional object
that is seamlessly incorporated into a three-dimensional scene. Typically, a billboard is a partially
transparent, textured quadrilateral. The texture map is an image of the object represented. The quad-
rilateral is partially transparent in cases when the object's image does entirely cover the quadrilateral;
5 The penumbra is the area partially shadowed by an opaque body; it receives partial illumination from a light source. The
umbra is the area completely shadowed by an opaque body; it receives no illumination from the light source.