Rendering Issues - Computer Animation: Algorithms and Techniques - page 399

A.4 Drop shadows
The shadow cast by an object onto a surface is an important visual cue in establishing the distance
between the two. Contact shadows, or shadows produced by an object contacting the ground, are espe-
cially important. Without them, objects appear to float just above the ground plane. In high-quality
animation, shadow decisions are prompted by lighting, cinematography, and visual understanding con-
siderations. However, for most other animations, computational expense is an important concern and
computing all of the shadows cast by all objects in the scene onto all other objects in the scene is over-
kill. Much computation can be saved if the display system supports the user specification of which
objects in a scene cast shadows onto which other objects in a scene. For example, the self-shadowing 4
of an object is often not important to understanding the scene visually. Shadows cast by moving objects
onto other moving objects are also often not of great importance. These principles can be observed in
traditional hand-drawn animation in which only a select set of shadows is drawn.
By far the most useful type of shadow in animated sequences is the drop shadow. The drop shadow
is the shadow that an object projects to the ground plane. The drop shadow lets the viewer know how far
above the ground plane an object is as well as the object's relative depth and therefore relative size (see
Figures A.11 and A.12 ) .
Drop shadows can be produced in several different ways. When an object is perspectively projected
from the light source to a flat ground plane, an image of the object can be formed (see Figure A.13 ). If
this image is colored dark and displayed on the ground plane (as a texture map, for example), then it is
an effective shortcut.
Another inexpensive method for creating a drop shadow is to make a copy of the object, scale it flat
vertically, color it black (or make it dark and transparent), and position it just on top of the ground plane

FIGURE A.11
Scene without drop shadows; without shadows, it is nearly impossible to estimate relative heights and distances if
the sizes of the objects are not known.

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Self-shadowing refers to an object casting shadows onto its own surface.