Shadows for Cel Animation
Author(s): Lena Petrovic, Brian Fujito, Lance Williams, Adam Finkelstein.
Proceedings: Proceedings of SIGGRAPH, Kurt Akeley, pp. 511--516, July, ACM Press / ACM SIGGRAPH / Addison Wesley Longman,
2000.
[BibTeX] 
Abstract:
We present a semi-automatic method for creating shadow mattes in
cel animation. In conventional cel animation, shadows are drawn by
hand, in order to provide visual cues about the spatial relationships
and forms of characters in the scene. Our system creates shadow
mattes based on hand-drawn characters, given high-level guidance
from the user about depths of various objects. The method employs
a scheme for “inflating” a 3D figure based on hand-drawn art. It
provides simple tools for adjusting object depths, coupled with an
intuitive interface by which the user specifies object shapes and
relative positions in a scene. Our system obviates the tedium of
drawing shadow mattes by hand, and provides control over complex
shadows falling over interesting shapes.
URL: http://www.cs.princeton.edu/gfx/proj/cel_shadows