One key function of graphics systems is to present information about
the 3-D structure of modeled environments.  For real-time simulations,
conveying a sense of contact between touching surfaces and relative
position and motion between proximate objects is particularly critical.
Neither stereo nor occlusion cues are completely effective for such fine
judgments. Conventional wisdom often argues that shadows play a critical
role.  Less often, it is argued that interreflection also contributes to
the sense that two surfaces are touching.  This paper explores the
actual utility of shadows and interreflection in signaling contact and
suggests how this information can be exploited in real-time rendering
systems to glue objects to surfaces.