Importance of shadow effects

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As discussed in the introduction, shadows play an important role in our understanding of 3D geometry:

  - Shadows help to understand relative object position and size in a scene^{49, 38, 27, 30, 31}. For example, without a cast shadow, we are not able to determine the position of an object in space (see Figure 1).
This point is explained in the following Animation: it shows how shadows can remove ambiguity about trajectory of a sliding object, which can be the same in 2D but corresponding to two different paths in 3D.


  - Shadows can also help us understanding the geometry of a complex receiver³⁸ (see Figure 2a, 2b and Animation).


  - Finally, shadows provide useful visual cues that help in understanding the geometry of a complex occluder³⁸: shadows can reveal occluded geometry (as the teapot held by the robot, see Figure 3), or even geometry that is out of the field of view, as a plane passing above the robot (see Animation).