] We present a novel photographic technique called dual
] photography, which exploits Helmholtz reciprocity to
] interchange the lights and cameras in a scene. With a
] video projector providing structured illumination,
] reciprocity permits us to generate pictures from the
] viewpoint of the projector, even though no camera was
] present at that location. The technique is completely
] image-based, requiring no knowledge of scene geometry or
] surface properties, and by its nature automatically
] includes all transport paths, including shadows,
] interreflections and caustics.

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