... problem¹

The Aperture problem arises due to uniformly colored surfaces in the scene. In the absence of strong lighting effects, a uniform surface in the scene appears nearly uniform in projection. It is then impossible to determine correspondences within these regions.

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... silhouettes²

The visual hull represents a conservative shell that envelops the true geometry of the objects, consisting in the shape obtained from silhouette image data. The visual hull techniques require that foreground objects in the input images can be segmented from the background.

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