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A visual hull is an approximation of a three-dimensional (3-D) object generated by the shape-from-silhouettes (SfS) technique. Because a visual hull is calculated from silhouettes, a visual hull can be represented by silhouette images, encoded by a small number of bits. However, it is challenging to represent the concave regions of a visual hull. In this paper, we model voxel data with a set of visual hulls, thereby handling the concave regions of voxel data. To accomplish this, silhouettes are generated from input voxel data using virtual cameras, and a visual hull is computed by SfS using the silhouettes. To handle the concave regions, we calculate the residuals of visual hulls, and the residuals are represented by visual hulls again. This process is repeated until all concave regions are processed, and a hierarchical data structure, i.e., a visual hull tree, is generated. Because the visual hull tree is constituted from a set of visual hulls, it can represent the details of the voxel data even in the root node. Also, because a set of visual hulls can be represented by silhouettes, a visual hull tree has a small number of bits. From the experiments, we compare our method to the octree-based representation, and our method demonstrates good encoding performance. |
