Surface reconstruction from dense range data has been an active area of research for several decades. The strategies have proceeded along two basic directions: reconstruction from unorganized points, and reconstruction that exploits the underlying structure of the acquired data. These two strategies can be further subdivided according to whether they operate by reconstructing parametric surfaces or by reconstructing an implicit function.
A major advantage of the unorganized points algorithms is the fact that they do not make any prior assumptions about connectivity of points. In the absence of range images or contours to provide connectivity cues, these algorithms are the only recourse. Among the parametric surface approaches, Boissanat  describes a method for Delaunay triangulation of a set of points in 3-space. Edelsbrunner and Mücke  generalize the notion of a convex hull to create surfaces called alpha-shapes. Examples of implicit surface reconstruction include the method of Hoppe, et al  for generating a signed distance function followed by an isosurface extraction. More recently, Bajaj, et al  used alpha-shapes to construct a signed distance function to which they fit implicit polynomials. Although unorganized points algorithms are widely applicable, they discard useful information such as surface normal and reliability estimates. As a result, these algorithms are well-behaved in smooth regions of surfaces, but they are not always robust in regions of high curvature and in the presence of systematic range distortions and outliers.
Among the structured data algorithms, several parametric approaches have been proposed, most of them operating on range images in a polygonal domain. Soucy and Laurendeau  describe a method using Venn diagrams to identify overlapping data regions, followed by re-parameterization and merging of regions. Turk and Levoy  devised an incremental algorithm that updates a reconstruction by eroding redundant geometry, followed by zippering along the remaining boundaries, and finally a consensus step that reintroduces the original geometry to establish final vertex positions. Rutishauser, et al  use errors along the sensor's lines of sight to establish consensus surface positions followed by a re-tessellation that incorporates redundant data. These algorithms typically perform better than unorganized point algorithms, but they can still fail catastrophically in areas of high curvature, as exemplified in Figure 8.
Several algorithms have been proposed for integrating structured data to generate implicit functions. These algorithms can be classified as to whether voxels are assigned one of two (or three) states or are samples of a continuous function. Among the discrete-state volumetric algorithms, Connolly  casts rays from a range image accessed as a quad-tree into a voxel grid stored as an octree, and generates results for synthetic data. Chien, et al  efficiently generate octree models under the severe assumption that all views are taken from the directions corresponding to the 6 faces of a cube. Li and Crebbin  and Tarbox and Gottschlich  also describe methods for generating binary voxel grids from range images. None of these methods has been used to generate surfaces. Further, without an underlying continuous function, there are no mechanism for representing range uncertainty or for combining overlapping, noisy range surfaces.
The last category of our taxonomy consists of implicit function methods that use samples of a continuous function to combine structured data. Our method falls into this category. Previous efforts in this area include the work of Grosso, et al , who generate depth maps from stereo and average them into a volume with occupancy ramps of varying slopes corresponding to uncertainty measures; they do not, however, perform a final surface extraction. Succi, et al  create depth maps from stereo and optical flow and integrate them volumetrically using a straight average. The details of his method are unclear, but they appear to extract an isosurface at an arbitrary threshold. In both the Grosso and Succi papers, the range maps are sparse, the directions of range uncertainty are not characterized, they use no time or space optimizations, and the final models are of low resolution. Recently, Hilton, et al  have developed a method similar to ours in that it uses weighted signed distance functions for merging range images, but it does not address directions of sensor uncertainty, incremental updating, space efficiency, and characterization of the whole space for potential hole filling, all of which we believe are crucial for the success of this approach.
Other relevant work includes the method of probabilistic occupancy grids developed by Elfes and Matthies . Their volumetric space is a scalar probability field which they update using a Bayesian formulation. The results have been used for robot navigation, but not for surface extraction. A difficulty with this technique is the fact that the best description of the surface lies at the peak or ridge of the probability function, and the problem of ridge-finding is not one with robust solutions . This is one of our primary motivations for taking an isosurface approach in the next section: it leverages off of well-behaved surface extraction algorithms.
The discrete-state implicit function algorithms described above also have much in common with the methods of extracting volumes from silhouettes    . The idea of using backdrops to help carve out the emptiness of space is one we demonstrate in section 4.