Creating Visual Hull Models Using Only Two Mirrors and an Uncalibrated Perspective Camera

We present closed form solutions for the focal length and pose of a camera and two mirrors that are computed directly from the silhouette outlines that appear in a single image. In the noisy case, we show how these equations can be used to form an initial parameter estimate that can be refined with a nonlinear iterative minimisation. This allows five-view visual hulls to be constructed from a single image. We show how these five-view visual hulls can be used to form initial estimates of the similarity transforms that relate multiple five-view silhouette sets of a rigid object in different poses. A nonlinear iterative minimisation is then used to refine the solution so that all silhouettes are specified in a common reference frame, and visual hulls can be constructed from an arbitrary number of silhouettes. Experimental results demonstrating the reconstruction of a toy horse are presented.