400 according to various implementations of the present disclosure fig4 shows two views 402 and 404 of model 400 in view 402 a spherical object 406 having a radius r and modeling a human head appears viewed side on with respect to a view plane 408 representing for instance an image plane of depth image 108 in view 404 object 406 is shown from the perspective of the view plane 410 in both views 402 and 404 an ordinate z orthogonal to view plane 410 represents increasing depth or distance away from view plane 410 fig4 depicts several points a b and c on the surface of object 406 and corresponding projections of those points a u2032 b u2032 and c u2032 on view plane 410 point c represents a closest point on object 406 with respect to view plane 410 point a represents a furthest visible point and point b represents a location having an intermediate depth or distance between point a and point c the distance between the projection points b u2032 and c u2032 is denoted by r assuming that the distance aa u2032 is m then the distance cc u2032 is m r and a depth value associated with b u2032 may.