1. A computer-implemented method, comprising the steps of: developing from objects for a first set of at least two image cells a Zfar T value indicating a depth beyond which all cells of subsequently received objects are known to be occluded, at least to the extent such cells are within said first set of cells; developing a mask which covers less than all cells of said first set of cells, and a Zfar M value indicating a depth not farther than Zfar T but beyond which all cells of subsequently received objects are known to be occluded if covered by said mask.