  We show that a digital sphere, constructed by the circular sweep of a digital
semicircle (generatrix) around its diameter, consists of some holes
(absentee-voxels), which appear on its spherical surface of revolution. This
incompleteness calls for a proper characterization of the absentee-voxels whose
restoration will yield a complete spherical surface without any holes. In this
paper, we present a characterization of such absentee-voxels using certain
techniques of digital geometry and show that their count varies quadratically
with the radius of the semicircular generatrix. Next, we design an algorithm to
fill these absentee-voxels so as to generate a spherical surface of revolution,
which is more realistic from the viewpoint of visual perception. We further
show that covering a solid sphere by a set of complete spheres also results in
an asymptotically larger count of absentees, which is cubic in the radius of
the sphere. The characterization and generation of complete solid spheres
without any holes can also be accomplished in a similar fashion. We furnish
test results to substantiate our theoretical findings.
