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C00002 00002 5.0 HIDDEN LINE ELIMINATION FOR COMPUTER VISION.
C00003 00003 5.0 Introduction.
C00004 00004 5.1 Hiding a Topologically Coherent Object.
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5.0 HIDDEN LINE ELIMINATION FOR COMPUTER VISION.
5.0 Introduction.
5.1 Hiding a Topologically Coherent Object.
5.2 Two simple hidden line eliminators that almost work:
Edge-Edge and Face-Vertex.
5.3 2-D Face, Edge, Vertex Sorting.
5.X Propagating Underfaces.
5.X Shadows.
5.X Photometric Modeling and Video Generation.
5.X Performance
5.0 Introduction.
Hidden line elimination refers to the process of simulating
the appearance of a collection of opaque objects. In computer
graphics the results of a hidden line elimination are intended for
humans, while in computer vision the output is intended for further
machine processing, namely for the image comparison which is
verification vision.
Three reasons why
hidden line elimination is a perenial research problem
that has yet been adequately solved
solutions are quite lacking in both realism, economy and flexibility.
5.1 Hiding a Topologically Coherent Object.
OCCULT marks the faces, edges and vertices of a polyhedral
scene as either visible or hidden with respect to a camera model.
Edges that were at first partially visible are split into pieces so
that each piece is either fully visible or fully hidden.
Like Warnock's algorithm, OCCULT has two parts: a recursive outer
part that splits an image until there are only afew edges in it or it
has become too small; the inner part is the real hidden line
eliminator. The inner part of OCCULT is based on the idea of
comparing each edge with all the other edges and hiding as much as
possible when a pair of intersecting edges is discovered.
OCCULT is a hidden line eliminator; it is neither a Watkins
nor a Warnock algorithm but is rather a throw-back to the naive idea
of comparing each edge with all the other edges and having ways to
dampen the potentially large number of comparisons that might occur.
There are three kinds of dampening in OCCULT. The first (used
in other hidden eliminators) is to get rid of the faces that have
their backs to the camera and to consider for comparision only the
edges with one potentially visible face. These edges are called
"folds". The second kind of dampening, is to hide everything
connected to the hidden portion of an edge when a fold crossing is
discovered, this is made possible by the winged edge primitives which
allow polyhedron surfaces to be easily traversed topologically; and
by the Euler primitives which allows the edges to be quickly broken
into visible and hidden portions without losing their topology. The
third kind of dampening involves having a raster of edge buckets to
localize the comparisons.
The reason for doing hidden line elimination in this fashion
is to get the topology of the image regions and edges in a modeled
scene including the shadows. OCCULT was used to make some of the
figures that appeared earlier in this paper; for example the arm
model in figure 1.2, (which required twelve seconds of PDP-10 compute
time).