We describe a three dimensional graphics editor called PG
(Polygon Groper) and its use by artists for defining and viewing
solid objects. A simple set of commands in PG are used to manipulate
3D models. The models are stored as a hierarchy of objects, where each
object represents a geometrical transformation of another object or a
primitive polygon. It has been found that a hierarchical structure
provides a more powerful way of building and manipulating 3D objects
than conventional linear structures. The object hierarchy also allows
recursion to be used to describe pictures which are difficult or impossible
to define in other graphics systems without considerable programming effort.
Although any recursive process may be expressed iteratively, a certain class of
pictures is more concisely defined and more easily visualised
recursively. Many tools are available to aid the artist manufacture primitive
These tools generate the surface polygons to describe different solids.
PG offers a number of alternative viewing algorithms, these provide the
artist with successively higher quality images at the cost of
correspondingly greater time for computation. PG is an integral part of
the Graphicsland, a film animation system under development at
the University of Calgary.
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