Browse Prior Art Database

Surface Flow Correction in Fast Drawing Systems

IP.com Disclosure Number: IPCOM000120906D
Original Publication Date: 1991-Jun-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 4 page(s) / 212K

Publishing Venue

IBM

Related People

Todd, S: AUTHOR [+2]

Abstract

Two techniques for correcting image flows in fast drawing graphics systems are disclosed. The first detects holes by counting the number of uses of each edge, proceeding on to repair the surface by generating polygons from the free edges. The second technique forces an approximate wire frame consisting of many not-quite-meeting polygons into a meshed polygon structure by coercing vertices and then uses topological information to make the meshing more efficient. These descriptions relate to the FAST DRAW derivative of WINSOM (Winchester Solid Modeller) rendering prototype and to WINSOM90, but could apply to similar systems such as CATIA.

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Surface Flow Correction in Fast Drawing Systems

      Two techniques for correcting image flows in fast drawing
graphics systems are disclosed.  The first detects holes by counting
the number of uses of each edge, proceeding on to repair the surface
by generating polygons from the free edges.  The second technique
forces an approximate wire frame consisting of many not-quite-meeting
polygons into a meshed polygon structure by coercing vertices and
then uses topological information to make the meshing more efficient.
These descriptions relate to the FAST DRAW derivative of WINSOM
(Winchester Solid Modeller) rendering prototype and to WINSOM90, but
could apply to similar systems such as CATIA.

      In a system to convert constructive solid geometry models to an
approximate graphics representation as fast as possible, there will
be flaws in the generated graphics. One kind of flaw takes the form
of 'holes' in the surface of pictures.  These are not intrusive when
creating wire frame output, but are when generating smooth polygons.
Another kind of flaw occurs where several polygons are used to
generate a faceted surface.  The polygons should mesh together, with
three or so polygons meeting at each polygon corner.  In practice,
short-cuts in the high level processing cause polygons to be
generated that do not quite meet at the corners.  FASTDRAW consists
of two major layers of processing.  The higher level breaks down a
complete model valid in all space to a set of simple models, each
valid in a particular voxel.  This level operates by recursive
subdivision and is most common code to WINSOM.

      The lower level looks at the simplified models in each voxel
and generates edges on the model by DOTLINE.  These edges may be
drawn directly and the picture improved in various ways.  Lines may
be collected into polygons which may be drawn directly, or collected
and "meshed" to fit better together.  There are still flaws in the
picture even with meshed polygons, and these become obtrusive when
smooth (e.g., Gouraud) shading is used to give fair quality graphical
rendering of FASTDRAW output.  Holes on a surface appear because
FASTDRAW generates planar approximations to each primitive
independently in each voxel.  It does not attempt to force these
planar approximations to meet exactly at the voxel boundaries.  To do
so would be impossible if each primitive is to be replaced by just
one plane in each voxel.  Complications arise where adjacent voxels
have different sizes, but varying resolution is important to the
speed of FASTDRAW.

      Flaws in the picture may also be due to many coincident, or
nearly coincident, surfaces confusing the topology.  For most models,
these other flaws will be localized to small maximum resolution
voxels and are not obtrusive, nor are they addressed in this article
which describes the generation of extra polygons to repair the holes.
Method 1: Generating Polygons from Free Edges

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