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System for the Display of Three Dimensional Objects with Hidden Lines Eliminated

IP.com Disclosure Number: IPCOM000108039D
Original Publication Date: 1992-Apr-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 6 page(s) / 240K

Publishing Venue

IBM

Related People

McCabe, DH: AUTHOR

Abstract

Three-dimensional objects can be displayed on graphics screens by drawing the edges which define those objects. However, if the objects are sufficiently complex, the resultant image can be confusing because too many edges are shown. This confusion can be eliminated by only displaying the visible edges of the object. The following describes how to remove the hidden edges in a simple and efficient manner.

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System for the Display of Three Dimensional Objects with Hidden Lines Eliminated

       Three-dimensional objects can be displayed on graphics
screens by drawing the edges which define those objects. However, if
the objects are sufficiently complex, the resultant image can be
confusing because too many edges are shown.  This confusion can be
eliminated by only displaying the visible edges of the object.  The
following describes how to remove the hidden edges in a simple and
efficient manner.

      Three-dimensional objects can be displayed on graphics screens
by drawing the edges of the objects.  This view of an object is
usually called the wire frame view.  The wire frame view is
relatively easy to create.  However, for a complex object with many
edges, the human perceptual system has difficulties understanding the
resultant images.  It is very difficult to differentiate edges which
are on the back of the object from edges which are on the front.
This effect is well demonstrated by the Necker illusion.

      The perceptual confusion can be removed by eliminating the
edges which are hidden and displaying only the edges which are
visible.  This system describes hardware which permits the rapid
generation and display of such an image.

      The literature is rich with methods for eliminating hidden
lines.  However, these methods tend to be very expensive because they
are based on complex geometrical calculations.  An excellent survey
of these methods can be found in I.E. Sutherland, R.F. Sproull, and
R.A. Schumacker, "A Characterization of Ten Hidden-Surface
Algorithms," ACM Computing Surveys 6, 1, 1-55 (March 1974).

      Whereas the above algorithms are based on geometrical
calculations, this system is based on image processing techniques.
The basic method was suggested by S.D. Roth, "Ray Casting for
Modeling Solids," Computer Graphics and Image Processing 18, 2,
109-144 (1982).  Roth generates his images using a costly ray tracing
technique.  This determines which object is visible at any given
pixel.  On a pixel by pixel basis, the object visible at each pixel
is compared with its neighbors.  If one of them differs from the
pixel in question, that pixel is drawn in the color of the edge.

      This system differs from the aforementioned in that this system
uses a much more efficient method for determining the visible object
at each pixel.  In addition, this system compares multiple pixels in
a single operation, gaining further efficiency over Roth's method.
Finally, this system has the ability to differentiate hidden lines
from visible lines.  Roth's method only displays visible lines.

      The principles behind the hidden line elimination algorithm
are:
1.   Determine which object is visible at each pixel.
2.   Compare the object at each pixel with those of its neighbors.
           If the object differs from its neighbors, mark the pixel
as an edge pixel.
3.   Draw the li...