Computer graphics processing and selective visual display system – Computer graphics processing – Three-dimension
Reexamination Certificate
1999-03-09
2001-11-13
Zimmerman, Mark (Department: 2671)
Computer graphics processing and selective visual display system
Computer graphics processing
Three-dimension
C345S421000, C345S427000, C345S581000, C345S589000
Reexamination Certificate
active
06317126
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the field of computer graphics and, more particularly, to associating a pixel with one of a plurality of regions for evaluating the pixel light values.
Rendering of realistic images is one of the main goals of graphics system designers. Rendering images of real or imaginary objects typically involves generating geometric models (e.g., polygons) of the objects and applying lighting effects to the polygonal surfaces. In computer graphics, surfaces of an object are often modeled by a polygonal mesh, which is a collection of vertices, edges, and/or polygons. A mesh of polygons may be produced from a variety of sources such as an application, tesselated NURBS surfaces, spheres, cones, etc. The vertices may be connected by edges and a sequence of edges or vertices may define one or more polygons.
Rendering of realistic 3D graphics requires accurate and efficient modeling of 3D surfaces based upon the position, orientation, and characteristics of the surfaces and the light sources illuminating them. In particular, the interaction between lights and surfaces must be modeled for rendering. To accurately model lighting effects, conventional computer graphics systems have typically implemented a variety of lighting models and shading techniques to generate light values at individual pixels of a graphics primitive such as a polygon. A co-pending U.S. patent application Ser. No. 09/265507 and entitled “Method and Device for Generating Per-Pixel Values,” by inventor David Tannenbaum et al., describes several exemplary lighting models and shading techniques.
Conventional lighting models typically model one or more lighting effects such as ambient light, diffuse reflection, specular reflection, and spotlighting, each of which is well known in the art. The ambient light accounts for a lighting effect resulting from multiple reflections of light from the surfaces present in a scene. On the other hand, the diffuse reflection models reflection of light off a dull, matte surface. In this model, the reflected light from the surface falls off uniformly as a function of the angle between N and L, where N is the outward normal vector at a surface point and L is a light vector. The light vector L has a direction from the surface point to a light source. The diffuse light fall-off in the diffuse reflection model is typically modeled by using a dot product term N·L.
Similarly, the specular reflection accounts for reflection of light off a shiny surface. When light from a light source is reflected off a surface, the reflected light is typically modeled as falling off exponentially from the direction of reflection vector R as seen from the direction of view vector V. For example, the fall-off may be modeled by cos
s
&agr;, where s is a surface material's specular-reflection coefficient and a is an angle between vectors R and V. In practice, a dot product term (N·H)
s
is often used in place of cos
s
&agr; to model specular reflection at a surface point, where N is a normal vector and H is a half-angle vector. Both the diffuse and specular reflection models assume that a light source radiates light uniformly in all directions.
The spotlight model, on the other hand, adds a direction to a positional light source to allow modeling of directional lights. That is, a spotlight is a special type of local or infinite light source that has a direction as well as a position. For example, a positional light source may function as a spotlight by restricting the shape of the light to exist within a cone. The direction of the spotlight is the direction in which the light points. The spotlight thereby simulates a cone of light, which may have a fall-off in intensity based upon distance from the center of the cone of light.
The spotlight and other conventional lighting models are described in
Computer Graphics: Principles and Practice
by James D. Foley et al., Addison-Wesley (1996), ISBN 0-201-84840-6, which is incorporated herein by reference and constitutes a portion of the background against which the present invention was developed. Additionally, the OpenGL™ (versions 1.1 and 1.2) graphics application programming interface, which is commercially available from Silicon Graphics, Inc., the assignee of the present application, describes various lighting models such as spotlight, diffuse light reflection, specular light reflection, and related parameters for implementing such models. The OpenGL™ (versions 1.1 and 1.2) graphics application programming interface also is incorporated herein by reference.
FIG. 1A
illustrates a perspective view of a conventional spotlight
100
being radiated from a light source
102
onto a surface
104
. In this configuration, the light source
102
radiates the spotlight
100
in a cone shape that is defined by edge
108
of the cone and a center axis
106
of the cone. The angle between the center axis
106
and the edge
108
of the cone defines a cutoff angle &phgr;. The cutoff angle &phgr; defines a boundary
110
of the spotlight
100
. That is, the spotlight value is set to zero in an outer region
114
lying outside of the boundary
110
. In an inner region
112
of the boundary
110
, the spotlight value is computed in accordance with a basic spotlight equation, spotlight=(S·L)
exp
, where S represents a spotlight source direction vector, L represents the vector from a point on the surface to the light source, and exp is a spotlight exponent. The spotlight equation may also include other well-known variables such as attenuation, shadow, etc.
FIG. 1B
shows a vector diagram
150
depicting the relationship between the light source direction vector S and spotlight vector L of the spotlight equation at an arbitrary point P on a surface
152
. The vector S represents the direction of light from the light source
102
. The anti-parallel vector −L, on the other hand, represents the direction from the spotlight to point P. By definition, the vector L has the same magnitude but opposite direction as its anti-parallel vector, −L. The angle between the vectors S and −L is designated as angle &thgr;.
Unfortunately, conventional spotlighting techniques using the spotlight equation often exhibit aliasing effects such as jaggies or stairsteps near the boundary of the cutoff angle (e.g., boundary
110
in FIG.
1
A). These aliasing effects are caused by the abrupt transition into the cutoff region where the light values are zero. In other words, the transition from the lighted cone area to the outer region is not smooth because the light contribution from a light source is applied in an all-or-nothing fashion based on the relationship of point P with respect to the lighted cone's projection on the surface
104
or
152
.
To reduce the aliasing effects, a conventional method provided a transition region between an inner, lighted region and an outer, dark region.
FIG. 1C
illustrates a perspective view of a spotlight
170
being radiated from a light source
102
onto a surface
176
. The light source
102
radiates the spotlight
170
in a cone shape having an angle &phgr;, which is the angle between the edge
172
of the cone and the center axis
174
of the cone. This method provides three regions: an inner region
178
, a transition region
180
, and an outer region
182
. The inner and transition regions
178
and
180
are defined with respect to an inner boundary
184
. The transition and outer regions
180
and
182
are defined with respect to an outer boundary
186
defined by the cutoff angle &phgr;.
In this arrangement, the light values in the inner region
178
are computed in accordance with a spotlight equation while the light values in the outer region are set to zero. The light values in the transition region, on the other hand, are computed to provide a gradual fall-off in intensity. The gradual intensity fall-off in the transition region thereby reduces aliasing effects.
One of the drawbacks in implementing the conventional approaches in a computer system is the complexity and cost
Baker & Botts L.L.P.
Nguyen Kimbinh T.
Silicon Graphics Inc.
Zimmerman Mark
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