Rendering pipeline for surface elements

Details Rendering pipeline for surface elements Rendering pipeline for surface elements

2000-02-28

2003-06-24

Razavi, Michael (Department: 2672)

Computer graphics processing and selective visual display system

Computer graphics processing

Shape generating

C345S442000

Reexamination Certificate

active

06583787

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to graphic rendering, and more particularly to rendering zero-dimensional surface elements of graphic objects using a pipelined rendering engine.
Introduction to Computer Graphics
Three-dimensional computer graphics have become ubiquitous at the consumer level. There is a proliferation of affordable 3D graphics hardware accelerators, from high-end PC workstations to low-priced game stations. However, interactive computer graphics have still not reached the level of realism that allows a true immersion into a virtual world. For example, typical foreground characters in real-time games are extremely minimalistic polygon models that often exhibit annoying artifacts, such as angular silhouettes.
Various sophisticated modeling techniques, such as implicit surfaces, or subdivision surfaces, allow the creation of 3D graphics models with increasingly complex shapes. Higher order modeling primitives, however, are eventually decomposed into triangles before being rendered by the graphics subsystem. The triangle as a rendering primitive seems to meet the right balance between descriptive power and computational burden. To render realistic, organic-looking models requires highly complex shapes with ever more triangles, or, as stated by Smith in “
Smooth Operator
,” The Economist, pp. 73-74, Mar. 6, 1999, “reality is 80 million polygons.”
Processing many small triangles leads to bandwidth bottlenecks and excessive floating point number calculations and rasterization requirements. To increase the apparent visual complexity of objects, texture mapping has been introduced. Textures convey more detail inside a polygon, thereby allowing larger and fewer triangles to be used. Today's graphics engines are highly tailored for high texture mapping performance. However, texture maps have to follow the underlying geometry of the polygon model and work best on flat or slightly curved surfaces. Realistic or “organic” surfaces frequently require a large number of textures that have to be applied in multiple passes during rasterization.
Advanced rendering techniques, such as Phong shading, bump mapping, and displacement mapping, are not handled by most current consumer graphics systems. Graphic phenomena such as smoke, fire, or water are difficult to render using textured triangles.
Graphical Representations
In computer graphics, one can represent objects in 3D space in many different ways using various primitive graphic elements. The known representations that are commonly used to represent graphic objects are implicit, geometric, volumetric, and point sample.
Implicit Representation
In an implicit representation, the graphic object can be generated from arbitrary mathematical and/or physical functions. For example, to draw the outline of a hollow sphere one simply supplies the rendering engine with the function (in Cartesian coordinates) x
2
+y
2
+z
2
=r, and for a solid sphere the function is x
2
+y
2
+z
2
≦r. Color and other material properties can similarly be synthetically generated. Functions can be used to describe various geometric shapes, physical objects, and real or imaginary models. Implicit functions are not suitable for synthezising complex objects, for example, a human figure.
Geometric Representation
Classically, 3D objects have been geometrically modeled as a mesh of polygonal facets. Usually, the polygons are triangles. The size of each facet is made to correspond mostly to the degree of curvature of the object in the region of the facet. Many polygons are needed where the object has a high degree of curvature, fewer for relatively flat regions. Polygon models are used in many applications, such as, virtual training environments, 3D modeling tools, and video games. As a characteristic, geometric representations only deal with the surface features of graphic objects.
However, problems arise when a polygon model is deformed because the size of the facets may no longer correspond to local degrees of curvature in the deformed object, after all, a triangle is flat. Additionally, deformation may change the relative resolution of local regions. In either case, it becomes necessary to re-mesh the object according to the deformed curvature. Because re-meshing (polygonization) is relatively expensive in terms of computational time, it is usually done as a preprocessing step. Consequently, polygon models are not well suited for objects that need to be deformed dynamically.
Volumetric Representation
In an alternative representation, the object is sampled in 3D space to generate a volumetric data set, for example, a MRI or CT scan. Each sample is called a voxel. A typical data set may include millions of voxels. To render a volumetric data set, the object is typically segmented. Iso-surfaces can be identified to focus on specific volumetric regions. For instance, a volumetric data set of the human head may segment the voxels according to material properties, such as bone and soft tissue.
Because of the large number of voxels, physically-based modeling and the deformation of volumetric data sets is still a very computationally expensive operation. Often, one is only interested in surface features, and the interior of the object can effectively be ignored.
Point Sample Representation
A point sample representation of objects is often used to model fluid flows, for example, in wind tunnel simulations. Certain attributes, such as orientation velocity, are given to point samples in order to track individual point samples through the fluid flow, or to visualize the complete flow.
Another application of point sample representation is in the visualization of “cloud-like” objects, such as smoke, dust or mist. A shading model can be applied to point samples that emit light to render cloud-like objects. Also, point samples can be constrained to subspaces with the help of energy functions to model surfaces. An advantage of point sample clouds is that the clouds are very deformable. As a disadvantage, the point samples in the cloud are unconnected and behave individually when exposed to forces. Furthermore, prior art point samples are quite unsuitable for representing surfaces of solid objects or models.
Rendering Considerations
The rendering time for these conventional primitives depends on the complexity of the objects modeled. For example, with a geometric representation of a complex object, the polygons are typically very small in size, in the order of a very small number of pixels, and the object is represented by many polygons. The polygons are usually represented with vertices that define a triangle.
To render a polygon, the projection of the triangle is scan-converted (rasterized) to calculate the intensity of each pixel that falls within the projection. This is a relatively time consuming operation when only a few pixels are covered by each polygon. Replacing the polygons with point samples and projecting the point samples to the image can be a more efficient technique to render objects.
A number of techniques are known for rendering volumes. In general, volume rendering is quite complex. Unless the number of voxels is limited, real-time rendering can be time consuming, or impractical for real-time applications.
Discrete Particles
A real-time rendering system, described in U.S. Pat. No. 5,781,194 “Real-time Projection of Voxel-based Objects,” issued to Ponomarov et al. on Jul. 14, 1998, constructs a chain of surface voxels using incremental vectors between surface voxels. That representation succeeds in modeling and displaying objects showing highly detailed surface regions. The modeling of rigid body motion is done with the aid of scripting mechanisms that lack realism because physically-based methods are not used.
The use of points as rendering primitives has a long history in computer graphics. Catmull, in “
A Subdivision Algorithm for Computer Display of Curved Surfaces
,” Ph.D. thesis, University of Utah, December 1974, observed that geometric subdivision may ultimately lead to points on surfaces. Particles

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