VISIBLE-OBJECT DETERMINATION FOR INTERACTIVE VISUALIZATION

Abstract: A system and method for performing visible object determination based upon a dual search of a cone hierarchy and a bounding (e.g. hull) hierarchy. Visualization software running on a host processor represents space with a hierarchy of cones constructed by recursive refinement, and generates a hierarchy of bounding hulls from a given collection of objects by recursively grouping clusters of objects. Hull nodes in the hull hierarchy reflect cluster membership. Each hull node stores parameters which characterize a bounding hull for the corresponding cluster or object. The visualization software searches the cone and hull hierarchies starting with the root cone and the root hull. Each leaf-cone is assigned a visibility distance value which represents the distance to its closest known object. The visibility distance value of a non-leaf cone is set equal to the maximum of the visibility distance values for its subcone children. Before exploring a given cone-hull pair, a cone-restricted minimum distance between the cone and the hull is measured and compared to the visibility distance value of the cone. Only when the former is smaller than the latter will the cone be searched against the hull. Assuming the search condition is satisfied, subhulls of the given hull are again conditionally explored against the given cone (or its subcones) in ascending order of their cone-hull distances from the cone (or its subcones). When the recursive search of the two trees is completed, visible objects will have been assigned to each leaf-cone.

Description: TITLE: VISIBLE-OBJECT DETERMINATION FOR INTERACTIVE VISUALIZATION BACKGROUND OF THE INVENTION 1 TECHNICAL FIELD The present invention relates generally to the field of computer graphics, and more particularly, to the problem of determining the set of objects/surfaces visible from a defined viewpoint in a graphics environment 2 BACKGROUND ART Visualization software has proven to be very useful in evaluating three-dimensional designs long before the physical realization of those designs In addition, visualization software has shown its cost effectiveness by allowing engineering companies to find design problems early in the design cycle, thus saving them significant amounts of money Unfortunately, the need to view more and more complex scenes has outpaced the ability of graphics hardware systems to display them at reasonable frame rates As scene complexity grows, visualization software designers need to carefully use the rendeπng resource provided by graphic hardware pipelines A hardware pipeline wastes rendeπng bandwidth when it discards triangle work Rendeπng bandwidth waste can be decreased by not asking the pipeline to draw triangles that it will discard Vaπous software methods for reducing pipelme waste have evolved over time Each technique reduces waste at a different point within the pipelme As examples, software frustum culling can significantly reduce discards m a pipeline's clipping computation while software backface culling can reduce discards in a pipeline's lighting computation The z-buffer is the final part of the graphics pipeline that discards work In essence, the z-buffer retains visible surfaces and discards those not visible As scene complexity increases, especially in walk through and CAD environments, the number of occluded surfaces πses rapidly and as a result the number of surfaces that the z-buffer discards rises as well A frame's average depth complexity determines roughly how much work (and thus rendeπng bandwidth) the z-buffer discards In a frame with a per-pixel depth complexity of d the pipeline's effectiveness is 1/d As depth complexity πses, the hardware pipeline thus becomes proportionally less and less effective Software occlusion culling has been proposed as an additional tool for improving rendeπng effectiveness A visualization program which performs occlusion culling effectively mcreases the graphic hardware's overall rendering bandwidth by not asking the hardware pipelme to draw occluded objects Computing a scene's visible objects is the complementary problem to that of occlusion culling Rather than removmg occluded objects from the set of objects m a scene or even a frustum culled scene, a program mstead computes which objects are visible and draws just those A simple visualization program can compute the set of visible objects and draw those objects from the current viewpoint, allowing the pipeline to remove backfacing polygons and the z-buffer to remove any non-visible surfaces One technique for computmg the visible object set uses ray casting RealEyes [Sowizral, H A , Zikan, K , Esposito, C , Janm, A , Mizell, D "RealEyes A System for Visualizing Very Large Physical Structures", SIGGRAPH '94, Visual Proceedmgs, 1994, p 228], a system that implemented the ray casting technique, was demonstrated in SIGGRAPH 1994's BOOM room. At interactive rates, visitors could "walk" around the mteπor of a Boemg 747 or explore the structures compπsmg Space Station Freedom's lab module The intuition for the use of rays m determining visibility relies on the properties of light. The first object encountered along a ray is visible smce it alone can reflect light into the viewer's eye. Also, that object interposes itself between the viewer and all succeedmg objects along the ray making them not visible. In the discrete world of computer graphics, it is difficult to propagate a continuum of rays. So a discrete subset of rays is invariably used. Of course, this implies that visible objects or segments of objects smaller than the resolution of the ray sample may be missed and not discovered. This is because rays guarantee coπect determination of visible objects only up to the density of the ray-sample. Figure 1 illustrates the ray-based method of visible object detection. Rays that mteract with one or more objects are marked with a dot at the point of their first contact with an object It is this point of first contact that determines the value of the screen pixel corresponding to the ray Also observe that the object denoted A is small enough to be entirely missed by the given ray sample Visible-object determination has its roots in visible-surface determination. Foley et al. [Foley, J., van Dam, A , Femer, S. and Hughes, J Computer Graphics Principles and Practice, 2nd ed , Addison- Wesley, Chapter 15, pp.649-718, 1996] divide visible-surface determination approaches mto two broad groups: image-precision and object-precision algonthms. Image precision algonthms typically operate at the resolution of the display device and tend to have supeπor performance computationally. Object precision approaches operate in object space — usually performing object to object comparisons. A prototypical image-precision visible-surface-determination algoπthm casts rays from the viewpomt through the center of each display pixel to determine the nearest visible surface along each ray. The list of applications of visible-surface ray casting (or ray tracing) is long and distinguished Appel ["Some Techniques for Shading Machine Rendeπng of Solids", SJCC'68, pp 37-45, 1968] uses ray castmg for shading. Goldstein and Nagel [Mathematical Applications Group, Inc., "3-D Simulated Graphics Offered by Service Bureau," Datamation, 13(1), February 1968, p 69.; see also Goldstein, R A and Nagel, R "3-D Visual Simulation", Simulation. 16(1), pp.25-31, 1971] use ray castmg for boolean set operations. Kay et al [Kay, D S and Greenberg, D., "Transparency for Computer Synthesized Images," SIGGRAPH'79, pp.158-164] and Whitted ["An Improved Illumination Model for Shaded Display", CACM, 23(6). pp.343-349, 1980] use ray tracing for refraction and specular reflection computations. Arrey et al. [Airey, J.M., Rohlf, J.H.. and Brooks, Jr. F P. Towards Image Realism with Interactive Update Rates in Complex Virtual Building Environments ACM SIGGRAPH Symposium on Interactive 3D Graphics, 24, 2(1990), pp 41-50] uses ray castmg for computmg the portion of a model visible from a given cell. Another approach to visible-surface determination relies on sending beams or cones mto a database of surfaces [see Dadoun et al., "Hierarchical approachs to hidden surface intersection testing " Proceeedings of Graphics Interface '82, Toronto, May 1982, 49-56; see also Dadoun et al., "The geometry of beam tracmg." In Joseph O'Rourke, ed., Proceeedings of the Symposium on Computational Geometry, pp.55-61, ACM Press, New York, 1985]. Essentially, beams become a replacement for rays. The approach usually results in compact beams decomposmg mto a set of possibly non-connected cone(s) after interacting with an object A vaπety of spatial subdivision schemes have been used to impose a spatial structure on the objects m a scene. [The following four references pertain to spatial subdivision schemes- (a) Glassner, "Space subdivision for fast ray tracmg," IEEE CG&A, 4(10).15-22, Oct 1984, (b) Jevans et al , "Adaptive voxel subdivision for ray tracmg," Proceedmgs Graphics Interface '89, 164-172, June 1989; (c) Kaplan, M. "The use of spatial coherence in ray tracmg," m Techniques for Computer Graphics..., Rogers, D. and Earnshaw R.A. (eds), Springer- Verlag, New York, 1987; and (d) Rubm, S M and Whitted, T. "A 3-dιmensιonal representation for fast rendeπng of complex scenes," Computer Graphics, 14(3):110-116, July 1980 ] Kay et al. [Kay, T.L and Kajiya, J.T. "Ray Tracing Complex Scenes", SIGGRAPH 1986, pp. 269—278,1986], concentrating on the computational aspect of ray castmg, employed a hierarchy of spatial bounding volumes in conjunction with rays, to determine the visible objects along each ray. Of course, the spatial hierarchy needs to be precomputed. However, once in place, such a hierarchy facilitates a recursive computation for finding objects. If the environment is stationary, the same data-structure facilitates finding the visible object along any ray from any ongrn. Teller et al. [Teller, S. and Sequm, C H "Visibility Preprocessing for Interactive Walkthroughs," SIGGRAPH '91, pp 61-69] use preprocessmg to full advantage m visible-object computation by precomputmg cell-to-cell visibility. Their approach is essentially an object precision approach and they report over 6 hours of preprocessmg time to calculate 58 Mbytes of visibility information for a 250,000 polygon model on a 50 MIP machme [Teller, S. and Sequm. C. H "Visibility computations m polyhedral three-dimensional environments," U.C. Berkeley Report No. UCB/CSD 92/680, Apnl 1992]. In a different approach to visibility computation, Greene et al [Greene, N., Kass, M., and Miller, G. "Hierarchical z-Buffer Visibility," SIGGRAPH '93, pp.231-238] use a vaπety of hierarchical data structures to help exploit the spatial structure inherent m object space (an octree of objects), the image structure inherent m pixels (a Z pyramid), and the temporal structure inherent m frame-by-frame rendeπng (a list of previously visible octree nodes). The Z-pyramid permits the rapid culling of large portions of the model by testing for visibility usmg a rapid scan conversion of the cubes m the octree The depth complexity of graphical environments continues to mcrease in response to consumer demand for realism and performance Thus, the efficiency of an algorithm for visible object determination has a direct impact on the marketability of a visualization system. The computational bandwidth required by the visible object determination algoπthm determines the class of processor required for the visualization system, and thereby effects overall system cost Thus, a system or method for improvmg the efficiency of visible object determination is greatly desired. DISCLOSURE OF INVENTION The present invention comprises a system and method for displaying visible objects in a graphics environment. In particular, a system and method for performing visible object determination based upon a dual search of a cone hierarchy and a boundmg hierarchy is herein disclosed The system includes a processor, a display device, system memory, and optionally a graphics accelerator The processor executes visualization software which provides for visualization of a collection of three-dimensional objects on the display device. The objects reside m a three-dimensional space and thus admit the possibility of occluding one another. The visualization software represents space in terms of a hierarchy of cones emanating from the viewpomt. In one embodiment, the leaf-cones of the hierarchy, i.e the cones at the highest level of refinement, subtend an area which corresponds to a fraction of a pixel in screen area. For example, two cones may conveniently fill the area of a pixel. Alternatively, the leaf-cone may subtend areas which mclude one or more pixels An initial view frustum or neighborhood of the view frustum is recursively tessellated (i.e. refined) to generate a cone hierarchy Alternatively, the entire space around the viewpomt may be recursively tessellated to generate the cone hierarchy. In this case, the cone hierarchy does not need to be recomputed for changes in the viewpomt and view-direction. The visualization software also generates a hierarchy of bounds from the collection of objects. In particular, the boundmg hierarchy is generated by: (a) recursively groupmg clusters starting with the objects themselves as order-zero clusters, (b) boundmg each object and cluster (of all orders) with a corresponding bound, e.g. a polytope hull, (c) allocating a node m the boundmg hierarchy for each object and cluster, and (d) organizing the nodes m the boundmg hierarchy to reflect cluster membership. For example if node A is the parent of node B, the cluster corresponding to node A contams a subcluster (or object) coπesponding to node B. Each node stores parameters which characterize the bound of the coπesponding cluster or object The visualization software performs a search of the cone and boundmg hierarchies starting with the root cone and the root bound. Each leaf-cone is assigned a visibility distance value which represents the distance to the closest known object as perceived from within the leaf-cone. Each leaf-cone is also assigned an object attπbute which specifies the closest known object withm view of the leaf-cone. Similarly, each non-leaf cone is assigned a visibility distance value. However, the visibility distance value of a non-leaf cone is set equal to the maximum of the visibility distance values for its subcone children. This implies that the visibility distance value for each non-leaf cone equals the maximum of the visibility distance values of its leaf-cone descendents. The visibility software operates on cone-bound parrs. Before exploring a given cone-bound parr, the distance between the cone and the bound is measured This mvolves determmmg the mmimum distance to pomts residing m both the bound and the cone from the vertex of the cone This cone-bound distance is then compared to the visibility distance value of the cone. If the cone-bound distance is larger than the visibility distance value of the cone, all of the leaf-cone descendents of the given cone have known visible objects closer than the given bound by definition of the visibility distance value. Thus, no benefit can be gamed from exploring the cone-bound pair. In contrast, if the cone-bound distance is smaller than the visibility distance value of the cone, the bound may contain objects which will affect the visibility distance values of one or more leaf-cone descendents of the given cone. The cone-bound pair must be searched. Accordmg to the present invention, cone- bound pairs are advantageously searched only when there is a possibility that the given bound may affect the visibility of the cone's descendents. Thus, the search algonthm of the present invention avoids unnecessary cone-bound explorations and thereby saves considerable computational bandwidth. Supposmg that the search condition is satisfied, the bound is explored with respect to the given cone. If the cone and bound are both leaves of their respective hierarchies, the bound specifies an object which is closer than the closest known object for the leaf-cone. Thus, the visibility distance value of the leaf-cone is updated with the cone-bound distance between the cone and bound Also, the object attπbute for the cone is updated to pomt to the given bound In the case that the cone is a leaf-cone and the bound is a non-leaf bound, the search algonthm examines subbounds of the given bound, and conditionally explores these subbounds m ascendmg order of their cone-bound distance from the given cone Agam, exploration of a subbound is conditioned upon the subbound achievmg a cone-bound distance to the given cone which is smaller than the cone's visibility distance value In the case that the cone is a non- leaf cone and the bound is a leaf bound (1 e one which bounds a smgle object), the search algonthm conditionally explores subcones of the given cone with respect to the given bound Exploration of a subcone is conditioned upon the subcone achievmg a cone-bound distance to the given bound which is smaller than the subcone 's visibility distance value In the case that the cone is a non-leaf cone and the bound is a non-leaf bound, the search algonthm conditionally explores subbounds of the given bound agamst the subcones of the given cone Consider a particular subcone of the given cone for the sake of discussion The subbounds of the given bound are conditionally explored agamst the subcone in ascending order of their cone-bound distances from the subcone Because the closest subbound is searched first, and potentially decreases the visibility distance value of the given subcone, succeedmg (more distant) subbounds will have more difficulty passmg the search condition, I e of having a cone-bound distance to the given subcone which is less than the visibility distance value of the subcone Thus, the probability is maximized that the fewest number of subbounds will need to be explored by ordering the conditional explorations according to cone-bound distance When the search of the two trees is completed, the object attπbute of each leaf-cone pomts to the object which is visible to the leaf-cone, and the visibility distance value of the leaf-cone specifies the distance to the visible object This visibility information is provided to the graphics accelerator so that the graphics accelerator may render the visible objects (or visible portions of visible object) on the display device In one embodiment, the visualization software provides for interactive visualization by readmg user inputs to control the cuπent viewpoint and view-direction in the graphics environment Additional software ensures efficient computation through the use of careful state management and parallelism In one alternative embodiment, the cone hierarchy and boundmg hierarchy are searched iteratively In a second alternative embodiment, a level order search is performed on the cone hierarchy and the boundmg hierarchy The present invention contemplates a wide variety of techniques for measuring the extent of separation or proximity between a bound and a cone One set of embodiments focus of minimizing an mcreasmg function of separation distance between the vertex of the cone and pomts m the intersection of the cone and the bound Another set of embodiments involve maximizing a decreasmg function of separation distance between the vertex of the cone and pomts in the mtersection of the cone and the bound In general, any wavefront with a boundary that obeys a mild "star shape" condition may provide the basis for a measurement of separation between a bound and a cone BRIEF DESCRIPTION OF DRAWINGS FIGURE 1 illustrates the ray-based method of visible object detection accordmg to the pπor art, FIGURE 2A illustrates a graphical computmg system accordmg to the present invention; FIGURE 2B is a block diagram illustrating one embodiment of the graphical computing system of the present invention; FIGURE 3 illustrates several main phases of visualization software accordmg to the present invention; FIGURE 4A illustrates a collection of objects m a graphics environment; FIGURE 4B illustrates a first step in the first formation of a hull hierarchy, I e. the step of boundmg objects with containing hulls and allocating hull nodes for the contammg hulls, FIGURE 4C illustrates the process of groupmg together hulls to form higher order hulls, and allocating nodes m the hull hierarchy which correspond to the higher order hulls, FIGURE 4D illustrates the culmination of the recursive groupmg process wherem all objects are contamed m a universal contammg hull which corresponds to the root node of the hull hierarhcy; FIGURE 5A illustrates the mathematical expressions which descπbe lines and half-planes m two dimensional space; FIGURE 5B illustrates the descπption of a rectangular region as the intersection of four half-planes m a two dimensional space, FIGURE 6 illustrates a two-dimensional cone partitioned mto a number of subcones which mteract with a collection of objects by means of wavefronts propagatmg withm each of the subcones; FIGURE 7 illustrates polyhedral cones with rectangular and triangular cross-section emanating from the oπgm; FIGURE 8A illustrates the mathematical expressions which descπbe a line through the ongm and a corresponding half-plane given a normal vector in two-dimensional space; FIGURE 8B illustrates the specification of a two-dimensional conic region as the intersection of two half-planes; FIGURES 9A-9C illustrate the formation of a cone hierarchy based on repeated subdivision of an initial cone with rectangular cross-section; FIGURE 10 illustrates a visibility search algorithm accordmg to the present invention; FIGURE 11 illustrates a method for displaying visible objects in a graphics environment; FIGURE 12 illustrates the process of recursively clustermg a collection of objects to form a boundmg hierarchy; FIGURE 13 illustrates the processmg steps to be performed when the visibility search amves at a terminal cone and a terminal bound; FIGURE 14 illustrates the processing steps to be performed when the visibility search amves at a terminal cone and a non-terminal bound; FIGURE 15 further elaborates step 712 of FIGURE 14, I e. the step of conditionally exploring subbounds of the first bound with respect to the first cone, FIGURE 16 further elaborates step 722 of FIGURE 15, i.e. the step of conditionally searchmg the subbounds of the first bound with respect to the first cone, by means of a geneπc subbound as an example, FIGURE 17 illustrates the processmg steps to be performed when the visibility search amves at a terminal bound and a non-terminal cone; FIGURE 18 further elaborates step 742 of FIGURE 17, i.e. the step of conditionally searchmg subcones of the first cone with respect to the first bound, by means of a genenc subcone example; FIGURE 19 illustrates the processmg steps to be performed when the visibility search arnves at a non-terminal bound and a non-terminal cone; FIGURE 20 further elaborates step 762 of FIGURE 19, l e. the step of conditionally exploring subbounds of the first bound with respect to the first subcone; and FIGURE 21 further elaborates step 772 of FIGURE 20, i.e. the step of conditionally searchmg the subbounds of the first bound with respect to the first subcone, by means of a generic subhull example MODE(S) FOR CARRYING OUT THE INVENTION Figure 2A presents a graphical computmg system 80 accordmg to the prefeπed embodiment of the present mvention. Graphical computmg system 80 compnses a system unit 82, and a display device 84 coupled to the system unit 82. The display device 84 may be realized by any various types of video momtors or graphical displays Graphics computer system 80 also mcludes a keyboard 86 and preferably a mouse 88 Figure 2B is a block diagram illustrating one embodiment of graphical computing system 80 As shown, graphical computmg system 80 includes a host central processmg unit (CPU) 102 coupled to a high speed bus or system bus 104. A system memory 106 is also preferably coupled to system bus 104 The system memory 106 may include any of vanous types of memory subsystems mcludmg random access memory, read only memory, and/or mass storage devices The host processor 102 executes a visualization software program which determines the set of visible objects m a scene. The visualization software program is stored in system memory 106. In an alternative embodiment, the visualization software program executes on a processor compπsed withm graphics accelerator 112. An optional 3-D graphics accelerator 112 may be coupled to system bus 104 If 3D accelerator 112 is not mcluded in graphical computmg system 80, then display device 84 may couple directly to system bus 104. It is assumed that various other peπpheral devices, or other buses, may be connected to system bus 104, as is well known m the art. Display device 84 couples to 3-D graphics accelerator 112. CPU 102 may transfer information to and from the graphics accelerator 112 accordmg to a programmed input output (I/O) protocol over the system bus 104 Furthermore, the graphics accelerator 112 may access system memory 106 accordmg to a direct memory access (DMA) protocol or through intelligent bus mastering. 3-D graphics accelerator 112 is a specialized graphics rendering subsystem which is designed to offload the 3-D rendering functions from the host processor, thus providing improved system performance. In a system with a 3-D graphics accelerator, a graphics application program executmg on the host processor 102 generates three-dimensional geometry data that defines three-dimensional graphics elements for output on a display device 84 The application program transfers the geometry data to the graphics accelerator 112. The graphics accelerator 112 receives the geometry data and renders the coπesponding graphics elements on the display device. A fundamental goal of computer graphics is to render objects that are realistic to the user. The problem of visual realism, however, is quite complex Objects m the "real world" include an incredible amount of detail, mcludmg subtle vaπations m color, reflections, and surface irregulanties. Applications which display three-dimensional graphics require a tremendous amount of processmg bandwidth For example, for a computer system to generate smooth 3-D motion video, the computer system is required to mamtam a frame rate or update rate of between 20 to 30 frames per second. This typically requires a 3-D graphics accelerator capable of processmg over a million tnangles per second. The computer system 80 mcludes visualization software accordmg to the present mvention. The visualization software is preferably executed by the host processor 102, or alternatively may be executed by a processor compπsed on the graphics accelerator 112 Visualization Software Architecture As illustrated m Figure 3, the visualization software of the present mvention compπses three mam phases. In an initial step 210, the visualization software loads a scene The scene mcludes a plurality of objects. Scene loadmg mcludes constructmg an object hierarchy — if it has not already been constructed. In step 220 the visualization software discovers the set of visible objects in the scene with respect to a cuπent viewpomt. In the prefeπed embodiment, the visualization software is designed to compute visibility for three-dimensional objects from a view pomt m a three-dimensional coordmate space. However, the methodologies herem descnbed naturally generalize to spaces of arbitrary dimension In one embodiment of the visualization software, the viewpomt m the graphical environment may be changed by user input. For example, by manipulating the mouse 88 and/or depressmg keys on keyboard 86, the user may cause the viewpoint and view oπentation to change Thus, the visualization software preferably recomputes the set of visible objects whenever the viewpomt or view oπentation changes In step 230 the visualization software m conjunction with graphics accelerator 112 displays the visible objects, i.e. commands the graphics accelerator to 112 to render those objects in the display screen. By performing visible object determination m software on the host computer, the graphics accelerator 112 may operate with greatly mcreased efficiency Thereby, the overall processmg capacity of the graphics system 80 is improved. Discovery and display of the visible-object-set occur repeatedly as the viewpomt or view direction (i.e. oπentation) changes. The discovery and display steps are preferably performed as concuπent processes However, m an alternative embodiment, they are performed sequentially Unless otherwise stated, it is assumed that all objects in the model are opaque convex polytopes. A three-dimensional solid is said to be convex if any two pomts in the solid (or on the surface of the solid) may be connected with a lme segment which resides entirely withm the solid Thus a solid cube is convex, while a donut is not. A polytope is an object with planar sides (e.g. cube, tetrahedron, etc.). The methodologies descnbed herem for opaque objects naturally extend to transparent or semi-transparent objects by not allowmg such objects to terminate a cone computation. The convexity assumption presents more of a problem. However, every object can be approximated as a union of convex polytopes. It is helpful to note that the visible-object-set computation does not require an exact computation, but rather a conservative one. In other words, it is permissible to over-estimate the set of visible objects Constructmg the Object Hierarchy Initially, the objects in the scene are organized into a hierarchy that groups objects spatially. An octree is one possibility for generating the object hierarchy However, m the prefeπed embodiment, a clustermg algonthm is used which groups nearby objects then recursively clusters pairs of groups mto larger contammg 8 spaces. The clustermg algonthm employs a simple distance measure and thresholding operation to achieve the object clustermg. Figures 4A-4D illustrate the clustermg process for a collection of four objects J00 through Jl l. The objects are mdexed in a fashion which anticipates their ultimate position m a binary tree of object groups. The objects are depicted as polygons situated m a plane (see Figure 4A) However, the reader should imagine these objects as arbitrary three-dimensional objects. In one embodiment, the objects are three- dimensional polytopes. Each object is bounded, 1 e. enclosed, by a coπespondmg boundmg surface refeπed to herem as a bound. In the prefeπed embodiment, the bound for each object is a polytope hull (i.e. a hull having planar faces) as shown m Figure 4B The hulls H00 through HI 1 are given labels which are consistent with the objects they bound. For example, hull H00 bounds object J00. The hulls are illustrated as rectangles with sides parallel to a parr of coordinate axes. These hulls are mtended to represent rectangular boxes (parallelopipeds) in three dimensions whose sides are normal to a fixed set of coordmate axes. For each hull a coπespondmg node data structure is generated The node stores parameters which characterize the coπespondmg hull. Smce a hull has a surface which is compnsed of a finite number of planar components, the descπption of a hull is mtimately connected to the descπption of a plane in three-space. In Figure 5A, a two dimensional example is given from which the equation of an arbitrary plane may be generalized A unit vector n [any vector suffices but a vector of length one is convenient for discussion] defines a lme L through the ongm of the two dimensional space. By takmg the dot product of a vector v with the unit vector n, denoted V • n , one obtams the length of the projection of vector v in the direction defined by unit vector n. Thus, given a real constant c it follows that the equation x ■ n — c , where x is a vector variable, defines a lme M peφendicular to lme L and situated at a distance c from the ongm along lme L. In the context of three-dimensional space, this same equation defines a plane peφendicular to the lme L, agam displaced distance c from the ongm along lme L. Observe that the constant c may be negative, m which case the line (or plane) M is displaced from the ongm distance c along lme L in the direction opposite to unit vector n. The lme x ■ n = c divides the plane into two half-planes. By replacmg the equality m the above equation with an inequality, one obtains the descnption of one of these half-planes. The equality x - n < c defines the half-plane which contams the negative infinity end of lme L. [The unit vector n defines the positive direction of lme L ] In three dimensions, the plane x - n = c divides the three-dimensional space mto two half- spaces. The inequality x • n < c defines the half-space which contams the negative infinity end of lme L. Figure 5B shows how a rectangular region may be defined as the intersection of four half-planes. Given four normal vectors «, through « 4 , and four coπespondmg constants c l through c 4 , a rectangular region is defined as the set of pomts which simultaneously satisfy the set of inequalities x • n t < C ( , where i ranges from one to four This system of inequalities may be summarized by the matrix- vector expression N • X < c , where the rows are matπx N are the normal vectors «, through n 4 , and the components of vector c are the coπespondmg constants c, through c 4 . If the normal vectors are chosen so as to e m the positive and negative axial directions (as shown m Figure 5B), the resultmg rectangular region has sides parallel to the axes. It is noted that the rectangular hulls H00 through HI 1 shown in Figure 4B all use a common set of normal vectors Thus, each hull is characterized by its unique c vector 9 In three-dimensional space, a rectangular box may be analogously defined as the intersection of six half-spaces. Given six normal vectors n through n 6 , oriented in each of the three positive and three negative axial directions, and six coπesponding constants c, through c 6 , the simultaneous solution of the inequalities X • n t < c where / runs from one to six, defines a rectangular box with sides parallel to the coordinate planes. Thus, a rectangular box may be compactly represented with the same matrix-vector expression NX < c , where matrix N now has six rows for the six normal vectors, and vector c has six elements for the six coπesponding constants. To construct an object hierarchy, object hulls H00 through HI 1 are paired together as shown in Figure 4C. Each pair of object hulls is bounded by a containing hull. For example, hulls H00 and HOI are paired together and bounded by containing hull HO. Containing-hull HO efficiently contains the two component hulls H00 and HOI. Likewise, object hulls H10 and HI 1 are paired together and bounded by containing-hull HI. In addition, two parent nodes are generated in the object hierarchy, one for each of the containing-hulls HO and HI. For simplicity, the parent nodes are commonly labeled as their coπesponding containing-hulls. Thus, parent node HO points to its children nodes H00 and HOI, while parent node HI points to its children nodes H10 and HI 1. Each parent node contains the characterizing c vector for the coπesponding containing-hull. The containing-hulls HO and HI may be refeπed to as first order containing-hulls since they are the result of a first pairing operation on the original object hulls. A second pairing operation is applied to the first- order containing hulls to obtain second-order containing-hulls. Each second-order containing-hull efficiently contains two first-order hulls. For each of the second-order containing-hulls a parent node is generated in the object hierarchy. The parent node reflects the same parent-child relationship as the coπesponding second-order containing-hull. For example, in Figure 4D, second-order containing-hull H efficiently contains first-order containing-hulls HO and HI. Thus, parent node H in the object hierarchy points to children nodes HO and HI. Parent node H stores the characterizing vector c for the containing-hull H. In the example presented in Figures 4A-4D, the object hierarchy is complete after two pairing operations since the original object collections contained only four objects. In general, a succession of pairing operations is performed. At each stage, a higher-order set of containing-hulls and coπesponding nodes for the object hierarchy are generated. Each node contains the describing vector c for the coπesponding containing-hull. At the end of the process, the object hierarchy comprises a binary tree with a single root node. The root node coπesponds to a total containing-hull which contains all sub-hulls of all orders including all the original object-hulls. The object hierarchy, because it comprises a hierarchy of bounding hulls, will also be refeπed to as the hull hierarchy. In the prefeπed embodiment, the pairing operations are based on proximity, i.e. objects (and hulls of the same order) are paired based on proximity. Proximity based pairing results in a more efficient visible object determination algorithm. This tree of containing hulls provides us with a computationally efficient, hierarchical representation of the entire scene. For instance, when a cone completely misses a node's containing-hull, none of the node's descendents need to be examined. Bounding hulls (i.e. containing hulls) serve the puφose of simplifying and approximating objects. Any hierarchy of containing hulls works in principle. However, hierarchies of hulls based on a common set of 10 normal vectors are particularly efficient computationally. A collection of hulls based on a common set of normal vectors will be refeπed to herem as a fixed-direction or commonly-generated collection. As descnbed above, a polytope hull is descnbed by a boundmg system of lmear inequalities {x : NX ≤ c} , where the rows of the matnx N are a set of normal vectors, and the elements of the vector c define the distances to move along each of the normal vectors to obtam a coπespondmg side of the polytope In a fixed-direction collection of hulls, the normal matnx N is common to all the hulls in the collection, while the vector c is unique for each hull in the collection. The problem of calculating the coefficient vector c for a contammg hull given a collection of subhulls is greatly simplified when a common set of normal vectors is used. In addition, the nodes of the hull hierarchy may advantageously consume less memory space since the normal matrix N need not be stored m the nodes. In the prefeπed embodiment of the mvention, the hull hierarchy compπses a fixed-direction collection of hulls. In a first embodiment, six normal vectors onented m the three positive and three negative axial directions are used to generate a fixed-direction hierarchy of hulls shaped like rectangular boxes with sides parallel to the coordmate planes These axis-aligned boundmg hulls provide a simple representation that has excellent local computational properties. It is easy to transform or compare two axis-aligned hulls However, the approximation provided by axis-aligned hulls tends to be rather coarse, often proving costly at more global levels. In a second embodiment, eight normal vectors directed towards the corners of a cube are used to generate a hierarchy of eight-sided hulls For example, the eight vectors (± 1, ± 1, ± l) may be used to generate the eight-sided hulls. The octahedron is a special case of this hull family In a third embodiment, fourteen normal vectors, i.e. the six normals which generate the rectangular boxes plus the eight normals which generate the eight-sided boxes, are used to generate a hull hierarchy with fourteen-sided hulls. These fourteen-sided hulls may be descnbed as rectangular boxes with corners shaved off. It is noted that as the number of normal vectors and therefore side increases, the accuracy of the hull's approximation to the underlymg object increases. In a fourth embodiment, twelve more normals are added to the fourteen normals just descnbed to obtam a set of twenty-six normal vectors. The twelve additional normals serve to shave off the twelve edges of the rectangular box m addition to the corners which have already been shaved off This results m twenty-six sided hulls. For example, the twelve normal vectors (± 1, ± 1, 0) , (± 1, 0, ± l) , and (0, ± 1, ± l) may be used as the additional vectors. In the examples given above, hulls are recursively grouped in pairs to generate a binary tree. However, in other embodiments, hulls are grouped together m groups of size G, where G is larger than two. In one embodiment, the group size varies from group to group. This may be particularly advantageous for scenes which have non-uniform object density. For example, if a large number of small objects are clustered in a scene, it may be advantageous to include these m a smgle group, i.e. bound them with a smgle contammg hull. Larger objects may be assembled mto groups with fewer members. Although the above discussion has focussed on the use of polytope hulls as bounds for object and clusters, it is noted that any type of boundmg surfaces may be used, thereby generatmg a hierarchy of bounds refeπed to herem as a boundmg hierarchy. Each node of the boundmg hierarchy coπesponds to an object or cluster and stores parameters which characterize the coπespondmg bound for that object or cluster. For 1 1 example, polynomial surfaces such as quadratic surfaces may be used to generate bounds for objects and/or clusters. Spheres and ellipsoids are examples of quadratic surfaces Cones m Visible Object Determination In addition to the bounding hierarchy (e.g. hull hierarchy) discussed above, the present mvention makes use of a hierarchy of spatial cones. An initial cone which represents the view frustum is recursively subdivided mto a hierarchy of sub-cones. Then a simultaneous double recursion is performed through the pair of trees (the object tree and cone tree) to rapidly determine the set of visible objects. This cone-based method provides a substantial computational gam over the pπor art method based on ray-casting. This is partially due to the fact that the ray-castmg methods require multiple unrelated visibility quenes to cover a region equivalent to a cone. Cones discretize the spatial continuum differently than rays. Consider the simultaneous propagation of all possible rays from a pomt and the ensumg sphencal wavefront The first object encountered by each ray is visible. If consideration is restncted to those rays that form a cone, the same observation still applies. The first object encountered by the cone's wavefront is visible. Now, if the view frustum is partitioned mto some number of cones, the objects visible from the viewpomt can be determined up to the resolution of the cones. Figure 6 illustrates a two-dimensional cone C in a two-dimensional environment. Cone C is defined by the region mteπor to the rays Rl and R2 (and inclusive of those rays). The cone C is partitioned mto a number of sub-cones. The ambient space is populated with a collection of two-dimensional objects. A wavefront propagates withm each of the subcones Each wavefront terminates upon its first interaction with an object. Three of the subcones are especially highlighted, i.e. subcones Cl, C2 and C3. Subcone Cl mteracts with two objects, i.e an ellipse and a star shaped object. However, the wavefront propagatmg withm subcone Cl terminates, by definition, upon its first mteraction with an object. Smce the wavefront interacts with the ellipse first, the wavefront terminates on the ellipse, and the star object is defined to be invisible with respect to this subcone. If higher resolution is desired, subcone Cl may be further partitioned mto a set of smaller cones. Some of these smaller cones would then terminate on the ellipse, while others would terminate on the star- shaped object Subcone C2 mteracts with only one object, l e. object B. The wavefront propagatmg with subcone C2 terminates at the pomt of first contact with object B. It is noted that the cone-based object visibility query (modeled on the wavefront propagation concept) is an inherently spatial computation. Thus, the object visibility query for subcone C3 detects the small object A even though this object lies entirely m the mtenor of subcone C3. The visibility query computation will be explamed m detail later. Polyhedral Cones The spatial cones used in the prefeπed embodiment are polyhedral cones. The genenc polyhedral cone has a polygonal cross-section. Figure 7 give two examples of polyhedral cones. The first polyhedral cone PCI has a rectangular cross-section, while the second polyhedral cone PC2 has a tnangular cross-section. The view frustum is a cone with rectangular cross-section like cone PCI. Polyhedral cones are defined by homogeneous lmear inequalities. Given a normal vector n, the equation n ■ x = 0 mvolvmg vector argument x defines a plane passmg through the ongm and peφendicular to the normal vector n This plane divides space mto two half- spaces. The linear inequality n ■ x < 0 defines the half-space from which the normal vector n pomts outward. 12 Figure 8A gives a two-dimensional example As shown, the equation n • x = 0 specifies the set of pomts (rnteφreted as vectors) which are peφendicular to normal n This peφendicular line L divides the plane mto two half-planes The half-plane defined by the inequality n ■ X < 0 is denoted by shadmg Observe that the normal vector n pomts out of this half-plane A polyhedral cone is constructed by intersection of multiple half-spaces For example, solid cone PC2 of Figure 7 is the mtersection of three half-spaces Similarly, solid cone PCI is the intersection of four half- spaces Figure 8B provides a two-dimensional example of intersecting half-planes to generate a conic area The two normal vectors «, and n 2 define peφendicular lmes Z, and L 2 respectively The inequality «, • X < 0 specifies the half-plane which is south-west (I e left and below) of the lme L χ The inequality n 2 ■ X < 0 defines the half-plane which is to the πght of line L 2 The solution to the simultaneous system of mequalities n χ ■ X < 0 and n 2 • x < 0 is the mtersection region denoted m shadmg This system of mequalities may be summarized by the matrix equation Sx < 0 , where the rows of matrix S are the normal vectors From this discussion, it may be observed that solid cone PCI of Figure 7 is determined by four normal vectors The normal matrix S would then have four rows (for the four normal vectors) and three columns coπespondmg to the dimension of the ambient space Thus, a polyhedral cone emanatmg from the ongm is defined as the set of pomts satisfying a system of lmear mequalities Sx < 0 [There is no loss of generality m assuming the ongm to be the viewpomt ] Accordmg to this definition, half-spaces, planes, rays, and the ongm itself may be considers as polyhedral cones In addition, the entire space may be considered to be a polyhedral cone, I e that cone which is defined by an empty matnx 5 Distance Measurement In view of the discussion concerning wave propagation, the distance of a object, hull, or bound from a particular viewpomt is defined to be the mmimum distance to the object, hull, or bound from the viewpomt So, assuming a viewpomt at the ongm, the distance of the object, hull, or bound X from the viewpomt is defined as /(N) = mm|xj where X is the norm of vector x When the object, hull, or bound X is empty, the distance is taken to be positive infinity Any vector norm may be chosen for the measurement of distance In one embodiment, the Euclidean norm is chosen for distance measurements The Euclidean norm results m a sphencally shaped wavefront Any wavefront shape may be used as long as it satisfies a mild "star-shape" cπtenon the entire boundary of the wavefront must be unobstructed when viewed from the ongm All convex wavefronts satisfy this condition, and many non-convex ones do as well In general, the level curves of a norm are recommended as the wavefront shapes From a computational standpoint, the spherical wavefront shape given by the L 2 norm, and the piecewise-lrnear wavefront shapes given by the L„ and L ∞ norms provide good choices for visibility detection 13 Not suφnsmgly, piecewise- near approximations of such norms, for example the approximation to the L 2 norm also works well: such an approximation is illustrated m Figure 6 Cones and Visibility From a viewpoint located withm a large set of objects, there exists at least one pomt (on some object) nearest to the viewpomt. Smce that point (or set of pomts) is closest to the viewpomt, nothing can occlude the view of that pomt (or those pomts). This implies that the object (or objects) contammg the nearest pomt (or pomts) is (are) at least partially visible. Now, consider an arbitrary cone K emanating from the ongm as a viewpomt. The unobstructed visibility argument holds even if all distance measurements are restricted to points that fall within the cone. Define the distance of an object, hull, or bound X relative to the cone K as Λ (N) = xmeλinnK" x " . If the distance f κ is computed for each object m a scene, the nearest object, l e. the object which achieves a mmimum distance value, is visible with respect to cone K. In fact, an object X is visible if and only if there exists a cone K such that (a) f κ (X) is finite and (b) f κ (X) ≤ f κ (Y) holds for all objects Y m the set of objects. It is noted that rays may be viewed as degenerate cones that emanate from the viewpomt and pass through the center of each pixel. The nearest object along each ray is visible and thus determines the value of the coπespondmg pixel. Similarly, it is possible to construct cones which emanate from the viewpomt and cover each pixel. For example, two or more cones with triangular cross-section may neatly cover the area of a pixel. The nearest object withm each cone is visible and generates the value of the corresponding pixel. As discussed above, the ray-based methods of the prior art are able to detect objects only up the resolution of the ray sample. Small visible objects or small portions of larger objects may be missed entirely due to msufficient ray density. In contrast, cones can completely fill space. Thus, the cone-based method of the present mvention may advantageously detect small visible objects or portions of objects that would be missed by a ray-based method with equal angular resolution Generalized Separation Measurement For the puφoses of performing a visibility search procedure, it is necessary to have a method for measurmg the extent of separation (or conversely proximity) of objects, bounds, or hulls with respect to cones. There exists a great vanety of such methods m addition to those based on minimizing vector norms defined above As alluded to above, a measurement value indicating the extent of separation between a set X and a cone K may be obtamed by propagatmg a wavefront internal to the cone from the vertex of the cone and observing the radius of first mteraction of the internal wavefront with the set X. As mentioned above, the wavefront must satisfy a mild "star shape" condition: the entire boundary of the wavefront must be visible from the vertex of the cone. In one embodiment, the measurement value is obtamed by computmg a penalty of separation between the set X and the cone K The penalty of separation is evaluated by minimizing an increasing function of 14 separation distance between the vertex of the cone K and points in the mtersection of the cone K and set X For example, any positive power of a vector norm gives such an mcreasmg function In another embodiment, the measurement value is obtamed by computmg a meπt of proximity between the set X and the cone K The merit of separation is evaluated by maximizing a decreasmg function of separation distance between the vertex of the cone K and pomts m the mtersection of the cone K and set X For example, any negative power of a vector norm gives such a decreasmg function A Cone Hierarchy The visibility determination method of the present mvention relies on the use of a hierarchy of cones m addition to the hierarchy of hulls descnbed above The class of polyhedral cones is especially well suited for generating a cone hierarchy polyhedral cones naturally decompose mto polyhedral subcones by the insertion of one or more separatmg planes The ability to nest cones mto a hierarchical structure allows a very rapid examination of object visibility As an example, consider two neighbonng cones that share a common face By taking the union of these two cones, a new composite cone is generated The composite cone neatly contains its children, and is thus capable of bemg used m querying exactly the same space as its two children In other words, the children cones share no intenor pomts with each other and they completely fill the parent without leavmg any empty space A typical display and its associated view frustum has a rectangular cross-section There are vast aπay of possibilities for tessellating this rectangular cross-section to generate a system of sub-cones For example, the rectangle naturally decomposes mto four rectangular cross-sections, or two tnangular cross-sections Although these examples illustrate decompositions usmg regular components, iπegular components may be used as well Figures 9A-9C illustrate a hierarchical decomposition of an initial view frustum C Figure 9A depicts the rectangular cross-section of the view frustum and its bisection mto two cones with triangular cross-section, l e cones CO and Cl The view frustum C coπesponds to the root node of a cone tree Cones and their coπespondmg nodes in the cone tree are identically labeled for simplicity Each node of the cone tree stores the matnx S of normal vectors which generates the coπespondmg cone The root node pomts to two children nodes coπespondmg to cones CO and Cl Figure 9B illustrates a second decomposition stage Each of the cones CO and Cl is bisected mto two sub-cones (agam with tnangular cross-section) Cone CO decomposes into the two sub-cones COO and C01 Likewise, cone Cl is bisected mto two sub-cones CIO and Cl l Nodes are added to the cone tree to reflect the structure of this decomposition The parent-child relation of nodes m the cone tree reflects the superset-subset relation of the respective cones in space Figure 9C illustrates the pattern of successive cone bisections accordmg to one embodunent Each cone m the hierarchy may be decomposed mto two sub-cones by means a bisectmg plane Figure 9C illustrate several successive descending bisections which generate cones CO, CIO, C110, and Cl l 10, and so on The initial cone C (l e the view frustum) may be decomposed to any desired resolution In one embodunent, the bisections terminate when the resultant cones mtercept some fraction of a pixel such as, for example, V a pixel The coπespondmg terminal nodes of the cone tree are called leaves Alternate embodiments are contemplated where the bisections terminate when the resultant leaf-cones mtercept areas which subtend (a) a portion of pixel such as 1/N where N is a positive mteger, or (b) areas mcludmg one or more pixels 15 The triangular hierarchical decomposition shown m Figures 9A-9C has a number of useful properties. By decomposmg the oπgmal rectangular cone based on recursive bisection, a binary tree of cones of arbitrary depth is generated. Triangular cones have the fewest sides making them computational more attractive. Lastly, tnangular cones can also tessellate the unit cube (or unit sphere) Thus, it is possible to create a hierarchical cone representation for the entire space suπoundmg the viewpomt. It is noted that any cone decomposition strategy may be employed to generate a cone hierarchy. In a second embodiment, the view frustum is decomposed mto four similar rectangular cones; each of these subcones is decomposed mto four more rectangular subcones, and so on This results m a cone tree with four-fold branches Discovering the Set of Visible Objects Once the hull hierarchy and the cone hierarchy have been constructed, the set of visible objects is computed from the cuπent viewpomt. In one embodiment, the visible object set is repeatedly recomputed for a succession of viewpoints and viewmg directions. The successive viewpomts and viewmg directions may be specified by a user through an mput device such as a mouse, joystick, keyboard, trackball, or any combmation thereof. The visible object determination method of the present mvention is organized as a simultaneous search of the hull tree and the cone tree. The search process mvolves recursively performing cone-hull queries Given a cone node K and a hull node H, a cone-hull query on cone K and hull H mvestigates the visibility of hull H and its descendent hulls with respect to cone K and its descendent cones. The search process has a computational complexity of order log M, where M equals the number of cone nodes times the number of hull nodes. In addition, many cone-hull queπes can occur m parallel allowmg aggressive use of multiple processors m constructmg the visible-object-set. Viewmg the Scene Independently, and also concurrently, the set of visible objects from the cuπent viewpomt may be rendered on one or more displays. The rendeπng can occur concuπently because the visible-object-set remams fairly constant between frames m a walkthrough environment. Thus the previous set of visible objects provides an excellent approximation to the cuπent set of visible objects. Managmg the Visible-Object-Set The visualization software must manage the visible-object-set Over time, as an end-user navigates through a model, just inserting objects into the visible-object-set would result m a visible-object-set that contams too many objects. To ensure good rendeπng performance, the visualization process must therefore remove objects from the visible-object-set when those objects no longer belong to the set — or soon thereafter. A vanety of solutions to object removal are possible. One solution is based on object aging. The system removes any object from the visible-object-set that has not been rediscovered by the cone query withm a specified number of redraw cycles. 16 Computmg Visibility Using Cones Substantial computation leverage is provided by recursively searching the hierarchical tree of cones m conjunction with the hierarchical tree of hulls. Whole groups of cones may be tested agamst whole groups of hulls m a single query. For example, if a parent cone does not mtersect a parent hull, it is obvious that no child of the parent cone can mtersect any child of the parent hull. In such a situation, the parent hull and all of its descendants may be removed from further visibility considerations with respect to the parent cone Visibility Search Algonthm In the prefeπed embodiment, the visibility search algonthm of the present mvention is realized m a visibility search program stored m memory 106. Thus, heremafter the terms visibility search program and visibility search algonthm will be used mterchangeably. The visibility search program performs a recursive search of the two trees (the object tree and the cone tree) to assign visible objects to each of the leaf cones of the cone tree. The host processor 102 executes the visibility search program. In an alternate embodiment, the visibility search program executes on a processor compnsed withm the graphics accelerator 112 The recursive search of the two trees provides a number of opportunities for aggressive pruning of the search space. Central to the search is the object-cone distance measure defined above, i.e given a cone K and an object (or hull) X, the object-cone distance is defined as It is noted that this minimization is m general a nonlmear programmmg problem smce the cones and object hulls are defined by constramt equations, i.e. planes in three-space. If the vector norm x is the L norm (i.e. the norm defined as the sum of absolute values of the components of vector x), the nonlmear programmmg problem reduce to a lmear programming problem If the vector norm is the Euclidean norm, the nonlmear programmmg problem reduce to a quadratic programmmg problem Given a collection of objects, the object X which achieves the smallest distance f κ (X) is closest to the cone's viewpomt, and therefore must be at least partially visible. The recursive search starts with the root H of the hull tree and the root C of the cone tree (see Figures 4 and 9). Remember that each node of the hull tree specifies a boundmg hull which contams the hulls of all its descendant nodes. Initially the distance between the root cone and the root hull is computed. If that distance is infinite, then no cone in the cone hierarchy mtersects any hull m the hull hierarchy and there are no visible objects. If the distance is finite, then further searchmg is required. Either tree may be refined at this pomt. In the prefeπed embodiment, both trees are refined m a predefined order. The pruning mechanism is built upon several basic elements. A distance measurement function computes the distance f κ (X) of a hull X from the viewpomt of a cone K In other words, the function determines the cone-restricted distance to the hull X In the prefeπed embodiment, the minimization associated with evaluatmg the distance measurement function is implemented solvmg an associated lmear programmmg problem. 17 To facilitate the search process, each leaf-cone, 1 e each terminal node of the cone tree, is assigned an extent value which represents its distance to the closest known object-hull Thus, this extent value may be refeπed to as the visibility distance The visibility distance of a leaf-cone is non-increasing, 1 e it decreases as closer objects (1 e object hulls) are discovered m the search process Visibility distances for all leaf-cones are initialized to positive infinity [An object-hull is a hull that directly bounds an object Thus, object-hulls are terminal nodes of the hull tree ] In addition to a visibility distance value, each leaf-cone node is assigned storage for an cuπently visible object This object attπbute is initialized to NO OBJECT or BACKGROUND dependmg upon the scene context In addition, each non-leaf cone, 1 e each cone at a non-final refinement level, is assigned an extent value which equals the maximum of its sub-cones Or equivalently, the extent value for a non-leaf cone equals the maximum of its leaf-cone descendents These extent values are also refeπed to as visibility distance values The visibility distance values for all non-leaf cones are initialized to positive infinity also (consistent with initialization of the leaf-cones) Suppose a given non-leaf cone K and a hull H achieve a cone-object distance f κ (X) If this distance f κ (X) is greater than the visibility distance value of the cone K, then all of the leaf- cone descendents of cone K already have known objects closer than the hull H Therefore, no benefit can be gamed by searchmg hull H agamst cone K and its descendents In contrast, if a hull H achieves a distance f κ (X) from cone K which is less than the visibility distance value of cone K, it is possible that hull H contams objects which will stnctly decrease the visibility distance of some leaf-cone descendent of cone K Thus, the hull H and its descendents should be searched agamst cone K and its descendents The followmg code fragment illustrates the beginning of the search process The vanables hullTree and coneTree pomt to the root nodes of the hull tree and cone tree respectively mam(hullTree, coneTree) { cone extent = infinity, distance = DIST(hullTree, coneTree), ιf(dιstance < infinity) findVιsιble(hullTree, coneTree, distance), } The DIST function evaluates the distance between the root hull and the root cone If this distance is less than positive infinity, the function findVisible is called with the root hull, root cone, and their hull-cone distance as arguments The function findVisible performs the recursive search of the two trees Figures 10A-10C illustrate one embodiment of the findVisible function (also refeπed to by the contraction FV) Refer to the Appendix for a source code embodiment of the findVisible function The findVisible function receives several mput vaπable from a callmg routme as illustrated by the followmg function prototype fιndVιsιble( H, C, d) where H is a hull node to be explored agamst the cone node C The value d represents the cone-hull distance between cone C and hull H In step 300, the findVisible function determines whether the cone C and hull H are both leaf nodes, I e terminal nodes of their respective trees If so, the visibility distance value d c for the leaf- 18 cone C is set equal to the cuπent distance to object hull H In addition, the object attribute of leaf-cone C is assigned the value of hull H Then control is returned to the callmg routme. If the hull H and cone C are not both leaves, step 302 is performed. In step 302, the findVisible function determines if the cone C is itself a leaf node of the cone tree. If so, steps 304 through 310 are performed. In step 304, the children of hull H are mvestigated, i.e. the distances of cone C to the subhulls HO and HI are compared. [The children of hull H will be geneπcally refeπed to as HO and HI.] If the distance to subhull HO is smaller than the distance to subhull HI, steps 306 and 308 are performed Otherwise, step 310 is performed. Step 304 is performed m order to determine which subhull will be allowed the opportunity of first mteraction with cone C. In the prefeπed embodiment of the mvention, the closer subhull is searched first. In step 306, the findVisible function determines the cone-hull distance of subhull HO from cone C is less than the visibility distance value dc of cone C. If so, the findVisibility function is called agam m order to search subhull HO agamst cone C. The cone-hull distance between cone C and subhull HO is provided as the distance argument for the function call. If the distance to subhull HO is not less than the cuπent visibility distance, step 308 is performed. In step 308, the findVisible function determines if the cone-hull distance of subhull HI from cone C is smaller than the visibility distance value of cone C. If it is smaller, the findVisibility function is called agam m order to search subhull HI agamst cone C. The cone-hull distance between cone C and subhull HI is provided as the distance argument for the function call. If the distance to subhull HI is not less than the cuπent visibility distance, program control returns to the callmg program. It is noted that the call to the function findVisible in step 306 may cause the visibility distance value for cone C to decrease. Thus, the visibility distance value for cone C may actually have a smaller value when tested m step 308 than when tested m step 306. This underscores the importance of the test performed m step 304 to determine the subhull which is closer to cone C The closer hull is searched first and allowed to preempt the more distant subhull In other words, after searchmg the closer subhull, the more distant subhull may not need to be searched. Thus, the distance conditioned search order for subhulls advantageously improves the efficiency of the visibility search algorithm In step 310, the subhulls are searched in the opposite order m response to the determination of step 304 that subhull HO achieves a cone-hull distance from cone C which is not less than the coπespondmg distance for subhull HI. Agam, the closer subhull HI is searched first and allowed to preempt the search of the farther subhull HO. If, m step 302, it is determined that cone C is not a leaf, step 320 is performed (see Fig. 10B). In step 320, the findVisibility function determines if the hull node H is a leaf node, i.e. an object hull, of the hull hierarchy If so, the children of cone C are explored with respect to object hull H m steps 322 through 326. In step 322, the findVisibility function determines if the cone-hull distance between hull H and child subcone CO is less than the visibility distance value of child subcone CO. If so, the findVisibility function is called agam m order to explore hull H with respect to subcone CO Thus, hull H, subcone CO and the cone-hull distance between these two are supplied to the findVisibility function as arguments When the findVisibility function returns, control passes to step 324 Also, if the inequality test of step 322 fails, control passes to step 324 19 In step 324, the findVisibility function determmes if the cone-hull distance of hull H from subcone Cl is less than the visibility distance value for subcone Cl. If so, the findVisibility function is called again in order to explore hull H with respect to subcone Cl. Thus, hull H, subcone Cl and the cone-hull distance between these two are supplied to the findVisibility function as arguments. When the findVisibility function returns, control passes to step 326. Also, if the inequality test of step 324 fails, control passes to step 326. Smce the visibility distance values for subcones CO and Cl may have decreased by the interactions with hull H m steps 322 and 324 respectively, the visibility distance value for cone C must be updated Thus, m step 326, the visibility distance value for cone C is set equal to the maximum of the visibility distance values of subcones CO and Cl. After step 326, program control returns to the callmg routme. If, m step 320, the hull node H is determined not to be a leaf node, then step 330 is performed (see Fig IOC). Thus, step 330 and succeedmg steps may assume that neither hull H nor cone C are leaf nodes m their respective hierarchies. Steps 332, 334, 335 and 336 explore the children of hull H with respect to cone CO conditioned upon the result of testmg step 330. Similarly, step 340 explores the children of hull H with respect to subcone Cl conditioned upon the result of testmg step 338 In step 330, the findVisibility function determmes if the cone-hull distance between hull H and subcone CO is less than the cuπent visibility distance value of subcone CO If so, step 332 is performed. In step 332, the cone-hull distances of subhulls HO and HI from subcone CO are compared The closer subhull is given the opportunity of first mteraction with subcone CO. Thus, if the cone-hull distance of subhull HO from subcone CO is smaller than the cone-hull distance of subhull HI from subcone CO, step 334 is performed. Otherwise step 336 is performed. In step 334, the findVisibility function determmes if the cone-hull distance of subhull HO from subcone CO is less than the cuπent visibility distance value of subcone CO If so, the findVisibility function is called agam in order to explore subhull HO agamst subcone CO. Thus, subhull HO, subcone CO, and the cone-hull distance between these two are provided as arguments to the findVisibility function When this call to the findVisibility function returns, step 335 is performed. If, m step 334, it is determined that the cone-hull distance of subhull HO from subcone CO is not smaller than the visibility distance value of subcone CO, step 335 is performed. In step 335, the findVisibility function determmes if the cone-hull distance of subhull HI from subcone CO is smaller the visibility distance value of subcone CO. If so, the findVisibility function is called agam m order to explore subhull HI with respect to subcone CO. Thus, subhull HI, subcone CO, and the cone-hull distance between them are provided as arguments to the findVisibility function When this call to the findVisibility function returns, program control passes to step 338. Additionally, if the inequality test of step 335 fails, step 338 is performed. Smce the closer subhull HO is allowed the first mteraction with cone CO, it may alter (l e. decrease) the visibility distance value of subcone CO. Thus, the farther subhull HI may have more difficulty in passmg the inequality test of step 335 due to the pnor mteraction of subhull HO In other words, the exploration of subhull HI with respect to subcone CO may not need to be performed, and advantageously skipped, m view of the pnor interaction with the closer subhull HO Thus, the search efficiency of the visibility determination method is improved by avoidmg unnecessary subhull explorations. 20 Step 336 is performed in response to a determination m step 332 that the cone-hull distance of subhull HO from subcone CO is not less than the cone-hull distance of subhull HI from subcone CO. In other words, step 336 is performed when subhull HI is closer to subcone CO than subhull HO Step 336 mvolves operations similar to those performed m steps 334 and 335. However, the conditional search of subhull HI to performed before the conditional search of subhull HO, i.e. the closer subhull is searched first. After step 336 control passes to step 338. If, in step 330, it is determined that the cone-hull distance of hull H from cone CO is not less than the visibility distance of subcone CO, control passes to step 338. In step 338, the findVisibility function determmes if the cone-hull distance of hull H from subcone Cl is less than the visibility distance value of subcone Cl. If so, step 340 is performed. Step 340 performs conditional exploration of subhulls HO and HI with respect to subcone Cl. Thus, step 340 mvolves operations similar to those descnbed m steps 332, 334, 335 and 336 with the roles of subcone CO and subcone Cl are mterchanged After step 340, step 342 is performed. In addition, if the inequality test of step 338 fails, step 342 is performed In step 342, the visibility distance value of cone C is set equal to the maximum of the visibility distance values of subcones CO and Cl . After step 342, program control returns to the callmg routme As explamed above, the visibility distance value assigned to each cone in the cone tree equals the maximum of the visibility distance values assigned to its subcone children. Thus, if a given hull node achieves a distance to a cone which is larger than the cone's cuπent visibility distance value, it is immediately apparent that none of the hulls descendents will be of mterest to any of the cone's descendents The given hull node may be skipped so far this cone is concerned. A cone's visibility distance value decreases as the recursion tests more and more object-cone leaf parrs. As nearby objects are discovered, a cone's visibility distance value decreases and the probability of skipping unpromising hull nodes mcreases. A leaf in the hull tree bounds the volume of the associated object and also approximates that object's contents. Thus, cone visibility distance values, set during recursion, are usually not the real distances to objects but a conservative approximation of those distances. If the conservative approximation is inadequate for use in an application, then that application can invoke an exact-computation for the visibility distance values. Throughout the above discussion of the visibility search algorithm it has been assumed that the DIST function used to compute cone-hull separation distance is based on minimizing an increasmg function of separation distance between the vertex of the given cone and pomts m the mtersection of the given cone and the given bound/hull. However, it is noted that the DIST function may be programmed to compute a ment of proximity between a given cone and given bound hull. The resulting ment value increases with mcreasmg proximity and decreases with mcreasmg separation, converse to the typical behavior of a distance function. In this case, the visibility search algorithm performs a search of bound/hull H against cone K only if the ment value of separation between cone K and bound hull H is greater than the cuπent merit value associated with cone K. Furthermore, after a search of subcones of cone K is completed, the ment value associated with the cone K is updated to equal the mmimum of the merit values of its subcone children. In general, the DIST function determmes a cone-hull measurement value of separation by computmg the extremum (i.e. mmimum or maximum) of some monotonic (mcreasmg or decreasing) function of separation 21 between the vertex of the cone K and pomts in the mtersection of cone K and bound/hull H. The search of cone K agamst a bound/hull H is conditioned on the bound/hull H achievmg a cone-hull measurement value with respect to cone K which satisfies an inequality condition with respect to measurement value assigned to cone K. The sense of the inequality, i.e. less than or greater than, depends on the whether the DIST function uses an mcreasmg or decreasmg function of separation. While the search of the hull and cone hierarchies descnbed above assumes a recursive form, it is noted that any search strategy may be employed. In one alternate embodiment, the hull and/or cone hierarchies are searched iteratively. Such a brute force solution may be advantageous when a large aπay of processor is available to implement the iterative search. In another embodiment, a level-order search is performed on the hull and/or cone hierarchies. Method for Displaying Visible Obiects A method for displaying visible objects m a graphics environment is descnbed m the flowchart of Figure 11. A visibility search algonthm executing on CPU 102 or optionally graphics accelerator 112 controls the determination of a set of visible objects for display on display screen 84. In step 501, a collection of objects is accessed from memory 106. In step 502, a boundmg hierarchy is constructed from the collection of objects. The construction of the bounding hierarchy will be descnbed m detail below m conjunction with Figure 12. In step 503 a cone hierarchy is constructed by refining an initial cone. Refer to the discussion above in connection with Figures 6-9 for more information on construction of the cone hierarchy. In one embodiment, the initial cone is the entire space. In a second embodiment, the initial cone coπesponds to a neighborhood which contams the view frustum and a neighborhood around the view frustum. Thus, if the viewmg onentation (i.e. direction) changes so that the new view frustum remains withm the neighborhood of the initial cone, the cone hierarchy may not need to be reconstructed, thereby savmg computational bandwidth. Each cone the cone hierarchy is allocated a measurement value which coπesponds to totally un-occluded visibility In step 504, a search of the cone hierarchy and the boundmg hierarchy is performed. The search procedure determmes a nearest object for each of the leaf cones of the cone hierarchy. It is noted that some leaf cones may not mteract with any object m the given collection In this case, the nearest object may be rnteφreted to be the background. The search procedure performs conditional explorations of cone-bound pairs starting with the root cone of the cone hierarchy and the root bound of the boundmg hierarchy Given a first cone in the cone hierarchy and a first bound m the bounding hierarchy, the search algonthm determines a first measurement value which mdicates an extent of separation between the first cone and the first bound as shown in step 504A In step 504B, the first measurement value is compared to a measurement value associated with the first cone. If the first measurement value satisfies an inequality condition with respect to the measurement value associated with the first cone, the first bound is searched with respect to the first cone as shown m step 504C. In general, the measurement value associated with a non-leaf cone reflects the maximum of the measurement values of its leaf-cone descendents, and the measurement value associated with a leaf-cone mdicates the extent of separation between the leaf-cone and its nearest known object Therefore, no benefit can be gamed by searchmg the first bound with respect to the first cone if the comparison of step 504B mdicates that the first 22 bound is farther than the nearest known object for each of the leaf-cone descendents of the first cone Steps 504B and 504C are together refeπed to as a conditioned search Thus, the first bound and first cone are explored only if there is evidence that the first bound is closer than the nearest known object for at least one leaf descendent of the first cone In step 506, the nearest objects for a subset of cones of the cone hierarchy are displayed on display device 84 In the prefeπed embodiment, the subset of cones compnses the leaf-cones of the cone hierarchy, l e the nearest object for each leaf-cone m the cone hierarchy is displayed on display device 84 Tremendous processmg bandwidth is saved by requestmg rendeπng computations only for the objects which are visible The nearest object to a leaf cone is the visible object for that cone It is noted that the visibility search performed m step 504 may be performed repeatedly for different viewpomts and or viewmg directions Thus, the display step 506 may advantageously be performed m parallel with step 504 While, the visibility search 504 is bemg performed for a cuπent viewpoint/view-direction, the results from a previous visibility search may be concuπently displayed m step 506 In one embodiment, the first measurement value comprises a penalty of separation between the first cone and the first bound which is determined by minimizing an mcreasmg function of separation distance between the vertex of the first cone and pomts m the mtersection of the first cone and the first bound In this case, step 504B determmes whether the first measurement value is smaller than the measurement value associated with the first cone smce smaller measurement values mdicate closer proximity In other words, the inequality condition of step 504B is satisfied when the first measurement value is less than the measurement value associated with the first cone The mcreasmg function of separation distance may be specified by a vector norm For example, the expression s defines an mcreasmg function of separation distance, where s is a displacement vector representing the vector difference between the vertex of the first cone and an arbitrary pomt m the mtersection of the first cone and the first bound, and denotes a vector norm Examples, of vector norms mclude the L norm, the L ~ norm, the 14 norm, and m general, any of the If norms where p is any real number m the range from one to infinity inclusive The L norm of a vector is defined as the sum of the absolute values of its components The L norm of a vector is defined as the square root of the sum of the squares of the vector components The 14 norm of a vector is defined as the maximum of the absolute values of the vector components In the prefeπed embodiment, the first measurement value of separation between the first bound and the first cone is computed by solving a lmear programmmg problem usmg the lmear constramts given by normal matnx S for the first cone, and the lmear constramts given by the normal matrix N and the extent vector c for the first bound Recall the discussion m connection with Figures 9A-9C In an alternate embodiment, the first measurement value compπses a merit of proximity (I e closeness) between the first cone and the first bound which is determined by maximizing a decreasmg function of separation distance between the vertex of the first cone and pomts m the intersection of the first cone and the first bound In this case, step 504B determmes whether the first measurement value is larger than the measurement value of associated with the first cone since larger measurement values mdicate closer proximity 23 In other words, the inequality condition of step 504B is satisfied when the first measurement value is greater than the measurement value associated with the first cone. Figure 12 illustrates step 502, l e. the construction of the boundmg hierarchy from the collection of objects. In step 602, the objects in the graphics scene are recursively clustered. Objects are assembled mto clusters preferably based on proximity These first order clusters are themselves assembled mto second order clusters Clusters of successively higher order are formed until all the objects are contamed in one universal cluster. Objects may be considered as order zero clusters. In step 604, each cluster of all orders is bounded with a coπespondmg bound. The bounds are preferably polytope hulls as descnbed above m connection with Figures 4 and 5 However, other types of bounds are contemplated such as, e.g., quadratic surfaces, generalized polynomial bounds, etc. In step 606, a hierarchical tree of bounds is generated by allocating a node for each of the objects and clusters In step 608, each node is assigned parameters which descπbe (characterize) the coπespondmg bound. In one embodiment this parameter assignment comprises storing the extent vector c which locates the polytope hull faces as descnbed in connection with Figures 5A and 5B. In step 610, the nodes are organized so that node relationships coπespond to cluster membership For example, if node A is the parent of node B m the boundmg hierarchy, then the cluster coπespondmg to node A contams a subcluster coπespondmg to node B, and the bound for node A contams the bound for node B. Although the construction of the cone hierarchy above has been descnbed m terms of recursive clustermg, it is noted alternative embodiments are contemplated which use other forms of clustermg such as iterative clustermg. A Terminal Cone-Bound Pair Figure 13 illustrates a first portion of step 504C, i.e. the step of searchmg the first bound with respect to the first cone, when both the first bound and the first cone are leaves of their respective hierarchies In step 702, the visibility search algonthm determmes whether the first cone and the first bound are leaves of their respective hierarchies. If they are both leaves, the visibility search algonthm (a) sets the measurement value associated with the first cone equal to the first measurement value of separation between the first bound and the first cone as shown m step 704, and (b) sets a visible object attnbute associated with the first cone equal to the first bound as shown m step 706. It is noted that steps 704 and 706 may be performed m any order or m parallel. A Terminal Cone with a Non-Terminal Bound Figure 14 illustrates a second portion of step 504C, i.e. the step of searchmg the first bound with respect to the first cone, when the first cone is a leaf of the cone hierarchy, and the first bound is not a leaf of the boundmg hierarchy In step 710, the visibility search algonthm determines whether the first cone is leaf-cone of the cone hierarchy, and the first bound is a non-leaf bound of the boundmg hierarchy, I e. a non-termmal bound. In response to an affirmative determination that the first cone is a leaf cone and the first bound is not a leaf bound, the sub-bounds of the first bound are conditionally explored with respect to the first cone. Figure 15 illustrates step 712, i.e. the step of conditionally exploring the sub-bounds of the first bound with respect to the first cone. In step 720, the visibility search algonthm computes a cone-bound separation 24 value for each of the sub-bounds of the first bound with respect to the first cone. In one embodiment, the cone- bound separation values are computed in the same way as the first measurement value. In step 722, the visibility search algonthm conditionally searches sub-bounds of the first bound with respect to the first cone m ascendmg order of their separation from the first cone The cone-bound separation values are used to determine the ascendmg order In other words, if sub-bound A is closer to the first cone than sub-bound B, sub-bound A is conditionally searched before sub-bound B. Figure 16 illustrates step 722, l e. the conditional search of the sub-bounds of the first bound with respect to the first cone by means of a generic first subbound of the first bound. In step 730, the visibility search algonthm determines whether the cone-bound separation value of the first subbound satisfies an inequality condition with respect to the measurement value associated with the first cone. The sense of the inequality tested m step 730 may be the same as the sense of the inequality tested m step 504B. In step 732, the visibility search algonthm searches the first subbound with respect to the first cone m response to an affirmative determination that the cone-bound separation value of the first subbound satisfies the inequality condition with respect to the first cone's measurement value A Terminal Hull with a Non- Terminal Cone Figure 17 illustrates a third portion of step 504C, i.e. the step of searchmg the first bound with respect to the first cone when the first bound is a leaf-bound of the boundmg hierarchy, and the first cone is a non-leaf cone of the cone hierarchy In step 740, the visibility search algonthm determines if the first bound is a leaf of the boundmg hierarchy and the first cone is a non-leaf of the cone hierarchy In step 742, visibility search algonthm conditionally searches the subcones of the first cone with respect to the first bound m response to an affirmative determination that the first bound is a leaf bound and the first cone is a non-leaf cone Figure 18 illustrates step 742, i.e. the step of conditionally searchmg subcones of the first cone with respect to the first bound by means of a genenc first subcone. The generic representative is mtended to typify the process of conditionally searching any of the subcones of the first cone. In step 749 the visibility search algonthm computes a cone-bound separation value for the first bound with respect to the first subcone. In step 750, the visibility search algorithm determmes whether the cone-bound separation value satisfies an inequality condition with respect to a measurement value associated with the first subcone. In one embodiment, the cone- bound separation value is computed usmg the same method as used to compute the first measurement value m step 504A. In this case, the sense of the inequality tested m step 750 may be consistent with the sense of the inequality tested m step 504B In step 752, the visibility search algonthm searches the first subcone with respect to the first bound m response to an affirmative determination that the cone-bound separation value satisfies the inequality condition with respect to the measurement value associated with the first subcone After the subcones of the first cone have been searched agamst the first bound, the measurement value associated with the first cone is updated Namely, the measurement value associated with the first cone may be set equal to an extremum (i.e. maximum or minimum) of the measurement values associated with the subcones of the first cone. The choice of the maximum as the extremum is associated with embodiments which compute cone-bound separation based on an increasing function of separation. The choice of minimum as the extremum 25 is associated with embodiments which compute cone-bound separation based on a decreasmg function of separation. A Non- Terminal Cone with a Non-Terminal Bound Figure 19 illustrates a fourth portion of step 504C, i.e. the step of searchmg the first bound with respect to the first cone when neither the first cone nor the first bound are leaf nodes of their respective hierarchies. In step 759, the visibility search algorithm computes a first cone-bound separation value for the first bound with respect to a first subcone of the first cone. In step 760, the visibility search algonthm determmes whether the first cone-bound separation value satisfies an inequality condition with respect to a measurement value associated with the first subcone. In one embodiment, the first cone-bound separation value is computed usmg the same method used to compute the first measurement value m step 504A. In this case, the sense of the inequality tested m step 760 may be consistent with the sense of the inequality tested m step 504B. In step 762, the visibility search algonthm conditionally explores subbounds of the first bound with respect to the first subcone m response to an affirmative determination that the first cone-bound separation value satisifes the inequality condition with respect to the measurement value associated with the first subcone Figure 20 illustrates step 762, i.e. the step of conditionally exploring the subhulls of the first hull with respect to the first subcone. In step 770, the visibility search algonthm computes a second cone-bound separation value for each of the subbounds of the first bound with respect to the first subcone. The second cone- bound separation values may be computed usmg the same method as used to compute the first measurement value m step 504A. In step 772, the visibility search algonthm conditionally searches the subbounds of the first bound with respect to the first subcone m ascendmg order of their separation from the first subcone, i.e. subbounds which are closer to the first subcone are searched first. This ordermg of the search advantageously allows closer subbounds to preempt the search of farther subbounds, and thus unneeded searches are avoided. Figure 21 illustrates step 772, I e the step of conditionally searchmg subbounds of the first bound with respect to the first subcone. A generic subhull of the first hull is used to illustrate the conditioned search. In step 780, the visibility search algonthm determines whether the second cone-hull separation value for a first subbound among the subbounds of the first bound satisfies an inequality condition with respect to the measurement value associated with the first subcone. It is noted that the sense of the inequality tested m step 780 may be similar to the sense of the inequality tested m step 504B, especially if the method used to compute the second cone-hull separation value is the same as that used to compute the first measurement value. In step 782, the visibility search algonthm searches the first subbound with respect to the first subcone m response to an affirmative determination that the second cone-hull separation value for the first subbound satisfies the inequality condition with respect to the measurement value associated with the first subcone. After the subbounds of the first bound have been searched agamst the first subcone, a second subcone of the first cone is tested. In particular, the visibility search algonthm computes a measure of the separation of the first bound from the second subcone If this measure of separation satisfies an inequality condition with respect to a measurement value associated with the second subcone, then subbounds of the first bound may be conditionally explored with respect to the second subcone. The conditional exploration of the second subcone is similar to the conditonal exploration of the first subcone descnbed m conjunction with Figures 20 and 21 26 After all subcones of the first cone have been tested and conditionally searched as described above, the measurement value associated with the first cone is set equal to an extremum of the measurement values associated with the subcones. As noted above, the maximum is chosen as the extremum m embodiments which use an mcreasmg function of separation to compute cone-bound separation values, and the mmimum is chosen m embodiments which use a decreasmg function of separation to compute the cone-bound separation values. In one alternate embodiment, the leaf-cones subtend angular sectors larger than one pixel. Thus, after termination of the visibility search algonthm described above, the leaf-cones may be further processed by a ray- based exploration method to determine the values for individual pixels withm leaf-cones. Although the search of the boundmg hierarchy and the cone hierarchy descnbed above assumes a recursive form, alternate embodiments are contemplated where a level-order search or iterative search is performed on one or both of the bounding hierarchy and cone hierarchy Computmg the Cone Restncted Distance Function Recall that evaluation of the cone-hull distance f c (H) of a hull H from a cone C calls for minimizing X subject to the hull constraints Ax ≤ b and the cone constramts Sx < 0 . The rows of matnx A compnse normals for the hull surfaces The rows of matnx S comprise normals for the cone surfaces. This minimization may be formulated as a nonlmear programmmg problem For example, the nonlmear programmmg problem reduces to a quadratic programmmg problem when a Euclidean norm is used, and a lmear programmmg problem when the L norm is used. The cone-hull distance computation is herem refeπed to as a geometric query It is also noted that cone-hull separation may be measured by maximizing an mcreasmg function separation such as x for pomts x satisfymg the bound/hull constramts and the cone constramts. Thus, m general a cone-hull separation value may be computed by determmmg an extremal (i.e. minimal or maximal) value of the separation function subject to the cone constramts and the bound/hull constramts. The use of a hierarchy of cones instead of a collection of rays is motivated by the desire for computational efficiency. Thanks to early candidate pruning that results from the double recursion illustrated earlier, fewer geometπc queries are performed. These quenes however are more expensive than the quenes used m the ray castmg method. Therefore, the cone query calculation must be designed meticulously. A sloppy algonthm could end up wastmg most of the computational advantage provided by improvements in the dual tree search. For the lmear programming case, a method for achievmg a computationally tight query will now be outlmed. A piecewise-lmear formulation of distance f c leads to the following lmear program- min( r x) subject to Ax ≤ b , Sx ≤ 0 . 27 The vector v is some member of the cone that is polar to the cone C For instance, v = — S e , where e is the vector of all ones. [It is noted that the matnx 5 of cone normals S are outward normals to the cone surfaces. Thus, the negation of the sum of the normal vectors gives a polar vector ] The condition Ax < b implies that the pomt x is withm the boundmg hull. The condition Sx < 0 implies that the pomt x is withm the cone C. For an efficient solution method, the lmear program problem is restated in term of its dual: max(b r >) subject to A τ y + S r z = v, O ≤ y, O ≤ z The dual objective value, b y is infinite when the cone and boundmg hull do not intersect (the vanables y and z are the Lagrange multipliers of the previous problem's constramts) In the prefeπed embodiment, the boundmg hulls have sides normal to a fixed set of normal vectors. Thus, the matnx A is the same for all hulls. For a given cone, the matnx S and the vector v are also fixed From this observation, it is apparent that the multi-dimensional polyhedron {(y,z) : A τ y + S τ z = v, 0 ≤ y, 0 ≤ z} is associated with the cone. (In one embodunent, this polyhedron has seventeen dimensions. Fourteen of those dimensions come from the type of the fixed-direction boundmg hull and an three additional dimensions come from the cone.) Smce the polyhedron depends only on the cone matnx S, it is feasible to completely precompute the extremal structure of the polygon for each cone in the cone hierarchy. By complementary slackness, the vertices of the polyhedron will have at most three elements. The edges and extremal rays will have at most four non-zero elements. An abbreviated, simplex-based, hill-climbing technique can be used to quickly solve the query this setting To establish the setting, the onentation of the cone hierarchy needs to be fixed. This is not feasible if the cone hierarchy changes onentation with changes m onentation of the view frustum. Thus, the entire space is tessellated with cones, and visible objects are detected withm the entire space. Only after this entire space computation is the set of visible of objects culled to conform to the cuπent view frustum In an alternative embodiment, a less aggressive approach may be pursued. Namely, by noting which subsets of the cones coπespond to the cuπent onentation of the view frustum, only this subset may be mcluded in the visible-object-set computation Memory Media As descnbed above, the visibility software and visibility search program of the present invention are preferably stored in memory 106 for access by processor 102. In addition, the visibility software and visibility search program may be stored in any desired memory media such as an installation media (e.g. CD-ROM, floppy disk, etc ), a non-volatile memory (e g hard disk, optical storage, magnetic tape, zip drive storage, ROM, 28 etc.), vanous kmds of volatile memory such as RAM, or any combmation thereof The present mvention contemplates the deposition and storage of the visibility software and visibility search program on memory media for distnbution to end users and/or customers. Multiple Objects Per Cone Accordmg to the visibility search algonthm, a single nearest object is identified for each leaf cone (i.e. terminal cone). If each of the leaf cones have the ultimate resolution, 1 e. the resolution of a pixel, then the strategy of identifying the nearest object m each leaf cone is guaranteed to detect all visible objects. However, the visibility search of the fully resolved cone hierarchy is computationally expensive The computational expense may be decreased by having fewer levels of refinement in the cone hierarchy But fewer levels of refinement implies that the size of the leaf cones is larger. As the size of the leaf cones mcreases, there is an mcreasmg probability that two or more objects will be visible to a single leaf cone, i.e. that the nearest object is not the only object visible to the cone. Therefore, there is an increased probably of reporting less than the full set of visible objects as the size of the leaf-cones increases, or equivalently, when fewer levels of cone refinement as used m the cone hierarchy. In order to increase the probability of capturing the full set of visible objects, the visibility search algonthm may be modified to identify the first K nearest objects for each leaf cone, where K is an integer greater than or equal to two. Advantageously the mteger K may be a function cone size. Thus, if the cone hierarchy is close to ultimate resolution K may be close to one. Conversely, if the cone hierarchy is poorly resolved, i.e mcludes only a few levels of refinement, the mteger K may be larger. The present mvention contemplates maximizing computational efficiency along the axis of high-cone- resolution/low-K-value on the one hand and low-cone-resolution/high-K-value on the other Adaptive Refinement of the Cone Hierarchy In the foregomg discussion, the cone hierarchy is descnbed as being constructed pnor to initiation of the search for visible objects by the visibility search algonthm, and remams static duπng the search Another alternative is to adaptively refine the cone hierarchy during the search procedure. In this fashion, the cone hierarchy may not waste storage for cones which will never interact with any objects The cone hierarchy may be refined m response to user mputs. For example, cones which coπespond to the user's cuπent direction of gaze may waπant additional refinement A given cone may remam unrefined until the search procedure discovers a bound which mteracts with the given cone, at which time the cone may be refined. The refinement of a given cone may be further refined as additional mteractmg objects bounds are discovered in order to more adequately distinguish the objects. In the context where objects are in motion, the movement of an object mto a given cone's field of view may mduce mcreased refinement of the given cone If the user in a virtual environment stops to look at a given object, the cones defining that object may be increasingly refined. Refinement of the cone hierarchy may be subject to the availability of computational cycles. Accordmg to the paradigm of successive warming, the initial cone tree may have only one or a few cones allowmg a crude initial estimate of visible object set to be immediately displayed. As computational cycles become available the 29 cone hierarchy may be successively refined and searched in order to provide an mcreasmgly accurate display of the visible object set. In general the cones of the cone hierarchy may be at differing levels of refinement. Cone refinement is allowed only if the cone mteracts with an object or bound (e.g. hull). Adaptive refinement of a cone terminates when the cone resolution equals that of a pixel or when no object occurs in the cone. It is noted that a combmation of fixed refinement and adaptive refinement of the cone hierarchy may be used. The present mvention also contemplates adaptively refining the cone hierarchy and identifying the K nearest objects/bound for each cone, where K changes the refinement level changes. Non-Occluding Objects Non-occludmg objects are objects which do not totally occlude (i.e. block visibility) of other objects For example, a transparent, semi-transparent, or translucent object may be a non-occluder. A screen door, anted glass, a wmdow with slats may be classified as non-occluders Objects behmd a non-occluder may be partially visible. The present invention contemplates certam modifications to the visibility search algonthm to allow for the presence on non-occludmg objects (NOOs) m the collection of objects to be searched. In particular, the visibility search algonthm may be configured to search for the first K nearest occludmg objects and any NOO closer than the K* occluder m each leaf cone, where K may be a function of leaf cone size. INDUSTRIAL APPLICABILITY This mvention is applicable to a number of different fields, mcludmg, but not limited to: computer graphics, computer visualization, computer aided design and engineering, digital image processmg, virtual reality modelmg, and parallel processmg. Numerous vanations and modifications will become apparent to those skilled m the art once the above disclosure is fully appreciated It is mtended that the followmg claims be mteφreted to embrace all such vanations and modifications 30 Appendix Source Code for the findVisibility Function The followmg source code illustrates one embodiment for the recursive search of the hull tree and cone tree is order to identify visible objects for each leaf-cone findVιsιble(hull, cone, distance) { ιf(leaf(hull) && leaf(cone)) { cone extent = dιstance(hull, cone), cone object = hull, return, } ιf(leaf(cone)) { leftD = dιstance(left(hull), cone), nghtD = dιstance(nght(hull), cone), ιf(leftD < nghtD) { ιf(leftD < cone extent) findVιsιble(left(hull), cone, leftD), ιf(nghtD < cone extent) fϊndVιsιble(πght(hull), cone, nghtD), } else { ιf(nghtD < cone extent) findVιsιble(nght(hull), cone, nghtD), ιf(leftD < cone extent) findVisible(left(hull), cone, leftD), } return, } ιf(leaf(hull)) { leftD = dιstance(hull, left(cone)), nghtD = dιstance(hull, nght(cone)), ιf(leftD < left(cone) extent) findVιsιble(hull, left(cone), leftD), ιf(πghtD < nght(cone) extent) findVιsιble(hull, πght(cone), nghtD), cone extent = max(left(cone) extent, πght(cone) extent), return, } hullLeftConeD = dιstance(hull, left(cone)), hullRightConeD = dιstance(hull, πght(cone)), ιf(hullLeftConeD < left(cone) extent) { lHullLConeD = dιstance(left(hull), left(cone)), rHullLConeD = dιstance(πght(hull), left(cone)), ιf(lHullLConeD < rHullLConeD) { ιf(lHullLConeD < left(cone) extent) findVιsιble(left(hull), left(cone), lHullLConeD), ιf(rHullLConeD < left(cone) extent) findVιsιble(πght(hull), left(cone), rHullLConeD), } else { ιf(rHullLConeD < left(cone) extent)) findVιsιble(πght(hull), left(cone), rHullLConeD), ιf(lHullLConeD < left(cone) extent)) fmdVιsιble(left(hull), left(cone), lHullLConeD), 31 if(hullRightConeD < right(cone).extent) { lHullRConeD = distance(left(hull), right(cone)); rHullRConeD = distance(right(hull), right(cone)); if(lHullRConeD < rHullRConeD) { if(lHullRConeD < right(cone).extent) findVisible(left(hull), right(cone), lHullRConeD); if(rHullRConeD < right(cone).extent) findVisible(right(hull), right(cone), rHullRConeD); } else { if(rHullRConeD < right(cone).extent) fmdVisible(right(hull), right(cone), rHullRConeD); if(lHullRConeD < right(cone).extent) findVisible(left(hull), right(cone), lHullRConeD); } } cone. extent = max(left(cone). extent, right(cone). extent); return } 32

Claims: Claims 1. A computer software program embodied on a earner medium, wherem the program compπses a plurality of mstructions configured to perform a method for displaymg visible objects on a display device, the method comprising- accessmg a collection of objects, constructmg a boundmg hierarchy from the collection of objects, constructmg a cone hierarchy; searchmg the cone hierarchy and the boundmg hierarchy to determme nearest objects for a subset of cones of the cone hierarchy; and displaymg the nearest objects for the subset of cones of the cone hierarchy. 2. The computer software program as recited in claim 1, wherem the earner medium is a transmission medium 3 The computer software program as recited m claim 1, wherein the earner medium is a computer-readable medium. 4 A method for displaymg visible objects on a display device, the method compnsmg: accessmg a collection of objects; constructmg a boundmg hierarchy from the collection of objects; constructmg a cone hierarchy; searchmg the cone hierarchy and the bounding hierarchy to determme nearest objects for a subset of cones of the cone hierarchy; and displaymg the nearest objects for the subset of cones of the cone hierarchy 5. The method of claims 2, 3 or 4, wherem said subset of cones comprises the leaf cones of the cone hierarchy. 6. The method of claims 2, 3 or 4, wherein, for a first cone m the cone hierarchy and a first bound m the boundmg hierarchy, said searching includes- determmmg a first measurement value of separation between the first cone and the first bound; determmmg whether the first measurement value satisfies an mequality condition with respect to a measurement value associated with the first cone; searchmg the first bound with respect to the first cone in response to an affirmative determmation that the first measurement value satisfies the mequality condition with respect to the measurement value associated with the first cone. 7. The method of claim 6, wherein said constructing the bounding hierarchy compπses 33 clustermg said objects to form clusters; boundmg each object and each cluster with a coπespondmg bound; allocating a node m the boundmg hierarchy for each object and each cluster; associating parameters with each node, wherem the parameters associated with the node descπbe the coπespondmg bound; organizing the nodes so that node relationships represent cluster membership. 8. The method of claim 6, wherem said determmmg the first measurement of separation between the first cone and the first bound compnses performmg computations which minimize an mcreasmg function of separation distance between the vertex of the first cone and pomts m the mtersection of the first cone and the first bound, and wherem said determmmg whether the first measurement value satisfies the mequality condition compnses determining whether the first measurement value is smaller than the measurement value associated with the first cone. 9 The method of claim 6, wherem said determining the first measurement of separation between the first cone and the first bound comprises performmg computations which maximize a decreasing function of separation distance between the vertex of the first cone and pomts in the intersection of the first cone and the first bound, and wherem said determmmg whether the first measurement value satisfies the mequality condition compnses determining whether the first measurement value is larger than the measurement value associated with the first cone. 10. The method of claim 6, wherein said determmmg the first measurement value of separation between the first cone and the first bound comprises solvmg a nonlinear programmmg problem usmg a first set of constramts defining the first cone and a second set of constramts defining the first bound m order to determme an extremal distance between the vertex of the first cone and pomts in the mtersection of the first cone and the first bound. 11 The method of claim 6, wherem said cone hierarchy is generated by refining an initial cone. 12. The method of claim 6, wherem said searchmg the first bound with respect to the first cone compnses- determining whether or not said first cone is a leaf of the cone hierarchy and said first bound is a leaf of the boundmg hierarchy; setting the measurement value associated with the first cone equal to the first measurement value of separation between the first bound and the first cone; setting a visible object attribute associated with the first cone equal to the first bound, wherem said setting of the measurement value associated with the first cone and said setting of the visible object attnbute associated with the'first cone are performed m response to an affirmative determmation that the first cone is a leaf of the cone hierarchy and the first bound is a leaf of the hull hierarchy. 34 13 The method of claim 6, wherem said searchmg the first bound with respect to the first cone compπses. determining whether said first cone is a leaf of the cone hierarchy and said first bound is not a leaf of the boundmg hierarchy, conditionally exploring sub-bounds of said first bound with respect to said first cone m response to an affirmative determmation that said first cone is a leaf of the cone hierarchy and said first bound is not a leaf of the boundmg hierarchy 14 The method of claim 13, wherem said conditionally exploring sub-bounds of said first bound mcludes: computmg a cone-bound separation value for each of the sub-bounds of the first bound with respect to said first cone; conditionally searchmg said sub-bounds of said first bound with respect to said first cone m ascendmg order of separation from the first cone. 15 The method of claim 14, wherem said conditionally searchmg said sub-bounds of said first bound mcludes determmmg whether the cone-bound separation value of a first subbound among said subbounds satisfies the mequality condition with respect to the measurement value associated with the first cone; searchmg said first sub-bound with respect to said first cone m response to an affirmative determmation that said cone-bound separation value of said first sub-bound satisfies the mequality condition with respect to the measurement value associated with the first cone. 16. The method of claim 6, wherem said searchmg of the first bound with respect to the first cone includes: determmmg if the first bound is a leaf of the bounding hierarchy and the first cone is not a leaf of the cone hierarchy; conditionally searching subcones of the first cone with respect to the first bound m response to an affirmative determmation that said first bound is a leaf of the bounding hierarchy and said first cone is not a leaf of the cone hierarchy 17. The method of claim 16, wherein said conditionally searchmg subcones of the first cone with respect to the first bound mcludes computmg a cone-bound separation value for the first bound with respect to a first subcone of the first cone; determmmg whether the cone-bound separation value satisfies the mequality condition with respect to a measurement value associated with the first subcone; 35 searchmg said first subcone with respect to said first bound m response to an affirmative determination that said cone-bound separation value satisfies the inequality condition with respect to the measurement value associated with the first subcone, wherem said searchmg the first bound with respect to the first cone further comprises setting the measurement value associated with the first cone equal to an extremum of the measurement values associated with said subcones after said conditionally searchmg said subcones with respect to the first bound 18 The method of claim 6, wherem said searching the first bound with respect to the first cone mcludes computmg a first cone-bound separation value for the first bound with respect to a first subcone of the first cone, determmmg whether the first cone-bound separation value satisfies the mequality condition with respect to a measurement value associated with said first subcone, conditionally exploring subbounds of said first bound with respect to said first subcone m response to an affirmative determmation that said first cone-bound separation value satisfies the mequality condition with respect to the measurement value associated with the first subcone 19 The method of claim 18, wherein said conditionally exploring subbounds of said first bound with respect to said first subcone mcludes computmg a second cone-bound separation value for each of said subbounds with respect to said first subcone, conditionally searching said subbounds of said first bound with respect to said first subcone m ascendmg order of their separation from the first subcone 20 The method of claim 19, wherem said conditionally searchmg said subbounds of said first bound with respect to said first subcone mcludes determmmg whether the second cone-hull separation value for a first subbound among said subbounds satisfies the mequality condition with respect to the measurement value associated with the first subcone, searchmg said first subbound with respect to said first subcone m response to an affirmative determmation that said second cone-hull separation value for the first subbound satisfies the mequality condition with respect to the measurement value associated with the first subcone, wherem said searchmg the first bound with respect to the first cone further compπses setting the measurement value associated with the first cone equal to an extremum of the measurement values associated with subcones of the first cone mcludmg said first subcone 21 A computer system which displays visible objects, the computer system compnsmg a display device, a memory for storing a collection of objects, wherein the memory also stores a visibility search program, 36 a processor coupled to the memory which executes the visibility search program, wherem, under the control of the visibility search program, the processor is configured to access the collection of objects; construct a bounding hierarchy from the collection of objects, construct a cone hierarchy; search the cone hierarchy and the boundmg hierarchy to determme nearest objects for a subset of cones of the cone hierarchy, wherem the display device is operable to display the nearest objects for the subset of cones m the cone hierarchy. 22. The computer system of claim 21, wherein the subset of cones compnses the leaf cones of the cone hierarchy 23. The computer system of claim 21, wherem, in performing said search of the cone hierarchy and the boundmg hierarchy, said processor is configured to: determine a first measurement value of separation between a first cone of the cone hierarchy and a first bound of the boundmg hierarchy; determme whether the first measurement value satisfies an mequality condition with respect to a measurement value associated with the first cone; search the first bound with respect to the first cone m response to an affirmative determmation that the first measurement value satisfies the mequality condition with respect to the measurement value associated with the first cone. 24. The computer system of claim 23 further compnsmg a graphics accelerator coupled to the processor, wherem said processor is configured to transmit visibility information mcludmg a specification of the nearest object for each leaf cone of the cone hierarchy to the graphics accelerator, wherem the graphics accelerator is configured to render an image on the display screen based on the nearest object for each leaf cone of the cone hierarchy. 25. The computer system of claim 23, wherem, m order to construe, the boundmg hierarchy, the processor is operable to. cluster said objects to form clusters; bound each object and each cluster with a coπespondmg bound, allocate a node m the boundmg hierarchy for each object and each cluster, associate parameters with each node, wherem the parameters associated with each node descπbe the coπespondmg bound; organize the nodes so that node relationships represent cluster membership. 37 26 The computer system of claim 23, wherem, in order to determme the first measurement of separation between the first cone and the first bound, the processor performs computations which minimize an mcreasmg function of separation distance between the vertex of the first cone and pomts m the mtersection of the first cone and the first bound, and wherem, m determmmg whether the first measurement value satisfies the mequality condition, the processor determmes whether the first measurement value is smaller than the measurement value associated with the first cone 27. The computer system of claim 23, m determmmg the first measurement of separation between the first cone and the first bound, the processor performs computations which maximize a decreasmg function of separation distance between the vertex of the first cone and pomts m the mtersection of the first cone and the first bound, and wherem, m determmmg whether the first measurement value satisfies the mequality condition, the processor determmes whether the first measurement value is larger than the measurement value associated with the first cone. 28. The computer system of claim 23, wherem, in determmmg the first measurement value of separation between the first cone and the first bound, the processor solves a nonlinear programmmg problem usmg a first set of constramts defining the first cone and a second set of constramts defining the first bound m order to determme an extremal distance between the vertex of the first cone and pomts m the mtersection of the first cone and the first bound. 29. The computer system of claim 23, wherem the processor constructs the cone hierarchy by refining an initial cone. 30. The computer system of claim 23, wherem, m searchmg the first bound with respect to the first cone, the processor is configured to determme whether or not said first cone is a leaf of the cone hierarchy and said first bound is a leaf of the bounding hierarchy; set the measurement value associated with the first cone equal to the first measurement value of separation between the first bound and the first cone, set a visible object attribute associated with the first cone equal to the first bound, wherem said setting of the measurement value associated with the first cone and said setting of the visible object attribute associated with the first cone are performed in response to an affirmative determmation that the first cone is a leaf of the cone hierarchy and the first bound is a leaf of the hull hierarchy 31. The computer system of claim 23, wherem, m searching the first bound with respect to the first cone, the processor is configured to: determme whether said first cone is a leaf of the cone hierarchy and said first bound is not a leaf of the boundmg hierarchy; 38 conditionally explore sub-bounds of said first bound with respect to said first cone m response to an affirmative determmation that said first cone is a leaf of the cone hierarchy and said first bound is not a leaf of the boundmg hierarchy 32 The computer system of claim 31, wherem, m conditionally explonng sub-bounds of said first bound, the processor is configured to compute a cone-bound separation value for each of the sub-bounds of the first bound with respect to said first cone, conditionally search said sub-bounds of said first bound with respect to said first cone m ascendmg order of separation from the first cone 33 The computer system of claim 32, wherem, m conditionally searching said sub-bounds of said first bound, the processor is configured to determme whether the cone-bound separation value of a first subbound among said subbounds satisfies the mequality condition with respect to the measurement value associated with the first cone, search said first sub-bound with respect to said first cone m response to an affirmative determmation that said cone-bound separation value of said first sub-bound satisfies the mequality condition with respect to the measurement value associated with the first cone 34 The computer system of claim 23, wherem, m searchmg the first bound with respect to the first cone, the processor is configured to determme if the first bound is a leaf of the boundmg hierarchy and the first cone is not a leaf of the cone hierarchy, conditionally search subcones of the first cone with respect to the first bound in response to an affirmative determmation that said first bound is a leaf of the boundmg hierarchy and said first cone is not a leaf of the cone hierarchy 35 The computer system of claim 34, wherein, m conditionally searchmg subcones of the first cone with respect to the first bound, the processor is configured to computmg a cone-bound separation value for the first bound with respect to a first subcone of the first cone, determmmg whether the cone-bound separation value satisfies the mequality condition with respect to a measurement value associated with the first subcone, searchmg said first subcone with respect to said first bound in response to an affirmative determmation that said cone-bound separation value satisfies the mequality condition with respect to the measurement value associated with the first subcone, wherem, m searchmg the first bound with respect to the first cone, the processor sets the measurement value associated with the first cone equal to an extremum of the measurement values associated with said subcones after said conditionally searchmg said subcones with respect to the first bound 39 36. The computer system of claim 23, wherem, m searchmg the first bound with respect to the first cone, the processor is configured to- compute a first cone-bound separation value for the first bound with respect to a first subcone of the first cone; determme whether the first cone-bound separation value satisfies the inequality condition with respect to a measurement value associated with said first subcone; conditionally explore subbounds of said first bound with respect to said first subcone m response to an affirmative determmation that said first cone-bound separation value satisfies the mequality condition with respect to the measurement value associated with the first subcone 37. The computer system of claim 36, wherem, m conditionally explonng subbounds of said first bound with respect to said first subcone, the processor is configured to: compute a second cone-bound separation value for each of said subbounds with respect to said first subcone; conditionally search said subbounds of said first bound with respect to said first subcone m ascendmg order of their separation from the first subcone. 38. The computer system of claim 37, wherem, m conditionally searchmg said subbounds of said first bound with respect to said first subcone, the processor is configured to: determme whether the second cone-hull separation value for a first subbound among said subbounds satisfies the mequality condition with respect to the measurement value associated with the first subcone; search said first subbound with respect to said first subcone m response to an affirmative determination that said second cone-hull separation value for the first subbound satisfies the mequality condition with respect to the measurement value associated with the first subcone; wherem, m order to search the first bound with respect to the first cone, the processor sets the measurement value associated with the first cone equal to an extremum of the measurement values associated with subcones of the first cone mcludmg said first subcone. 40

Inventors: SOWIZRAL HENRY, ZIKAN KAREL, SOWIZRAL, HENRY, ZIKAN, KAREL
Applicants: SUN MICROSYSTEMS INC, SUN MICROSYSTEMS, INC.
Application year: 1999
Main classification: G06T15
IPC codes: G06T15