Rendering highly complex models can be time and space prohibitive, and decimation is an important tool in providing simplifications. A decimated model may replace the original entirely or provide level-of-detail approximations. We present and evaluate, quantitatively and qualitatively, methods for rapidly decimating volumetric data defined on a tetrahedral grid. Results are compared using both direct volume rendering and isosurface rendering. A mass-based and a density-based decimation error metric are compared, and the mass-based metric is found to be superior. Grid surface vertices are decimated using a geometric error metric, as well as one of the data-based error metrics. Images produced using direct volume rendering and isosurface extraction on grids that are decimated approximately 80% are nearly indistinguishable from similar images using the non-decimated grids, and even at 95% decimation, the rendered images have few artifacts. Rendering speed-up depends upon the renderer used.

The authors describe a new hybrid approach to modeling and animating animals. A pre-defined skin is modeled as a triangle mesh, such as may be purchased from a digital model vendor, or generated with a typical modeling program. This skin is then attached to the underlying bone, muscle, and tissue model. The rest shape of the skin is exactly as given in the surface model of the animal. Internal components-bones, muscles, and general tissue-are directly modeled with triangle meshes or ellipsoids. Changes in joint angle result in changes to the position of bones and generalized tissue, and in changes to the shapes of muscles. The attached skin vertices move with their underlying components, resulting in natural-looking deformations to the animal modeled. This alternative approach to modeling skin allows for greater realism or detail without requiring more accurate internals, and it allows pre-existing animal models to be realistically deformed using an anatomically based method.

We describe an improved, anatomically based approach to modeling and animating animals.Underlying muscles, bones, and generalized tissue are modeled as triangle meshes or ellipsoids.Muscles are deformable discretized cylinders lying between fixed origins and insertions on specific bones.Default rest muscle shapes can be used, or the rest muscle shape can be designed by the user with a small set of parameters.Muscles automatically change shape as the joints move.Skin is generated by voxelizing the underlying components, filtering, and extracting a polygonal isosurface.Isosurface skin vertices are associated with underlying components and move with them during joint motion.Skin motion is consistent with an elastic membrane model.All components are parameterized and can be reused on similar bodies with non-uniformly scaled parts.This parameterization allows a non-uniformly sampled skin to be extracted, maintaining more details at the head and extremities.

A general volume rendering technique is described that efficiently produces images of excellent quality from data defined over irregular grids having a wide variety of formats. Rendering is done in software, eliminating the need for special graphics hardware, as well as any artifacts associated with graphics hardware. Images of volumes with about 1,000,000 cells can be produced in one to several minutes on a workstation with a 150-MHz processor. A significant advantage of this method for applications such as computational fluid dynamics is that it can process multiple intersecting grids. Such grids present problems for most current volume rendering techniques. Also, the wide range of cell sizes does not present difficulties, as it does for many techniques. A spatial hierarchical organization makes it possible to access data from a restricted region efficiently. The tree has greater depth in regions of greater detail, determined by the number of cells in the region. It also makes it possible to render useful "preview" images very quickly by displaying each region associated with a tree node as one cell. Previews show enough detail to navigate effectively in very large data sets. The algorithmic techniques include use of a k-d tree, with prefix-order partitioning of triangles, to reduce the number of primitives that must be processed for one rendering, coarse-grain parallelism for a shared-memory MIMD architecture, a new perspective transformation that achieves greater numerical accuracy, and a scanline algorithm with depth sorting and a new clipping technique.

No abstract available.

A popular technique for rendition of isosurfaces in sampled data is to consider cells with sample points as corners and approximate the isosurface in each cell by one or more polygons whose vertices are obtained by interpolation of the sample data. That is, each polygon vertex is a point on a cell edge, between two adjacent sample points, where the function is estimated to equal the desired threshold value. The two sample points have values on opposite sides of the threshold, and the interpolated point is called an intersection point . When one cell face has an intersection point in each of its four edges, then the correct connection among intersection points becomes ambiguous. An incorrect connection can lead to erroneous topology in the rendered surface, and possible discontinuities. We show that disambiguation methods, to be at all accurate, need to consider sample values in the neighborhood outside the cell. This paper studies the problems of disambiguation, reports on some solutions, and presents some statistics on the occurrence of such ambiguities. A natural way to incorporate neighborhood information is through the use of calculated gradients at cell corners. They provide insight into the behavior of a function in well-understood ways. We introduce two gradient consistency heuristics that use calculated gradients at the corners of ambiguous faces, as well as the function values at those corners, to disambiguate at a reasonable computational cost. These methods give the correct topology on several examples that caused problems for other methods we examined.

Fast techniques for direct volume rendering over curvilinear grids (common to computational fluid dynamics and finite element analysis) are developed. Three new projection methods that use polygon-rendering hardware for speed are presented and compared with each other and with previous methods for tetrahedral grids and rectilinear grids. A simplified algorithm for visibility ordering, based on a combination of breadth-first and depth-first searches, is described. A new multi-pass blending method is described that reduces visual artifacts that are introduced by linear interpolation in hardware where exponential interpolation is needed. Visualization tools that permit rapid data banding and cycling through transfer functions, as well as region restriction, are described.>

The large size of many volume data sets often prevents visualization algorithms from providing interactive rendering. The use of hierarchical data structures can ameliorate this problem by storing summary information to prevent useless exploration of regions of little or no current interest within the volume. This paper discusses research into the use of the octree hierarchical data structure when the regions of current interest can vary during the application, and are not known a priori . Octrees are well suited to the six-sided cell structure of many volumes. A new space-efficient design is introduced for octree representations of volumes whose resolutions are not conveniently a power of two; octrees following this design are called branch-on-need octrees (BONOs). Also, a caching method is described that essentially passes information between octree neighbors whose visitation times may be quite different, then discards it when its useful life is over. Using the application of octrees to isosurface generation as a focus, space and time comparisons for octree-based versus more traditional “marching” methods are presented.

Direct volume rendering offers the opportunity to visualize all of a three-dimensional sample volume in one image. However, processing such images can be very expensive and good quality high-resolution images are far from interactive. Projection approaches to direct volume rendering process the volume region by region as opposed to ray-casting methods that process it ray by ray. Projection approaches have generated interest because they use coherence to provide greater speed than ray casting and generate the image in a layered, informative fashion. This paper discusses two topics: First, it introduces a projection approach for directly rendering rectilinear, parallel-projected sample volumes that takes advantage of coherence across cells and the identical shape of their projection. Second, it considers the repercussions of various methods of integration in depth and interpolation across the scan plane. Some of these methods take advantage of Gouraud-shading hardware, with advantages in speed but potential disadvantages in image quality.

When several objects are moved about by computer animation, there is the chance that they will interpenetrate. This is often an undesired state, particularly if the animation is seeking to model a realistic world. Two issues are involved: detecting that a collision has occurred, and responding to it. The former is fundamentally a kinematic problem, involving the positional relationship of objects in the world. The latter is a dynamic problem, in that it involves predicting behavior according to physical laws. This paper discusses collision detection and response in general, presents two collision detection algorithms, describes modeling collisions of arbitrary bodies using springs, and presents an analytical collision response algorithm for articulated rigid bodies that conserves linear and angular momentum.