Jane Wilhelms

Collision Detection and Response for Computer Animation

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.

Octrees for faster isosurface generation

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.

Anatomically based modeling

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 coherent projection approach for direct volume rendering

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.

A 'Notion' for interactive behavioral animation control

Behavioral animation is a means for automatic motion control in which animated objects are capable of sensing their environment and determining their motion within it according to certain rules. An interactive method of behavioral animation in which the user controls motion by designing a network mapping sensor information to effectors is described. The system is called Notion. The network consists of sensors (such as distance to other objects and recognition of their qualities) and effectors (here, jet motors propel objects) connected by nodes and connections. Nodes, which embody such responses as attraction, avoidance, arbitration, and their outputs, map sensory stimuli to effector responses. Sensors, nodes, and effectors are given set limits. Such limits, together with an interactive window environment for altering the network, make it possible to explore a variety of motions quickly.<<ETX>>

Using Dynamic Analysis for Realistic Animation of Articulated Bodies

A major problem in computer animation is creating motion that appears natural and realistic, particularly in such complex articulated bodies as humans and other animals. At present, truly lifelike motion is produced mainly by copying recorded images, a tedious and lengthy process that requires considerable external equipment. An alternative is the use of dynamic analysis to predict realistic motion. Using dynamic motion control, bodies are treated as masses acting under the influence of external and internal forces and torques. Dynamic control is advantageous because motion is naturally restricted to physically realizable patterns, and many types of motion can be predicted automatically. Use of dynamics is computationally expensive and specifying controlling forces and torques can be difficult. However, there is evidence that dynamics offers hope for more realistic, natural, and automatic motion control. Because such motion simulates real world conditions, an animation system using dynamic analysis is also a useful tool in such related fields as robotics and biomechanics.

Multi-dimensional trees for controlled volume rendering and compression

This paper explores the use of multi-dimensional trees to provide spatial and temporal efficiencies in imaging large data sets. Each node of the tree contains a model of the data in terms of a fixed number of basis functions, a measure of the error in that model, and a measure of the importance of the data in the region covered by the node. A divide-and-conquer algorithm permits efficient computation of these quantities at all nodes of the tree. The flexible design permits various sets of basis functions, error criteria, and importance criteria to be implemented easily. Selective traversal of the tree provides images in acceptable time, by drawing nodes that cover a large volume as single objects when the approximation error and/or importance are low, and descending to finer detail otherwise. Trees over very large datasets can be pruned by the same criterion to provide data representations of acceptable size and accuracy. Compression and traversal are controlled by a user-defined combination of modeling error and data importance. For imaging decisions additional parameters are considered, including grid location, allowed time, and projected screen area. To analyse results, two evaluation metrics are used: the first compares the hierarchical model to actual data values, and the second compares the pixel values of images produced by different parameter settings.

Put: language-based interactive manipulation of objects

Our approach to scene generation capitalizes the expressive power of natural language by separating its aptness in specifying spatial relations from the difficulties of understanding text. We are implementing an object-placement system called Put that uses a combination of linguistic commands and direct manipulation. The system is language-based, meaning that its design and structure are guided by natural language. Our approach (inspired by research in cognitive linguistics) is to analyze the natural use of spatial relations, define a well-understood class of fundamental relationships, and gradually build a coherent and natural spatial-manipulation system. Just a few simple spatial relationships, such as in, on, and at, parameterized by a limited number of environmental variables can provide comfortable object manipulation. These natural commands can be used to quickly prototype a complex scene and constrain object placement. We believe that we have an extensible, predictable, and computationally feasible environment for object manipulation. We have focused first on spatial relationships because they are fundamental to many conceptual domains beyond object placement, including motion and time. These particular domains are very important to areas of computer graphics such as animation. Uses of spatial relationships in these areas can be quite complex. We briefly introduce the complexities of understanding spatial relations and summarize related work. Then we describe the core of the Put placement system, followed by its linguistic, procedural, and interactive interfaces. We conclude by discussing future enhancements to the system.

Toward Automatic Motion Control

Motion control for computer animation is a rich area for new research. The trend toward greater complexity in animation makes the development of more convenient and automatic methods of motion control important. Most commonly used motion control methods, such as keyframing and scripts, require a great deal of user effort to design acceptable animations. More automic methods will allow the production of sophisticated animation with less user effort. These methods include dynamic analysis, path planning and collision avoidance, stimulus-response control, and learning algorithms.

Animals with anatomy

In general, computer graphics achieves greater realism using methods that simulate the real world rather than ad hoc methods that just appear somewhat realistic. To this end, we need to develop modeling and animation approaches based on anatomical and physiological principles. The method presented in this article models any animal that has a jointed endoskeleton moved by muscles and covered by a flexible skin. This semi-automated method permits user-defined parameters and achieves comfortable interactive speeds on graphics workstations.