James S. Painter

Parallel volume rendering using binary-swap compositing

We describe a parallel volume-rendering algorithm, which consists of two parts: parallel ray tracing and parallel compositing. In the most recent implementation on Connection Machines CM-5 and networked workstations, the parallel volume renderer evenly distributes data to the computing resources available. Without the need to communicate with other processing units, each subvolume is ray traced locally and generates a partial image. The parallel compositing process then merges all resulting partial images in depth order to produce the complete image. The compositing algorithm is particularly effective for massively parallel processing, as it always uses all processing units by repeatedly subdividing the partial images and distributing them to the appropriate processing units. Test results on both the CM-5 and the workstations are promising. They do, however, expose different performance issues for each platform.<<ETX>>

Radioptimization: goal based rendering

This paper presents a method for designing the illumination in an environment using opti mization techniques applied to a radiosity based image synthesis system An optimization of lighting parameters is performed based on user speci ed constraints and objectives for the illumination of the environment The system solves for the best possible settings for light source emissivities element re ectivities and spot light directionality parameters so that the design goals such as to minimize energy or to give the the room an impression of privacy are met The system absorbs much of the burden for searching the design space allowing the user to focus on the goals of the illumination design rather than the intricate details of a complete lighting speci cation A software implementation is described and some results of using the system are reported The system employs an object space perceptual model based on work by Tumblin and Rushmeier to account for psychophysical e ects such as subjective brightness and the visual adaptation level of a viewer This provides a higher delity when comparing the illumination in a computer simulated environment against what would be viewed in the real world Optimization criteria are based on subjective impressions of illumination with qualities such as pleasantness and privateness The qualities were selected based on Flynn s work in illuminating engineering These criteria were applied to the radiosity context through an experiment conducted with subjects viewing rendered images and the respondents evaluated with a Multi Dimensional Scaling analysis Radioptimization Goal Based Rendering John K Kawai James S Painter Department of Computer Science University of Utah Michael F Cohen Department of Computer Science Princeton University

A data distributed, parallel algorithm for ray-traced volume rendering

This paper presents a divide-and-conquer ray-traced volume rendering algorithm and a parallel image compositing method, along with their implementation and performance on the Connection Machine CM-5, and networked workstations. This algorithm distributes both the data and the computations to individual processing units to achieve fast, high-quality rendering of high-resolution data. The volume data, once distributed, is left intact. The processing nodes perform local raytracing of their subvolume concurrently. No communication between processing units is needed during this locally ray-tracing process. A subimage is generated by each processing unit and the final image is obtained by compositing subimages in the proper order, which can be determined a priori. Test results on the CM-5 and a group of networked workstations demonstrate the practicality of our rendering algorithm and compositing method.

Interactive texture-based volume rendering for large data sets

To employ direct volume rendering, TRex uses parallel graphics hardware, software-based compositing, and high-performance I/O to provide near-interactive display rates for time-varying, terabyte-sized data sets. We present a scalable, pipelined approach for rendering data sets too large for a single graphics card. To do so, we take advantage of multiple hardware rendering units and parallel software compositing. The goals of TRex, our system for interactive volume rendering of large data sets, are to provide near-interactive display rates for time-varying, terabyte-sized uniformly sampled data sets and provide a low-latency platform for volume visualization in immersive environments. We consider 5 frames per second (fps) to be near-interactive rates for normal viewing environments and immersive environments to have a lower bound frame rate of l0 fps. Using TRex for virtual reality environments requires low latency - around 50 ms per frame or 100 ms per view update or stereo pair. To achieve lower latency renderings, we either render smaller portions of the volume on more graphics pipes or subsample the volume to render fewer samples per frame by each graphics pipe. Unstructured data sets must be resampled to appropriately leverage the 3D texture volume rendering method.

Volume seedlings

Recent advances in software and hardware technology have made direct ray-traced volume rendering of 3-d scalar data a feasible and effective method for imaging of the data’s contents. The time costs of these rendering techniques still do not permit full interaction with the data, and all of the parameters effecting the resulting images. This paper presents a set of real-time interaction techniques which have been developed to permit exploration of a volume data set. Within the limitation of a static viewpoint, the user is able to interactively alter the position and shape of an area of interest, and modify local viewing parameters. A run length encoded cache of volume rendering samples provides the means to rerender the volume at interactive rates. The user locates and plants “seeds” in areas of interest through the use of data slicing and isosurface techniques. Image processing techniques applied to volumes ( i.e. volume processing), can then automatically form regions of interest which in turn modify the rendering parameters. This “region growing” of “seedlings” incrementally alters the image in real-time providing further visual cues concerning the contents of the data. These tools allow interactive exploration of internal structures in the data which may be obscured by other imaging algorithms. Magnetic Resonance Angiography (MRA) provides a driving application for this technology. Results from preliminary studies of MRA data are included.

Volume seeds: A volume exploration technique

Ray-traced volume rendering has been shown to be an effective method for visualizing 3D scalar data. However, with currently available workstation technology, interactive volume exploration using conventional volume rendering is still too slow to be attractive. This paper describes an enhanced volume rendering method which allows interactive changes of rendering parameters such as colour and opacity maps. An innovative technique is provided which allows the user to plant a ‘seed’ in the volume to rapidly modify local shading parameters. For a fixed viewing position, the user can interactively explore specific regions of interest. Furthermore, a virtual cutting technique with the exploratory seed allows the user to remove surfaces and see the internal structure of the volume. Examples demonstrate these techniques as an attractive option in many applications.

PARALLEL VOLUME VISUALIZATION ON WORKSTATIONS

Abstract This paper discusses the use of general-purpose graphics workstations for interactive high-resolution volume visualization. We survey previous research results in parallel volume rendering as well as commercial products that take advantage of parallel processing to make volume rendering a practical visualization method. Our focus is on developing distributed computation methods that can distribute the memory and computational demands of volume visualization across a network of general purpose workstations. We describe three distributed computation strategies based on ray-casting volume rendering that can be implemented on either shared-memory multiprocessor workstations or on a network of ordinary workstations. Multiple views of real-time feature extraction give tremendous insight to the volume data. Multiple variable visualization helps scientists to capture the interaction between important variables in a simulation. Divide-and-conquer rendering allows interactive high-resolution volume visualization of large data sets on a network of midrange workstations, even when the data set is too large for available memory on any single workstation. Several examples in medical imaging and computational fluid dynamics are shown illustrating the practicality of these methods.

Case study: mantle convection visualization on the Cray T3D

The recent years have seen rapid advancement towards viewing the Earth as an integrated system. This means that we have come to understand the interdependence of the major planetary subsystems-atmosphere, biosphere, oceans and the deep earth interior-on a large range of time and length scales. One of the longest time scales of the planet is imposed by solid state convection within the silicate Earth mantle. Mantle convection modeling, and other earth science modeling efforts, now are producing simulation data on grids that are large enough to strain the memory and processing power of even the largest high-end graphics workstations. Another alternative is to use parallel visualization tools running on the massively parallel computers that generated the data. This is the approach that we have taken for the visualization of mantle convection simulation data.

Parallel sphere rendering

Sphere rendering is an important method for visualizing molecular dynamics data. This paper presents a parallel algorithm that is almost 90 times faster than current graphics workstations. To render extremely large data sets and large images, the algorithm uses the MIMD features of the supercomputers to divide up the data, render independent partial images, and then finally composite the multiple partial images using an optimal method. The algorithm and performance results are presented for the CM-5 and the T3D.

State of the Art in Image Synthesis

Image Synthesis is not a new idea. Human have been creating pictures of non-existent worlds for as long as they have had the capacity to imagine them. These efforts have ranged from cave drawings to fantastically rendered scenes of great complexity. They have depicted scenes of great horror in the paintings of Hironymous Bosch, to the impossible worlds of M. C. Escher.