Time, data-time, and real-time interactive visualization

ime-varying simulations are becoming increasingly common in computational physics (see Fig. 1). These simulations are used in the study of many kinds of phenomena , including unsteady fluid flows, the deformation and fracture of materials, and molecular dynamics. Visualization plays an important role in the analysis of time-varying simulations. To be useful for this purpose, a visualization system must properly handle time evolution of both the simulation data and the visualizations extracted from those data. A superior visualization system would have the ability to control time, making it flow slower, faster, or backwards or stopping time altogether. In this issue I dwell on the implications of these requirements, particularly for real-time interactive visualization systems. The desire for graceful control of time flow in a visuali-zation system is driven by user requirements, and the implementation of such a system with real-time interactive performance has proven challenging. The solutions to the various problems that have arisen force significant choices in the architecture of the visualization system. Here I describe the requirements in as general a manner as possible, giving the most detail to the solutions that Sandy Johan and I implemented in the virtual windtunnel. For a real-time interactive visualization system to work, it must keep track of how far the simulation has proceeded and whether or not the visualization of a given stage of the simulation is complete. The user must be able to stop the proceedings and look at a particular phenomenon in greater detail or trace its evolution by going back to an earlier stage in the simulation. Our approach to providing these capabilities involves setting up various counters to measure the steps that occur in the simulation and its visualization. The values of the counters we associate in a natural manner with various senses of " time. " Much of the discussion in this article is designed to motivate the introduction of these senses of time and to explore their meanings and utility. By the end of the article, we shall have introduced a half dozen senses of time. They are summarized in the table. Time step, an integer, is the current reading of a counter that is incremented by one unit whenever a new frame of data is loaded into the visualization system. For simplicity, assume that the simulation proceeds in a sequence of discrete steps, and that there is only one data clock, that is, only …