Defining, modeling, and measuring system lag in virtual environments

This paper considers the issue of total system lag in real-time interactive computer graphics environments. In these systems, such as virtual environments and simulators, system lag dramatically effects the usability of the system. There are two types of lag discussed in this paper: transmission lag time, the time difference between the moving of a sensing device (such as a position tracker) and the display of that device's motion on a graphic display; and position lag, the difference between the actual position of a tracker in motion compared and the displayed position of the tracker at the same time. Using the Virtual Interactive Environment Workstation being developed at NASA Ames Research Center as the system to be measured, a method of measuring these types of lag using a video technique is described. The relationship between the two types of lag is observed and modeled, as well as a relationship between system lag and graphic update rate. It is found that the position lag can be understood in terms of the transmission lag, so that optimizing a system for small transmission lag will also optimize for small position lag. Using the results described in this paper the lag in other systems can be estimated and the effect of graphics performance on system lag can be predicted.