One of the great advantages of three dimensional animation methods is the ability to move around in a simulated 3D environment interactively, or to create a programmed tour or flyby. For the ash cloud visualization, we wanted to follow the moving ash clouds in relation to the terrain and to look at them from different distances and different directions. AVS allows fully interactive manipulation of the views of the ash cloud, but the rendering process is too slow (minutes per frame) to allow for realtime animation. For this reason we decided to create a flyby of the events on December 15 by combining camera animation with the time dependent animations of the plume and jet flight path.
In our initial attempts, we used the AVS Animator module and found that it worked well for the linear time dependent portions of the animation. However, the camera animation was problematic because camera motion in a flyby situation is seldom linear. When we attempted to use the Animator in its "smooth" mode, we found it was only possible to control the camera accurately when we introduced dozens of key frames in order to tightly constrain the frame acceleration introduced by the sinusoidal interpolation technique used in the Animator. Having to use a large number of key frames makes it very time consuming to construct a flight path, because changing the flight path requires all the key frames near the change to be modified in a consistent manner. We eventually realized that a minor extension to the flight path spline algorithm already developed could easily provide the desired camera coordinates. In essence, the camera coordinates could be determined from the flight path of the viewer in the same manner that we computed the flight path of the jetliner.
The first version of the Spline Animator used a text file for input which contained the key frame information. The output was a sequence of camera coordinates which were edited into a CLI script which could be played back interactively or in batch mode. In this first effort we were able to define a flight path using about a half dozen key frames in place of the dozens required by the AVS Animator and the smoothness and predictability of results were far superior. After the video animation of the eruption visualization was completed, a second version of the Spline Animator was created with a Motif interface and submitted to the International AVS Center (IAC) for distribution via anonymous ftp. The ftp site for the IAC is avs.ncsc.org. More information about AVS and the IAC can be obtained from the MCNC web server at http://www.mcnc.org. For more information about this module, the reader is referred to [1].