Recall from Section 3.1 that the virtual world is composed of geometric primitives, which are usually 3D triangles arranged in a mesh. The chain of transformations and rasterization process must be applied for each triangle, resulting in a computational cost that is directly proportional to the number of triangles. Thus, a model that contains tens of millions of triangles will take orders of magnitude longer to render than one made of a few thousand. In many cases, we obtain models that are much larger than necessary. They can often be made much smaller (fewer triangles) with no perceptible difference, much in the same way that image, video, and audio compression works. Why are they too big in the first place? If the model was captured from a 3D scan of the real world, then it is likely to contain highly dense data. Capture systems such as the FARO Focus3D X Series capture large worlds while facing outside. Others, such as the Matter and Form MFSV1, capture a small object by rotating it on a turntable. As with cameras, systems that construct 3D models automatically are focused on producing highly accurate and dense representations, which maximize the model size. Even in the case of purely synthetic worlds, a modeling tool such as Maya or Blender will automatically construct a highly accurate mesh of triangles over a curved surface. Without taking specific care of later rendering burdens, the model could quickly become unwieldy. Fortunately, it is possible to reduce the model size by using mesh simplification algorithms; see Figure 7.16. In this case, one must be careful to make sure that the simplified model will have sufficient quality from all viewpoints that might arise in the targeted VR system. In some systems, such as Unity 3D, reducing the number of different material properties across the model will also improve performance.
In addition to reducing the rendering time, a simplified model will also lower computational demands on the Virtual World Generator (VWG). For a static world, the VWG does not need to perform any updates after initialization. The user simply views the fixed world. For dynamic worlds, the VWG maintains a simulation of the virtual world that moves all geometric bodies while satisfying physical laws that mimic the real world. It must handle the motions of any avatars, falling objects, moving vehicles, swaying trees, and so on. Collision detection methods are needed to make bodies react appropriately when in contact. Differential equations that model motion laws may be integrated to place bodies correctly over time. These issues will be explained in Chapter 8, but for now it is sufficient to understand that the VWG must maintain a coherent snapshot of the virtual world each time a rendering request is made. Thus, the VWG has a frame rate in the same way as a display or visual rendering system. Each VWG frame corresponds to the placement of all geometric bodies for a common time instant. How many times per second can the VWG be updated? Can a high, constant rate of VWG frames be maintained? What happens when a rendering request is made while the VWG is in the middle of updating the world? If the rendering module does not wait for the VWG update to be completed, then some objects could be incorrectly placed because some are updated while others are not. Thus, the system should ideally wait until a complete VWG frame is finished before rendering. This suggests that the VWG update should be at least as fast as the rendering process, and the two should be carefully synchronized so that a complete, fresh VWG frame is always ready for rendering.
Steven M LaValle 2016-12-31