Standard volume rendering techniques typically employ only a local illumination model for shading, and therefore produce images without global effects. Including a global illumination model within a visualization system has several advantages. First, global effects are often desirable in scientific applications. For example, by placing mirrors in the scene, a single image can show several views of an object in a natural, intuitive manner, leading to a better understanding of the 3D nature of the scene (see Fig. 8). Also, complex geometric surfaces are often easier to render when represented volumetrically than when represented by high-order functions or geometric primitives, and global effects using ray tracing or radiosity are desirable for such
applications, called volume graphics applications . Volumetric ray tracing is described in Section 6.1 and volumetric radiosity is discussed in Section 6.2.
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