Advanced_Renderman_Book[torrents.ru]

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15.4 Conclusion

map technique to make the machine look like it was embedded inside the gas, as described in Section 15.2.4.

15.3.4 Saturn

The last shot I'd like to discuss is the effect produced for Saturn's rings that also appeared in the opening shot (Figure 15.15). Rodney Iwashina created the CG effects for this shot using Houdini and RenderMan. The Saturn rings used a number of elements. The long shots showed the rings looking like rings. This was done with simple geometry but with a shader that varied the opacity according to the various belts that make up Saturn's rings. These values were taken from the specs of the actual rings. Different shaders were applied to the top and bottom rings to account for the fact that the rings looked quite different depending on whether you were looking at the top at a grazing angle to the Sun or looking at the bottom with the Sun backlighting the ice particles that make up the rings.

There were about 40,000 large ice "rocks" instanced with the Houdini particle system with procedurally varying sizes and a displacement shader to put midand high-frequency detail in them. The surface shader was fairly simple, coloring them dark against the backlit small ice particles. The small ice particles that gave the rings the volumetric feel used a lot of stacked planes, approximating a ray march or discrete sampling of continuous noise space. About eight octaves of fBm with sharpened high frequencies gave the rings their gritty look. The "God rays" were created by some judicious tweaking with moire patterns, and the lighting was given a falloff based on the angle of the Sun object behind the planes and the I vector.

15.4 Conclusion

Realistic rendering of volumetric gases and clouds is not usually justified. Particles and stacking planes can often work faster and provide more control over important aspects like overall shape and motion. However, each new show raises the bar, and it won't be long before fully shadowed volumetric effects are the norm rather than the exception in visual effects. The ray marching technique really is at its best when no self-shadowing is needed (for example, flames for jet engines).

You should not underestimate the power and convenience of texture maps. Although it is a joy to write beautiful algorithms to describe a scene, if a texture map can be used, the result is more controllable and much faster to produce and change in 2D.

It is possible to expand on the basic volume shader idea to use depth maps, or to write plug-ins to Pixar's RenderMan to define volumes and include voxel intersection for faster density function integrations. Refraction is also not difficult to add, but when doing so it is important to do your specular calculations only at the surface of your solid.

438 15 Volumetric Shaders for Visual Effects

Figure 15.14 Rendering of the machine glow sequence. (CONTACT © 1997 Warner Bros., a division of Time Warner Entertainment Company L.P. All Rights Reserved.) See also color plate 15.14.