Autodesk 12812-051462-9011 User Guide 3 - Page 983

page 6295, and one each in the Caustics and Global Illumination (GI) groups of the Indirect Illumination panel > Caustics And Global Illumination rollout on page 6306. These apply to all lights in the scene. Reducing the Multiplier values can eliminate washout. If a single light object is causing the problem, you can reduce the Energy multiplier's value in that light object's mental ray Indirect Illumination rollout on page 5112, available on the Modifier panel. ■ To improve the quality of caustics, go to the Caustics group on page 6309 of the Caustics And Global Illumination rollout on page 6306 and increase the Max Num. Photons Per Sample setting. ■ Be careful of the total number of photons you're emitting: A very high number (100,000 and above) can dramatically increase your rendering time. Then again, for some simple scenes, you might actually be able to set these to 1,000,000 and still render in an acceptable amount of time. WARNING The number of photons specified for each light indicates the number of photons that need to be stored for each light, not the number of photons to be shot. This is an important distinction: If a light points in a direction where there is no surface, the mental ray renderer might shoot photons forever. In the Messages Window on page 6244, the mental ray renderer displays warnings that no photons are being stored. To avoid the slowdowns related to this issue, make sure that every light points in the direction of a surface (this is sometimes impossible to do with omni lights). Another way to avoid this problem is to add a big sphere around your entire model. ■ In general, use an exposure control. The mr Photographic Exposure Control on page 6744 works particularly well for adjusting overall exposure. Coincident Faces When it encounters coincident faces, the mental ray renderer can produce artifacts, because it can't decide which face is nearer the camera (neither is). To fix this, move or scale one of the objects so faces are no longer coincident. Backface Culling mental ray rendering correctly performs backface culling, and renders one-sided faces much as the scanline renderer does. 6238 | Chapter 20 Rendering