The Rays Panel

Enable Ray Tracing - Global control for enabling and disabling ray tracing. This causes MTOR to tag each object with visibility attributes required by PRMan's ray tracing subsystem. Only objects made visible to this subsystem will appear in ray traced reflections and shadows.

Trace Breadth Sample Factor - A global multiplier for all multi-sampling ray tracing operations. Setting this to a value greater that 1 will increase sampling quality and slow down the rendering. Set this to a value less than 1 to speed up your renderings. Currently, this setting will only affect ray tracing shaders that are "trace sample factor savvy." This includes all factory slim shaders.

Trace Depth Sample Factor - A global multiplier for all multi-sampling ray tracing operations that varies the requested ray sample count according to the current ray depth. During preview rendering, a value less than 1 should be used. Smaller values result in a coarser approximation but yield significant speed improvements, and the gritty artifacts aren't entirely unpleasant. Currently, this setting will only affect ray tracing shaders that are "trace sample factor savvy." This includes all factory slim shaders.

Max Ray Depth - This sets a limit to the number of bounces any ray can travel regardless of its type.

Specular Threshold - This angle controls the categorization of a ray path. Rays scattered in regions smaller than this value are specular. Larger regions yield diffuse ray paths.

Irradiance Cache 

This panel controls aspects related to irradiance caching. The irradiance cache can be used to accelerate global illumination calculations and can be stored in irradiance cache files for future reuse. You can have any number of independent irradiance cache contexts to override the scene-wide default established here.

Max Error - An error metric to control quality/speed tradeoff for indirect illumination. Larger numbers result in a coarser approximation but speed up the rendering. Set this value to 0 if you wish to force a recalculation of global illumination at every point. Set this value to a million to force complete reliance on indirect seed file, if provided. This value acts as the default and you can override this value on a per-primitive basis via the Slim Ensemble. 

Max Pixel Distance - A secondary error metric to control quality/speed tradeoff for indirect illumination. Larger numbers result in a coarser approximation but speed up the rendering. Set this value to 0 if you wish to force a recalculation of global illumination at every point. Set this value to a million to force complete reliance on indirect seed file, if provided. This value acts as the default and you can override this value on a per-primitive basis via the Slim Ensemble. 

Cache File - An irradiance cache file is used to accelerate the indirect diffuse calculations. During image generation, the renderer continuously updates the various internal irradiance caches and these results can be reused - either to speed up test renders, or to "bake" the irradiance calculations once and for all for a final model or scenario. As your set and lighting design begins to settle out, you can save lots of rendering time by setting the file mode to Read and using a cached file. Refer to the help for Irradiance File Mode for more. 

Cache File Mode - The file mode associated with the Irradiance Cache, which controls how a cache file is used. When set to Ignore, the cache files are read and no cache is saved to disk. When set to Read, cached data will be referenced. When set to Write, any existing cache file will be ignored, effectively starting with a blank slate. The state of the cache will be written to the file when rendering completes.