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. Set 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 improvement. 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.

 Photon Mapping 

This panel controls aspects related to photon map generation and usage. Photon maps are computed in a separate rendering pass and are then used to calculate caustic and other global illumination effects.

Shading Model - Specifies a simplified shading model to employ when tracing photons. If empty, the shader will be analyzed to determine photon scattering. The shader analysis phase is slower than the simplified shading model but captures complex shader-specific behavior. This value acts as the scene default and you can override this value on a per-primitive basis via the Slim Ensemble. 

Caustic Map - Specifies the name of a caustic photon map to access when a shader that requires this information is attached to a particular primitive. Note that some shaders with support for caustics may choose to ignore this value. This value acts as the scene default and you can override this value on a per-primitive basis via the Slim Ensemble. Note that this string will be interpretted as an expression allowing you to use MTOR/Slim scripting variables and procedures to contruct your file name. 

Global Map - Specifies the name of a global photon map to access when gathering soft indirect illuminance (color bleeding, etc.). This value acts as the scene default and you can override this value on a per-primitive basis via the Slim Ensemble. Note that this string will be interpretted as an expression allowing you to use MTOR/Slim scripting variables and procedures to contruct your file name. 

Estimator - The number of photons to consider when calculating indirect illumination via caustic or global photon maps. Larger numbers result in more blurry approximation. This value acts as the scene default and you can override this value on a per-primitive basis via the Slim Ensemble.