Advanced Options > Show
The Show checkbox allows you to display or hide the advanced final gathering options. The advanced options are: Refinement Passes, Fast Lookup, Presample Density, Filter Size, Sampling Contrast, Maximum Sample Normal Deviation, Trace Depth, Primary Bounce Color, and Secondary Bounce Color.
The advanced options are hidden by default because they are infrequently used and in most cases their default settings work fine as is.
Refinement Passes
When enabled, Refinement Passes provide a progressive update (in three passes) of how the final gathering is being rendered. The passes are rendered using the mental ray Tile Order, instead of the usual final gathering tile order. Refinement Passes are most useful when used in conjunction with the final gathering Visualize option.
Maximum Sample Normal Deviation
Specifies how many degrees a final gathering point's surface normal can deviate from the sampling point’s surface normal and still be used for interpolation when calculating the shading.
The default value of 25.842 degrees corresponds to the mental ray default and is equal to ArcCos(0.9).
The example scene below shows two geometric planes, with one lying flat on the XZ plane, and the other rotated 45 degrees downward. Both have local environment shaders, set to blue and red, respectively, which the final gathering sampling picks up.
Autodesk Softimage v.7.5