Spherical Harmonics GI Engine
Spherical Harmonics GI Engine is used to very quickly render SH, baked by the spherical harmonics baking engine. For more information see Spherical Harmonics Baking Engine.
File name - this is the name of the *.vrsh file or one of the sequence *XXXX.vrsh of files which contain the precomputed SH for this scene.
Use single vrsh - this option should be enabled when you render one frame or animation from single vrsh file. If you render animation from sequence of vrsh files, you should turn it off.
Precalc light per frame - reasonable when rendering animations. Depending on this option V-Ray calculates the lighting either once at the beginning of the rendering or precalculates it before every frame.
On - turns on the environment sampling to add environment light contribution.
Upper hemisphere only - depending on this option V-Ray samples either the whole sphere or only the upper hemisphere of the environment.
Subdivs - the square of this parameter is proportional to the number of rays, sampled in the environment.
On - turns on the filtering of the spherical harmonics. This is useful to reduce the ringing artifacts (known as Gibbs phenomena in signal processing) by suppressing the high frequencies. This produces blurred SH which result in a smoother image.
Filter Strength - the strength of high frequencies' suppression. Values near 0.0 slightly change the image while values near 1.0 smooth it a lot.