​@yfei 

Object Properties: Volume Physics vs Coat/Scatter

• Volume Physics: this is where you can specify your scattering function within the volume of interest
• Coat/Scatter: this is where you can apply a coating or scatter function to a given surface of the object

NSC 3D Layout: Scatter NSC Rays vs NSTR Merit Function Operand: Scat?

• Scatter NSC Rays in 3D Layout (and Shaded Model): this enables scattering inside the 3D Layout only (as you mentioned)
• Scat? in NSTR: this enables scattering during the raytraces of the optimization

To render the 3D Layout, a raytrace is performed, and you enable it in the settings of the 3D Layout (Scatter NSC Rays). During optimization, multiple raytraces are performed, and if you want to consider scattering, you need to enable (set it to value 1.0) the Scat? flag of the NSTR operand.

One clarification to this:

My understanding is the Scatter NSC Rays are only for show in 3D layout, and NSTR will consider the volume physics I set in component editor. Is this correct?

This is correct, but I want to emphasize that the Volume Physics should be also considered in the 3D Layout if Scatter NSC Rays is enabled and Scat? in NSTR is 0.

If I don’t input any model in volume physics and turn on Scat? in merit function, how would the calculation be done? 

If you have defined scattering surfaces in Coat/Scatter, some scattering might happen at those surfaces. Otherwise, if you don’t have any scattering surface/volume, then no scattering will happen. The Scat? flag is only there to allow for scattering, it doesn’t mean scattering will happen. Scattering only happens is Scat? is turned on AND you have a scattering volume (Volume Physics) or surface (Coat/Scatter).

Let me know if that answers your question.

Take care,

David