59th example file, to illustrate the user-defined scattering option

The geometry is that of the 3rd benchmark test.  The rays start at z = -0.8, in a field-free region.

Between z=-0.5 and z=0.5 the rays interact with the molecules of a background gas.

The routine that defines the scattering function is in file sct3back.cpp, which is supplied with the CPO3D package.  It should be copied to scatter3.cpp and used to create scatter3.dll, as explained in Help.

This routine calls the data file sct3back.dat (also supplied with the package), which is named by the User in the databuilder.

Please see sct3back.cpp for more detailed information.