This is not possible and not likely to be something we will be able to support in the near future. Terragen uses its own proprietary renderer which we continue to develop. We are working toward Path Tracing and other more advanced rendering functions. Exporting the entire scene with enough fidelity to match or exceed the Terragen rendered result would require a lot of development work and would create massive many-GB (if not TB) files.
We do have options for exporting basic terrain geometry however, as well as render elements. You can use these for scene matching, shadow casting/catching, occlusion, and final compositing.
- Oshyan