The ever capable Chris Maynard says some very fine things about Terragen as he demonstrates building a sunset sky in the video: 3D Digital Environments. However, the real star of the video is the app 'Clarisse' from isotropix.
https://cmivfx.com/store/572-3d+digital+environments+masterclass
Clarisse is a monster. Check out the teaser for a set of Clarisse tutorials: https://www.youtube.com/watch?v=y2___L_NxOE&list=PLOZhw3kXDRQfqZOTCBCP9qrzCmZVRkQus
While Clarisse is pricy at $999, there is a 'mostly' fully functional PLE (watermarked, export type restricted) for free. This is a CPU/realtime machine with Alembic support that will rock your socks!
I still have problems to file/classify Clarisse and it's possibilities and position in (let's say) a "typical production pipeline" properly.
Clarisse "sounds an looks promising" (and may got hyped eligible maybe) but is really confusing me pipeline/workflow wise..
Seems to be a soft for bring in all assets you want like backgrounds, animations, models and stuff in all or most known industries standarts you want for mash them up in realtime without problems, right?
Then you can modify, rearrange, paint and multiply assets, lights and animations procedurally for rendering out precomped final scenes with new passes for a final post then?
(http://athome.readinghorizons.com/community/blog/wp-content/uploads/2012/07/confused-child.jpg)