I apologize that I had forgotten about posting this.
1. Here's how I used it.. I rendered a planet using POV-Ray (or you can render using Terragen or even find a photo).
2. Then I render a Terragen scene (designed with traditional Terragen in mind).
3. I re-render the scene as a "mask", by setting surface color to white, turning off all shadows from clouds and terrain, and setting sun to Zenith. I turn off clouds too. I reverse the colors of the Decay in the sky, and set the "blue" to 0%.
4. I optionally have a land and cloud mask, setting all atmosphere parts to 0%, setting cloud color to white (if they are not already white). Again, sun at Zenith with no shadows.
I use the Merge feature in TerragenTools to put the parts together to produce the final image, a landscape scene with a planet in the background.
This current version requires NetPBM because I was too lazy to process the images myself. The next future version won't require NetPBM.
Also I intend to add more features to it.