I thought this might be useful to people wanting to be able to comp and grade Terragen AOV's properly. The proper way to build a Terragen render out of AOV's only worked once I finally figured out how to use the alpha, atmoAlpha, cloudAlpha and surfAlpha elements the correct way. The atmo, cloud and surface elements need to be divided by their respective alpha, then they can be graded. Then after, they get multiplied back with the alpha, and finally all plussed together. I had been wanting to figure this out for years, as I was just doing it the 'quick and dirty' way by using one of the individual RGB channels from the alpha elements, instead of using all the channels chromatically.
Then at the very top are some latlong elements (stars, planetary rings and moons) mapped onto a sphere in Nuke behind the Terragen renders, all properly combined using the chromatic alpha the correct way.
When I compare the beauty render to my AOV break out I finally get a 1 to 1 perfect comparison, and now I want to share in case others want to know the same.
Thanks!