It's ironic you're asking me how it's done, and you did half the work. lol
As far as writing tut's goes, I don't know if that's in the cards, and it's far from perfected. Even this scene I scrapped together in about 5 mins (and thus, did not test it under a lot of different circumstances). Although at this point, I'm confident it will work the way I want it to (finally). But I can tell you what I did and it's actually quite a bit less than what's obvious and so simple you'll kick yourself.
I know Hollywood does this sort of thing all the time, and the 3-D purists will scoff, I'm sure. And even though I'm very impressed with Terragen's cloud engine, for some things, I just want to use my own photographs that I've saved specifically for a project I've been working on for years. I could easily slide this into photoshop and do the same thing (with even more options), however, in the case of having something in front of the sky backdrop -foliage, for instance - that process is a little difficult and tends to leave annoying artifacts between the branches and leaves and is not ideal.
Essentially, there are 4 of the mapped plane objects in the scene that you provided me with. The first three are obvious, they are the 3 nonsensically hanging in mid-air above the lake (done to make sure I can reproduce what I wanted). The 4th is the large sky object in the background. The *only difference* between the one in the background and the ones in the front is the scale.
It's the lighting I'm sure you're most curious about, and the magic of that is, there is none. Well, obviously the scene is lit, but I stripped away any and all lighting effects. Turn Off Shadows on the mapped plane objects and crank the Translucency to 1 for the mapped plane and viola! They will very much appear in the render as the original files do when browsing them in your folder (although these are a little bit brighter, I'm confident I can correct later, somehow).
I can definitely offer a few tips for avoiding issues, however. In most cases, animation is out, if that's your thing. Otherwise you're looking at a forced perspective nightmare. For the same reason, you kind of need some generic, high-quality photographs (royalty free or self-created, if you want to avoid trouble) of skies, clouds, or anything else you want to use this technique for. And these pictures will sort of have to 'go' with your image. This is a subjective, artistic thing you sort of have to 'feel' out. A good rule of thumb to (almost) always go by, is the sky is brighter than the ground in (almost) all cases. Also, the angle at which you render your scene should be very close to the angle that the photograph was taken. In my case, clouds that are directly above you, will appear different than clouds at the horizon.
The single biggest problem I imagine in getting things to mesh together nicely, it the atmosphere in Terragen. Removing your mapped plane from the effects of light is easy, but if you place your mapped plane too far away from the camera, it will begin to collect atmosphere in front of it and then things can deteriorate quickly as far as maintaining the illusion. This is the problem I was getting when I was using the plane object last night. I didn't save any except the one I posted a few comments ago (however that was a quick test scene, and not a good example, because the light is in front). It is possible, if your atmosphere is constant, to have atmosphere in front of the plane, but much of TG's atmosphere is not (which is it's strength), as it tapers away with height and increases with distance. Still you might get away with it, if you're particularly clever and can match up the distant haze and atmosphere to the bottom part of your sky (maybe they're both white at such a great distance).
In any event, I still have a lot more testing to do. Certainly it won't work for every scene, and the simpler the scene, and the closer the plane to the camera, the better. If I run some more tests in the next few days, I'll post them to this thread.
I'm looking forward to modeling, I just hope texturing isn't another nightmare beyond that.
And I'd like to know more about the photo compositing you're doing. And what's a Hero object?
Also, the TG2 plane object and I are no longer on speaking terms...and the future doesn't look good either.
