lol
zsouthbody, you can't know it, but months ago, before this *really* started, I chatted with Ono in the Indigochat and talked with him about this idea.
He told me, exr is far faster, which is true, obviously, but I (not sure, if *only* I...) asked for this, because of several interesting cases, which you can't really get with an exr map.
For example, you can make a shot from the universe down the planet (this sky simulation is (or was?) also supposed to once support stars and different sun - earth - moon constellations, like full and semi eclipses.
Or what if one once models a full featured air plane, that flies?
Sure, you could use an exr map for it, but there wont be too many exr maps from very high in the air... - and what's about parts of clouds, which might cross the air plane.
In even farther future, where Indigo will also have particle- and volume emitter support (now, that I mention it: what's about the particles, Ono?), there might also be a limited physical weather simulation (Indigo'd have a very complex but good GUI, 'till then, else, it wouldn't be tweakable, I guess), like snow, rain and thunderstorm (and tornado with randomly created stuff and cows, flying around

)
Especially the last part might never happen. - Except Ono is once thinking of making a full featured physics prog out of the current physics-of-light-renderer-only
