True area lights would be a lot faster than using IDL for this. And a word of caution is called for; the level of detail/calculation required to sample the environment well enough to do a perfect job of lighting via a relatively small, self-lit rectangle is extraordinary. The recent optimizations (compromises) that led to a significant speedup for IDL also led to some restrictions in how crazy you can go with this.

SM will be working on improving the utility of IDL in the coming weeks. These improvments will likely show up in a service release. By that I do not mean that it isn't useful as it is right now. I've shown you plenty of renders where the quality was good enough that you could use less aggressive render settings, giving us usable GI in a few minutes.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)