Yes - the irradiance cache avoids calculating the irradiance of the environment for every single pixel. Instead, calculations are performed on relatively widely spaced points and stored in the cache. Then every individual point is "lit" by interpolating the nearest cache values. The fewer calculated entries in the cache, the faster it is, but it's also less accurate.

Those red dots you see accumulating during IDL pre-calc are the cache entries.

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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)