The problem I'm having with the "distant" light isn't just about luminance. I solved that by simply using a different multiplier depending on the integrator.

But if the integrator is anything other than bidirectional, the light doesn't work correctly. Shadows are fine, but specular highlights are inverted - they appear darker instead of lighter.

This is really weird. Well I'm wasting a lot of time. I'll move on to the sun light and we'll figure out distant light later.

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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)