I get the same a lot of the time. I know an unbiased renderer has to have a lot of samples, but I think there's something wrong with LuxRender. It just doesn't seem logical that after 50 or more samples, you'd still have really strong fireflies. There's one in there just to the right of the brown barrel that looks like it is only one sample.

Perhaps the metropolis sampler is broken. Is it possible it does not make any attempt to evenly distribute samples? It would seem to be a trivial matter to notice that a pixel has not gotten much attention and is wildly different from its neighbors and so it should get a little more attention for a while.

I also thought that it would be cool if in the Lux UI you could click on a spot or sweep out an area and it would do a little extra area rendering there for a while before going back to the image as a whole. With a human watching and directing its attention here and there, it should be possible to do better than the random stuff it seems to be doing.

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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)