I want to show light reflected from a flat, mirror-like surface onto a diffuse surface. I noticed that even when everything else looks good, the light reflected from the mirror is 100% fireflies. When the roughness is increased a bit, they smooth out quickly to what I would expect. NOTE: mirror is flat - this is not a caustic issue. However, that is the direction I'm going next. Also, I'd like to see how well Blender can do - "just use another rendering program" is not helpful here.
Is there a concise yet accurate way to explain why this happens? I'd like to understand as much as possible.
Is there some particular setting (beyond just "Samples") that I can use to improve the reflected light pattern smoothness (for a given render time)?
Could Light Portals be used to reduce render time for smooth reflected light?
The images below are 100 samples. 1000 doesn't really fix it.
I have found that this answer by @gandalf3 seems to address part of my question already - but I'm still interested in trying everything I can within Blender, before resorting to those measures.
So a ray from the camera hits the "floor" at a certain point (pixel) indicated by a tiny cube. Actually, in the case of a mirror (specular reflection) a similar number of rays from that point would reach the light directly and by reflection. So we can't just say there are "fewer rays". There is something more here.
DIfferent areas require very different number of samples. Words describe my impression of how much noise in the direct lit areas, direct shadows, and reflected lit areas.
The .blend file is here.