Yet another question regarding rendering artifacts -- tiny black squares (Solved?)

stevejackhammer

See the attached image.  What are those black artifacts?!  Not fireflies, as I understand them.  They seem to cluster near inset corners.  In the shot below, that's a fireplace, and above it a shelf.  The dots are where the mantle and the shelf meet the far wall.  But as you can see, the dotrs are not exlusively there (many appear on the perfectly flat floor, for example), but that seems to be a hint.  This is a canvas-based render, and we're looking at the beauty layer, but all the canvases seem to have them, even depth, distance, and material ID canvases.  I've been messing with different pixel filters at different levels, adding more lights, especially largish floodlights, fiddling with the tone mapping, the render quality, and so on.  I'm about to embark on a organized walkthrough the various settings to see how each affects the appearance of the artifacts (some of my random pokes have been awash in those darn dots) but I'm hoping someone will look and recognize these for what they are.

The lighting is from photometric spotlights, multiple point source lights using the ALR Lighting product, and a couple of emissive textures (for example, a TV screen).  Iray render with a Titan GPU.

The render was stopped at 350 iterations, with the progress stuck at about 35% (I know not to read much into that number).  The dots were actually getting worse over time.  Can someone attest to the problem going away with patience and more samples?

One more thing that's weird about them.  They're all perfectly square.  They aren't tiny multi-coloured dots, but identifcally sized black squares.

[UPDATE]

Solved?  I was running through the IRay forums at NVIDIA and someone described a similar artifact and blamed a bad shader that was throwing a divide-by-0 exception and that suddenly made me think of checking all the shaders.  (I don't think that person was using Daz, but was definitely using the IRay engine to render.)  And wouldn't you know it that I had several 3Delight shaders in the scene, including the entire ceiling.  Doing a new render, and so far so good.  If it turns out to be the problem, then sorry for wasting bandwidth on this one.