My final render came back from Pixelplow. Always exciting, and luckily, they're só fast! A few hours, while my i7 would have taken 24 hrs or so. Recommended (again)!
But, I saw the artifacts came back that I complained about earlier, and I now get the impression they 'grow worse' with higher detail/AA settings. I didn't see them this clear/large when I made my last crop tests, but the farm settings were a bit higher (0.8 AA8).
The culprit is a snow layer with displacement intersection or favor depressions, but also a height constraint. I needed the latter for snow remains in the lower region only, but I think that's making TG compute colors wrongly.
Does anyone have a clue about this behavior and what to do about it? I might have to clone the grey patches out or rerun the low part with different settings, but I hate to fiddle like that. And snow ill be quite different.