OS: Win10 x64
GeForce GTX 960/PCIe/SSE2
Blender Version: 2.82 (sub 7), branch: master, commit date: 2020-03-12 05:06, hash: 375c7dc4caf4, type: Release
build date: 2020-03-12, 15:41:08
"Short description of error" - yeah not really
- tiled errors on water surface on multiple frames
- missing denoise on multiple frames
while rendering on SheepIT's distributed renderfarm, so I decided to investigate what's causing it. (blocks visible on water on the frame's lower side)
There are various other frames exhibiting the problem, rendered on various people's machines, so I decided to do a test render myself, and while not as pronounced the results did have a sort of similar error to them: (small error visible in the center, slightly upwards)
Second test to see if it was deterministic: (it's not, now the error is at the center penisula region both times)
It was caused by the Denoise Normal node providing part of a tile of misinformation:
Since the Denoise Normal data contains information on caustic and transparent surface's normals, and the error only occured on the edges of the water surface where I'm playing with transparency mixing, it made sense to try and disconnect the transparency part from the node tree, and lo and behold the error disappeared, along with the transparency, obviously.
Tested it out with the node hooked up with the diffuse mix, but the factor was 000.
And then with the opposite; 1:
Redid the original value of 0.525 but it rendered out correctly...
Re-unzipped the entire thing and did a second render of the original values, sure enough there were artifacts.
Re rendered and it was fine.
Re rendered and it was there again.
Dialed the mix node to 0.5 again and there was the error again.
I'm guessing it's some sort of stupid float rounding error yielding a nan or null or zero value during the mixed transparency calculation of the normals behind the object.
As the file uses image sequences, I dont really want to start stripping out the parts and testing if I screw up, this is random enough as is.
Frame 44 has produced errors on the farm and several times on my pc aswell, so to test just dial in frame 44 and hit f12
If it's not very apparent just hook up the Denoise Normal node to the view in the compositor and look for the big white splotch.