VCM rendering artifacts?

[TomClabault]

So this is VCM rendered for something like 20 minutes on the CPU and there seems to be some artifacts at the edge of the right wing and at the top of the left wing?

Initial radius 0, decay 256.

Also a question on VCM: why can we see circle patterns? Is it because the paths are "shared"/"gathered" spatially between vertices?

[serhii-rieznik]

Yes, similar to photon mapping - they are gathered in certain radius.
For preview / faster rendering you can open "Camera and Film" options and set larger pixel size

Also try adjusting initial radius. For this scene 0.01 - 0.001 should work. Otherwise (when it is zero) it is selected automatically based on scene bounding box and image size