Blender Git Loki

Cycles: hide particles with broken motion blur traces.

Currently cycles cannot correctly render motion blur for objects that appear or
disappear during the shutter window. Until that can be fixed properly, it may be
better to hide such particles rather than let them render as if they were
stationary for half of the frame.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D2125