Suspicious conditional found by PVS-Studio T48917
Original code (from Blender’s initial open-source commit!) looks like
it’s testing inputs to atan2f, to avoid undefined function values.
Passing xn=0 does *not* always evaluate to 0 though… I’m not sure if
this is a coding error or was done for a desired visual result.
Also changed xn==0 to xn>=0 to avoid function call in more cases.
Good description and visualization of atan2f function:
http://en.cppreference.com/w/c/numeric/math/atan2
bd11d917c1