This number (default value = 100.0) controls the behaviour of the
z-sorting algorithm. When an edge is very close to anothers polygon
plane, there is an interval of uncertainty in which you cannot tell which
object is visible (which z is smaller). This is due to cumulative
numerical errors for edges that have undergone a lot of transformations
The default value means that if the 1/z values (in projected space)
differ by only 1/100 (one percent), they are considered to be equal and
the x-slopes of the planes are used to find out which plane is getting
closer when we move to the right.
Larger values means narrower margins, and increasing the chance of
missing true adjacent edges/planes. Smaller values means larger margins,
and increasing the chance of mistaking close polygons for adjacent ones.
The value of 100 is close to the optimum. However, the optimum shifts
slightly with resolution, and may be application-dependent. It is here
for you to fine-tune.