Am 10.05.2018 um 22:20 schrieb pkoning:
> I was trying to make a rounded rectangle pane of glass by union { } of some
> cylinders and boxes.  That of course is a bad idea because the interior
> surfaces.  Replaced it by merge, and got a very ugly result.  It appears that
> the ray tracing on a merged object gets very confused if the ray strikes a
> surface that is coincident on more than one element.

Yes, that's a well-know issue. Wherever there are somewhat coincident
surfaces in a merge, intersection or difference, you can expect trouble.

The reason for those so-called coincident surface artifacts is (in those
case) that POV-Ray needs to test each and every ray-surface intersection
with any single element and see if the intersection is inside or outside
the other elements, to decide whether the surface should be visible or
suppressed. However, intersection points jitter a bit due to the
imperfect nature of floating-point maths, so if the intersections are on
a surface that is coincident with that of another element, the
intersections jitter in and out of that other element, toggling them
from visible to suppressed and back.

It's essentially the raytracing equivalent of Z-fighting.

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