clipka <ano### [at] anonymousorg> wrote:
> 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.

That's a great explanation.  I saw the same sort of messy thing in FreeCAD.
Offsetting one surface by a hair is a workaround; an extra intersection
operation as I did in the example is another.

What I found curious is that merge shows the artifacts while union doesn't.

Given that this is a well known issue with a straightforward explanation, it
would be great to have it covered in the documentation.

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