"Chris B"  wrote in message <458bd48e$1@news.povray.org>:
> I may be missing something obvious to everyone else here, but why not just 
> use merge instead of union to get rid of the internal coincident surfaces?

Because a merge does not avoid the problem of coincident surfaces. In fact,
this is quite the opposite: using a merge instead of an union makes the
problem worst.

More precisely, coincident surfaces are cases where two objects share a same
boundary surface. Due to calculations made with floating point arithmetic
and not perfect mathematical objects, rounding errors occur, and the ray
tracer may consider that the ray hit one object first and the other next, or
the other way around.

If both objects have exactly the same texture, it is not a problem at all.

With a merge in the situation I am describing, on the other hand, it changes
everything. If the rounding errors tell that the ray first enters the second
object and then leaves the first, then the merge takes place, and no
boundary is seen. On the other hand, if the rays first leaves the first
object and then enters the second, then the boundary must be drawn.

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