"Nicolas George" <nicolas$george@salle-s.org> wrote in message 
news:458c2171@news.povray.org...
> "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.
>

Hi Nicolas,
The example I posted did not manifest this problem (even using different 
colours).

> 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.
>

Yes. I understood that bit.

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

A few things here:
  o  Firstly, substituting different colours for each of the two parts of 
the merge in the example I posted didn't show a problem.
  o  Secondly, because the coincident surface is the one you don't want to 
display, then why would you need to use a different texture for each 
instance of it.
  o  Thirdly, if it causes a problem in the particular circumstances you 
have and you really want to use a union, then why not just make the cutting 
surfaces transparent as in the following example:

camera {location <0,0,-1> look_at 0}
light_source { <-1, 0.3, -2> color rgb 1}
light_source { <-1, 0.3,  2> color rgb 1}

union {
  difference {
    cylinder {-y,y*0.2,0.2}
    plane {y,0 rotate z*225 pigment {color rgbt 1}}
    pigment {color rgbt <1,1,0,0.7>}
  }
  difference {
    cylinder {-x,x*0.2,0.2}
    plane {y,0 rotate z*45 pigment {color rgbt 1}}
  }
  texture {
    pigment {color rgbt <0,1,1,0.4>}
    normal {agate}
  }
  rotate y*45
}

If you still get a problem, why not post a snippet that illustrates your 
problem.

Regards,
Chris B.

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