The 'sphere' is definately a volitle primitive from what I have seen.  The
bug notion sounds very plausible, at least in the sense that maybe it could
be hardened against such defects maybe.  Possibly not a true bug in the
sense of what is normally expected in everyday use.

Bob

Margus Ramst <mar### [at] peakeduee> wrote in message
news:37F9F793.C93F696C@peak.edu.ee...
> The code itself is OK. The problem seems to be with automatic bounding.
Add
> manual bounding to the union and the spheres reappear.
> Also, if you replace the central cylinder with a sphere, this sphere also
> disappears upon scaling. So I don't think the problem is with bounding
boxes not
> getting transformed (you can also check this by placing the camera along
the X
> axis - the spheres still won't appear).
> Anyway I'm pretty confident it's a bug.
>
> Margus
>
> Lutz Kretzschmar wrote:
> >
> > Hi All,
> >
> > while trying to track down a bug I found that negative scaling can
> > make objects disappear and was wondering (not being too well-versed in
> > POV) whether this is legal or not:
> >
> > default { pigment { color rgb <1,1,0> }}
> >
> > union {
> >   sphere {
> >     <-1,0,0>,1
> >   }
> >   sphere { // Sphere101
> >     <1,0,0>,1
> >   }
> >   cylinder { // Cylndr034
> >     <-1,0,0>, <1,0,0>, 0.7
> >   }
> >   scale 1 // <- Change to -1 and see the spheres disappear
> > }
> >
> >
> > light_source
> > {
> >   0*x // light's position (translated below)
> >   color red 1.0  green 1.0  blue 1.0  // light's color
> >   translate <20, 40, 50>
> > }
> >
> > camera { location <0,0,6> look_at 0 }
> >
> > The docs do not say anything about negative scaling. It is
> > inconsistent in that the cylinder does not disappear, though.
> >
> > Thoughts?
> >
> > - Lutz
> >   email : lut### [at] stmuccom
> >   Web   : http://www.stmuc.com/moray

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