Mac Mega Pov-Ray 3.0(MPW MrC version) can see two yellow spheres.
See this old bugreport.
Y.Tanabe
Kobe,Japan

On Wed, 6 Oct 1999 01:17:27 -0400, Mark Wagner wrote:
>The problem only occurs when scaling by <-1,-1,-1>.
Scaling by <-1,1,1>,><1,-1,1>, <1,1,-1>, or anything else doesn't cause
problems.

(Crossposted to bugreports)

Ah, now it begins to make sense.
See, when you scale a sphere it multiplies the radius and the center by the
scale factor unless it
either already has a nonuniform transform in its list or the scale factor
itself is nonuniform;
in those two cases it uses the usual,more sophisticated methods.

Obviously, making the radius negative is a bad thing.
Here's the fix, in the function Scale_Sphere in spheres.c:
   if(Sphere->Trans == NULL)
   {
      VScaleEq(Sphere->Center, Vector[X]);
-     Sphere->Radius *= Vector[X];
+     Sphere->Radius *= fabs(Vector[X]);
      Compute_Sphere_BBox(Sphere);
   }

Ken wrote:

> I have waited a few days but have not seen this appear in .general or
> in .bugreports so I am moving it from c.g.r.r. so that it is not
> overlooked by the POV-Team. If someone could confirm that this is
> indeed a bug let me know and I will post a report to .bugreports.
>
> Thanks,
>
> Ken
>
> Yannick Patois wrote:
> >
> > Hi,
> >
> > I'm quite new to povray, and I'm having problems rendering a scene(I put
> > the script below). Note that I greatly simplified my original script in
> > order to focus on the problem (the scene by itself has no real meaning).
> >
> > Harware & Software specifications :
> >
> > - Linux Mandrake 6.1 on an AMD K6II 450MHz
> > $ uname -a : Linux 2.2.13-7mdk #1 Wed Sep 15 18:02:18 CEST 1999 i586
> >
> > - "Persistence of Vision(tm) Ray Tracer Version 3.1g.Linux.gcc
> >   This is an official version prepared by the POV-Ray Team(tm)"
> > (I didnt recompile it).
> >
> > - povray called with the command :
> > $ x-povray +D +P -W640 -H480 -Q2 -i pov_problem.pov
> > (It doesnt seems to depend on the specific rendering switchs I use).
> >
> > Description of the problem :
> > The objet 'TheTest' is not drawn properly when a negative scale is
> > applied (I use negative scale to generate symetrical objects, like the
> > left hand from the description of a right hand). When a negative scale
> > is applyed, the included sphere is not drawn any more. Removing any of
> > the cylinder from the description of TheTest will increase the rendering
> > time by a factor 6 (!) but the sphere will be drawn. Note that in the
> > original scene when I noticed the problem, the cylinder and sphere where
> > not one over the other, it's only to simplify the script. There is not
> > any difference in the behavior if I move the cylinders around.
> >
> > I would be very happy if someone could help me to solve the problem
> > either by telling me what I am doing wrong or by giving me some
> > workaround. If there is a more appropriated place to ask this question,
> > please inform me too. Thanx.
> >
> >         Yannick
> >
> > Script is below :
> >
> > #include "colors.inc"
> >
> > light_source { <0, -10, -20> color White}
> >
> > camera
> > {
> >    location <0, -10, -100>
> >    look_at  <-5,  -2,  2>
> > }
> >
> > #declare TheTest=
> >     union
> >     {
> > // Removing any of this cylinders will solve the problem
> >       cylinder{<0, 0, 0>,<1, 0, 0>,1}
> >       cylinder{<0, 0, 0>,<1, 0, 0>,1}
> >       cylinder{<0, 0, 0>,<1, 0, 0>,1}
> >       cylinder{<0, 0, 0>,<1, 0, 0>,1}
> >       cylinder{<0, 0, 0>,<1, 0, 0>,1}
> >       cylinder{<0, 0, 0>,<1, 0, 0>,1}
> >       cylinder{<0, 0, 0>,<1, 0, 0>,1}
> >       cylinder{<0, 0, 0>,<1, 0, 0>,1}
> >       cylinder{<0, 0, 0>,<1, 0, 0>,1}
> >       cylinder{<0, 0, 0>,<1, 0, 0>,1}
> >       cylinder{<0, 0, 0>,<1, 0, 0>,1}
> >       cylinder{<0, 0, 0>,<1, 0, 0>,1}
> >       sphere { <3,2.5,0> 5}
> >       texture {pigment { color Yellow }}
> >     }
> >
> > // this objet is not drawn properly
> > object
> > {
> >         TheTest
> >         scale -1
> >         translate <0,20,0>
> > }
> >
> > // this one is
> > object
> > {
> >         TheTest
> >         scale 1
> >         translate <0,-20,0>
> > }
>
>       Subject: BUG + FIX: Strange povray behavior : 'forget' to draw sphere
>                object when negative scaling is applied
>          Date: Sun, 23 Jan 2000 11:10:30 +1000
>          From: Darius Davis <see### [at] studentuqeduau>
>  Organization: King's College
>            To: Yannick Patois <pat### [at] ganilfr>
>    Newsgroups: comp.graphics.rendering.raytracing
>   References:  1
>
> Hi Yannick,
>
> > The objet 'TheTest' is not drawn properly when a negative scale is
> > applied.
>
> You've found a bug!
>
> If:
> - the object is a sphere
> - there are no other complex transformations (matrix, etc) in effect
> - automatic bounding box creation is ON (eg. >= 25 objects in scene)
> - the object is scaled equally in all three axes, and
> - the scaling factor is negative
>
> the object will 'disappear' due to a bug in the bounding box
> calculations.
>
> > Removing any of
> > the cylinder from the description of TheTest will increase the rendering
> > time by a factor 6 (!) but the sphere will be drawn.
>
> The slowdown you noticed is also due to Automatic Bounding.  Put +MB1 on
> the end of the command line and it'll trace a lot faster.  (With 11
> cylinders in the union, there are 24 objects in the scene, with 12
> cylinders there are 26 objects, and the threshold for auto-bounding is
> 25 objects.)  Incidentally, adding +MB1 makes the sphere disappear even
> when there are *no* cylinders in the scene (since auto-bounding will be
> turned on, and all the other conditions for the bug to occur are also
> true).
>
> By the way, to make a left hand from a right hand, you probably want to
> only mirror it in one axis anyway, eg:
>         scale <-1, 1, 1>
> to mirror in the x-axis.  Doing this, the problem won't happen anyway!
>
> If you really do need to apply a negative uniform scale to such an
> object, I know of two workarounds:
>
> If precision is not essential, scale the object slightly unevenly, ie.
>
>         scale <-2, -2, -2.0001>
>
> If precision is required, just put an identity 'matrix' transformation
> before the negative scaling, as in:
>
> object
> {
>         TheTest
>         matrix <1, 0, 0,   0, 1, 0,  0, 0, 1,  0, 0, 0>
>         scale -2
>         translate <0,20,0>
> }
>
> (Look up 'matrix' in the documentation if you're not familiar with it)
>
> Or put
>         scale <-2, -2, 2> scale <1, 1, -1>
>
> There's probably another simpler transformation you could use, but
> that's what springs to mind.
>
> Alternatively, you could disable Automatic Bounding (put -MB on the end
> of the command line), but that will slow down the tracing considerably.
>
> For those of you who may wish to fix it yourself, here's the offending
> line of source:
>
> SPHERES.C, function Scale_Sphere, approx line 652:
> Change
>     Sphere->Radius *= Vector[X];
> to
>     Sphere->Radius *= fabs(Vector[X]);
>
> I'll send this to the POV-Team ASAP.
>
> Darius
>
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> | s328919 at |      WYSIWYD - What you see is what you desire.
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