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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