Pretty strange. Negative scaling is perhaps the most useful undocumented
feature of pov.
Could be an epsilon problem + float-to-vector promotion problem ????
scale -1*<1,1.0001,1> works all right
Gilles

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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