Lutz Kretzschmar wrote in message <37f9d4e7.14580615@194.174.214.110>...
>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 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.

Mark

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