Wasn't it Carl Hoff who wrote:
>This is cut from a post of mine over in the images area that I think might
>serve me better here.
>
>I'm still playing with the Draw_Vistas option and I'm still not
>sure if I need the -UR flag to be able to use my manual bounding
>or not.
>
>With these options (+UD +MB0) try the following:
>
>  #include "colors.inc"
>  camera {location <-500, 0, 0> look_at <0, 85, 0> angle 36}
>  light_source { <110, 5, -500> White}
>
>  #declare rear_tire = difference {
>    sphere {<0, 85, 0>, 85 scale <1,1,.2>}
>    cone {<0, 85, 20>, 50 <0, 85, -20>, 60}
>    cone {<0, 85, -20>, 50 <0, 85, 20>, 60}
>    bounded_by { box {<-85, 0, -13>, <85, 170, 13>} }
>  }
>
>  object {rear_tire pigment {Blue}}
>
>Now comment out the bounded_by line. You'll be able to
>see the difference even without the -UR flag.

One of the things that is going on is that POV-Ray is reasonably smart
about knowing when it is likely to be helpful to remove manual bounding.
+UR doesn't remove all manual bounds, only manual bounds where POV-Ray
thinks it can do better. In this case, it has trusted you to provide
better manual bounds than it would have applied automatically.

>
>However while playing with this code using the same options:
>
>  #include "colors.inc"
>  camera {location <0, 0, -500> look_at <0, 85, 0> angle 36}
>  light_source { <110, 5, -500> White}
>
>  #declare rear_tire = difference {
>    sphere {<0, 85, 0>, 85 scale <1,1,.2>}
>    cone {<0, 85, 20>, 50 <0, 85, -20>, 60}
>    cone {<0, 85, -20>, 50 <0, 85, 20>, 60}
>    bounded_by { box {<-85, 0, -13>, <85, 170, 13>} }
>  }
>
>  #declare rear_axle = sphere {<0, 85, 0>, 17}
>
>  #declare rear_hub = difference {
>    cylinder {<0, 85, 1>, <0, 85, -1>, 60}
>    object {rear_tire}
>    object {rear_axle}
>    bounded_by { box {<-56, 29, -2>, <56, 141, 2>} }
>  }
>
>  object {rear_hub pigment {Blue}}
>
>I note that even with the -UR flag on, the bounding slab
>size doesn't visual change from the default.

In this case, the automatic bounding box calculated by POV-Ray is
*considerably* better than the one you provided manually. I would have
though that -UR would force it to use the manual bounding. However, the
documented reason why you would want to keep inefficient manual bounds
doesn't apply in this case, and perhaps POV-Ray is smart enough to have
worked that out.

Your manual bounding box has a volume of 50,176 cubic units, whereas the
automatic bounds calculated by POV-Ray has a volume of 28800 cubic
units. A quick experiment shows that POV-Ray is choosing the bounds that
have the lower volume irrespective of the UR setting.

Try making the Z values of your manual bounding box a closer fit.

-- 
Mike Williams
Gentleman of Leisure

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