Wasn't it Francesco who wrote:
>I need to generate only a portion of an ellipsoid (a surface, not a solid). The
>best way to proceed is by declaring it as a difference between an ellipsoid and
>a box? Thanks in advance for help. I am trying to learn by myself but I am
>afraid it will require a lot of time, so any help is useful.

Using a difference makes it look like a solid rather than a surface. Try
clipped_by instead of difference.

Note that you can make an ellipsoid by scaling a sphere.

sphere {0,2
  scale <2,1,0.5>
  clipped_by {box {<-5,-5,-0.2><5,5,0.7>}}
  pigment {rgb 1}
}

-- 
Mike Williams
Gentleman of Leisure

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