>> Depends on exactly which way it isn't perfectly cylindrical in.
> 
> How about simply a stretched cylinder, ie an extruded ellipse.  You 
> can't simply scale the torus or cone otherwise the radius/bevel will not 
> be constant around the perimeter.

Yeah. In that case you'd have to use a sphere-sweep [which is 
inexplicably slow for no defined reason].

> As well as having elliptical buttons, the top surface of the buttons on 
> my phone are also curved (concave inwards).  I suppose you could model 
> this in POV by subtracting a big sphere from the extruded oval.

Indeed, that's exactly how you'd do it. Much like if you wanted a convex 
top instead.

> But how 
> to do the rounded edge then?  Matching up a round with the vertical 
> sides (actually they're slightly off-vertical) of the button with the 
> curved top surface doesn't sound very easy to me.

It does to me, but hey...

>> Really, you can go a seriously long way using only quadratics and CSG.
> 
> Do you have any examples?

Not to hand, no.

>> Think about it - if you wanted to model the Natural History Museum, 
>> would you built it out of a few quadrillion triangles? Or just cut a 
>> few solids out of each other? I know what I'd choose...
> 
> And I know which one would look like a photo-realistic model and which 
> one would look like a few solids cut out of each other :-)

Yes, because crude approximations to cylindrical columns look so much 
more photo-realistic than actually cylindrical columns. Oh, wait...

>> Mmm, interesting. In all the years I've been using POV-Ray, I've never 
>> actually noticed that before...
> 
> I didn't realise it either until I got into designing stuff that people 
> will actually see (as opposed to internal components).  Most companies 
> will reject any curvature discontinuities.

Ah, well, presumably if you're doing CAD rather than just pretty 
pictures, you have *real* modelling tools. ;-)

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