In article <3f3696fe@news.povray.org>, Wolfgang Wieser <wwi### [at] gmxde> 
wrote:

> A paraboloid is essentially y=x^2 rotated around the Y axis (isn't it??)
> So, using a quadratic spline and modelling just that y=x^2 curve 
> seems to me the most natural approach to me. 

A more precise description would be "y = x^2 + z^2", though it is 
equivalent to sweeping a 2D curve as you described.


> Why should I use a quadric? Is it really faster than a lathe?

A quadric would be a paraboloid, while a lathe would just look like one. 
As for speed, a lathe is a far more complex object. I would expect 
slower rendering and much higher probability of artifacts. Also, 
although there is a quadratic spline, it may not give the expected 
result, and would be a bit more awkward to control...splines are 
generally not used for precise mathematical curves. A lathe *can* be 
used for this, but it is certainly not the best tool for the job. An 
isosurface or quadric would be a perfect match for this task. A quadric 
is more difficult to use than an isosurface, but the isosurface will be 
less precise. Either one may be faster, it depends on the situation.

And finally, there are already predefined paraboloid quadrics in the 
includes...

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: chr### [at] tagpovrayorg
http://tag.povray.org/

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