> I would like to create prisms with a combination of straight lines and curves -
> including circular.
>
> For a very very simple example - a 4*4 box shape but with one corner rounded off
> with a circular radius of 1.
>
> I am struggling to work out how to do this with either a cubic or quadratic
> spline and cannot find an example.  Should I be using beziers?    I've tried not
> to be lazy but would appreciate some kind experienced Pov user showing me the
> way!
>
> I appreciate my simple example could be achieved with some bits in a union, but
> I am looking for a more generic approach for more complex shapes combining
> straight lines and curves and want to make sure I'm not chasing something
> impossible / impractical.
>
> Excuse my lack of mathematical understanding, but is it also the case that
> bezier curves cannot truly implement a circular curve?
>
> Many thanks for any help.
>
>
>
>
>
Bezier spline looks your best bet. Then, even with the added control it 
gives you, you may need to subdivide the curved parts so that they 
follow the desired curve well enough. You'll need to experiment with the 
length of the control lines to get the desired curvature.

Second to that is the cubic spline.
I can't recomend the use of the quadratic spline as the control ot the 
curvature is only effective at one end of every segments.

Another possibility would be to increase the number of points for your 
prism and use a cubic spline. You need at least 3 points, placed very 
close together, at each corners to make them sharp when you use a cubic 
spline. Using that approach, there will be a small chamfer at those 
corners. It can be a good thing in several cases, but it may not be 
exactly what you want in this case.


Alain

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