Am 18.04.2016 um 18:52 schrieb Le_Forgeron:

> Prism can have more than one loop/fold.
> 
> I can reproduce the issue with :
> 
> #declare P0=<0,0>;
> #declare P1=<1,0>;
> #declare P2=<2,1>;
> #declare P3=P1;
> #declare P4=<4,1>;
> #declare P5=<5,0>;
> 
> prism{cubic_spline 0, 1, 7, P0, P1, P2, P3, P4, P1, P5}
> 
> Notice that the problem is between the keyboard and the chair:

Hm... I don't think it's that easy. A reasonable case can be made for
single loops with coincident control points; think e.g. of an hourglass
shape in an animation, where the narrow portion is supposed to reach
zero in one frame. The case is even easier to make for Bezier splines.

So I think we should consider this a flaw in the prism syntax, and think
of how this could be remedied.

One possible solution would be to introduce an alternative syntax like so:

    prism {
      cubic_spline 0, 1
      loop { 7, P0, P1, P2, P3, P4, P1, P5 }
    }

This would make it clear that a single loop is desired, even if control
points match. Obviously, multiple loops would be specified using
multiple "loop" blocks.

Lathe should get the same extended syntax of course.

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