Am 19.04.2016 um 19:47 schrieb Le_Forgeron:
> Le 19/04/2016 15:58, clipka a écrit :
>> 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.
> 
> It's only the first point of the curve which is special.
> 
> If you expect the prism to have a concentration of points, do not choose that region
for the first point.

What if it isn't known beforehand what points will coincide?
What if there's some external tool generating the points, and the user
of that tool isn't aware of this limitation?

I'm sure we /can/ make the syntax safer in this respect, with reasonable
effort, so I think we /should/ do that.

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