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Re: Displaying cylinders

  • To: mathgroup at smc.vnet.net
  • Subject: [mg110844] Re: Displaying cylinders
  • From: Albert Retey <awnl at gmx-topmail.de>
  • Date: Thu, 8 Jul 2010 06:50:09 -0400 (EDT)
  • References: <i13trd$7hu$1@smc.vnet.net>

Am 08.07.2010 09:13, schrieb S. B. Gray:
> I have displays like this:
> 
> th = .01;
> cp1 = {  1, -1, 2};
> cp2 = {  2,   0, 1};
> cp3 = {-1,   3, 2};
> Graphics3D[{Opacity[.4],
>    Yellow, Cylinder[{cp1, cp1 + {th, -th, th}}, 12],
>    Green,  Cylinder[{cp2, cp2 - {th, th, -th}}, 15],
>    Red,    Cylinder[{cp3, cp3 + {-th, -th, -th}}, 12]},
>   Boxed -> False]
> 
> but the cylinders are stand-ins for what I really want, which is circles 
> in 3D. (I would like a 3D circle primitive with circles defined by three 
> points, rather than the way cylinders are specified.)
> 
> The smallest number of circles in the actual application is 20, so it is 
> almost impossible to see how they interact, no matter what Opacity is 
> set to. It would be much better if I could make the edges thicker and 
> opaque (with EdgeForm), and have the cylinder ends or sides transparent. 
> That way the edges (rings or circles) would occlude each other so it 
> would be clear what is in front of what. So far as I know, the edges 
> cannot be contolled separately. If you set "th" to a higher value, you 
> get a double ring which is really the cylinder side, which also cannot 
> be given its own Opacity.

I think I am missing your point, the following does what you described
and seems to work, is it something else that you want?

th = .01;
cp1 = {1, -1, 2};
cp2 = {2, 0, 1};
cp3 = {-1, 3, 2};
Graphics3D[{
  Opacity[0],
  EdgeForm[Directive[Opacity[1], Thickness[th], Yellow]],
  Cylinder[{cp1, cp1 + {th, -th, th}}, 12],
  EdgeForm[Directive[Opacity[1], Thickness[th], Green]],
  Cylinder[{cp2, cp2 - {th, th, -th}}, 15],
  EdgeForm[Directive[Opacity[1], Thickness[th], Red]],
  Cylinder[{cp3, cp3 + {-th, -th, -th}}, 12]
  }, Boxed -> False]

I have included Opacity in EdgeForm to show that this works, but of
course with a value of 1 you could just as well omit it...

hth,

albert


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