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MathGroup Archive 2007

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Re: Anomolous behaviour of Penrose Triangle Demonstration

  • To: mathgroup at smc.vnet.net
  • Subject: [mg81103] Re: [mg81085] Anomolous behaviour of Penrose Triangle Demonstration
  • From: Curtis Osterhoudt <cfo at lanl.gov>
  • Date: Wed, 12 Sep 2007 03:51:04 -0400 (EDT)
  • Organization: LANL
  • References: <200709110923.FAA26497@smc.vnet.net>
  • Reply-to: cfo at lanl.gov

Hi, Syd, 

   I have no real insight. I confirm that changing the ViewPoint prefactor 
causes the same behavior on my machine. I did not try to evaluate the code 
which allows "vp" to be changed via a slider; just rotating the graphic 
produced by the first chunk of code causes part of it to disappear (somewhat 
randomly).
    Of course, I've been having lots of problems with my ATI drivers not being 
compatible with Mathematica 6.0's Graphics3D[] objects, so this may be a 
symptom of that.

   This is on "6.0 for Linux x86 (32-bit) (June 19, 2007)".



On Tuesday 11 September 2007 03:23:52 you wrote:
> I would appreciate some insight regarding the Penrose Triangle
> Demonstration which can be found at:-
>
> 		http://demonstrations.wolfram.com/PenroseTriangle/
>
> The original source I downloaded when run on my machine displayed a
> blank panel!
> (The demonstration ran fine on my machine in "watch web preview" mode!)
>
>
> $Version    Out[1]= "6.0 for Mac OS X x86 (32-bit) (June 19, 2007)"
>
> The demonstration source was;-
>
> Manipulate[
>   cu[x_, y_, z_, d_] :=
>    Cuboid[ {x - d/2, y - d/2, z - d/2}, {x + d/2, y + d/2, z + d/2} ];
>   Graphics3D[
>    {Opacity[o],
>     cu[0, 15, 30, 7.5], cu[15, 45, 0, 7.5],
>     Table[cu[0, 0, 15 n, 7.5], {n, 0, 3}],
>     Table[cu[0, 15 n, 0, 7.5], {n, 3}]
>     } ,
>          ViewPoint -> 200 {-1, -1, 1},
>          Boxed -> False,
>          PlotRange -> 50 {{-1, 0.8}, {-0.6, 1}, {-0.08, 1}},
>          ImageSize -> {400, 400}
>    ],
>         {{o, 1, "opacity"}, 0, 1}, TrackedSymbols :> Manipulate]
>
>
> I changed the "viewpoint factor" in  ViewPoint -> 200 {-1, -1, 1}
> above to 100{-1, -1, 1} on a hunch and the panel
> displayed correctly.
>
> After failing to find any documentation for the use of the "viewpoint
> factor" I tried setting a range for vp and generating a slider in
> addition to opacity to see the effects.
>
> Manipulate[
>   cu[x_, y_, z_, d_] :=
>    Cuboid[ {x - d/2, y - d/2, z - d/2}, {x + d/2, y + d/2, z + d/2} ];
>   Graphics3D[
>    {Opacity[o],
>     cu[0, 15, 30, 7.5], cu[15, 45, 0, 7.5],
>     Table[cu[0, 0, 15 n, 7.5], {n, 0, 3}],
>     Table[cu[0, 15 n, 0, 7.5], {n, 3}]
>     } ,
>          ViewPoint -> vp {-1, -1, 1},
>          Boxed -> False,
>          PlotRange -> 50 {{-1, 0.8}, {-0.6, 1}, {-0.08, 1}},
>          ImageSize -> {400, 400}
>    ],
>         {{o, 1, "opacity"}, 0, 1}, {{vp, 10, "ViewPoint Factor"}, 10,
> 200, 10},
>    TrackedSymbols :> Manipulate]
>
> The effects of using the vp slider are very odd. Bits of the graphic
> start disappearing!
>
> Any clues as to what is happening here would be appreciated.
>
>
> Yet another strange "feature" showed up when I tried to "Save As" the
> modified demonstration code.
>
> I cannot successfully save the modified demonstration.
>
> I suspect that is a WRI safeguard for protecting the integrity of the
> Demonstrations Project (which is fine by me).
>
> I can of course save the modified code in a new blank notebook.
>
> Syd Geraghty



-- 
==========================================================
Curtis Osterhoudt
cfo at remove_this.lanl.and_this.gov
PGP Key ID: 0x4DCA2A10
Please avoid sending me Word or PowerPoint attachments
See http://www.gnu.org/philosophy/no-word-attachments.html
==========================================================


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