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MathGroup Archive 2007

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Re: Problems updating Graphics3D Polygons

  • To: mathgroup at smc.vnet.net
  • Subject: [mg77625] Re: Problems updating Graphics3D Polygons
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Thu, 14 Jun 2007 05:19:42 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <f4ol3r$6uj$1@smc.vnet.net>

chuck009 wrote:
> Hello guys.  Can you help me write a function which takes a Graphics3D object and multiplies the z-coordinate of each point of all the polygons by a ratio.  This is what I have so far but it's not working:
> 
> In[473]:=
> ExpandZ[g_,ratio_]:=
>     g/.Polygon[pts_]:>Polygon[{#[[1]],#[[2]],#[[3]]*ratio}& /@pts];
> 
> When I test it with this simple Graphics3D object:
> 
> gobject=Graphics3D[{Polygon[{{0.,0.5,1.28},{0.,0.,1.},{0.5,0.,1.28},{0.5,
>             0.5,1.64}}],Polygon[{{
> 0.5,0.5,1.64},{0.5,0.,1.28},{1.,0.,2.71},{1.,0.5,3.49}}]}];
> 
> It doesn't do anything:
> 
> g2=Expand[gobject,2]
-----^^^^^^
Should have been *ExpandZ*

> InputForm[g2]
> 
> Out[475]=
> â??Graphics3Dâ??
> 
> Out[476]//InputForm=
> Graphics3D[{Polygon[{{0., 0.5, 1.28}, {0., 0., 1.}, 
>     {0.5, 0., 1.28}, {0.5, 0.5, 1.64}}], 
>   Polygon[{{0.5, 0.5, 1.64}, {0.5, 0., 1.28}, 
>     {1., 0., 2.71}, {1., 0.5, 3.49}}]}]
> 
> I'll keep working on it.  Thanks!
> 

Your code is fine. However, you have made a spelling mistake: you do not 
want Expand bur *ExpandZ* (that is why it is useful to use a lowercase 
letter at the beginning of the name of a user define function :-)

In[1]:= ExpandZ[g_, ratio_] := g /. Polygon[pts_] :>
          Polygon[({#1[[1]], #1[[2]], #1[[3]]*ratio} & ) /@ pts];
gobject = Graphics3D[{Polygon[{{0., 0.5, 1.28}, {0., 0., 1.},
              {0.5, 0., 1.28}, {0.5, 0.5, 1.64}}],
          Polygon[{{0.5, 0.5, 1.64}, {0.5, 0., 1.28}, {1., 0., 2.71},
              {1., 0.5, 3.49}}]}];
g2 = ExpandZ[gobject, 2]
InputForm[g2]

Out[4]//InputForm=
Graphics3D[{Polygon[{{0., 0.5, 2.56}, {0., 0., 2.}, {0.5, 0., 2.56},
     {0.5, 0.5, 3.28}}], Polygon[{{0.5, 0.5, 3.28}, {0.5, 0., 2.56},
     {1., 0., 5.42}, {1., 0.5, 6.98}}]}]

Regards,
Jean-Marc


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