Services & Resources / Wolfram Forums / MathGroup Archive
-----

MathGroup Archive 2007

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: is the visibility of a Polygon[] exposed?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg83749] Re: is the visibility of a Polygon[] exposed?
  • From: m.r at inbox.ru
  • Date: Thu, 29 Nov 2007 06:37:51 -0500 (EST)
  • References: <figuou$foo$1@smc.vnet.net> <fijick$io6$1@smc.vnet.net>

On Nov 28, 5:11 am, m... at inbox.ru wrote:
> On Nov 27, 5:24 am, congruentialumina... at yahoo.com wrote:
>
>
>
> > Hello Mathematica UG:
>
> > Is it possible for my code to tell if a Polygon (in my graphics
> > object) is visible?
>
> > I assume that this is based upon (or is affected by) the ViewPoint->
> > setting.
>
> > Further, I assume that the kernel is deriving this somewhere to do
> > rendering. Do I have any access to this?
>
> > If I evaluate:
>
> > q = PolyhedronData["SnubCube"]
>
> > I can see the component Polygon[] and GraphicsComplex[] objects that
> > make up the Graphics3D[]. I want to make a code decision based on
> > whether a face is showing.
>
> > TIA.
>
> > Regards..Roger W.
>
> This doesn't look perfect because it draws every edge twice, but it
> gives a general idea:
>
> DynamicModule[{gr, p, cent},
>  {p, cent} = {Max@ #, Mean@ #}&@
>    N@ PolyhedronData["SnubCube", "VertexCoordinates"];
>  p *= 2 {1.3, -2.4, 2.};
>  gr = PolyhedronData["SnubCube", "Faces"] // N // Normal;
>  gr = gr /. Polygon[L_?MatrixQ, ___] :>
>     {Dynamic@ If[Det@
>        {L[[2]] - L[[1]], L[[3]] - L[[1]], p - cent - L[[1]]} >= 0,
>       Thick, Dashed],
>      Line@ Append[L, L[[1]]]};
>  Graphics3D[gr, Boxed -> False,
>   ViewCenter -> Dynamic[Automatic, None],
>   ViewVector -> Dynamic@ {p, cent}]
> ]
>
> Maxim Rytin
> m... at inbox.ru

Except I made a mistake, p - cent - L[[1]] should be just p - L[[1]].
cent can be anywhere:

DynamicModule[{gr, p, cent, cent1, cent2, r},
 {p, cent1, cent2} = {Max@ Abs@ #, Mean@ #, First@ #}&@
   N@ PolyhedronData["SnubCube", "VertexCoordinates"];
 p *= 2 {1.3, -2.4, 2.};
 cent = cent1;
 r = Norm[p - cent];
 gr = PolyhedronData["SnubCube", "Faces"] // N // Normal;
 gr = gr /. Polygon[L_?MatrixQ, ___] :>
  {Dynamic@ If[Det@
     {L[[2]] - L[[1]], L[[3]] - L[[1]], p - L[[1]]} >= 0,
    Thick, Dashed],
   Line@ Append[L, L[[1]]]};
 Manipulate[
  Graphics3D[{gr,
    {Red, AbsolutePointSize[5], Point[Dynamic@ cent]}},
   Boxed -> False, ViewCenter -> Dynamic[Automatic, None],
   ViewVector -> Dynamic[{p, cent},
    (p = r Normalize[#[[1]] - #[[2]]] + #[[2]])&]],
  {{s, False, "Fixed Vertex"},
   Checkbox[Dynamic[s, (cent = If[s = #, cent2, cent1])&]]&}]
]

Maxim Rytin
m.r at inbox.ru


  • Prev by Date: Re: PlotLabel - SubscriptBox - Vers. 6: FontColor not working?
  • Next by Date: Beginner, plot with multiple rules depending on parameters
  • Previous by thread: Re: is the visibility of a Polygon[] exposed?
  • Next by thread: File reading and writing within for loop