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

*To*: mathgroup at smc.vnet.net*Subject*: [mg83709] Re: is the visibility of a Polygon[] exposed?*From*: m.r at inbox.ru*Date*: Wed, 28 Nov 2007 05:48:50 -0500 (EST)*References*: <figuou$foo$1@smc.vnet.net>

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.r at inbox.ru