Re: PolygonIntersections, MeshRange: What do they do?(Amendment)
- To: mathgroup at smc.vnet.net
- Subject: [mg20678] Re: PolygonIntersections, MeshRange: What do they do?(Amendment)
- From: "Allan Hayes" <hay at haystack.demon.co.uk>
- Date: Sun, 7 Nov 1999 02:10:11 -0500
- References: <7vrcg0$2nq@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Further to my previous posting: It seems that with PolygonIntersetions -> False, *all* polygons are triangulated in a way that gives non intersecting triangles. I had also misinterpreted the coloring For convenience I give an amended version of the original posting: PolygonIntersections I must just take the book's statement that "PolygonIntersections -> False is useful in creating graphics objects which can be sent to certain external threedimensional rendering programs" Perhaps someone else will enlarge on this, but here are some experiments: gr = Show[Graphics3D[{Polygon[{{0, 0, 0}, {2, 2, 2}, {0, 2, 2}}], Polygon[{{0, 2, 1}, {0, 0, 1}, {3, 2, 1}}], Polygon[{{1, 0, 0}, {3, 0, 0}, {3, 2, 1}, {1, 2, 0}}]}] ] InputForm[gr] gr2 = Show[gr, PolygonIntersections -> False] Why no display? Maybe for convenience in exporting to external 3D rendering programs? But we have Show[gr2] It looks as if the non-planar quadrilateral has been triangulated. But there is more: Looking at the input form we see that: - *all* polygons have been triangulated, and in a way that gives non intersecting triangles. - colors are given explicitly using FaceForm (it turns out that thes are the colors as seen in the original) - the original polygon edges are suppressed with EdgeForm[ ] and are provided by explicit Line objects However, the colors given by FaceForm are not used unless we set Lighting ->False, hence the effect of triangulation on the non-planar quadrilateral . InputForm[gr2] Turning the lighting off. removing lines and EdgeForm[ ] shows up the triangulation and the (original) coloring from FaceForm. Show[DeleteCases[gr2, _EdgeForm | _Line, Infinity], Lighting -> False] Allan --------------------- Allan Hayes Mathematica Training and Consulting Leicester UK www.haystack.demon.co.uk hay at haystack.demon.co.uk Voice: +44 (0)116 271 4198 Fax: +44 (0)870 164 0565 Ersek, Ted R <ErsekTR at navair.navy.mil> wrote in message news:7vrcg0$2nq at smc.vnet.net... > > When would I want to use the options below? I checked the documentation and > I still don't understand. > > In[1]:= > Options[Graphics3D, PolygonIntersections] > > Out[1]= > {PolygonIntersections -> True} > > ------------------- > > In[2]:= > Options[SurfaceGraphics, MeshRange] > > Out[2]= > {MeshRange -> Automatic} > > -------------------- > Regards, > Ted Ersek > > For Mathematica tips, tricks see > http://www.dot.net.au/~elisha/ersek/Tricks.html > > >