Difference between revisions of "Geometry package"
m (→Meshing Octave) 

Line 25:  Line 25:  
P = bsxfun (@minus, P, centroid (P));  P = bsxfun (@minus, P, centroid (P));  
</pre>}}  </pre>}}  
−  Now we have our SVG as a polygon compatible with the Geometry package format. You can plot the polygon using the function {{CodelinedrawPolygon}}. the next step is to mesh the interior of the polygon. To do this we could just call {{Codelinedelaunay}} on the polygon and be done with it, but in general such mesh wont be so nice (you will need to add interior points). A very effective way of generating a good mesh is to use the package {{Forgemsh}}, which requires [http://geuz.org/gmsh/ Gmsh] installed in your system. The function {{Codelinedata2geo}} in the Geometry package makes our work very easy:  +  Now we have our SVG as a polygon compatible with the Geometry package format. You can plot the polygon using the function {{CodelinedrawPolygon}}. 
+  {{CodePlotting a polygon compatible with geometry package<pre>  
+  drawPolygon (P,'o');  
+  </pre>}}  
+  As you can see the polygon has lots of points. We need to simplify the polygon in order to obtain a mesh of reasonable size. Otherwise gmsh will have problems meshing and the result could be huge (or a segmentation fault :( ). THe package geometry comes with a simplification function but as of version 1.4.0, this function is very naive and wont fix this problem. If you know how to simplify polygons you can contribute!  
+  We are going to reduce the amount of points of th epolygon in a drastic and destructive way (i.e. the shape may be considerably damaged). The codes to do so follows, in the future a better procedure will be published here.  
+  {{CodeSymplification of a polygon compatible with geometry package<pre>  
+  
+  </pre>}}  
+  
+  The next step is to mesh the interior of the polygon. To do this we could just call {{Codelinedelaunay}} on the polygon and be done with it, but in general such mesh wont be so nice (you will need to add interior points). A very effective way of generating a good mesh is to use the package {{Forgemsh}}, which requires [http://geuz.org/gmsh/ Gmsh] installed in your system. The function {{Codelinedata2geo}} in the Geometry package makes our work very easy:  
{{CodeGenerating mesh for plot with msh package<pre>  {{CodeGenerating mesh for plot with msh package<pre>  
Line 32:  Line 42:  
meshsize = sqrt (mean (sumsq (diff (P, 1, 1), 2)))/2;  meshsize = sqrt (mean (sumsq (diff (P, 1, 1), 2)))/2;  
data2geo (P, meshsize, "output", [filename ".geo"]);  data2geo (P, meshsize, "output", [filename ".geo"]);  
−  T = msh2m_gmsh (filename);  +  T = msh2m_gmsh (filename); 
</pre>}}  </pre>}}  
Revision as of 07:16, 8 March 2012
The geometry package is multipackage providing functions to manipulate geometrical entities in 2D and 3D.
Relation to matGeom
octCLIP
Piecewise 2D polynomial polygons
Tutorials
Loading SVG files
Meshing Octave
Under construction
This tutorial shows the workflow to generate a triangular mesh inside an arbitrary region.
This tutorial requires that you install the package fpl and msh (which requires Gmsh installed in your system). Alternatively, the core function delaunay
could be used (the tutorial explains how) but the result aren't so pretty.
The first part of the tutorial requires an interesting shape. If you have Inkscape you can use the previous tutorial to load it into octave. Here I will be using this SVG.
Code: Loading the file as polygon compatible with geometry package 
octavesvg = svg ("octave.svg").normalize(); ids = octavesvg.pathid(); P = octavesvg.path2polygon (ids{1}, 12)(1:end1,:); P = bsxfun (@minus, P, centroid (P)); 
Now we have our SVG as a polygon compatible with the Geometry package format. You can plot the polygon using the function drawPolygon
.
Code: Plotting a polygon compatible with geometry package 
drawPolygon (P,'o'); 
As you can see the polygon has lots of points. We need to simplify the polygon in order to obtain a mesh of reasonable size. Otherwise gmsh will have problems meshing and the result could be huge (or a segmentation fault :( ). THe package geometry comes with a simplification function but as of version 1.4.0, this function is very naive and wont fix this problem. If you know how to simplify polygons you can contribute! We are going to reduce the amount of points of th epolygon in a drastic and destructive way (i.e. the shape may be considerably damaged). The codes to do so follows, in the future a better procedure will be published here.
Code: Symplification of a polygon compatible with geometry package 

The next step is to mesh the interior of the polygon. To do this we could just call delaunay
on the polygon and be done with it, but in general such mesh wont be so nice (you will need to add interior points). A very effective way of generating a good mesh is to use the package msh, which requires Gmsh installed in your system. The function data2geo
in the Geometry package makes our work very easy:
Code: Generating mesh for plot with msh package 
pkg load msh filename = tmpnam (); meshsize = sqrt (mean (sumsq (diff (P, 1, 1), 2)))/2; data2geo (P, meshsize, "output", [filename ".geo"]); T = msh2m_gmsh (filename); 
To plot the generated mesh we use the function pdemesh
from the fpl package.
Code: Plotting mesh with fpl package 
pkg load fpl pdemesh (T.p, T.e, T.t) 