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From Luc Maisonobe <Luc.Maison...@free.fr>
Subject Re: [math] Polygon Difference Question
Date Wed, 07 Sep 2011 19:15:53 GMT
```Le 07/09/2011 00:02, Curtis Jensen a écrit :
> Below is a simpler example.  Here, set2 is a polygon completely
> encompassed by the set1 polygon.  Yet the difference function produces
> a single polygon that doesn't seem to be a difference in any sense
> that I can understand.  How are the verticies of a polygon suppose to
> be interpreted?

Using these values, I see set1 as a square and set2 as a triangle which
overlaps set1 right boundary. that is to say it is partly inside set1
and partly outside set1 (for the part with x lying between -4.2 and -4.0).

So the resulting polygon with one simply connected boundary which look
like a dented square seems fine to me. The boundary vertices I get are:

-7.2 0.10000000000000002
-4.2 0.10000000000000002
-4.2 1.0705882352941176
-5.699999999999999 1.5999999999999996
-4.2 1.9529411764705877
-4.2 3.1
-7.199999999999999 3.1000000000000005

I also tried to truncate the triangle to the right by changing the
abscissas of the last to points from -4.0 to -4.3 so the triangle lies
completely inside the square also gives a result I would consider
correct: two loops defining a square with a triangular hole.

What result do you get ?

Luc

>
> Thanks,
> Curtis
>
>
>      public void testdifferenceWithHole() {
>
>      	Vector2D[][] vertices1 = new Vector2D[][] {
>          		new Vector2D[] {
>                      new Vector2D(-7.2, 3.1),
>                      new Vector2D(-7.2, 0.1),
>                      new Vector2D(-4.2, 0.1),
>                      new Vector2D(-4.2, 3.1)
>                  }
>              };
>      	PolygonsSet set1 = buildSet(vertices1);
>
>      	Vector2D[][] vertices2 = new Vector2D[][] {
>          		new Vector2D[] {
>                      new Vector2D(-5.7, 1.6),
>                      new Vector2D(-4.0, 1.0),
>                      new Vector2D(-4.0, 2.0)
>                  }
>              };
>      	PolygonsSet set2 = buildSet(vertices2);
>
>      	PolygonsSet setDiff  = (PolygonsSet) new
> RegionFactory<Euclidean2D>().difference(set1.copySelf(),
> set2.copySelf());
>      	Vector2D[][] diffVerts = setDiff.getVertices();
>      	for (int i = 0; i<  diffVerts.length; i++) {
>      		System.out.println("Verts: " + i);
>
>      		Vector2D[] set = diffVerts[i];
>          	for (Vector2D vertex : set) {
>          		System.out.println("\t" + vertex);
>          	}
>          }
>      }
>
>
> On Fri, Aug 26, 2011 at 11:41 AM, Curtis Jensen<curtis@the-jensens.org>  wrote:
>> Using math 3.0, I have two polygons with many points.  One is
>> completely contained within the other.  When I do a difference on the
>> two, I expected to get a polygon with a hole in it.  However, I get 86
>> polygons, that roughly make up a polygon with a hole in it.  If I
>> scale the points by a factor of 0.1, I get 7 polygons, and if I scale
>> it differently in the two directions, I get a different number of
>> polygons.  Sometimes the resultant polygons don't seem to make a shape
>> resembling a polygon with a hole in it.
>>
>> How should I interpret the results of the difference method?  i.e. How
>> do I process the 86 or 7 or however many polygons so that it resembles
>> 1 polygon with 1 hole in it?
>>
>> Thanks,
>> Curtis
>>
>>
>> Attached are two csv files with the points in CCW order.  Also
>> attached is a plot of the points in the two files.  Below is code I
>> class to test with (It uses the Apache Common FileUtils too)
>>
>>
>>
>>    @Test
>>     public void testDifferenceManyPoints() throws IOException {
>>         PolygonsSet set1 = csv2set(new File("src_ccw.csv"));
>>         PolygonsSet set2 = csv2set(new File("inner_ccw.csv"));
>>
>>         PolygonsSet set  = (PolygonsSet) new
>> RegionFactory<Euclidean2D>().difference(set1.copySelf(),
>> set2.copySelf());
>>         Vector2D[][] verts = set.getVertices();
>>         System.out.println(verts.length);
>>     }
>>
>>     private PolygonsSet csv2set(File file) throws IOException {
>>
>>                 Vector2D[][] verts = new Vector2D[1][linesObj.size()];
>>                 for (int i = 0; i<  linesObj.size(); i++) {
>>                         String line = (String)linesObj.get(i);
>>                         String[] tokens = line.split(",");
>>
>>                         double x = Double.valueOf(tokens[0]);
>>                         double y = Double.valueOf(tokens[1]);
>>
>>                         verts[0][i] = new Vector2D(x, y);
>>                 }
>>
>>                 return buildSet(verts);
>>         }
>>
>
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