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From Curtis Jensen <curtis.jen...@gmail.com>
Subject Re: [math] Polygon Difference Question
Date Fri, 09 Sep 2011 20:27:48 GMT
Ooopps, I got my signs mixed up when trying to make a simple example.
This example works as expected even when I move the triangle
completely in the square.

Please disregard this example (user error).

--
Curtis


On Wed, Sep 7, 2011 at 12:15 PM, Luc Maisonobe <Luc.Maisonobe@free.fr> wrote:
> 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
>>> added to the
>>> org.apache.commons.math.geometry.euclidean.twod.PolygonsSetTest
>>> 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 {
>>>                List linesObj = FileUtils.readLines(file);
>>>
>>>                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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>>
>
>
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