Java polygons and Area

Question

Ok so I have a number of polygons (outlined in white in the image).

In an attempt to add all the polygons together so that I get one polygon, which is the outer bounds of all of them, I have converted each Polygon (java class) to an Area(java class) and then added the areas together with the add(Area a) method provided by the Area class.

From there I converted the area back to a polygon and drew it as a filled Polygon (the purple area in the image).

This however is not what I was expecting. Can anyone think of a was where I can do this correctly, or at least just add the triangle area to my polygon?

When I just draw all the separate polygons with a fill I get what I want (drawn) but I need a Polygon representation of it.

What I want

Might you be colorblind? That doesn't look purple to me! Also, if that wasn't what you were expecting, what _were_ you expecting? — cheeken, Sep 05 '12 at 04:21
I wasn't expecting what appears in the first image (top one). I WAS expecting the second image (bottom one, without the triangle gap in the middle that appears in the top one). — user1135469, Sep 05 '12 at 04:26
Could be a winding issue...unfortunately that's about all I can say :P — MadProgrammer, Sep 05 '12 at 05:03
What happens if you fill the added Area? (before converting back to polygon) — lbalazscs, Sep 05 '12 at 17:56

Lars · Answer 1 · 2017-03-15T15:05:25.410

I can recommend to use JTS. Just create your geometries you want to combine and use the union method. Afterwards you can use the new geometry and paint it using your previous code. Simple code example for union:

// build polygon p1
Polygon p1 = new GeometryFactory().createPolygon(new Coordinate[]{new Coordinate(0,0), new Coordinate(0,10), new Coordinate(10,10), new Coordinate(10,0), new Coordinate(0,0)});
// build polygon p2
Polygon p2 = new GeometryFactory().createPolygon(new Coordinate[]{new Coordinate(0,0), new Coordinate(0,30), new Coordinate(5,30), new Coordinate(5,0), new Coordinate(0,0)});
// calculate polygon3 as the union of p1 and p2
Polygon p3 = (Polygon) p1.union(p2);
// print simple WKT
System.out.println(p3.toText());

Output in this case is (like expected):

POLYGON ((0 0, 0 10, 0 30, 5 30, 5 10, 10 10, 10 0, 5 0, 0 0))

Java polygons and Area

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