Polygon union give a different result depending of the order

[flamaros.xavier@…]

I found a precision issue with union between polygons and the result vary depending of the order.

We add consecutive polygons to a multi-polygon that contains the previous result. The union of polygons (that have only one outer contour) in the following order give an empty union:

Polygon[(-5.180416107177734375,0.46142292022705078125),(-2.6576340198516845703125,0.46142292022705078125),(-2.6576340198516845703125,4.818223476409912109375),(-2.6576340198516845703125,7.19262790679931640625),(-7.35814762115478515625,7.19262790679931640625),(-7.35814762115478515625,4.818223476409912109375),(-7.35814762115478515625,0.46142292022705078125)]
Polygon[(-7.35814762115478515625,-1.8333890438079833984375),(-7.35814762115478515625,-5.162303924560546875),(-3.343097686767578125,-5.162303924560546875),(-3.3430979251861572265625,-1.19531953334808349609375),(-3.343098163604736328125,0.4614227116107940673828125),(-2.6576340198516845703125,0.4614226818084716796875),(-2.6576340198516845703125,4.79442691802978515625),(-5.180416107177734375,4.79442691802978515625),(-7.35814762115478515625,4.79442691802978515625),(-7.35814762115478515625,0.46142292022705078125)]
Polygon[(4.4465618133544921875,-5.162303924560546875),(4.4465618133544921875,-1.195319652557373046875),(3.5822856426239013671875,-1.195319652557373046875),(-3.3430979251861572265625,-1.19531953334808349609375),(-3.343097686767578125,-5.162303924560546875)]
Polygon[(3.5822856426239013671875,-1.195319652557373046875),(3.5822856426239013671875,0.4614227116107940673828125),(-2.6576340198516845703125,0.4614226818084716796875),(-3.343098163604736328125,0.4614227116107940673828125),(-3.3430979251861572265625,-1.19531953334808349609375)]
Polygon[(-0.0815036296844482421875,-5.162303924560546875),(-0.0815036296844482421875,-4.110321521759033203125),(-2.6576340198516845703125,-4.110321521759033203125),(-2.6576340198516845703125,-5.162303924560546875)]
Polygon[(-0.0815036296844482421875,0.46142292022705078125),(-2.6576340198516845703125,0.46142292022705078125),(-2.6576340198516845703125,-0.438062191009521484375),(-0.0815036296844482421875,-0.438062191009521484375)]
Polygon[(-2.6576340198516845703125,0.46142292022705078125),(-2.6576340198516845703125,-0.438062191009521484375),(-2.6576340198516845703125,-4.110321521759033203125),(-2.6576340198516845703125,-5.162303924560546875),(-0.0815036296844482421875,-5.162303924560546875),(-0.0815036296844482421875,-4.110321521759033203125),(-0.0815036296844482421875,-0.438062191009521484375),(-0.0815036296844482421875,0.46142292022705078125)]

Same polygons in a different order give the expected result:

Polygon[(4.4465618133544921875,-5.162303924560546875),(4.4465618133544921875,-1.195319652557373046875),(3.5822856426239013671875,-1.195319652557373046875),(-3.3430979251861572265625,-1.19531953334808349609375),(-3.343097686767578125,-5.162303924560546875)]
Polygon[(-2.6576340198516845703125,0.46142292022705078125),(-2.6576340198516845703125,-0.438062191009521484375),(-2.6576340198516845703125,-4.110321521759033203125),(-2.6576340198516845703125,-5.162303924560546875),(-0.0815036296844482421875,-5.162303924560546875),(-0.0815036296844482421875,-4.110321521759033203125),(-0.0815036296844482421875,-0.438062191009521484375),(-0.0815036296844482421875,0.46142292022705078125)]
Polygon[(-0.0815036296844482421875,-5.162303924560546875),(-0.0815036296844482421875,-4.110321521759033203125),(-2.6576340198516845703125,-4.110321521759033203125),(-2.6576340198516845703125,-5.162303924560546875)]
Polygon[(3.5822856426239013671875,-1.195319652557373046875),(3.5822856426239013671875,0.4614227116107940673828125),(-2.6576340198516845703125,0.4614226818084716796875),(-3.343098163604736328125,0.4614227116107940673828125),(-3.3430979251861572265625,-1.19531953334808349609375)]
Polygon[(-0.0815036296844482421875,0.46142292022705078125),(-2.6576340198516845703125,0.46142292022705078125),(-2.6576340198516845703125,-0.438062191009521484375),(-0.0815036296844482421875,-0.438062191009521484375)]
Polygon[(-7.35814762115478515625,-1.8333890438079833984375),(-7.35814762115478515625,-5.162303924560546875),(-3.343097686767578125,-5.162303924560546875),(-3.3430979251861572265625,-1.19531953334808349609375),(-3.343098163604736328125,0.4614227116107940673828125),(-2.6576340198516845703125,0.4614226818084716796875),(-2.6576340198516845703125,4.79442691802978515625),(-5.180416107177734375,4.79442691802978515625),(-7.35814762115478515625,4.79442691802978515625),(-7.35814762115478515625,0.46142292022705078125)]
Polygon[(-5.180416107177734375,0.46142292022705078125),(-2.6576340198516845703125,0.46142292022705078125),(-2.6576340198516845703125,4.818223476409912109375),(-2.6576340198516845703125,7.19262790679931640625),(-7.35814762115478515625,7.19262790679931640625),(-7.35814762115478515625,4.818223476409912109375),(-7.35814762115478515625,0.46142292022705078125)]

The result:

Polygon[(-0.0815036296844482421875,-5.162303924560546875),(-0.0815036296844482421875,-4.110321521759033203125),(-2.6576340198516845703125,-4.110321521759033203125),(-2.6576340198516845703125,-5.162303924560546875)]

Notes:

1. We use float
2. This issue happens with mingw32, when tested with Visual 2015 (32bits compilation too) everything is good. Both compiler are used with default options for float precision strategy.
3. Prints are made with "%.64g" format option of printf to avoid financial round.
4. Those polygons are counter clockwise and open, but I also test them reversed and close, it gives the same result.

[flamaros.xavier@…]

Sorry this is a bad report the error is in our side, the issue comes from holes that have a polygon that don't respect the polygon rules. We have some holes that intersect themselves, we will find a way to avoid that.

Please accept my apologies.