id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
13215	Polygon union give a different result depending of the order	flamaros.xavier@…	Barend Gehrels	"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."	Bugs	new	To Be Determined	geometry	Boost 1.66.0	Not Applicable