Unify fails when polygons are wound in opposite directions

If two polygons are constructed, the first in a CCW direction and the second in a CW direction, the resulting union is not correct:

const polygon1 = new Polygon();
polygon1.addFace([point(0, 0), point(10, 0), point(10, 10), point(0, 10)]);

const polygon2 = new Polygon();
polygon2.addFace([point(8, 2), point(8, 8), point(20, 8), point(20, 2)]);

const result = unify(polygon1, polygon2);
// result is not the expected shape

[alexbol99]

Hi, David, Flatten-js support multi-polygons, comprised from a number of faces, some of them may be external, some internal. Internal faces represent holes - cut-off areas inside a polygon, and their orientation should be opposite to the outer polygon they belong. unify (and other boolean operations) assume that both polygons have same meaning of orientation, for example, in both polygons outer faces oriented counterclockwise, and inner faces oriented clockwise. Best regards, Alex

[alexbol99]

I have a method polygon.splitToIslands() which returns an array of polygons, where each polygon is ordered by inclusion of faces: the first face is an outer (we call it “island”) and the others are internal (we call them "holes). You can use it before and after unify , because unify indeed does not keep an order of faces. I can also add a method that rearrange faces in polygon in the way that island always precede holes, if it may help Best, Alex