/// <summary> /// Implements "A new algorithm for Boolean operations on general polygons" /// available here: http://liama.ia.ac.cn/wiki/_media/user:dong:dong_cg_05.pdf /// Merges two polygons, a subject and a clip with the specified operation. Polygons may not be /// self-intersecting. /// /// Warning: May yield incorrect results or even crash if polygons contain collinear points. /// </summary> /// <param name="subject">The subject polygon.</param> /// <param name="clip">The clip polygon, which is added, /// substracted or intersected with the subject</param> /// <param name="clipType">The operation to be performed. Either /// Union, Difference or Intersection.</param> /// <param name="error">The error generated (if any)</param> /// <returns>A list of closed polygons, which make up the result of the clipping operation. /// Outer contours are ordered counter clockwise, holes are ordered clockwise.</returns> private static List<Polygon> Execute(Polygon subject, Polygon clip, PolyClipType clipType, out PolyClipError error) { //Debug.Assert(subject.IsSimple() && clip.IsSimple(), "Non simple input!", "Input polygons must be simple (cannot intersect themselves)."); // Copy polygons Polygon slicedSubject; Polygon slicedClip; // Calculate the intersection and touch points between // subject and clip and add them to both CalculateIntersections(subject, clip, out slicedSubject, out slicedClip); // Translate polygons into upper right quadrant // as the algorithm depends on it Vector2D lbSubject = subject.GetCollisionBox().LowerBound; Vector2D lbClip = clip.GetCollisionBox().LowerBound; Vector2D translate; Vector2D.Min(ref lbSubject, ref lbClip, out translate); translate = Vector2D.One - translate; if (translate != Vector2D.Zero) { slicedSubject.Translate(ref translate); slicedClip.Translate(ref translate); } // Enforce counterclockwise contours slicedSubject.ForceCounterClockWise(); slicedClip.ForceCounterClockWise(); List<Edge> subjectSimplices; List<double> subjectCoeff; List<Edge> clipSimplices; List<double> clipCoeff; // Build simplical chains from the polygons and calculate the // the corresponding coefficients CalculateSimplicalChain(slicedSubject, out subjectCoeff, out subjectSimplices); CalculateSimplicalChain(slicedClip, out clipCoeff, out clipSimplices); List<Edge> resultSimplices; // Determine the characteristics function for all non-original edges // in subject and clip simplical chain and combine the edges contributing // to the result, depending on the clipType CalculateResultChain(subjectCoeff, subjectSimplices, clipCoeff, clipSimplices, clipType, out resultSimplices); List<Polygon> result; // Convert result chain back to polygon(s) error = BuildPolygonsFromChain(resultSimplices, out result); // Reverse the polygon translation from the beginning // and remove collinear points from output translate *= -1f; for (int i = 0; i < result.Count; ++i) { result[i].Translate(ref translate); SimplifyTools.CollinearSimplify(result[i].Vertices); } return result; }