//------------------------------------------------------------------------------
public static Polygons OffsetPolygons(Polygons pts, double delta)
{
double deltaSq = delta*delta;
Polygons result = new Polygons(pts.Count);
for (int j = 0; j < pts.Count; ++j)
{
int highI = pts[j].Count -1;
//to minimize artefacts, strip out those polygons where
//it's shrinking and where its area < Sqr(delta) ...
double a1 = Area(pts[j]);
if (delta < 0) { if (a1 > 0 && a1 < deltaSq) highI = 0;}
else if (a1 < 0 && -a1 < deltaSq) highI = 0; //nb: a hole if area < 0
Polygon pg = new Polygon(highI*2+2);
if (highI < 2)
{
result.Add(pg);
continue;
}
List<DoublePoint> normals = new List<DoublePoint>(highI+1);
normals.Add(GetUnitNormal(pts[j][highI], pts[j][0]));
for (int i = 1; i <= highI; ++i)
normals.Add(GetUnitNormal(pts[j][i-1], pts[j][i]));
for (int i = 0; i < highI; ++i)
{
pg.Add(new IntPoint(Round(pts[j][i].X + delta *normals[i].X),
(int)(pts[j][i].Y + delta *normals[i].Y)));
pg.Add(new IntPoint(Round(pts[j][i].X + delta * normals[i + 1].X),
(int)(pts[j][i].Y + delta *normals[i+1].Y)));
}
pg.Add(new IntPoint(Round(pts[j][highI].X + delta * normals[highI].X),
(int)(pts[j][highI].Y + delta *normals[highI].Y)));
pg.Add(new IntPoint(Round(pts[j][highI].X + delta * normals[0].X),
(int)(pts[j][highI].Y + delta *normals[0].Y)));
//round off reflex angles (ie > 180 deg) unless it's almost flat (ie < 10deg angle) ...
//cross product normals < 0 . reflex angle; dot product normals == 1 . no angle
if ((normals[highI].X *normals[0].Y - normals[0].X *normals[highI].Y) *delta > 0 &&
(normals[0].X *normals[highI].X + normals[0].Y *normals[highI].Y) < 0.985)
{
double at1 = Math.Atan2(normals[highI].Y, normals[highI].X);
double at2 = Math.Atan2(normals[0].Y, normals[0].X);
if (delta > 0 && at2 < at1) at2 = at2 + Math.PI*2;
else if (delta < 0 && at2 > at1) at2 = at2 - Math.PI*2;
Polygon arc = BuildArc(pts[j][highI], at1, at2, delta);
pg.InsertRange(highI * 2 + 1, arc);
}
for (int i = highI; i > 0; --i)
if ((normals[i-1].X*normals[i].Y - normals[i].X*normals[i-1].Y) *delta > 0 &&
(normals[i].X*normals[i-1].X + normals[i].Y*normals[i-1].Y) < 0.985)
{
double at1 = Math.Atan2(normals[i-1].Y, normals[i-1].X);
double at2 = Math.Atan2(normals[i].Y, normals[i].X);
if (delta > 0 && at2 < at1) at2 = at2 + Math.PI*2;
else if (delta < 0 && at2 > at1) at2 = at2 - Math.PI*2;
Polygon arc = BuildArc(pts[j][i-1], at1, at2, delta);
pg.InsertRange((i - 1) * 2 + 1, arc);
}
result.Add(pg);
}
//finally, clean up untidy corners ...
Clipper clpr = new Clipper();
clpr.AddPolygons(result, PolyType.ptSubject);
if (delta > 0){
if(!clpr.Execute(ClipType.ctUnion, result, PolyFillType.pftNonZero, PolyFillType.pftNonZero))
result.Clear();
}
else
{
IntRect r = clpr.GetBounds();
Polygon outer = new Polygon(4);
outer.Add(new IntPoint(r.left - 10, r.bottom + 10));
outer.Add(new IntPoint(r.right + 10, r.bottom + 10));
outer.Add(new IntPoint(r.right + 10, r.top - 10));
outer.Add(new IntPoint(r.left - 10, r.top - 10));
clpr.AddPolygon(outer, PolyType.ptSubject);
if (clpr.Execute(ClipType.ctUnion, result, PolyFillType.pftNonZero, PolyFillType.pftNonZero))
result.RemoveAt(0);
else
result.Clear();
}
return result;
}
