static mPoint intersection(mPoint a, mPoint b, mPoint c, mPoint d)
{
mPoint p = a;
double t = ((a.x - c.x) * (c.y - d.y) - (a.y - c.y) * (c.x - d.x)) / ((a.x - b.x) * (c.y - d.y) - (a.y - b.y) * (c.x - d.x));
p.x += (b.x - a.x) * t;
p.y += (b.y - a.y) * t;
return(p);
}
/// <summary>
/// ConvexPolygonIntersectArea
/// return the intersection area,
/// </summary>
/// <param name="a"></param>
/// <param name="b"></param>
/// <param name="intersectPolygon">full close loop polygon</param>
/// <returns></returns>
public static double CPIA(List <mPoint> a, List <mPoint> b, out List <mPoint> intersectPolygon)
{
int na = a.Count;
int nb = b.Count;
int tn, sflag, eflag;
a.Add(a[0]);
b.Add(b[0]);
intersectPolygon = new List <mPoint>();
List <mPoint> tmp = new List <mPoint>();
foreach (mPoint pa in a)
{
intersectPolygon.Add(pa);
}
for (int i = 0; i < na && nb > 2; i++)
{
sflag = SignofDouble(cross(a[i + 1], intersectPolygon[0], a[i]));
for (int j = tn = 0; j < nb; j++, sflag = eflag)
{
if (sflag >= 0)
{
tmp.Add(intersectPolygon[j]);
tn++;
}
eflag = SignofDouble(cross(a[i + 1], intersectPolygon[j + 1], a[i]));
if ((sflag ^ eflag) == -2)
{
mPoint intersets = intersection(a[i], a[i + 1], intersectPolygon[j], intersectPolygon[j + 1]); ///find the intersection point
tmp.Add(intersets);
tn++;
}
}
//memcpy(p, tmp, sizeof(Point) * tn);
intersectPolygon.Clear();
intersectPolygon = tmp;
//for(int k = 0; k < tn; k++)
//{
// p.Add(tmp[k]);
//}
nb = tn;
intersectPolygon.Add(intersectPolygon[0]);
}
if (nb < 3)
{
return(0.0);
}
return(PolygonArea(intersectPolygon, nb));
}
/// <summary>
/// /// SimplePolygonIntersectArea
/// Decompose to triangles and calculate the intersection area.
///
/// </summary>
/// <param name="a"></param>
/// <param name="b"></param>
/// <param name="intersectPolygon">full close loop polygon</param>
/// <returns></returns>
public static double SPIA(List <mPoint> a, List <mPoint> b, out List <List <mPoint> > intersectPolygon)///SimplePolygonIntersectArea
{
intersectPolygon = new List <List <mPoint> >();
int na = a.Count;
int nb = b.Count;
int i, j;
mPoint[] t1 = new mPoint[3];
mPoint [] t2 = new mPoint[3];
double res = 0, num1, num2;
a.Add(t1[0] = a[0]);
b.Add(t2[0] = b[0]);
for (i = 2; i < na; i++)
{
t1[1] = a[i - 1];
t1[2] = a[i];
num1 = SignofDouble(cross(t1[1], t1[2], t1[0]));
// area with directions. positive when sweeping towards the end, negative when backwards
if (num1 < 0)
{
GenericMethods.Swap <mPoint>(ref t1[1], ref t1[2]);
}
for (j = 2; j < nb; j++)
{
t2[1] = b[j - 1];
t2[2] = b[j];
num2 = SignofDouble(cross(t2[1], t2[2], t2[0]));
if (num2 < 0)
{
GenericMethods.Swap <mPoint>(ref t2[1], ref t2[2]);
}
List <mPoint> convexIntersectPolygon = new List <mPoint>();
res += CPIA(t1.ToList(), t2.ToList(), out convexIntersectPolygon) * num1 * num2;
intersectPolygon.Add(convexIntersectPolygon);
}
}
// merge all mergable convex polygons
intersectPolygon = mergeAllConvexPolygons(intersectPolygon);
return(res);//res is the intersection area
}
static double cross(mPoint a, mPoint b, mPoint c)
{
return((a.x - c.x) * (b.y - c.y) - (b.x - c.x) * (a.y - c.y));
}