private void GetStartEndPoints(out Point start, out Point end, out double startAngle)
{
start = null;
end = null;
startAngle = -1;
// Find the first point so we sweep in a counter-clockwise manner.
double angle1 = Point.GetRadianStandardAngleWithCenter(theCircle.center, endpoint1);
double angle2 = Point.GetRadianStandardAngleWithCenter(theCircle.center, endpoint2);
// Define to vectors: vect1: A----->B
// vect2: B----->C
// Cross product vect1 and vect2.
// If the result is positive, the sequence A-->B-->C is Counter-clockwise.
// If the result is negative, the sequence A-->B-->C is clockwise.
Point vect1 = Point.MakeVector(this.middlePoint, this.endpoint1);
Point vect2 = Point.MakeVector(this.endpoint2, this.middlePoint);
if (Point.CrossProduct(vect1, vect2) > 0)
{
start = endpoint1;
end = endpoint2;
startAngle = angle1;
}
else
{
start = endpoint2;
end = endpoint1;
startAngle = angle2;
}
}
public static bool CounterClockwise(Point A, Point B, Point C)
{
// Define two vectors: vect1: A----->B
// vect2: B----->C
// Cross product vect1 and vect2.
// If the result is negative, the sequence A-->B-->C is Counter-clockwise.
// If the result is positive, the sequence A-->B-->C is clockwise.
Point vect1 = Point.MakeVector(A, B);
Point vect2 = Point.MakeVector(B, C);
return(Point.CrossProduct(vect1, vect2) < 0);
}
//
// Return True if the polygon is convex.
// http://blog.csharphelper.com/2010/01/04/determine-whether-a-polygon-is-convex-in-c.aspx
//
protected static bool IsConcavePolygon(List <Point> orderedPts)
{
// For each set of three adjacent points A, B, C,
// find the dot product AB · BC. If the sign of
// all the dot products is the same, the angles
// are all positive or negative (depending on the
// order in which we visit them) so the polygon
// is convex.
bool got_negative = false;
bool got_positive = false;
int B, C;
for (int A = 0; A < orderedPts.Count; A++)
{
B = (A + 1) % orderedPts.Count;
C = (B + 1) % orderedPts.Count;
// Create normalized vectors and find the cross-product.
Point vec1 = Point.MakeVector(orderedPts[A], orderedPts[B]);
Point vec2 = Point.MakeVector(orderedPts[B], orderedPts[C]);
double cross_product = Point.CrossProduct(vec1, vec2);
if (cross_product < 0)
{
got_negative = true;
}
else if (cross_product > 0)
{
got_positive = true;
}
if (got_negative && got_positive)
{
return(true);
}
}
// If we got this far, the polygon is convex.
return(false);
}
