/// <summary>
/// Finds the points of intersection between a line and a circle. The results are two lambdas along the line, one
/// for each point, or NaN if there is no intersection.</summary>
public static void LineWithCircle(ref EdgeD line, ref CircleD circle,
out double lambda1, out double lambda2)
{
// The following expressions come up a lot in the solution, so simplify using them.
double dx = line.End.X - line.Start.X;
double dy = line.End.Y - line.Start.Y;
double ax = -line.Start.X + circle.Center.X;
double ay = -line.Start.Y + circle.Center.Y;
// Solve simultaneously for l:
// Eq of a line: x = sx + l * dx
// y = sy + l * dy
// Eq of a circle: (x - cx)^2 + (y - cy)^2 = r^2
//
// Eventually we get a standard quadratic equation in l with the
// following coefficients:
double a = dx * dx + dy * dy;
double b = -2 * (ax * dx + ay * dy);
double c = ax * ax + ay * ay - circle.Radius * circle.Radius;
// Now just solve the quadratic eqn...
double D = b * b - 4 * a * c;
if (D < 0)
{
lambda1 = lambda2 = double.NaN;
}
else
{
double sqrtD = Math.Sqrt(D);
lambda1 = (-b + sqrtD) / (2 * a);
lambda2 = (-b - sqrtD) / (2 * a);
}
}
private void AssertRayWithCircle(double frX, double frY, double toX, double toY, double cX, double cY, double cR, double expL1, double expL2)
{
EdgeD ray = new EdgeD(frX, frY, toX, toY);
CircleD cir = new CircleD(cX, cY, cR);
double l1, l2;
Intersect.RayWithCircle(ref ray, ref cir, out l1, out l2);
Assert.AreEqual(expL1, l1, 0.001);
Assert.AreEqual(expL2, l2, 0.001);
}
private void AssertLineWithCircle(double frX, double frY, double toX, double toY, double cX, double cY, double cR, double expL1, double expL2)
{
EdgeD lin = new EdgeD(frX, frY, toX, toY);
CircleD cir = new CircleD(cX, cY, cR);
double l1, l2;
Intersect.LineWithCircle(ref lin, ref cir, out l1, out l2);
Assert.AreEqual(MinTO(expL1, expL2), MinTO(l1, l2), 0.001);
Assert.AreEqual(MaxTO(expL1, expL2), MaxTO(l1, l2), 0.001);
}
private void AssertRayWithCircle(double frX, double frY, double toX, double toY, double cX, double cY, double cR, double expL1, double expL2)
{
EdgeD ray = new EdgeD(frX, frY, toX, toY);
CircleD cir = new CircleD(cX, cY, cR);
double l1, l2;
Intersect.RayWithCircle(ref ray, ref cir, out l1, out l2);
Assert.AreEqual(expL1, l1, 0.001);
Assert.AreEqual(expL2, l2, 0.001);
}
private void AssertLineWithCircle(double frX, double frY, double toX, double toY, double cX, double cY, double cR, double expL1, double expL2)
{
EdgeD lin = new EdgeD(frX, frY, toX, toY);
CircleD cir = new CircleD(cX, cY, cR);
double l1, l2;
Intersect.LineWithCircle(ref lin, ref cir, out l1, out l2);
Assert.AreEqual(MinTO(expL1, expL2), MinTO(l1, l2), 0.001);
Assert.AreEqual(MaxTO(expL1, expL2), MaxTO(l1, l2), 0.001);
}
/// <summary>
/// Given this circle and another circle, tries to find a third and fourth circle with a given target radius such
/// that the new circles are both tangent to the first two.</summary>
/// <param name="other">
/// The other circle.</param>
/// <param name="targetRadius">
/// Target radius for output circles.</param>
/// <returns>
/// The two output circles if they exist. If the input circles are further apart than twice the target radius, the
/// desires circles do not exist and null is returned.</returns>
public (CircleD, CircleD)? FindTangentCircles(CircleD other, double targetRadius)
{
double a = ((Center.X - other.Center.X) * (Center.X - other.Center.X)) /
((other.Center.Y - Center.Y) * (other.Center.Y - Center.Y)) + 1;
double t = Center.Y - (Radius * Radius - other.Radius * other.Radius + other.Center.X * other.Center.X
- Center.X * Center.X + other.Center.Y * other.Center.Y - Center.Y * Center.Y
+ 2 * targetRadius * (Radius - other.Radius)) / (other.Center.Y - Center.Y) / 2;
double b = -2 * Center.X - 2 * t * (Center.X - other.Center.X) / (other.Center.Y - Center.Y);
double c = Center.X * Center.X - (Radius + targetRadius) * (Radius + targetRadius) + t * t;
double q = b * b - 4 * a * c;
// At this point, Q < 0 means the circles are too far apart, so no solution
if (q < 0)
{
return(null);
}
double s = Math.Sqrt(q);
double xa = (-b + s) / (2 * a);
double xb = (-b - s) / (2 * a);
// These Sqrts should succeed, i.e. their parameter should never be < 0
double ya = Math.Sqrt(-other.Center.X * other.Center.X - xa * xa + targetRadius * targetRadius
+ 2 * other.Center.X * xa + other.Radius * other.Radius + 2 * other.Radius * targetRadius);
double yb = Math.Sqrt(-Center.X * Center.X - xb * xb + targetRadius * targetRadius
+ 2 * Center.X * xb + Radius * Radius + 2 * Radius * targetRadius);
if (Math.Sign(Center.X - other.Center.X) != Math.Sign(Center.Y - other.Center.Y))
{
ya += other.Center.Y;
yb = Center.Y - yb;
}
else
{
ya = other.Center.Y - ya;
yb += Center.Y;
}
return(new CircleD(xa, ya, targetRadius), new CircleD(xb, yb, targetRadius));
}
/// <summary>
/// Finds the points of intersection between a ray and a circle. The resulting lambdas along the ray are sorted in
/// ascending order, so the "first" intersection is always in lambda1 (if any). Lambda may be NaN if there is no
/// intersection (or no "second" intersection).</summary>
public static void RayWithCircle(ref EdgeD ray, ref CircleD circle,
out double lambda1, out double lambda2)
{
LineWithCircle(ref ray, ref circle, out lambda1, out lambda2);
// Sort the two values in ascending order, with NaN last,
// while resetting negative values to NaNs
if (lambda1 < 0)
{
lambda1 = double.NaN;
}
if (lambda2 < 0)
{
lambda2 = double.NaN;
}
if (lambda1 > lambda2 || double.IsNaN(lambda1))
{
double temp = lambda1;
lambda1 = lambda2;
lambda2 = temp;
}
}
/// <summary>
/// Given this circle and another circle, tries to find a third and fourth circle with a given target radius such
/// that the new circles are both tangent to the first two.</summary>
/// <param name="other">
/// The other circle.</param>
/// <param name="targetRadius">
/// Target radius for output circles.</param>
/// <returns>
/// The two output circles if they exist. If the input circles are further apart than twice the target radius, the
/// desires circles do not exist and null is returned.</returns>
public Tuple<CircleD, CircleD> FindTangentCircles(CircleD other, double targetRadius)
{
double a = ((Center.X - other.Center.X) * (Center.X - other.Center.X)) /
((other.Center.Y - Center.Y) * (other.Center.Y - Center.Y)) + 1;
double t = Center.Y - (Radius * Radius - other.Radius * other.Radius + other.Center.X * other.Center.X
- Center.X * Center.X + other.Center.Y * other.Center.Y - Center.Y * Center.Y
+ 2 * targetRadius * (Radius - other.Radius)) / (other.Center.Y - Center.Y) / 2;
double b = -2 * Center.X - 2 * t * (Center.X - other.Center.X) / (other.Center.Y - Center.Y);
double c = Center.X * Center.X - (Radius + targetRadius) * (Radius + targetRadius) + t * t;
double q = b * b - 4 * a * c;
// At this point, Q < 0 means the circles are too far apart, so no solution
if (q < 0) return null;
double s = Math.Sqrt(q);
double xa = (-b + s) / (2 * a);
double xb = (-b - s) / (2 * a);
// These Sqrts should succeed, i.e. their parameter should never be < 0
double ya = Math.Sqrt(-other.Center.X * other.Center.X - xa * xa + targetRadius * targetRadius
+ 2 * other.Center.X * xa + other.Radius * other.Radius + 2 * other.Radius * targetRadius);
double yb = Math.Sqrt(-Center.X * Center.X - xb * xb + targetRadius * targetRadius
+ 2 * Center.X * xb + Radius * Radius + 2 * Radius * targetRadius);
if (Math.Sign(Center.X - other.Center.X) != Math.Sign(Center.Y - other.Center.Y))
{
ya += other.Center.Y;
yb = Center.Y - yb;
}
else
{
ya = other.Center.Y - ya;
yb += Center.Y;
}
return Tuple.Create(new CircleD(xa, ya, targetRadius), new CircleD(xb, yb, targetRadius));
}
/// <summary>
/// Finds the points of intersection between a line and a circle. The results are two lambdas along the line, one
/// for each point, or NaN if there is no intersection.</summary>
public static void LineWithCircle(EdgeD line, CircleD circle,
out double lambda1, out double lambda2)
{
LineWithCircle(ref line, ref circle, out lambda1, out lambda2);
}
/// <summary>
/// Returns the smallest circle that encloses all the given points. If 1 point is given, a circle of radius 0 is
/// returned.</summary>
/// <param name="points">
/// The set of points to circumscribe.</param>
/// <returns>
/// The circumscribed circle.</returns>
/// <remarks>
/// <list type="bullet">
/// <item><description>
/// Runs in expected O(n) time, randomized.</description></item></list></remarks>
/// <exception cref="InvalidOperationException">
/// The input collection contained zero points.</exception>
public static CircleD GetCircumscribedCircle(IList <PointD> points)
{
// Clone list to preserve the caller's data
List <PointD> shuffled = new List <PointD>(points).Shuffle();
// Progressively add points to circle or recompute circle
// Initially: No boundary points known
CircleD?circ = null;
for (int i = 0; i < shuffled.Count; i++)
{
PointD p = shuffled[i];
if (circ == null || !circ.Value.Contains(p))
{
circ = MakeCircleOnePoint(shuffled.GetRange(0, i + 1), p);
}
}
if (circ == null)
{
throw new InvalidOperationException("The input collection did not contain any points.");
}
return(circ.Value);
// One boundary point known
CircleD MakeCircleOnePoint(List <PointD> pts, PointD p)
{
CircleD c = new CircleD(p, 0);
for (int i = 0; i < pts.Count; i++)
{
PointD q = pts[i];
if (!c.Contains(q))
{
if (c.Radius == 0)
{
c = MakeDiameter(p, q);
}
else
{
c = MakeCircleTwoPoints(pts.GetRange(0, i + 1), p, q);
}
}
}
return(c);
}
// Two boundary pts known
CircleD MakeCircleTwoPoints(List <PointD> pts, PointD p, PointD q)
{
CircleD crc = MakeDiameter(p, q);
CircleD left = new CircleD(new PointD(0, 0), -1);
CircleD right = new CircleD(new PointD(0, 0), -1);
// For each point not in the two-point circle
PointD pq = q - p;
foreach (PointD r in pts)
{
if (crc.Contains(r))
{
continue;
}
// Form a circumcircle and classify it on left or right side
double cross = Cross(pq, r - p);
CircleD c = MakeCircumcircle(p, q, r);
if (c.Radius < 0)
{
continue;
}
else if (cross > 0 && (left.Radius < 0 || Cross(pq, c.Center - p) > Cross(pq, left.Center - p)))
{
left = c;
}
else if (cross < 0 && (right.Radius < 0 || Cross(pq, c.Center - p) < Cross(pq, right.Center - p)))
{
right = c;
}
}
// Select which circle to return
if (left.Radius < 0 && right.Radius < 0)
{
return(crc);
}
else if (left.Radius < 0)
{
return(right);
}
else if (right.Radius < 0)
{
return(left);
}
else
{
return(left.Radius <= right.Radius ? left : right);
}
}
CircleD MakeDiameter(PointD a, PointD b)
{
PointD c = new PointD((a.X + b.X) / 2, (a.Y + b.Y) / 2);
return(new CircleD(c, Math.Max(c.Distance(a), c.Distance(b))));
}
CircleD MakeCircumcircle(PointD a, PointD b, PointD c)
{
// Mathematical algorithm from Wikipedia: Circumscribed circle
double ox = (Math.Min(Math.Min(a.X, b.X), c.X) + Math.Max(Math.Min(a.X, b.X), c.X)) / 2;
double oy = (Math.Min(Math.Min(a.Y, b.Y), c.Y) + Math.Max(Math.Min(a.Y, b.Y), c.Y)) / 2;
double ax = a.X - ox, ay = a.Y - oy;
double bx = b.X - ox, by = b.Y - oy;
double cx = c.X - ox, cy = c.Y - oy;
double d = (ax * (by - cy) + bx * (cy - ay) + cx * (ay - by)) * 2;
if (d == 0)
{
return(new CircleD(new PointD(0, 0), -1));
}
double x = ((ax * ax + ay * ay) * (by - cy) + (bx * bx + by * by) * (cy - ay) + (cx * cx + cy * cy) * (ay - by)) / d;
double y = ((ax * ax + ay * ay) * (cx - bx) + (bx * bx + by * by) * (ax - cx) + (cx * cx + cy * cy) * (bx - ax)) / d;
PointD p = new PointD(ox + x, oy + y);
double r = Math.Max(Math.Max(p.Distance(a), p.Distance(b)), p.Distance(c));
return(new CircleD(p, r));
}
// Signed area / determinant thing
double Cross(PointD p, PointD q)
{
return(p.X * q.Y - p.Y * q.X);
}
}
/// <summary>
/// Finds the points of intersection between a line and a circle. The results are two lambdas along the line, one
/// for each point, or NaN if there is no intersection.</summary>
public static void LineWithCircle(ref EdgeD line, ref CircleD circle,
out double lambda1, out double lambda2)
{
// The following expressions come up a lot in the solution, so simplify using them.
double dx = line.End.X - line.Start.X;
double dy = line.End.Y - line.Start.Y;
double ax = -line.Start.X + circle.Center.X;
double ay = -line.Start.Y + circle.Center.Y;
// Solve simultaneously for l:
// Eq of a line: x = sx + l * dx
// y = sy + l * dy
// Eq of a circle: (x - cx)^2 + (y - cy)^2 = r^2
//
// Eventually we get a standard quadratic equation in l with the
// following coefficients:
double a = dx * dx + dy * dy;
double b = -2 * (ax * dx + ay * dy);
double c = ax * ax + ay * ay - circle.Radius * circle.Radius;
// Now just solve the quadratic eqn...
double D = b * b - 4 * a * c;
if (D < 0)
{
lambda1 = lambda2 = double.NaN;
}
else
{
double sqrtD = Math.Sqrt(D);
lambda1 = (-b + sqrtD) / (2 * a);
lambda2 = (-b - sqrtD) / (2 * a);
}
}
/// <summary>
/// Finds the points of intersection between a ray and a circle. The resulting lambdas along the ray are sorted in
/// ascending order, so the "first" intersection is always in lambda1 (if any). Lambda may be NaN if there is no
/// intersection (or no "second" intersection).</summary>
public static void RayWithCircle(ref EdgeD ray, ref CircleD circle,
out double lambda1, out double lambda2)
{
LineWithCircle(ref ray, ref circle, out lambda1, out lambda2);
// Sort the two values in ascending order, with NaN last,
// while resetting negative values to NaNs
if (lambda1 < 0) lambda1 = double.NaN;
if (lambda2 < 0) lambda2 = double.NaN;
if (lambda1 > lambda2 || double.IsNaN(lambda1))
{
double temp = lambda1;
lambda1 = lambda2;
lambda2 = temp;
}
}
/// <summary>
/// Finds the points of intersection between a line and a circle. The results are two lambdas along the line, one
/// for each point, or NaN if there is no intersection.</summary>
public static void LineWithCircle(EdgeD line, CircleD circle,
out double lambda1, out double lambda2)
{
LineWithCircle(ref line, ref circle, out lambda1, out lambda2);
}
