// Check if this polygon contains a point
public override bool Contains(Vector2 point)
{
// Suppose we have a polygon with n vertices. Let p be a point, and
// consider the ray emanating from p and extending toward the left.
// Lastly, let n be the number of times this ray intersects the polygon's
// perimeter. Then the following properties hold.
//
// 1.) If n is even, then p lies outside the polygon.
// 2.) If n is odd, then p is inside the polygon.
//
// Note: This expresion assumes that the (x_0, y_0) and (x_n, y_n) are
// identical, i.e. the polygon is closed.
// Entry logging
#if IS_LOGGING_METHODS
Log.Write(String.Format("Entering method for {0}", this.Name));
#endif
// Declare result
bool result;
#region [1] Broad phase: Bounding box test
// First, check if point is outside MinBoundingBox
if (!this.MinBoundingBox.Contains(point))
{
result = false; goto exit;
}
#endregion
#region [2] Narrow phase: Ray casting test
// Note: We have to be very careful here. Suppose our ray intersects
// with a vertex of our polygon. In this case, the ray will technically
// intersect with two different edges! This is really just one point of
// intersection, and so we have to be careful that it is not counted
// twice!
// Initialize intersection counter
int count = 0;
// Create a horizontal ray stemming from the point and extending to the left
LineSegment ray = new LineSegment(point, new Vector2(this.MinBoundingBox.X - 1, point.Y));
// if (Globals.TestBool) { Trace.WriteLine("ray = " + ray); }
// Get edges
List <LineSegment> edges = this.Edges;
// Loop through each edge
for (int i = 0; i < this.Vertices.Count; i++)
{
// Point to current edge
LineSegment edge = edges[i];
// if (Globals.TestBool) { Trace.WriteLine("edge[" + i + "] = " + edge + ", count = " + count); }
// Check if ray intersects with edge
if (ray.IntersectsWith(edge))
{
// If ray is coincident with ray, count once
if (edge.Point1.Y == ray.Point2.Y && edge.Point2.Y == ray.Point2.Y)
{
count++;
}
// If ray intersects with vertex, don't double count it
else if (edge.Point2.Y != ray.Point2.Y)
{
count++;
}
}
}
// Check if count is even
if (count % 2 == 0)
{
result = false; goto exit;
}
else
{
result = true; goto exit;
}
#endregion
// [*] Exit trap
exit:
// Exit logging
// if (Globals.TestBool) { Trace.WriteLine("final count = " + count); }
#if IS_LOGGING_METHODS
Log.Write(String.Format("Exiting method for {0}", this.Name));
#endif
// Return result
return(result);
}
// Get point of intersection between this line and another
public Vector2 IntersectionWith(LineSegment line)
{
// Suppose A = (x1, y1), B = (x2, y2), C = (x3, y3), D = (x4, y4). We
// may define vector (ua, ub) as follows.
//
// ua = na / d
// ub = nb / d
// na = (x4 - x3) * (y1 - y3) - (y4 - y3) * (x1 - x3)
// nb = (x2 - x1) * (y1 - y3) - (y2 - y1) * (x1 - x3)
// d = (y4 - y3) * (x2 - x1) - (x4 - x3) * (y2 - y1)
//
// The following results hold true.
//
// 1.) If d = 0, then the line segments are parallel
// 2.) If d, na, and nb all equal zero, then the line segments are
// coincident (overlap).
// 3.) If ua and ub lie between 0 and 1, then the intersection point
// lies within the line segment' end points. In this case, the
// point of intersection (x, y) may be written as follows.
//
// x = x1 + ua * (x2 - x1)
// y = y1 + ua * (y2 - y1)
// Note: If the line segments do not intersect, this function will
// return positive infinity. If they overlap, it will simply return
// (x1, y1).
// Entry logging
#if IS_LOGGING_METHODS
Log.Write("Entering method");
#endif
// Declare result
Vector2 result;
// Declare each coordinate
float x1 = this.Point1.X;
float x2 = this.Point2.X;
float x3 = line.Point1.X;
float x4 = line.Point2.X;
float y1 = this.Point1.Y;
float y2 = this.Point2.Y;
float y3 = line.Point1.Y;
float y4 = line.Point2.Y;
// Calculate the denominator and each numerator
float d = (y4 - y3) * (x2 - x1) - (x4 - x3) * (y2 - y1);
float na = (x4 - x3) * (y1 - y3) - (y4 - y3) * (x1 - x3);
float nb = (x2 - x1) * (y1 - y3) - (y2 - y1) * (x1 - x3);
// Check if lines are parallel
if (d == 0)
{
// If so, check if they are coincident
if (na == 0 && nb == 0)
{
float minA = x1; float maxA = x2; if (x2 < x1)
{
minA = x2; maxA = x1;
}
float minB = x3; float maxB = x4; if (x4 < x3)
{
minB = x4; maxB = x3;
}
Interval intervalA = new Interval(minA, maxA);
Interval intervalB = new Interval(minB, maxB);
Interval overlap = intervalA.IntersectionWith(intervalB);
if (!overlap.IsEmpty)
{
result = new Vector2(overlap.Min, y1);
}
else
{
result = Vector2.PositiveInfinity;
}
}
else
{
result = Vector2.PositiveInfinity; goto exit;
}
}
// Otherwise
else
{
// Divide each numerator by the denominator
float ua = na / d;
float ub = nb / d;
// Stop if ua is outside the interval (0, 1)
if (ua < 0 || ua > 1)
{
result = Vector2.PositiveInfinity; goto exit;
}
// Stop if ub is outside the interval (0, 1)
if (ub < 0 || ub > 1)
{
result = Vector2.PositiveInfinity; goto exit;
}
// If we haven't stopped yet, then the line segments intersect
float x = x1 + ua * (x2 - x1);
float y = y1 + ua * (y2 - y1);
result = new Vector2(x, y); goto exit;
}
// Exit trap
exit:
// Exit logging
#if IS_LOGGING_METHODS
Log.Write("Exiting method");
#endif
// Return result
return(result);
}
// Find the point on this polygon closest to a point
public override Vector2 ClosestPointTo(Vector2 point)
{
// Let e be an arbitrary edge with direction d. Furthermore, let [a, b] and p
// be the projections of e and our point onto the d-axis, respectively. Then
// there are two possible cases.
// 1.) p is in [a, b]
// Then there is a perpendicular line connecting our point to e. This
// edge might contain the closest point.
// 2.) p is not in [a, b]
// This edge definitely does not contain the closest point. We may
// disregard it.
//
// We check this criterion for each edge, compute the closest point to each
// edge, and that which yields the smallest distance. However, if no edges
// satisfy the above criterion, then the closest point must be a vertex.
// Note: If there are two equidistant, closest points, this method will only
// return one of them!
// Entry logging
#if IS_LOGGING_METHODS
Log.Write(String.Format("Entering method for {0}", this.Name));
#endif
// Declare result
Vector2 result = Vector2.Zero;
// Initialize minDistance as positive infinity
float minDistance = float.PositiveInfinity;
// First, we loop through the polygon's vertices
for (int i = 0; i < this.Vertices.Count; i++)
{
// Get current vertex
Vector2 vertex = this.Vertices[i];
// Get distance between vertex and point
float distance = Vector2.Distance(vertex, point);
// Update minimum distance and result
if (distance < minDistance)
{
minDistance = distance;
result = vertex;
}
}
// Get edges
List <LineSegment> edges = this.Edges;
// Loop through edges
for (int i = 0; i < this.Vertices.Count; i++)
{
// Get current edge
LineSegment edge = edges[i];
// Get edge's direction vector
Vector2 axis = edge.Direction;
// Project edge onto axis
Interval edgeProjection = edge.ProjectOnto(axis);
// Project point onto axis
float pointProjection = Vector2.Project(point, axis);
// Check if p is in [a, b]
if (edgeProjection.Contains(pointProjection))
{
// If so, check minimum distance between point and edge
Vector2 closestEdgePoint = edge.ClosestPointTo(point);
float distance = Vector2.Distance(closestEdgePoint, point);
// Update minimum distance and result
if (distance < minDistance)
{
minDistance = distance;
result = closestEdgePoint;
}
}
}
// Exit logging
#if IS_LOGGING_METHODS
Log.Write(String.Format("Exiting method for {0}", this.Name));
#endif
// Return result
return(result);
}