// Check if polygon A is going to collide with polygon B for the given velocity
public SvgCollision.PolygonCollisionResult PolygonCollision(Polygon polygonA, Polygon polygonB, Vector velocity)
{
SvgCollision.PolygonCollisionResult result = new SvgCollision.PolygonCollisionResult();
result.IsIntersecting = true;
result.WillIntersect = true;
int edgeCountA = polygonA.Edges.Count;
int edgeCountB = polygonB.Edges.Count;
float minIntervalDistance = float.PositiveInfinity;
Vector translationAxis = new Vector();
Vector edge;
// Loop through all the edges of both polygons
for (int edgeIndex = 0; edgeIndex < edgeCountA + edgeCountB; edgeIndex++) {
if (edgeIndex < edgeCountA) {
edge = polygonA.Edges[edgeIndex];
} else {
edge = polygonB.Edges[edgeIndex - edgeCountA];
}
// ===== 1. Find if the polygons are currently intersecting =====
// Find the axis perpendicular to the current edge
Vector axis = new Vector(-edge.Y, edge.X);
axis.Normalize();
// Find the projection of the polygon on the current axis
float minA = 0; float minB = 0; float maxA = 0; float maxB = 0;
ProjectPolygon(axis, polygonA, ref minA, ref maxA);
ProjectPolygon(axis, polygonB, ref minB, ref maxB);
// Check if the polygon projections are currentlty intersecting
float intervalDistance = IntervalDistance(minA, maxA, minB, maxB);
if (intervalDistance > 0) result.IsIntersecting = false;
// ===== 2. Now find if the polygons *will* intersect =====
// Project the velocity on the current axis
float velocityProjection = axis.DotProduct(velocity);
// Get the projection of polygon A during the movement
if (velocityProjection < 0) {
minA += velocityProjection;
} else {
maxA += velocityProjection;
}
// Do the same test as above for the new projection
intervalDistance = IntervalDistance(minA, maxA, minB, maxB);
if (intervalDistance > 0) result.WillIntersect = false;
// If the polygons are not intersecting and won't intersect, exit the loop
if (!result.IsIntersecting && !result.WillIntersect) break;
// Check if the current interval distance is the minimum one. If so store
// the interval distance and the current distance.
// This will be used to calculate the minimum translation vector
intervalDistance = Math.Abs(intervalDistance);
if (intervalDistance < minIntervalDistance) {
minIntervalDistance = intervalDistance;
translationAxis = axis;
Vector d = polygonA.Center - polygonB.Center;
if (d.DotProduct(translationAxis) < 0) translationAxis = -translationAxis;
}
}
// The minimum translation vector can be used to push the polygons appart.
// First moves the polygons by their velocity
// then move polygonA by MinimumTranslationVector.
if (result.WillIntersect) result.MinimumTranslationVector = translationAxis * minIntervalDistance;
return result;
}
// Calculate the projection of a polygon on an axis and returns it as a [min, max] interval
public void ProjectPolygon(Vector axis, Polygon polygon, ref float min, ref float max)
{
// To project a point on an axis use the dot product
float d = axis.DotProduct(polygon.Points[0]);
min = d;
max = d;
for (int i = 0; i < polygon.Points.Count; i++) {
d = polygon.Points[i].DotProduct(axis);
if (d < min) {
min = d;
} else {
if (d > max) {
max = d;
}
}
}
}
public static SvgCollision.PolygonCollisionResult PolygonCollision(PointF[] ptsA, PointF[] ptsB, Vector velocity)
{
Polygon polygonA, polygonB;
polygonA = new Polygon();
polygonB = new Polygon();
foreach (PointF pts in ptsA)
{
polygonA.Points.Add(new Vector(pts.X, pts.Y));
}
foreach (PointF pts in ptsB)
{
polygonB.Points.Add(new Vector(pts.X, pts.Y));
}
polygonA.BuildEdges();
polygonB.BuildEdges();
SeparatingAxisTheorem svgCollision = new SeparatingAxisTheorem();
return svgCollision.PolygonCollision(polygonA, polygonB, velocity);
}