public CollisionData(float time, Vector2 times, MovableEntity a, MovableEntity b)
{
this.time = time;
this.times = times;
this.a = a;
this.b = b;
}
/// <summary>
/// AABB Sweep in two dimensions
/// TODO: These need to be Entities, or new AABB type
/// </summary>
public static CollisionData AABBSweep(MovableEntity a, MovableEntity b)
{
//the problem is solved in A's frame of reference
//relative velocity (in normalized time)
Vector2 v = b.velocity - a.velocity;
float t0, t1;
//first times of overlap along each axis
Vector2 u0 = new Vector2(0.0f, 0.0f);
//last times of overlap along each axis
Vector2 u1 = new Vector2(0.0f, 0.0f);
//check if they were overlapping on the previous frame
if (overlaps(a, b))
{
return new CollisionData(0.0f, u0, a, b);
}
//find the possible first and last times of overlap along each axis
for( long i=0 ; i<2 ; i++ )
{
float aMax = max(a, i);
float aMin = min(a, i);
float bMax = max(b, i);
float bMin = min(b, i);
float vi = get(v, i);
// if axis is colliding and velocity along this axis is 0, this axis will always be colliding
if (((aMax >= bMin && aMax <= bMax) || (aMin >= bMin && aMin <= bMax)) && vi == 0)
set(ref u1, i, float.MaxValue);
// If axis is not colliding and no velocity on this axis, this axis will never be colliding
else if (vi == 0)
set(ref u1, i, 0);
if (aMax < bMin && vi < 0)
set(ref u0, i, (aMax - bMin) / vi);
else if (bMax < aMin && vi > 0)
set(ref u0, i, (aMin - bMax) / vi);
if (bMax > aMin && vi < 0)
set(ref u1, i, (aMin - bMax) / vi);
else if (aMax > bMin && vi > 0)
set(ref u1, i, (aMax - bMin) / vi);
}
//possible first time of overlap
t0 = Math.Max(u0.X, u0.Y);
//possible last time of overlap
t1 = Math.Min(u1.X, u1.Y);
// they could have only collided if the first time of overlap occurred before the last time of overlap
// t0 must not be zero because that implies an initial overlap which we already checked
// t0 must be less than 1 for the collision to occur during this timestep
if (t0 <= t1 && t0 != 0 && t0 < 1)
{
return new CollisionData(t0, u0, a, b);
}
return null;
}
/// <summary>
/// returns true if a was overlapping b in the previous frame
/// </summary>
private static Boolean overlaps(MovableEntity a, MovableEntity b)
{
//vector from A to B
Vector2 dist = VectorUtil.pointToVector2(b.prevBoundingRectangle.Center) - VectorUtil.pointToVector2(a.prevBoundingRectangle.Center);
// If either axis is disjoint, there is no overlap
return Math.Abs(dist.X) < (a.width / 2.0f + b.width / 2.0f) &&
Math.Abs(dist.Y) < (a.height / 2.0f + b.height / 2.0f);
}
/// <summary>
/// The top left extent of the entity
/// </summary>
/// <param name="i">The nth dimension of the entities position</param>
private static float min(MovableEntity e, long i )
{
switch (i) {
case 0: return e.prevPosition.X;
case 1: return e.prevPosition.Y;
default: return 0;
}
}
/// <summary>
/// The bottom right extent of the entity
/// </summary>
/// <param name="i">The nth dimension of the entities position</param>
private static float max(MovableEntity e, long i)
{
switch (i)
{
case 0: return e.prevPosition.X + e.width - 1;
case 1: return e.prevPosition.Y + e.height - 1;
default: return 0;
}
}
