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TheBroodian

Collision Between Dynamic Objects

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I've been writing a 2D platformer for some time now, and I'm getting to the point where I am preparing to add more objects to the world other than the player. This has led me to a position where I need to create some sort of collision system for dynamic world objects (NPC's/enemies, movable objects, floating platforms, etc). Up to this point the player has only utilized world-based collision detection, which works great, but world data is static, so I can't really use it to solve dynamic world object collisions. To start off with, I was just trying to form some sort of system so that the player could push, or be pushed by objects (be they enemies, or some other neutral type source) that would also sort of simulate a sort of weight between objects. It doesn't have to be perfect but I haven't been able to nail it down yet. Here is the code I've written so far to give an example about what in theory I'm trying to achieve:

[source lang="csharp"]//Clears the anti-duplicate buffer
collisionRecord.Clear();
//pick a thing
foreach (GameObject entity in entities)
{
//pick another thing
foreach (GameObject subject in entities)
{
//check to make sure both things aren't the same thing
if (!ReferenceEquals(entity, subject))
{
//check to see if thing2 is in semi-near proximity to thing1
if (entity.WideProximityArea.Intersects(subject.CollisionRectangle) ||
entity.WideProximityArea.Contains(subject.CollisionRectangle))
{
//check to see if thing2 and thing1 are colliding.
if (entity.CollisionRectangle.Intersects(subject.CollisionRectangle) ||
entity.CollisionRectangle.Contains(subject.CollisionRectangle) ||
subject.CollisionRectangle.Contains(entity.CollisionRectangle))
{
//check if we've already resolved their collision or not.
if (!collisionRecord.ContainsKey(entity.GetHashCode()))
{
//more duplicate resolution checking.
if (!collisionRecord.ContainsKey(subject.GetHashCode()))
{
//if thing1 is traveling right...
if (entity.Velocity.X > 0)
{
//if it isn't too far to the right...
if (subject.CollisionRectangle.Contains(new Microsoft.Xna.Framework.Rectangle(entity.CollisionRectangle.Right, entity.CollisionRectangle.Y, 1, entity.CollisionRectangle.Height)) ||
subject.CollisionRectangle.Intersects(new Microsoft.Xna.Framework.Rectangle(entity.CollisionRectangle.Right, entity.CollisionRectangle.Y, 1, entity.CollisionRectangle.Height)))
{
//Find how deep thing1 is intersecting thing2's collision box;
float offset = entity.CollisionRectangle.Right - subject.CollisionRectangle.Left;
//Move both things in opposite directions half the length of the intersection, pushing thing1 to the left, and thing2 to the right.
entity.Velocities.Add(new Vector2(-((offset * (float)gameTime.ElapsedGameTime.TotalMilliseconds)), 0));
subject.Velocities.Add(new Vector2(((offset * (float)gameTime.ElapsedGameTime.TotalMilliseconds)), 0));
}
}
//if thing1 is traveling left...
if (entity.Velocity.X < 0)
{
//if thing1 isn't too far left...
if (entity.CollisionRectangle.Contains(new Microsoft.Xna.Framework.Rectangle(subject.CollisionRectangle.Right, subject.CollisionRectangle.Y, 1, subject.CollisionRectangle.Height)) ||
entity.CollisionRectangle.Intersects(new Microsoft.Xna.Framework.Rectangle(subject.CollisionRectangle.Right, subject.CollisionRectangle.Y, 1, subject.CollisionRectangle.Height)))
{
//Find how deep thing1 is intersecting thing2's collision box;
float offset = subject.CollisionRectangle.Right - entity.CollisionRectangle.Left;
//Move both things in opposite directions half the length of the intersection, pushing thing1 to the right, and thing2 to the left.
entity.Velocities.Add(new Vector2(((offset * (float)gameTime.ElapsedGameTime.TotalMilliseconds)), 0));
subject.Velocities.Add(new Vector2(-((offset * (float)gameTime.ElapsedGameTime.TotalMilliseconds)), 0));
}
}
//Make record that thing1 and thing2 have interacted and the collision has been solved, so that if thing2 is picked next in the foreach loop, it isn't checked against thing1 a second time before the next update.
collisionRecord.Add(entity.GetHashCode(), subject.GetHashCode());
}
}
}
}
}
}
}
}[/source]

Unfortunately for me, this code doesn't exactly work. It... produces results, but not as intended. When an object pushes into another object, it's almost as though the object pushing has met a wall, which isn't entirely undesirable behavior in of itself, but then when the object stops pushing, both objects sort of rubberband in the opposite direction.

Sorry if the code looks like a mess. Any thoughts are immensely appreciated. Edited by TheBroodian

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