I'm having trouble with collision detection in XNA [closed]

Question

I've written a tile editor and game in XNA, with velocities and gravity, and am having trouble with some of my collision detection things. It works okay, but seems to glitch between just inside and outside the tile the player is on. Here's my collision detection function and the other functions it uses:

void SolveCollisions(List<Tile> tiles, Vector2 screensize)
    {
        if (position.X < 0)
        {
            position.X = 0;
        }
        if (position.X + 32 > screensize.X)
        {
            position.X = screensize.X - 32;
        }
        if (position.Y < 0)
        {
            position.Y = 0;
        }
        if (position.Y + 32 > screensize.Y)
        {
            position.Y = screensize.Y - 32;
        }

        foreach (Tile t in tiles)
        {
            if (absTilePos(position.X) == absTilePos(t.position.X))
            {
                if (intersect(position.Y, t.position.Y, 32, 32))
                {
                    if (position.Y > 0)
                    {
                        velocity.Y = 0;
                        position.Y = tilePos(position.Y);
                    }

                    if (position.Y < 0)
                    {
                        velocity.Y = 0;
                        position.Y = tilePos(position.Y) + 32;
                    }
                }
            }

            if (absTilePos(position.Y) == absTilePos(t.position.Y))
            {
                if (intersect(position.X, t.position.X, 32, 32))
                {
                    if (position.X > 0)
                    {
                        velocity.X = 0;
                        position.X = tilePos(position.X);
                    }
                    if (position.X < 0)
                    {
                        velocity.X = 0;
                        position.X = tilePos(position.X) + 32;
                    }
                }
            }
        }
    }

    bool intersect(float p1, float p2, float s1, float s2)
    {
        if (p1 < p2 && p1 + s1 > p2)
        {
            return true;
        }
        if (p2 < p1 && p2 + s2 > p1)
        {
            return true;
        }

        return false;
    }

    float tilePos(float f)
    {
        int i = (int)f;
        i /= 32;
        i *= 32;
        return (float)i;
    }

    float absTilePos(float f)
    {
        int i = (int)f;
        i /= 32;
        return (float)i;
    }

The player also glitches into blocks on it's top and right ( http://puu.sh/2cVQf ) when it hits them instead of bouncing off them - it just goes straight into them.

Does anyone know how to remedy this? Thanks!

Answer 1

Here's how I would solve this problem. The code written below is written out of memory, not tested - you will obviously have to make it legal C# code and adapt it to your project.

class AABB // make your game objects have an AABB
{
    Vector2 position, size; // position is the top-left position
    float getLeft() { return position.X; }
    float getRight() { return position.X + size.X; }
    float getTop() { return position.Y; }
    float getBottom() { return position.Y + size.Y; }

    bool isIntersecting(AABB other)
    {
        return getRight() > other.getLeft() && 
        getLeft() < other.getRight() && 
        (getBottom() > other.getTop() && 
        getTop() < other.getBottom());
    }
}

void SolveCollisions(AABB aabb, List<Tile> tiles, Vector2 screensize)
{
    if (aabb.getLeft() < 0) aabb.position.X = 0;
    if (aabb.getRight() > screensize.X) aabb.position.X = screensize.X - aabb.size.X;
    if (aabb.getTop() < 0) aabb.position.Y = 0;
    if (aabb.getBottom() > screensize.Y) aabb.position.Y = screensize.Y - aabb.size.Y;

    foreach (Tile t in tiles)
    {
        AABB other = t.getAABB(); // getAABB() would return the tile's axis aligned bounding box

        if (!aabb.isIntersecting(other)) continue; // no collision with this tile, check the next one

        // let's find the shortest distance to "push out" the object and resolve collision

        float left = other.getLeft() - aabb.getRight(); 
        float right = other.getRight() - aabb.getLeft();
        float minXPush = Math.Abs(left) < Math.Abs(right) ? left : right;

        float top = other.getTop() - aabb.getBottom(); 
        float bottom = other.getBottom() - aabb.getTop();
        float minYPush = Math.Abs(top) < Math.Abs(bottom) ? top : bottom;

        if(Math.Abs(minXPush) < Math.Abs(minYPush)) // x pushing distance is minor than y
        {
            aabb.position.X += minXPush;
        }
        else
        {
            aabb.position.Y += minYPush;
        }

        // collision should be resolved now
    }
}

This solution is the most basic one, and has many problems. I wrote some articles about 2D collision I advise you to read:

http://veegamedev.wordpress.com/2012/08/14/getting-2d-collisions-right-part-one/ http://veegamedev.wordpress.com/2012/08/14/getting-2d-collisions-right-part-two/