Collision Detection Implementation

Question

Hi

I have a collision detection class that works by finding the distance between the centres and whether that distance is small enough to be a collision (see Collision Detection error). My problem is trying to make this actually work, with ellipses colliding. I will explain more if necessary. Thx

Answer 1

The best way would to implement per pixel collision detection when the images are overlapping you can read more about this in the following links

http://www.codeproject.com/KB/game/collision3.aspx

http://stackoverflow.com/questions/793109/per-pixel-collision-problem-in-c

I also did a problem like this for a project a few years ago when I needed to detect if two circles overlapped where i used the following code

    public static bool Intersect(Rectangle rectangle1, Rectangle rectangle2)
    {
        if (((rectangle1.X < (rectangle2.X + rectangle2.Width)) && (rectangle2.X < (rectangle1.X + rectangle1.Width))) && (rectangle1.Y < (rectangle2.Y + rectangle2.Height)) && (rectangle2.Y < (rectangle1.Y + rectangle1.Height)))
        {
            Vector2 rect1Centre = new Vector2(rectangle1.X + rectangle1.Width / 2, rectangle1.Y + rectangle1.Height / 2);
            Vector2 rect2Centre = new Vector2(rectangle2.X + rectangle2.Width / 2, rectangle2.Y + rectangle1.Height / 2);
            double radius1 = ((rectangle1.Width / 2) + (rectangle1.Height / 2)) / 2;
            double radius2 = ((rectangle2.Width / 2) + (rectangle2.Height / 2)) / 2;

            double widthTri = rect1Centre.X - rect2Centre.X;
            double heightTri = rect1Centre.Y - rect2Centre.Y;
            double distance = Math.Sqrt(Math.Pow(widthTri, 2) + Math.Pow(heightTri, 2));

            if (distance <= (radius1 + radius2))
                return true;
        }
        return false;
    }

Not very nice code but I wrote it doing my first XNA game

Answer 2

I had the same problem recently. Circle overlap is easy to determine. With ellipses it's trickier, but not that bad. You play around with the ellipse equation for a while, and the result comes up:

//Returns true if the pixel is inside the ellipse
public bool CollisionCheckPixelInEllipse(Coords pixel, Coords center, UInt16 radiusX, UInt16 radiusY)
{
   Int32 asquare = radiusX * radiusX;
   Int32 bsquare = radiusY * radiusY;
   return ((pixel.X-center.X)*(pixel.X-center.X)*bsquare + (pixel.Y-center.Y)*(pixel.Y-center.Y)*asquare) < (asquare*bsquare);
}

// returns true if the two ellipses overlap
private bool CollisionCheckEllipses(Coords center1, UInt16 radius1X, UInt16 radius1Y, Coords center2, UInt16 radius2X, UInt16 radius2Y)
{
    UInt16 radiusSumX = (UInt16) (radius1X + radius2X);
    UInt16 radiusSumY = (UInt16) (radius1Y + radius2Y);

    return CollisionCheckPixelInEllipse(center1, center2, radiusSumX, radiusSumY);
}