Question

I want to do really simple collision detection in a pong like game.
The ball is a square and the paddle (bats) is rectangles.

I have two entities coming in where I can get the current X and Y position, and the bitmap height and width. Which is the simplest way to do this?

I have this code:

public void getCollision(Entity enitityOne, Entity enitityTwo){

    double eventCoordX = (enitityOne.getCenterX() - (enitityTwo.getBitmapWidth() / 2));
    double eventCoordY = (enitityOne.getCenterY() - (enitityTwo.getBitmapHeight() / 2));

    double X = Math.abs(enitityTwo.getxPos() - eventCoordX);
    double Y = Math.abs(enitityTwo.getyPos() - eventCoordY);

    if(X <= (enitityTwo.getBitmapWidth()) && Y <= (enitityTwo.getBitmapHeight())){
        enitityOne.collision();
        enitityTwo.collision();
    }
}

But I’m pretty blind, this only works in the middle of the paddle not on the sides.
The problem is I can’t see where the code is wrong.
Anybody?
Anybody have a better idea?

Answer 1

If all you want is to find whether or not 2 given rectangles somehow intersect (and therefore collide), here’s the simplest check (C code; feel free to use floating-point values):

int RectsIntersect(int AMinX, int AMinY, int AMaxX, int AMaxY,
                   int BMinX, int BMinY, int BMaxX, int BMaxY)
{
    assert(AMinX < AMaxX);
    assert(AMinY < AMaxY);
    assert(BMinX < BMaxX);
    assert(BMinY < BMaxY);

    if ((AMaxX < BMinX) || // A is to the left of B
        (BMaxX < AMinX) || // B is to the left of A
        (AMaxY < BMinY) || // A is above B
        (BMaxY < AMinY))   // B is above A
    {
        return 0; // A and B don't intersect
    }

    return 1; // A and B intersect
}

The rectangles A and B are defined by the minimum and maximum X and Y coordinates of their corners.

Um… This has been asked before.