Simple Collision Detection - Android

2
votes

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?

2
androidcollision-detection
I find drawing diagrams of the normal case and edge cases is really useful. – Skizz
Joakim, getBitmapWidth and getBitmapHeight return the real size of the entities? I'm asking this because the entities can has a border and this border doesn't was sum in this size. – Bruno Arueira
Bruno - yes, it return the real size of the bitmaps, I've double checked ;-) – Joakim Engstrom

2 Answers

4
votes

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.

0
votes

if you are dealing with rectangles then 

    /**
 * Check if two rectangles collide
 * x_1, y_1, width_1, and height_1 define the boundaries of the first rectangle
 * x_2, y_2, width_2, and height_2 define the boundaries of the second rectangle
 */
boolean rectangle_collision(float x_1, float y_1, float width_1, float height_1, float x_2, float y_2, float width_2, float height_2)
{
  return !(x_1 > x_2+width_2 || x_1+width_1 < x_2 || y_1 > y_2+height_2 || y_1+height_1 < y_2);
}

this is a good example...