Collision Detection between two images in Java

Question

I have two characters displayed in a game I am writing, the player and the enemy. defined as such:

public void player(Graphics g) {
    g.drawImage(plimg, x,         


        

Answer 1

First, use the bounding boxes as described by Jonathan Holland to find if you may have a collision.

From the (multi-color) sprites, create black and white versions. You probably already have these if your sprites are transparent (i.e. there are places which are inside the bounding box but you can still see the background). These are "masks".

Use Image.getRGB() on the mask to get at the pixels. For each pixel which isn't transparent, set a bit in an integer array (playerArray and enemyArray below). The size of the array is height if width <= 32 pixels, (width+31)/32*height otherwise. The code below is for width <= 32.

If you have a collision of the bounding boxes, do this:

// Find the first line where the two sprites might overlap
int linePlayer, lineEnemy;
if (player.y <= enemy.y) {
    linePlayer = enemy.y - player.y;
    lineEnemy = 0;
} else {
    linePlayer = 0;
    lineEnemy = player.y - enemy.y;
}
int line = Math.max(linePlayer, lineEnemy);

// Get the shift between the two
x = player.x - enemy.x;
int maxLines = Math.max(player.height, enemy.height);
for ( line < maxLines; line ++) {
    // if width > 32, then you need a second loop here
    long playerMask = playerArray[linePlayer];
    long enemyMask = enemyArray[lineEnemy];
    // Reproduce the shift between the two sprites
    if (x < 0) playerMask << (-x);
    else enemyMask << x;
    // If the two masks have common bits, binary AND will return != 0
    if ((playerMask & enemyMask) != 0) {
        // Contact!
    }

}

Links: JGame, Framework for Small Java Games