Hi guys!
I found this code for pixel-perfect collision, but there’s 1 line I can’t grasp…
Here’s the code:
// returns a HashSet of strings that list all the pixels in an image that aren't transparent
// the pixels contained in the HashSet follow the guideline of:
// x,y where x is the absolute x position of the pixel and y is the absolute y position of the pixel
public HashSet<String> getMask(GameObject go){
HashSet<String> mask = new HashSet<String>();
BufferedImage image = null;
try {
image = ImageIO.read(new File (go.getDefaultImageLocation()));
} catch (IOException e) {
System.out.println("error");
}
int pixel, a;
for(int i = 0; i < image.getWidth(); i++){ // for every (x,y) component in the given box,
for( int j = 0; j < image.getHeight(); j++){
pixel = image.getRGB(i, j); // get the RGB value of the pixel
a= (pixel >> 24) & 0xff;
if(a != 0){ // if the alpha is not 0, it must be something other than transparent
mask.add((go.xPos+i)+","+(go.yPos- j)); // add the absolute x and absolute y coordinates to our set
}
}
}
return mask; //return our set
}
// Returns true if there is a collision between object a and object b
public boolean checkCollision(GameObject a, GameObject b){
// This method detects to see if the images overlap at all. If they do, collision is possible
int ax1 = a.getxPos();
int ay1 = a.getyPos();
int ax2 = ax1 + a.getWidth();
int ay2 = ay1 + a.getHeight();
int bx1 = b.getxPos();
int by1 = b.getyPos();
int bx2 = bx1 + b.getWidth();
int by2 = by1 + b.getHeight();
if(by2 < ay1 || ay2 < by1 || bx2 < ax1 || ax2 < bx1)
{
return false; // Collision is impossible.
}
else // Collision is possible.
{
// get the masks for both images
HashSet<String> maskPlayer1 = getMask(player1);
HashSet<String> maskPlayer2 = getMask(player2);
maskPlayer1.retainAll(maskPlayer2); // Check to see if any pixels in maskPlayer2 are the same as those in maskPlayer1
if(maskPlayer1.size() > 0){ // if so, than there exists at least one pixel that is the same in both images, thus
System.out.println("Collision" + count);// collision has occurred.
count++;
return true;
}
}
return false;
}
This is the line I don’t get:
a= (pixel >> 24) & 0xff;
I have no experience with bitwise operators, but Google revealed that the 0xff notation is alpha-hex shorthand for fully transluscent. And the pixel >> 24 apparently moves the color data to the right 24 bits, but I don’t know what that means. No clue what & means either and what it does to the operands.
Could someone please explain what this line does?
