Pixel-Perfect Collision Detection Android

Question

Ok so I am working on a game on Android. I need to implement pixel perfect collision detection. I already have the bounding boxes set up around each of the images, each bounding box is transformed to match the current rotation of the image. That all works great. I also have the pixel data from each bitmap stored in an array. Can someone help me figure out the most efficient way to go about detecting if the pixels overlap? Thanks in advance for any help!

Answer 1

The basic idea is to create a bitmask for each object where you indicate in each pixel if the object is actually there or not. Then you compare each pixel of the bitmasks for the two objects.

You could minimize the number of pixels you need to check by calculating the rectangular area in which the two bounding boxes overlap. The pixels within this area are what you need to check.

Iterate through all of those pixels, and check if the pixel is filled in both objects. If any of them are, then you have a collision.

If your rectangles are aligned with the x/y axis, to find the overlap, find the left, right, top and bottom of the overlap. It would look something like this (I could have screwed up the edge cases, haven't tried this):

int left = max(obj1.left, obj2.left)
int right = min(obj1.right, obj2.right)
int top = min(obj1.top, obj2.top)
int bottom = max(obj1.bottom, obj2.bottom)

for (int x = left; x < right; x++) {
  for (int y = top; y < bottom; y++) {
     if (obj1.isFilled(x,y) && obj2.isFilled(x,y)) {
        return true;
     }
  }
}