Pixel level collision detection fails for small objects. (Java)

Question

For a project we were given a game engine off which to create a game. We, as part of this, have to implement pixel level collision detection after a possible collision has been found via a bounding box detection method. I have implemented both but my pixel level test fails for small objects (bullets in this case). I have checked if it works for slow bullets but that fails too.

For my pixel level implementation I create bitmasks for each texture using an the available IntBuffer (a ByteBuffer is available too?). The IntBuffer is in RGBA format and its size is width*height, I placed this in a 2D array and replaced all non-zero numbers with 1's to create the mask. After a collision of bounding boxes I find the rectangle represented by the overlap (using .createIntersection) and then check the maps of both sprites within this intersection for a nonzero pixel from both using bitwise AND.

Here is my code for the pixel level test:

/**
 * Pixel level test
 *
 * @param rect the rectangle representing the intersection of the bounding
 * boxes
 * @param index1 the index at which the first objects texture is stored
 * @param index the index at which the second objects texture is stored
 */
public static boolean isBitCollision(Rectangle2D rect, int index1, int index2)
{
    int height = (int) rect.getHeight();
    int width = (int) rect.getWidth();

    long mask1 = 0;
    long mask2 = 0;

    for (int i = 0; i < width; i++)
    {
        for (int j = 0; j < height; j++)
        {
            mask1 = mask1 + bitmaskArr[index1].bitmask[i][j];//add up the current column of "pixels"
            mask2 = mask2 + bitmaskArr[index2].bitmask[i][j];

            if (((mask1) & (mask2)) != 0)//bitwise and, if both are nonzero there is a collsion
            {
                return true;
            }
            mask1 = 0;
            mask2 = 0;
        }

    }


    return false;
}

I've been struggling with this for days and any help will be greatly appreciated.

Answer 1

I managed to solve my own issue and now it works properly. For anyone interested what I did was find the rectangle created by the overlap of the two bounding boxes of the two sprites. I then dropped each object to the origin along with it, relatively, the rectangle of intersection. It should be noted that I dropped each object to a "separate" origin - ie I effectively had two rectangle of intersection afterwards - one for each. The co-ordinates of each rectangle of intersection, now in bounds of the bitmask 2D arrays for both objects, were used to check the correct regions for overlap of both objects:

I loop bottom to top left to right through the bitmask as the image data provided in upside - apparently this is the norm for image data.

 /**
 * My Pixel level test - 2D
 *
 * @param rect the rectangle representing the intersection of the bounding
 * boxes
 * @param index1 the index at which the first objects texture is stored
 * @param index2 the index at which the second objects texture is stored
 * @param p1 the position of object 1
 * @param p2 the position of object 2
 * @return true if there is a collision at a pixel level false if not
 */
//public static boolean isPixelCollision(Rectangle2D rect, Point2D.Float p1, Bitmask bm1, Point2D.Float p2, Bitmask bm2)
public static boolean isPixelCollision(Rectangle2D rect, Point2D.Float p1, int index1, Point2D.Float p2, int index2)
{
    int height = (int) rect.getHeight();
    int width = (int) rect.getWidth();

    byte mask1 = 0;
    byte mask2 = 0;

    //drop both objects to the origin and drop a rectangle of intersection for each along with them
    //this allows for us to have the co-ords of the rect on intersection within them at number that are inbounds.
    Point2D.Float origP1 = new Point2D.Float((float) Math.abs(rect.getX() - p1.x), (float) Math.abs(rect.getY() - p1.y));//rect for object one
    Point2D.Float origP2 = new Point2D.Float((float) Math.abs(rect.getX() - p2.x), (float) Math.abs(rect.getY() - p2.y));//rect for object two

    //to avoid casting with every iteration
    int start1y = (int) origP1.y;
    int start1x = (int) origP1.x;
    int start2y = (int) origP2.y;
    int start2x = (int) origP2.x;

    //we need to loop within the rect of intersection
    //goind bottom up and left to right
    for (int i = height - 1; i > 0; i--)
    {
        for (int j = 0; j < width; j++)
        {
            mask1 = bitmaskArr[index1].bitmask[start1y + i][start1x + j];
            mask2 = bitmaskArr[index2].bitmask[start2y + i][start2x + j];
            if ((mask1 & mask2) > 0)
            {
                return true;
            }
        }

    }
    //no collsion was found
    return false;
}

Answer 2

The problem could be with this part of the code:

if (((mask1) & (mask2)) != 0)//bitwise and, if both are nonzero there is a collsion
{
    return true;
}

you seem to be using a bitwise and for checking whether both values are non zero - but this may not work in such a manner.

For instance the value of the following expression:

3 & 4 == 0

is true

This is because when you do a bitwise operation you need to think of the numbers as their bit representation and do the operation bit-by-bit.

So:

  3 = 0000 0011
& 4 = 0000 0100
---------------
  0 = 0000 0000

this is because of how the 1 bit values align to one another. For a bit in a bitwise and to have the value of 1 - two bits at the same location in the different numbers need to be 1.

Another example would be:

  3 = 0000 0011
& 2 = 0000 0010
---------------
  2 = 0000 0010

So in your case a better check would be:

if ( mask1>0 && mask2>0 )//logical and if both are nonzero there is a collsion
{
    return true;
}