Pixel perfect collision with Config.ALPHA_8

Question

I\'ve found this code of pixel perfect collision checking and used it in my code:

public  boolean isCollisionDetected(Bitmap bitmap1, int x1, int y1,
               


        

Answer 1

Calling getPixel() on a bitmap with ALPHA_8 configuration will always return zero. This seems to be a bug

You can work around this problem by storing the pixels of each bitmap as a byte array:

byte[] pixelData = getPixels(convert(bitmap, Bitmap.Config.ALPHA_8));

...

 public byte[] getPixels(Bitmap bmp) {
    int bytes = bmp.getRowBytes() * bmp.getHeight();
    ByteBuffer buffer = ByteBuffer.allocate(bytes);
    bmp.copyPixelsToBuffer(buffer);
    return buffer.array();
}

You will need to modify your collision detection function a little:

public  boolean isCollisionDetected(byte[] pixels1, Bitmap bitmap1, int x1, int y1,
            byte[] pixels2, Bitmap bitmap2, int x2, int y2) {

        int width1 =bitmap1.getWidth();
        int height1=bitmap1.getHeight();
        int width2 =bitmap2.getWidth();
        int height2=bitmap2.getHeight();

        Rect bounds1 = new Rect(x1, y1, x1 + width1, y1 + height1);
        Rect bounds2 = new Rect(x2, y2, x2 + width2, y2 + height2);

        if (Rect.intersects(bounds1, bounds2)) {
            Rect collisionBounds = getCollisionBounds(bounds1, bounds2);
            for (int i = collisionBounds.left; i < collisionBounds.right; i++) {
                for (int j = collisionBounds.top; j < collisionBounds.bottom; j++) {
                    byte bitmap1Pixel = pixels1[((j - y1) * width1) + (i - x1)];
                    byte bitmap2Pixel = pixels2[((j - y2) * width2) + (i - x2)];
                    if (isFilled(bitmap1Pixel) && isFilled(bitmap2Pixel)) {
                        return true;
                    }
                }
            }
        }
        return false;
    }

(... you might want to change the bitmap parameters into the respective width and height of the those bitmaps and call recycle().)

Answer 2

Android Config_Alpha_8 does not save color information, so when you draw the bitmap, adding the paint will do nothing, and all isFilled checks will return false.

Neat piece of code though.