Create Region from non-transparent pixels of a PNG image

Question

I need to extract from a PNG image a pixel perfect Region to detect collisions. I found the following code to be a solution:

public static Region create(Image image) {

    Region region = new Region();

    for (int i = 0; i < image.getWidth(); i++) {
        for (int j = 0; j < image.getHeight(); j++) {
            if (Color.alpha(image.getRGB(i, j)) == 255) {
                region.op(new Rect(i, j, 1, 1), Region.Op.UNION);
            }
        }
    }

    return region;

    }

The following is the implementation of the Image class:

public class AndroidImage implements Image {

    Bitmap bitmap;
    ImageFormat format;

    public AndroidImage(Bitmap bitmap, ImageFormat format) {
        this.bitmap = bitmap;
        this.format = format;
    }

    @Override
    public int getWidth() {
        return bitmap.getWidth();
    }

    @Override
    public int getHeight() {
        return bitmap.getHeight();
    }

    @Override
    public int getRGB(int x, int y) {
        return bitmap.getPixel(x, y);
    }

    @Override
    public ImageFormat getFormat() {
        return format;
    }

    @Override
    public void dispose() {
        bitmap.recycle();
    }

}

Collision detection doesn't work and when i call the method .isEmpty() from the returned region, it returns true. Any idea why?

Answer 1

You can just create a function inside of the AndroidImage class,

public Region getTranspRegion()
{
    BitmapDrawable bitmapChecker = new BitmapDrawable(bitmap);
    Region transpRegion = bitmapChecker.getTransparentRegion();
    return transpRegion;
}

and then in your main class:

Region imageRegion = new Region(imageX, imageY, image.getWidth(), image.getHeight());
Region collisionRegion = new Region(imageRegion);
collisionRegion.op(image.getTranspRegion(), Op.DIFFERENCE);

A lot easier than checking every pixel. I think your problem MIGHT have been the fact that you were calling getRGB, which seems like it would give you no Alpha value.

I don't know how to deal with the actual detection part with two regions, though...