Make a Transparent Canvas over a BitMap

Question

I have 2 ImageViews, 1 for Canvas and other for a ImageBackground.

The ImageViews have the size of the screen.

When I touch a Screen I need to redraw the Canvas like the example:

    private void reDraw() {
    Display display = getWindowManager().getDefaultDisplay();
    bmOverlay = Bitmap.createBitmap(display.getWidth(), display.getHeight(), oBitmap.getConfig());
    oCanvas = new Canvas(bmOverlay);

    //I need to erase this line
    oCanvas.drawBitmap(bmpBackGround, new Matrix(), null);

    //this code paint some bitmaps on the canvas
    for (ColorBall ball : colorballs) {
        oCanvas.drawBitmap(ball.getBitmap(), ball.getX(), ball.getY(), null);
    }
    //set the canvas on the image view
    getImageView2().setImageBitmap(bmOverlay);

}

But, on this example I redraw the Bitmap, when I touch the screen. I need to redraw a transparent Canvas because I don´t need to redraw the Bitmap on the background, because the bitmap never change. But if I don´t redraw the bitmap, the background stay black and the bitmap stay on the back. Any help?

Answer 1

Check to make sure the overlay bitmap you've created is actually transparent. Create it with config ARGB_8888 explicitly. http://developer.android.com/reference/android/graphics/Bitmap.Config.html

And you might want to move the creation of the bitmap out of the reDraw method and only create it once instead.