Canvasの外側を丸く塗る

/**
  * @param canvas
  */
private void setBlackEdge(Canvas canvas) {
	float w = canvas.getWidth();
	float h = canvas.getHeight();
	int[] colors = new int[] { 0x00000000, 0x00000000, 0xFF000000 };
        RectF rect = new RectF(0, 0, w, h);
        
        // create a paint with a RadialGradient
        RadialGradient shader = new RadialGradient(w / 2, h / 2, w / 2, colors, null, TileMode.CLAMP);
        
        Paint paint = new Paint();        
        paint.setDither(true);
        paint.setAntiAlias(true);
        paint.setFilterBitmap(true);
        paint.setShader(shader);
        
        // paint the rectangle with said gradient
        canvas.drawRect(rect, paint);
}