看很多例子里悬浮窗里放个canvas,为啥这么写背景是白色的

代码如下,要怎么把canvas 画布背景搞透明呢?
let window = floaty.rawWindow(
<vertical bg="#80000000">
<canvas id="board" h="300" w="300" />
</vertical>
);
window.setSize(500, 500);
window.setTouchable(false);
let paint = new Paint();
window.board.on("draw", function (canvas) {
log("canvas:" + canvas);
canvas.drawCircle(300,300,200,paint);
canvas.drawARGB(0, 0, 0, 0);a通道就算是0背景也是白色,不是0就会一直叠加,颜色越来越深...
});
setTimeout(() => { }, 5000);