HarmonyOS OffscreenCanvasRenderingContext2D使用问题咨询

OffscreenCanvasRenderingContext2D 实现一个文本的位移效果

ctx.clearRect(0,0,ctx.width,ctx.height)


let offContext = this.offCanvas.getContext("2d", this.settings)
// offContext.clearRect(0,0,ctx.width,ctx.height)
offContext.font = '60px sans-serif';
for (let i = 0; i < 30; i += 1) {
  offContext.fillStyle = `#ff0000`
  offContext.fillText("Hello World2", this.left, 0+i*30);
}
let image = this.offCanvas.transferToImageBitmap()
ctx.transferFromImageBitmap(image)

这断代码循环调用的时候，画布上并没有清除上一次的渲染的画面，我该如何清除