透明度一直不起作用啊

aneObj.prototype.draw = function(){

	ctx2.save();
	ctx2.globalAlpha = 0.6;
	for (var i = 0; i < this.num; i++) {
		//beginPath,moveTo,lineTo,stroke,strokeStyle ,lineWidth,
		ctx2.beginPath();
		ctx2.moveTo(this.x[i],canHeight);
		ctx2.lineTo(this.x[i],canHeight - this.len[i]);
		ctx2.lineWidth = 20;
		ctx2.lineCap = "round"
		ctx2.strokeStyle = "#3b154e";
		ctx2.stroke();
		

	}
	ctx2.restore();

}