鱼移到左上角，不会跟着鼠标动，求解答，代码如下：

momObj.prototype.init = function()

{

this.x = canWidth * 0.5;

this.y = canHeight * 0.5;

this.angle = 0;

this.bigEye.src = "./src/bigEye0.png" ;

this.bigBody.src = "./src/bigSwim0.png" ;

this.bigTail.src =  "./src/bigTail0.png" ;

}

momObj.prototype.draw = function()

{

//lerp x,y

this.x = lerpDistance(mx, this.x, 0.98);    //让大鱼的坐标一直趋向于鼠标的坐标

this.y = lerpDistance(my, this.y, 0.98);

//delta angle（角度差）

//Math.atan2(x,y)

var deltaY = my - this.y;

var deltaX = mx - this.x;

var beta = Math.atan2(deltaY ，deltaX);//返回值是[-PI，PI]

this.angle = lerpAngle(beta, this.angle, 0.6); 

ctx1.save();

ctx1.translate(this.x,this.y);  //大鱼的相对原点

ctx1.rotate(this.angle);    //旋转画布

ctx1.drawImage(this.bigEye, -this.bigEye.width * 0.5, -this.bigEye.height * 0.5);

ctx1.drawImage(this.bigBody, -this.bigBody.width * 0.5, -this.bigBody.height * 0.5);

ctx1.drawImage(this.bigTail, -this.bigTail.width * 0.5 + 30, -this.bigTail.height * 0.5);

ctx1.restore();

}