怎么样使用Html5 Canvas绘制放射性渐变色效果？

function Radia(bottom_x,bottom_y,bottom_r,top_x,top_y,top_r){
 this.bottom_x=bottom_x;
 this.bottom_y=bottom_y;
 this.bottom_r=bottom_r;
 this.top_x=top_x;
 this.top_y=top_y;
 this.top_r=top_r;
 this.gradient=can2_context.createRadialGradient(this.bottom_x,this.bottom_y,this.bottom_r,this.top_x,this.top_y,this.top_r)
}
Radia.prototype.addColor=function(){
 for(var i=0;i<arguments.length;i++){
  this.gradient.addColorStop(arguments[i].num,arguments[i].color)
 }
}
Radia.prototype.draw=function(x1,y1,x2,y2){
  can2_context.fillStyle=this.gradient;
  can2_context.fillRect(x1,y1,x2,y2)
}
var some1=new Radia(canvas_2.width/2, canvas_2.height-100, 0, canvas_2.width/2, 0, 300)
some1.addColor({num:0.2,color:"blue"},{num:1,color:"yellow"},{num:0.7,color:"white"})
some1.draw(0, 0, canvas_2.width, canvas_2.height)