js关于getImageData跨域问题的解决方法

function createGSCanvas(img){
 var canvas=document.createElement("canvas");
 canvas.width=img.width;
 canvas.height=img.height;
 var ctx=canvas.getContext("2d");
 ctx.drawImage(img,0,0);

 var c=ctx.getImageData(0,0,img.width,img.height);
 for(var i=0;i<c.height;i++){
  for(var j=0;j<c.width;j++){
   var x=(i*4)*c.height+(j*4);
   var r=c.data[x];
   var g=c.data[x+1];
   var b=c.data[x+2];
   c.data[x]=c.data[x+1]=c.data[x+2]=(r+g+b)/3;

  }
 }
 ctx.putImageData(c,0,0,0,0,c.width,c.height);
 return canvas.toDataURL();
}