h5标签canvas关于getImageData跨域的问题

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();
}