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();

}