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();}