灰色滤镜的问题

请问我这个灰色滤镜为什么无效

function greyEffect(){

var ImageData = contexta.getImageData(0,0,canvasa.width,canvasa.height)

var pixelData = ImageData.data

for(var i=0; i< canvasb.width * canvasb.height; i++){

var r = pixelData[i*4+0]

var g = pixelData[i*4+1]

var a = pixelData[i*4+2]

var grey = r*0.3 + g*0.59 + b*0.11

pixelData[i*4+0] = grey

pixelData[i*4+1] = grey

pixelData[i*4+2] = grey

}

contextb.putImageData(ImageData,0,0,0,0,canvasb.width,canvasb.height)

}