var imagedata = event.data.imgData,
intensity = event.data.intensity,
px = imagedata.data, // The line throwing the error. Could you help address this?
grey = null,
length = px.length;
for (var i = 0; i < length; i += 4) {
var k = px[i] * 0.3 + px[i + 1] * 0.59 + px[i + 2] * 0.11;
px[i] = ~~ (k * intensity + px[i] * (1 - intensity));
px[i + 1] = ~~ (k * intensity + px[i + 1] * (1 - intensity));
px[i + 2] = ~~ (k * intensity + px[i + 2] * (1 - intensity));
}
postMessage(imagedata);
};