HTML5 grayscale images

grayscale.js

function grayscale(src) {
	var canvas = document.createElement("canvas"),
		ctx = canvas.getContext("2d"),
		imgPixels, x, y, i, avg;

	var imgObj = new Image();
	imgObj.src = src;

	canvas.width = imgObj.width;
	canvas.height = imgObj.height; 

	ctx.drawImage(imgObj, 0, 0);
	imgPixels = ctx.getImageData(0, 0, canvas.width, canvas.height);

	for(y = 0; y < imgPixels.height; y++) {
		for(x = 0; x < imgPixels.width; x++) {
			i = (y * 4) * imgPixels.width + x * 4;
			avg = (imgPixels.data[i] + imgPixels.data[i + 1] + imgPixels.data[i + 2]) / 3;

			imgPixels.data[i] = avg;
			imgPixels.data[i + 1] = avg;
			imgPixels.data[i + 2] = avg;
		}
	}

	ctx.putImageData(imgPixels, 0, 0, 0, 0, imgPixels.width, imgPixels.height);
	return canvas.toDataURL();
}