Understanding how canvas converts an image to black and white

Question

I found this script for converting an image to black and white, which works great, but I was hoping to understand the code a little bit better. I put my questions in the code, i

Answer 1

function grayscale(src){ //Creates a canvas element with a grayscale version of the color image
    //create canvas
    var canvas = document.createElement('canvas');
    //get its context
    var ctx = canvas.getContext('2d');
    //create empty image
    var imgObj = new Image();
    //start to load image from src url
    imgObj.src = src;
    //resize canvas up to size image size
    canvas.width = imgObj.width;
    canvas.height = imgObj.height;
    //draw image on canvas, full canvas API is described here http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html
    ctx.drawImage(imgObj, 0, 0);
    //get array of image pixels
    var imgPixels = ctx.getImageData(0, 0, canvas.width, canvas.height);
    //run through all the pixels
    for(var y = 0; y < imgPixels.height; y++){
        for(var x = 0; x < imgPixels.width; x++){
            //here is x and y are multiplied by 4 because every pixel is four bytes: red, green, blue, alpha
            var i = (y * 4) * imgPixels.width + x * 4; //Why is this multiplied by 4?
            //compute average value for colors, this will convert it to bw
            var avg = (imgPixels.data[i] + imgPixels.data[i + 1] + imgPixels.data[i + 2]) / 3;
            //set values to array
            imgPixels.data[i] = avg; 
            imgPixels.data[i + 1] = avg; 
            imgPixels.data[i + 2] = avg;
        }
    }
    //draw pixels according to computed colors
    ctx.putImageData(imgPixels, 0, 0, 0, 0, imgPixels.width, imgPixels.height); 
    return canvas.toDataURL();
}

In this function coefficient equal to 1/3 are used, however the usually used are: 0.3R + 0.59G + 0.11B (http://gimp-savvy.com/BOOK/index.html?node54.html).