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Canvas Pixelation - Adjust pixel size

I've got a script that cycle's through images. The images start pixelated and then when they are in view, become unpixelated. I achieve that by calling this function x amount of times with requestAnimationFrame
Images.prototype.setPixels = function() {
var sw = this.imageWidth,
sh = this.imageHeight,
imageData = this.context.getImageData( 0, 0, sw, sh ),
data = imageData.data,
y, x, n, m;
for ( y = 0; y < sh; y += this.pixelation ) {
for ( x = 0; x < sw; x += this.pixelation ) {
var red = data[((sw * y) + x) * 4];
var green = data[((sw * y) + x) * 4 + 1];
var blue = data[((sw * y) + x) * 4 + 2];
for ( n = 0; n < this.pixelation; n++ ) {
for ( m = 0; m < this.pixelation; m++ ) {
if ( x + m < sw ) {
data[((sw * (y + n)) + (x + m)) * 4] = red;
data[((sw * (y + n)) + (x + m)) * 4 + 1] = green;
data[((sw * (y + n)) + (x + m)) * 4 + 2] = blue;
}
}
}
}
}
this.context.putImageData( imageData, 0, 0 );
}
Question: How can I make the individual pixels larger blocks than they are right now. Right now they are pretty small, and the effect is a little jarring. I'm hoping to fix this by having less pixel blocks on the screen, by making them bigger.
I hope this makes sense, I'm fairly green with canvas, so anything you could do to point me in the right direction would be great!

The best for this kind of effect is to simply use drawImage and let the browser handle the pixelation thanks to the nearest-neighbor anti-aliasing algorithm that can be set by changing the imageSmoothingEnabled property to false.
It then becomes a two step process to pixelate an image at any pixel_size:
draw the full quality image (or canvas / video ...) at its original size / pixel_size.
At this stage, each "pixel" is one pixel large.
draw this small image again but up-scaled by pixel_size. To do so, you just need to draw the canvas over itself.
Each pixel is now pixel_size large.
Instead of dealing with hard to read many parameters of drawImage, we can deal the scaling quite easily by just using ctx.scale() method.
const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
function drawPixelated( source, pixel_size ) {
// scale down
ctx.scale(1 / pixel_size, 1 / pixel_size)
ctx.drawImage(source, 0, 0);
// make next drawing erase what's currently on the canvas
ctx.globalCompositeOperation = 'copy';
// nearest-neighbor
ctx.imageSmoothingEnabled = false;
// scale up
ctx.setTransform(pixel_size, 0, 0, pixel_size, 0, 0);
ctx.drawImage(canvas, 0, 0);
// reset all to defaults
ctx.setTransform(1, 0, 0, 1, 0, 0);
ctx.globalCompositeOperation = 'source-over';
ctx.imageSmoothingEnabled = true;
}
const img = new Image();
img.onload = animeLoop;
img.src = "https://upload.wikimedia.org/wikipedia/commons/4/47/PNG_transparency_demonstration_1.png";
let size = 1;
let speed = 0.1;
function animeLoop() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
size += speed;
if(size > 30 || size <= 1) {
speed *= -1
}
drawPixelated( img, size );
requestAnimationFrame(animeLoop);
}
<canvas id="canvas" width="800" height="600"></canvas>
Now for the ones that come with a real need to use an ImageData, for instance because they are generating pixel-art, then know that you can simply use the same technique:
put your ImageData with each pixel being 1 pixel large.
scale your context to pixel_size
draw your canvas over itself upscaled
const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
function putPixelated( imageData, pixel_size ) {
ctx.putImageData(imageData, 0, 0);
// make next drawing erase what's currently on the canvas
ctx.globalCompositeOperation = 'copy';
// nearest-neighbor
ctx.imageSmoothingEnabled = false;
// scale up
ctx.setTransform(pixel_size, 0, 0, pixel_size, 0, 0);
ctx.drawImage(canvas, 0, 0);
// reset all to defaults
ctx.setTransform(1, 0, 0, 1, 0, 0);
ctx.globalCompositeOperation = 'source-over';
ctx.imageSmoothingEnabled = true;
}
const img = new ImageData(16, 16);
crypto.getRandomValues(img.data);
let size = 1;
let speed = 0.1;
animeLoop();
function animeLoop() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
size += speed;
if(size > 30 || size <= 1) {
speed *= -1
}
putPixelated( img, size );
requestAnimationFrame(animeLoop);
}
<canvas id="canvas" width="800" height="600"></canvas>

How to calculate actual width and height of rotated object in canvas?

Am trying to calculate width and height of object i loaded into canvas. When object is not rotated i get correct left right top bottom values, but when i load rotated object in canvas then i not get correct values , so i wonder what will be the logic or math formula to do achieve it.
how am doing.
initially load image into canvas
get image data from canvas
loop through image data to get only colored pixels by using alpha check
from colored pixel array find min max xy values
var temp_ray = []; // pixel array
for (var y = 0; y < imgData.height; ++y) {
for (var x = 0; x < imgData.width; ++x) {
var index = (y * imgData.width + x) * 4;
if(imgData.data[index+3]){
var xc = (index / 4) % imgData.width;
var yc = Math.floor((index / 4) / imgData.width);
temp_ray.push([xc,yc]);
}
}
}
if(temp_ray.length > 0){
var Xind = MaxMin2darray(temp_ray,0);
var Yind = MaxMin2darray(temp_ray,1);
var W = parseFloat(Xind['max']) - parseFloat(Xind['min']);
var H = parseFloat(Yind['max']) - parseFloat(Yind['min']);
var center_x = Xind['min'] + (W/2);
var center_y = Yind['min'] + (H/2);
// find corners of object
// find *min x , min y
let top_left = temp_ray[Xind['imin']]; // min X priority , min Y // top left
// find max x , *min y
let top_right = temp_ray[Yind['imin']]; // max X, min Y priority , // top right
// find *max x , min y
let bot_right = temp_ray[Xind['imax']]; // max X priority , min Y // bottom right
// find max x , *max y
let bot_left = temp_ray[Yind['imax']]; // max X , max Y priority // bottom left
var dim = {'W':W,'H':H,'CenterX':center_x,'CenterY':center_y,'top_left':top_left,'top_right':top_right,'bot_right':bot_right,'bot_left':bot_left,'Xend':Xind['max'],'Yend':Yind['max'],'Xstart':Xind['min'],'Ystart':Yind['min'],'Xend':Xind['max'],'Yend':Yind['max']};
console.log(dim);
}
and then using min max xy value find corners of object which works with none rotated objects but not work with rotated/tilted objects.
so any idea how to solve this problem
openpnp project is achieving this through opencv, but i think in js we do not have opencv library nor am that pro of java :(.
https://github.com/openpnp/openpnp/blob/develop/src/main/java/org/openpnp/vision/pipeline/stages/DrawRotatedRects.java
jsfiddle: http://jsfiddle.net/4L13vtaj/

In some simple cases (like rectangular objects), you could try to rotate the image until you minimize the number of uncolored pixels.
So you start with your image, and for each of the possible 360°, you compute the ratio. This is not perfect, but "doable" simply in pure js.
Here's a pseudoCode that might help you:
for degree in [0,365]{
rotateOriginalImageBy(degree);
cost[degree] = NemptyPixels/NfilledPixels;
}
predictedDegree = Math.min(cost);
rotateOriginalImageBy(predictedDegree);
compute 2 dimensions;
width = largerDimension;
height = shorterDimension;
Begining of an implementation (I edited your jsfiddle):
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var rotatioDegree = 45;
var imageObject = new Image();
imageObject.onload = function() {
var canvasWidth = imageObject.width;
var canvasHeight = canvasWidth; // not useful since width==height
document.getElementById('canvas').width = canvasWidth;
document.getElementById('canvas').height = canvasWidth;
ctx.clearRect(0, 0, canvasWidth, canvasWidth);
// Move registration point to the center of the canvas
ctx.translate(canvasWidth/2, canvasWidth/2)
ctx.rotate(rotatioDegree*3.1415/180);
ctx.translate(-canvasWidth/2,-canvasWidth/2)
ctx.drawImage(imageObject,0,0);
ctx.translate(canvasWidth/2, canvasWidth/2)
ctx.rotate(-rotatioDegree*3.1415/180);
ctx.translate(-canvasWidth/2,-canvasWidth/2)
var imgData = ctx.getImageData(0, 0, canvasWidth, canvasWidth);
http://jsfiddle.net/4L13vtaj/17/
If this doesn't work, you could implement some image detection techniques (Mathematical morphology for example). But i think this is outside the scope of stackoverflow.

If you work with some approximation, you can have something like that; I hope at least it can provide to you some ideas:
// some pixels in this image are not transparent, so we add a tollerance
// you can try to remove the second condition.
const isNotEmpty = (color) => color && color < 0xffaaaaaa;
function getTop(buff, w, h) {
for (let y = 0; y < h; y++) {
for (let x = 0; x < w; x++) {
let i = y * w + x;
if (isNotEmpty(buff[i])) {
return {x, y}
}
}
}
}
function getRight(buff, w, h) {
for (let x = w; x >=0; x--) {
for (let y = 0; y < h; y++) {
let i = y * w + x;
if (isNotEmpty(buff[i])) {
return {x, y}
}
}
}
}
function getBottom(buff, w, h) {
for (let y = h; y >= 0; y--) {
for (let x = 0; x < w; x++) {
let i = y * w + x;
if (isNotEmpty(buff[i])) {
return {x, y}
}
}
}
}
function getLeft(buff, w, h) {
for (let x = 0; x < w; x++) {
for (let y = 0; y < h; y++) {
let i = y * w + x;
if (isNotEmpty(buff[i])) {
return {x, y}
}
}
}
}
async function main(imageSource) {
const canvas = document.querySelector("canvas");
const ctx = canvas.getContext("2d");
const imageObject = new Image();
imageObject.src = imageSource;
await new Promise(r => imageObject.onload = r);
const w = canvas.width = imageObject.width;
const h = canvas.height = imageObject.height;
ctx.clearRect(0, 0, w, h);
ctx.drawImage(imageObject, 0, 0);
const imgData = ctx.getImageData(0, 0, w, h);
const buff = new Uint32Array(imgData.data.buffer);
const points = [
getTop(buff, w, h),
getRight(buff, w, h),
getBottom(buff, w, h),
getLeft(buff, w, h)
];
ctx.strokeStyle = "#0000ff"
ctx.beginPath();
ctx.moveTo(points[0].x, points[0].y);
ctx.lineTo(points[1].x, points[1].y);
ctx.lineTo(points[2].x, points[2].y);
ctx.lineTo(points[3].x, points[3].y);
ctx.closePath();
ctx.stroke();
}
main(/* image's url*/);
Here the link for testing: https://codepen.io/zer0/pen/zLxyQV
There are several problem with this approach: as said, with irregular images, it's not precise, in fact you will see the pin are making the image's bounding box a little bit smaller.
But the thing can be worse: try in the link above to use the 2nd image, that is quite irregular, and you will see.
Of course we can compensate, using also a bit more complex algorithm instead this simple one, but the question is: what the expected result for something like the 2nd image? Depends by that you can decide how to proceed.

Animating canvas to look like tv noise

I have a function named generateNoise() which creates a canvas element and paints random RGBA values to it; which, gives the appearance of noise.
My Question
What would be the best way to infinitely animate the noise to give the appearance of movement. So that it may have more life?
JSFiddle
function generateNoise(opacity) {
if(!!!document.createElement('canvas').getContext) {
return false;
}
var canvas = document.createElement('canvas'),
ctx = canvas.getContext('2d'),
x,y,
r,g,b,
opacity = opacity || .2;
canvas.width = 55;
canvas.height = 55;
for (x = 0; x < canvas.width; x++){
for (y = 0; y < canvas.height; y++){
r = Math.floor(Math.random() * 255);
g = Math.floor(Math.random() * 255);
b = Math.floor(Math.random() * 255);
ctx.fillStyle = 'rgba(' + r + ',' + b + ',' + g + ',' + opacity + ')';
ctx.fillRect(x,y,1,1);
}
}
document.body.style.backgroundImage = "url(" + canvas.toDataURL("image/png") + ")";
}
generateNoise(.8);

Update 1/2017: I rewrote the entire answer as it started to become rather messy, and to address some of the issues pointed out in the comments. The original answer can be found here. The new answer has in essence the same code but improved, and with a couple of new techniques, one utilizes a new feature available since this answer was first posted.
For a "true" random look we would need to use pixel-level rendering. We can optimize this using 32-bit unsigned buffers instead of 8-bit, and we can also turn off the alpha-channel in more recent browsers which speeds up the entire process (for older browsers we can simply set a black opaque background for the canvas element).
We create a reusable ImageData object once outside the main loop so the main cost is only putImageData() and not both inside the loop.
var ctx = c.getContext("2d", {alpha: false}); // context without alpha channel.
var idata = ctx.createImageData(c.width, c.height); // create image data
var buffer32 = new Uint32Array(idata.data.buffer); // get 32-bit view
(function loop() {
noise(ctx);
requestAnimationFrame(loop)
})()
function noise(ctx) {
var len = buffer32.length - 1;
while(len--) buffer32[len] = Math.random() < 0.5 ? 0 : -1>>0;
ctx.putImageData(idata, 0, 0);
}
/* for browsers wo/2d alpha disable support */
#c {background:#000}
<canvas id=c width=640 height=320></canvas>
A very efficient way, at the cost of some memory but reduced cost on the CPU, is to pre-render a larger off-screen canvas with the noise once, then place that canvas into the main one using random integer offsets.
It require a few extra preparation steps but the loop can run entirely on the GPU.
var w = c.width;
var h = c.height;
var ocanvas = document.createElement("canvas"); // create off-screen canvas
ocanvas.width = w<<1; // set offscreen canvas x2 size
ocanvas.height = h<<1;
var octx = ocanvas.getContext("2d", {alpha: false});
var idata = octx.createImageData(ocanvas.width, ocanvas.height);
var buffer32 = new Uint32Array(idata.data.buffer); // get 32-bit view
// render noise once, to the offscreen-canvas
noise(octx);
// main loop draw the offscreen canvas to random offsets
var ctx = c.getContext("2d", {alpha: false});
(function loop() {
var x = (w * Math.random())|0; // force integer values for position
var y = (h * Math.random())|0;
ctx.drawImage(ocanvas, -x, -y); // draw static noise (pun intended)
requestAnimationFrame(loop)
})()
function noise(ctx) {
var len = buffer32.length - 1;
while(len--) buffer32[len] = Math.random() < 0.5 ? 0 : -1>>0;
ctx.putImageData(idata, 0, 0);
}
/* for browsers wo/2d alpha disable support */
#c {background:#000}
<canvas id=c width=640 height=320></canvas>
Do note though that with the latter technique you may risk getting "freezes" where the new random offset is similar to the previous one. To work around this problem, set criteria for the random position to disallow too close positions in a row.

I tried to make a similar function a while ago. I set each pixel random value, and in addition to that, I overlayed a sinusodial wave that traveled upwards with time just to make it look more realistic. You can play with the constants in the wave to get different effects.
var canvas = null;
var context = null;
var time = 0;
var intervalId = 0;
var makeNoise = function() {
var imgd = context.createImageData(canvas.width, canvas.height);
var pix = imgd.data;
for (var i = 0, n = pix.length; i < n; i += 4) {
var c = 7 + Math.sin(i/50000 + time/7); // A sine wave of the form sin(ax + bt)
pix[i] = pix[i+1] = pix[i+2] = 40 * Math.random() * c; // Set a random gray
pix[i+3] = 255; // 100% opaque
}
context.putImageData(imgd, 0, 0);
time = (time + 1) % canvas.height;
}
var setup = function() {
canvas = document.getElementById("tv");
context = canvas.getContext("2d");
}
setup();
intervalId = setInterval(makeNoise, 50);
<canvas id="tv" width="400" height="300"></canvas>
I used it as a preloader on a site. I also added a volume rocker as a loading bar, here's a screenshot:

I re-wrote your code so each step is separate so you can re-use things without having to create and re-create each time, reduced in-loop calls and hopefully made it clear enough to be able to follow by reading it.
function generateNoise(opacity, h, w) {
function makeCanvas(h, w) {
var canvas = document.createElement('canvas');
canvas.height = h;
canvas.width = w;
return canvas;
}
function randomise(data, opacity) { // see prev. revision for 8-bit
var i, x;
for (i = 0; i < data.length; ++i) {
x = Math.floor(Math.random() * 0xffffff); // random RGB
data[i] = x | opacity; // set all of RGBA for pixel in one go
}
}
function initialise(opacity, h, w) {
var canvas = makeCanvas(h, w),
context = canvas.getContext('2d'),
image = context.createImageData(h, w),
data = new Uint32Array(image.data.buffer);
opacity = Math.floor(opacity * 0x255) << 24; // make bitwise OR-able
return function () {
randomise(data, opacity); // could be in-place for less overhead
context.putImageData(image, 0, 0);
// you may want to consider other ways of setting the canvas
// as the background so you can take this out of the loop, too
document.body.style.backgroundImage = "url(" + canvas.toDataURL("image/png") + ")";
};
}
return initialise(opacity || 0.2, h || 55, w || 55);
}
Now you can create some interval or timeout loop which keeps re-invoking the generated function.
window.setInterval(
generateNoise(.8, 200, 200),
100
);
Or with requestAnimationFrame as in Ken's answer
var noise = generateNoise(.8, 200, 200);
(function loop() {
noise();
requestAnimationFrame(loop);
})();
DEMO

Ken's answer looked pretty good, but after looking at some videos of real TV static, I had some ideas and here's what I came up with (two versions):
http://jsfiddle.net/2bzqs/
http://jsfiddle.net/EnQKm/
Summary of changes:
Instead of every pixel being independently assigned a color, a run of multiple pixels will get a single color, so you get these short, variable-sized horizontal lines.
I apply a gamma curve (with the Math.pow) to bias the color toward black a little.
I don't apply the gamma in a "band" area to simulate the banding.
Here's the main part of the code:
var w = ctx.canvas.width,
h = ctx.canvas.height,
idata = ctx.createImageData(w, h),
buffer32 = new Uint32Array(idata.data.buffer),
len = buffer32.length,
run = 0,
color = 0,
m = Math.random() * 6 + 4,
band = Math.random() * 256 * 256,
p = 0,
i = 0;
for (; i < len;) {
if (run < 0) {
run = m * Math.random();
p = Math.pow(Math.random(), 0.4);
if (i > band && i < band + 48 * 256) {
p = Math.random();
}
color = (255 * p) << 24;
}
run -= 1;
buffer32[i++] = color;
}

I happen to have just written a script that does just this, by getting the pixels from a black canvas and just altering random alpha values and using putImageData
Result can be found at http://mouseroot.github.io/Video/index.html
var currentAnimationFunction = staticScreen
var screenObject = document.getElementById("screen").getContext("2d");
var pixels = screenObject.getImageData(0,0,500,500);
function staticScreen()
{
requestAnimationFrame(currentAnimationFunction);
//Generate static
for(var i=0;i < pixels.data.length;i+=4)
{
pixels.data[i] = 255;
pixels.data[i + 1] = 255;
pixels.data[i + 2] = 255;
pixels.data[i + 3] = Math.floor((254-155)*Math.random()) + 156;
}
screenObject.putImageData(pixels,0,0,0,0,500,500);
//Draw 'No video input'
screenObject.fillStyle = "black";
screenObject.font = "30pt consolas";
screenObject.fillText("No video input",100,250,500);
}

Mine doesn't look identical to real TV static, but it's similar nonetheless. I'm just looping through all the pixels on canvas, and changing the RGB colour components of each pixel at a random coordinate to a random colour. The demo can be found over at CodePen.
The code is as follows:
// Setting up the canvas - size, setting a background, and getting the image data(all of the pixels) of the canvas.
canvas = document.getElementById("canvas");
ctx = canvas.getContext("2d");
canvas.width = 400;
canvas.height = 400;
canvasData = ctx.createImageData(canvas.width, canvas.height);
//Event listeners that set the canvas size to that of the window when the page loads, and each time the user resizes the window
window.addEventListener("load", windowResize);
window.addEventListener("resize", windowResize);
function windowResize(){
canvas.style.width = window.innerWidth + 'px';
canvas.style.height = window.innerHeight + 'px';
}
//A function that manipulates the array of pixel colour data created above using createImageData()
function setPixel(x, y, r, g, b, a){
var index = (x + y * canvasData.width) * 4;
canvasData.data[index] = r;
canvasData.data[index + 1] = g;
canvasData.data[index + 2] = b;
canvasData.data[index + 3] = a;
}
window.requestAnimationFrame(mainLoop);
function mainLoop(){
// Looping through all the colour data and changing each pixel to a random colour at a random coordinate, using the setPixel function defined earlier
for(i = 0; i < canvasData.data.length / 4; i++){
var red = Math.floor(Math.random()*256);
var green = Math.floor(Math.random()*256);
var blue = Math.floor(Math.random()*256);
var randX = Math.floor(Math.random()*canvas.width);
var randY = Math.floor(Math.random()*canvas.height);
setPixel(randX, randY, red, green, blue, 255);
}
//Place the image data we created and manipulated onto the canvas
ctx.putImageData(canvasData, 0, 0);
//And then do it all again...
window.requestAnimationFrame(mainLoop);
}

You can do it like this:
window.setInterval('generateNoise(.8)',50);
The 2nd arg 50 is a delay in milliseconds. Increasing 50 will slow it down and decreasing visa versa.
though.. this is going to severely affect web page performance. If it were me, I'd do the rendering server-side and render a handful of frame iterations and output as an animated gif. Not quite the same as infinite randomness, but would be a huge performance boost and IMO most people won't even notice.

Understanding how canvas converts an image to black and white

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, in the form of comments.
Can anyone explain in a little more detail what is happening here:
function grayscale(src){ //Creates a canvas element with a grayscale version of the color image
var canvas = document.createElement('canvas');
var ctx = canvas.getContext('2d');
var imgObj = new Image();
imgObj.src = src;
canvas.width = imgObj.width;
canvas.height = imgObj.height;
ctx.drawImage(imgObj, 0, 0); //Are these CTX functions documented somewhere where I can see what parameters they require / what those parameters mean?
var imgPixels = ctx.getImageData(0, 0, canvas.width, canvas.height);
for(var y = 0; y < imgPixels.height; y++){
for(var x = 0; x < imgPixels.width; x++){
var i = (y * 4) * imgPixels.width + x * 4; //Why is this multiplied by 4?
var avg = (imgPixels.data[i] + imgPixels.data[i + 1] + imgPixels.data[i + 2]) / 3; //Is this getting the average of the values of each channel R G and B, and converting them to BW(?)
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();
}

The canvas functions are, like most functions, described in an official specification. Also, MDC is helpful for more "informal" articles. E.g. the drawImage function on MDC is here.
The getImageData function returns an object, which contains an array with the byte data of all pixels. Each pixel is described by 4 bytes: r, g, b and a.
r, g and b are the color components (red, green and blue) and alpha is the opacity. So each pixel uses 4 bytes, and therefore a pixel's data begins at pixel_index * 4.
Yes, it's averaging the values. Because in the next 3 lines r, g and b are all set to that same value, you'll obtain a gray color for each pixel (because the amount of all 3 components are the same).
So basically, for all pixels this will hold: r === g, g === b and thus also r === b. Colors for which this holds are grayscale (0, 0, 0 being black and 255, 255, 255 being white).

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).

Disable anti-aliasing when drawing an image to a canvas-element in IE9

Our application loads a PNG-image via JavaScript and draws it to the 2D-context of a canvas-element in order to read the exact color values of the pixels (using getImageData).
This works fine in all browsers that support the canvas-API, except in IE9: ctx.drawImage(img, 0, 0) seems to apply some kind of anti-aliasing to the image. Is it possible to disable this behavior?
Our code kind of looks like this:
var img = document.createElement('IMG');
img.addEventListener('load', function(e) {
var w = img.width,
h = img.height,
ctx = canvas.getContext('2d');
ctx.drawImage(img, 0, 0);
var data = ctx.getImageData(0, 0, w, h),
pixels = data.data;
for (var y = 0, i = 0; y < h; y++) {
for (var x = 0; x < w; x++, i += 4) {
var r = pixels[i],
g = pixels[i+1],
b = pixels[i+2],
color = (r << 16) | (g << 8) | b;
// do something with x, y and color
}
}
});
img.src = 'images/source.png';

The approach to improve the crispness of lines depends on whether or not you are filling or drawing...see http://www.netmagazine.com/features/four-essential-html5-canvas-tips for approaches to each.