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Pure JavaScript HTML5 <canvas> to (Animated) GIF Conversion 03 January 2010

Based on as3gif Ported by antimatter15

This is the raw canvas element saved as a non-animated PNG
This is the GIF which was generated from the canvas.
This is the GIF which was generated from the canvas.

AS3GIF lets you play and encode animated GIF’s with ActionScript 3

Since web pages can usually natively play GIFs fine, it’s only a port of the GIFEncoder portions of the library.

Basic Usage

Since it pretty much is GIFEncoder, you could consult the as3gif how-to page

But there are some differences so I’ll cover it here anyway.

You first need to include the JS files. It’s probably best if you include it in this order, but it shouldn’t matter too much.

<script type="text/javascript" src="LZWEncoder.js"></script>
<script type="text/javascript" src="NeuQuant.js"></script>
<script type="text/javascript" src="GIFEncoder.js"></script>

If you want to render the gif through an inline <img> tag or try to save to disk or send to server or anything that requires conversion into a non-binary string form, you should probably include b64.js too.

<script type="text/javascript" src="b64.js"></script>

Simple enough right? Now to convert stuff to GIF, you need to have a working <canvas> or at least some imageData-esque array.

<canvas id="bitmap"></canvas>
  var canvas = document.getElementById('bitmap');
  var context = canvas.getContext('2d');
  context.fillStyle = 'rgb(255,255,255)';
  context.fillRect(0,0,canvas.width, canvas.height); //GIF can't do transparent so do white

  context.fillStyle = "rgb(200,0,0)";  
  context.fillRect (10, 10, 75, 50);   //draw a little red box

Now we need to init the GIFEncoder.

  var encoder = new GIFEncoder();

If you are making an animated gif, you need to add the following

  encoder.setRepeat(0); //0  -> loop forever
                        //1+ -> loop n times then stop
  encoder.setDelay(500); //go to next frame every n milliseconds

Now, you need to tell the magical thing that you’re gonna start inserting frames (even if it’s only one).


And for the part that took the longest to port: adding a real frame.


In the GIFEncoder version, it accepts a Bitmap. Well, that doesn’t exist in Javascript (natively, anyway) so instead, I use what I feel is a decent analogue: the canvas context. However, if you’re in a situation where you don’t have a real <canvas> element. That’s okay. You can set the second parameter to true and pass a array as your first argument. So in other words, you can do encoder.addFrame(fake_imageData, true) as an alternative. However, you must do an encoder.setSize(width, height); before you do any of the addFrames if you pass a array. If you pass a canvas context, then that’s all okay, because it will automagically do a setSize with the canvas width/height stuff.

Now the last part is to finalize the animation and get it for display.

  var binary_gif = //notice this is different from the as3gif package!
  var data_url = 'data:image/gif;base64,'+encode64(binary_gif);


Each of the files exposes a single global (see, at least it’s considerate!). But since there’s three files, that means that there’s three globals. But two of them are more of supporting libraries that I don’t totally understand or care about enough to document. So I’m just gonna document GIFEncoder.

new GIFEncoder() This is super parent function. You really don’t need the new keyword because It’s not really even using any special inheritance pattern. It’s a closure that does some var blah = exports.blah = function blah(){ for no good reason. Anyway, it returns an object with a bunch of methods that the section will be devoted to documenting. Note that I’ve never tested more than half of these, so good luck.

Boolean start() This writes the GIF Header and returns false if it fails.

Boolean addFrame(CanvasRenderingContext2D context) This is the magical magic behind everything. This adds a frame.

Boolean addFrame(CanvasPixelArray image, true) This is the magical magic behind everything. This adds a frame. This time you need you pass true as the second argument and then magic strikes and it loads your canvas pixel array (which can be a real array, I dont care and I think the program has learned from my constant apathy to also not care). But note that if you do, you must first manually call setSize which is happily defined just below this one.

void setSize(width, height) Sets the canvas size. It’s supposed to be private, but I’m exposing it anyway. Gets called automagically as the size of the first frame if you don’t do that crappy hacky hack.

void setDelay(int milliseconds) the number of milliseconds to wait on each frame

void setDispose(int code) Sets the GIF frame disposal code for the last added frame and any subsequent frames. Default is 0 if no transparent color has been set, otherwise 2. I have no clue what this means so I just copypasted it from the actionscript docs.

void setFrameRate(Number fps) Sets frame rate in frames per second. Equivalent to setDelay(1000/fps). I think that’s stupid.

void setQuality(int quality) Sets quality of color quantization (conversion of images to the maximum 256 colors allowed by the GIF specification). Lower values (minimum = 1) produce better colors, but slow processing significantly. 10 is the default, and produces good color mapping at reasonable speeds. Values greater than 20 do not yield significant improvements in speed. BLAH BLAH BLAH. Whatever

void setRepeat(int iter) Sets the number of times the set of GIF frames should be played. Default is 1; 0 means play indefinitely. Must be invoked before the first image is added.

void setTransparent(Number color) Sets the transparent color for the last added frame and any subsequent frames. Since all colors are subject to modification in the quantization process, the color in the final palette for each frame closest to the given color becomes the transparent color for that frame. May be set to null to indicate no transparent color.

ByteArray finish() Adds final trailer to the GIF stream, if you don’t call the finish method the GIF stream will not be valid.

String stream() Yay the only function that returns a non void/boolean. It’s the magical stream function which should have been a getter which JS does support but I didnt’ feel like making it a getter because getters are so weird and inconsistent. Like sure there’s the nice pretty get thing but I think IE9/8 doesn’t implement it because it’s non standard or something and replaced it with a hideously ugly blah blah. So Anyway, it’s a function. It returns a byteArray with three writeByte functions that you wouldn’t care about and a getData() function which returns a binary string with the GIF. There’s also a .bin attribute which contains an array with the binary stuff that I don’t care about.


The process isn’t really the fastest thing ever, so you should use WebWorkers for piecing together animations more than a few frames long.

I haven’t actually tried it yet, but here’s some incomplete mock-JS which should be able to do stuff once you add the boring stuff like serializing and deserializing the content (actually, i have most of the serializing done but you have to deserialize that and that’s really the boring part).

var frame_index,
    imageData; //get it from onmessage

var encoder = new GIFEncoder(); //create a new GIFEncoder for every new job
if(frame_index == 0){
  encoder.setProperties(true, true); //started, firstFrame
encoder.setSize(height, width);
encoder.addFrame(imageData, true);
if(frame_length == frame_index){
postMessage(frame_index + //on the page, search for the GIF89a to see the frame_index

var animation_parts = new Array(frame_length);
//on the handler side:

var worker = new WebWorker('blahblahblah.js');
worker.onmessage = function(e){
  //handle stuff, like get the frame_index
  animation_parts[frame_index] = frame_data;
  //check when everything else is done and then do animation_parts.join('') and have fun
var imdata = context.getImageData(0,0,canvas.width,canvas.height)
var len = canvas.width * canvas.height * 4;
var imarray = [];
for(var i = 0; i < len; i++){

worker.postMessage(frame_index + ';' + frame_length + ';' + canvas.height + ';' + canvas.width + ';' + imarray.join(','))