In this example we demonstrate how to interact with a REST API and how to draw images on the 2D canvas. We fetch the most popular images from the Instagram REST API and we draw them on the Canvas. The application also allow the user to save up to 2 compositions of images in the local storage.

Below there is the JS code for initializing the application. First we resize the canvas to fill the available space for the content. We also calculate the maximum number of images that we can visualize in the canvas.

var canvas = $('#canvas')[0];
var ctx = canvas.getContext('2d');

var header = $('div[data-role="header"]:visible');
var footer = $('div[data-role="footer"]:visible');
var content = $('div[data-role="content"]:visible');
var viewport_height = $(window).height();
var viewport_width = $(window).width();
var content_height = viewport_height - header.outerHeight() - footer.outerHeight();
/* Trim margin/border/padding height */
content_height -= (content.outerHeight() - content.height());

canvas.width = viewport_width;
canvas.height = content_height;
cols = Math.floor(canvas.width/104);
rows = Math.floor(canvas.height/104);
max = Math.min(cols*rows, 50);

if (!localStorage.index) {
  localStorage.index = 0;

The next step is to check if there are previously saved compositions and load them from the local storage. We load the images in the elements with id snap_0 and snap_1 of the dedicated page with id saved .

//Load the 3 saved images if they are available
$.each(['snap_0','snap_1'], function(index, value) {
  if(localStorage.getItem(value)) {

Below there is the code for fetching the images from Instagram. First we fetch a JSON object that contains the metadata of the most popular images on Instagram. Then we download the images and draw them on the canvas. We must use a workaround for fetching the images. The 2D canvas API follows the cross-origin policy which means that drawing on the canvas images that originate from a different host (than the one we are serving the main page) disables the possibility of taking snapshots of the canvas with the toDataURL method ( ). To workaround this limitation we use our Sinatra server as a proxy for fetching the images from the Instagram REST API through the endpoint: /fetch?url=...

$("#refresh").bind("click", function(e) {
  //Fetch the post popular images on istagram
    function(data) {
      $.each(,max), function(index, value) {
        //Use custom proxy for downloading and encoding the images in Base64
        //to avoid cross origin issues 
        $.get('/fetch?url=' + value.images.low_resolution.url, function (data) {
          //Create a new image to be placed on the canvas
          var img = new Image();
          img.onload = function () {
            //Copy the image to the canvas
            ctx.drawImage(this, 1 + (index % cols)*101, 1+(Math.floor(index/cols))*101, 100, 100);
          img.src = data;

Below there is the code for taking a snapshot of the canvas and saving it in the local storage. To stay within the typical memory limits of the local storage we don't store more than 2 images.

$("#save").bind("click", function(e) {
  //Take a snapshot of the canvas as base64 encoded data URL
  var snapshot = ctx.canvas.toDataURL('image/png');
  //Store it in the last used slot in the localStorage
  localStorage.setItem('snap_' + localStorage.index, snapshot);
  //Replace the image in the saved page
  $("#snap_" + localStorage.index).attr('src',snapshot);
  //Increment the index 
  localStorage.index = (JSON.parse(localStorage.index)+1) % 2;

Below there is the implementation of the Sinatra server whose only function is to server the main HTML file and act as a proxy for fetching the images from Instagram. The images are converted to Data URI with Base64 encoding before they are sent to the client.

get '/' do 
  #Renders the haml template index.html.haml
  #with the default layout layout.html.haml
  haml :index, :layout => :layout

get '/fetch' do
  #Fetch the image from the URL and encodes it as a data URI
  base_image = Net::HTTP.get(URI.parse(params['url']))
  puts "Fetched image: " + params['url']

You can try this example by starting the server and visiting the page http://localhost:4567 in your browser

ruby server.rb