For general information about using <canvas> see the canvas topic page.

Code usable from Web content

Getting the number of pixels of a certain color in a canvas

The following function will return the number of pixels in a canvas that have the RGB color of r, g and b. This can be very useful to compare for example if a user has painted over another area as explained in this blog post.

function getpixelamount(canvas, r, g, b) {
  var cx = canvas.getContext('2d');
  var pixels = cx.getImageData(0, 0, canvas.width, canvas.height);
  var all = pixels.data.length;
  var amount = 0;
  for (i = 0; i < all; i += 4) {
    if (pixels.data[i] === r &&
        pixels.data[i + 1] === g &&
        pixels.data[i + 2] === b) {
      amount++;
    }
  }
  return amount;
};

Getting the color of a pixel in a canvas

This following snippet returns an object with the RGBA values of the pixel at position x and y of the canvas. This can be used to determine if the mouse cursor is inside a certain shape or not.

function getpixelcolour(canvas, x, y) {
  var cx = canvas.getContext('2d');
  var pixel = cx.getImageData(x, y, 1, 1);
  return {
    r: pixel.data[0],
    g: pixel.data[1],
    b: pixel.data[2],
    a: pixel.data[3]
  };
}

Chaining methods

This class provides jQuery-style chained access to 2D context methods and properties.

function Canvas2DContext(canvas) {
  if (typeof canvas === 'string') {
    canvas = document.getElementById(canvas);
  }
  if (!(this instanceof Canvas2DContext)) {
    return new Canvas2DContext(canvas);
  }
  this.context = this.ctx = canvas.getContext('2d');
  if (!Canvas2DContext.prototype.arc) {
    Canvas2DContext.setup.call(this, this.ctx);
  }
}
Canvas2DContext.setup = function () {
  var methods = ['arc','arcTo','beginPath','bezierCurveTo','clearRect','clip',
    'closePath','drawImage','fill','fillRect','fillText','lineTo','moveTo',
    'quadraticCurveTo','rect','restore','rotate','save','scale','setTransform',
    'stroke','strokeRect','strokeText','transform','translate'];

  var getterMethods = ['createPattern','drawFocusRing','isPointInPath','measureText', // drawFocusRing not currently supported
    // The following might instead be wrapped to be able to chain their child objects
    'createImageData','createLinearGradient',
    'createRadialGradient', 'getImageData','putImageData'
  ];

  var props = ['canvas','fillStyle','font','globalAlpha','globalCompositeOperation',
    'lineCap','lineJoin','lineWidth','miterLimit','shadowOffsetX','shadowOffsetY',
    'shadowBlur','shadowColor','strokeStyle','textAlign','textBaseline'];

  for (let m of methods) {
    let method = m;
    Canvas2DContext.prototype[method] = function () {
      this.ctx[method].apply(this.ctx, arguments);
      return this;
    };
  }

  for (let m of getterMethods) {
    let method = m;
    Canvas2DContext.prototype[method] = function () {
      return this.ctx[method].apply(this.ctx, arguments);
    };
  }

  for (let p of props) {
    let prop = p;
    Canvas2DContext.prototype[prop] = function (value) {
      if (value === undefined)
        return this.ctx[prop];
      this.ctx[prop] = value;
      return this;
    };
  }
};

var canvas = document.getElementById('canvas');

// Use context to get access to underlying context
var ctx = Canvas2DContext(canvas)
  .strokeStyle("rgb(30,110,210)")
  .transform(10, 3, 4, 5, 1, 0)
  .strokeRect(2, 10, 15, 20)
  .context;

// Use property name as a function (but without arguments) to get the value
var strokeStyle = Canvas2DContext(canvas)
  .strokeStyle("rgb(50,110,210)")
  .strokeStyle();

Code usable only from privileged code

These snippets are only useful from privileged code, such as extensions or privileged apps.

Saving a canvas image to a file

The following function accepts a canvas object and a destination file path string. The canvas is converted to a PNG file and saved to the specified location. The function returns a promise which resolves when the file has been completely saved.

function saveCanvas(canvas, path, type, options) {
    return Task.spawn(function *() {
        var reader = new FileReader;
        var blob = yield new Promise(accept => canvas.toBlob(accept, type, options));
        reader.readAsArrayBuffer(blob);

        yield new Promise(accept => { reader.onloadend = accept });

        return yield OS.File.writeAtomic(path, new Uint8Array(reader.result),
                                         { tmpPath: path + '.tmp' });
    });
}

Loading a remote page onto a canvas element

The following class first creates a hidden iframe element and attaches a listener to the frame's load event. Once the remote page has loaded, the remotePageLoaded method fires. This method gets a reference to the iframe's window and draws this window to a canvas object.

Note that this only works if you are running the page from chrome. If you try running the code as a plain webpage, you will get a 'Security error" code: "1000' error.

RemoteCanvas = function() {
    this.url = "http://developer.mozilla.org";
};

RemoteCanvas.CANVAS_WIDTH = 300;
RemoteCanvas.CANVAS_HEIGHT = 300;

RemoteCanvas.prototype.load = function() {
    var windowWidth = window.innerWidth - 25;
    var iframe;
    iframe = document.createElement("iframe");
    iframe.id = "test-iframe";
    iframe.height = "10px";
    iframe.width = windowWidth + "px";
    iframe.style.visibility = "hidden";
    iframe.src = this.url;
    // Here is where the magic happens... add a listener to the
    // frame's onload event
    iframe.addEventListener("load", this.remotePageLoaded, true);
    //append to the end of the page
    window.document.body.appendChild(iframe);
    return;    
};

RemoteCanvas.prototype.remotePageLoaded = function() {
    // Look back up the iframe by id
    var ldrFrame = document.getElementById("test-iframe");
    // Get a reference to the window object you need for the canvas
    // drawWindow method
    var remoteWindow = ldrFrame.contentWindow;

    //Draw canvas
    var canvas = document.createElement("canvas");
    canvas.style.width = RemoteCanvas.CANVAS_WIDTH + "px";
    canvas.style.height = RemoteCanvas.CANVAS_HEIGHT + "px";
    canvas.width = RemoteCanvas.CANVAS_WIDTH;
    canvas.height = RemoteCanvas.CANVAS_HEIGHT;
    var windowWidth = window.innerWidth - 25;
    var windowHeight = window.innerHeight;

    var ctx = canvas.getContext("2d");
    ctx.clearRect(0, 0,
                  RemoteCanvas.CANVAS_WIDTH,
                  RemoteCanvas.CANVAS_HEIGHT);
    ctx.save();
    ctx.scale(RemoteCanvas.CANVAS_WIDTH / windowWidth,
              RemoteCanvas.CANVAS_HEIGHT / windowHeight);
    ctx.drawWindow(remoteWindow,
                   0, 0,
                   windowWidth, windowHeight,
                   "rgb(255,255,255)");
    ctx.restore();
};

Usage:

var remoteCanvas = new RemoteCanvas();
remoteCanvas.load();

Convert image files to base64 strings

The following code gets a remote image and converts its content to Data URI scheme.

var canvas = document.createElement("canvas");
var ctxt = canvas.getContext("2d");
function loadImageFile (url, callback) {
  var image = new Image();
  image.src = url;
  return new Promise((accept, reject) => {
    image.onload = accept;
    image.onerror = reject;
  }).then(accept => {
    canvas.width = this.width;
    canvas.height = this.height;
    ctxt.clearRect(0, 0, this.width, this.height);
    ctxt.drawImage(this, 0, 0);
    accept(canvas.toDataURL());
  });
}

Usage:

loadImageFile("myimage.jpg").then(string64 => { alert(string64); });

If you want to get instead the base64 content of a local file using the file <input> element, you must use the FileReader object.

Document Tags and Contributors

 Last updated by: SlawomirRodak,