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.