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Here you can Sed posuere consectetur est at lobortis. Donec ullamcorper nulla non metus auctor fringilla. Maecenas sed diam eget risus varius blandit sit amet non magna. Donec id elit non mi porta gravida at eget metus. Praesent commodo cursus magna, vel scelerisque nisl consectetur et.

            
              <div id="main">
        
<h1>"Scratch off" image to reveal a different one</h1>

<div><canvas id="maincanvas"></canvas></div>

<div><input id="resetbutton" type="button" value="Reset"></input></div>
    
    
</div>
            
          
!
            
              * {
  margin:0; padding:0;
}

h1 { 
  width:100%; text-align:center;
}

.cell {
	display: table-cell;
	vertical-align: middle;
	text-align: center;
}

.cell canvas {
	vertical-align: middle;
}

.canthumb {
	border: 0px solid #222;
}
#main { 
    margin:0 auto;
  width:600px;
}

#maincanvas {
	border: 0px solid #222;
	cursor: pointer;
  margin:0 auto;
}
            
          
!
            
              (function() {

var image = { // back and front images
  'back': { 'url':'http://lorempixel.com/600/400/nature/2', 'img':null },
	'front': { 'url':'http://lorempixel.com/600/400/nature/5', 'img':null }
};

var canvas = {'temp':null, 'draw':null}; // temp and draw canvases

var mouseDown = false;

/**
 * Helper function to get the local coords of an event in an element,
 * since offsetX/offsetY are apparently not entirely supported, but
 * offsetLeft/offsetTop/pageX/pageY are!
 *
 * @param elem element in question
 * @param ev the event
 */
function getLocalCoords(elem, ev) {
	var ox = 0, oy = 0;
	var first;
	var pageX, pageY;

	// Walk back up the tree to calculate the total page offset of the
	// currentTarget element.  I can't tell you how happy this makes me.
	// Really.
	while (elem != null) {
		ox += elem.offsetLeft;
		oy += elem.offsetTop;
		elem = elem.offsetParent;
	}

	if (ev.hasOwnProperty('changedTouches')) {
		first = ev.changedTouches[0];
		pageX = first.pageX;
		pageY = first.pageY;
	} else {
		pageX = ev.pageX;
		pageY = ev.pageY;
	}

	return { 'x': pageX - ox, 'y': pageY - oy };
}

/**
 * Recomposites the canvases onto the screen
 *
 * Note that my preferred method (putting the background down, then the
 * masked foreground) doesn't seem to work in FF with "source-out"
 * compositing mode (it just leaves the destination canvas blank.)  I
 * like this method because mentally it makes sense to have the
 * foreground drawn on top of the background.
 *
 * Instead, to get the same effect, we draw the whole foreground image,
 * and then mask the background (with "source-atop", which FF seems
 * happy with) and stamp that on top.  The final result is the same, but
 * it's a little bit weird since we're stamping the background on the
 * foreground.
 *
 * OPTIMIZATION: This naively redraws the entire canvas, which involves
 * four full-size image blits.  An optimization would be to track the
 * dirty rectangle in scratchLine(), and only redraw that portion (i.e.
 * in each drawImage() call, pass the dirty rectangle as well--check out
 * the drawImage() documentation for details.)  This would scale to
 * arbitrary-sized images, whereas in its current form, it will dog out
 * if the images are large.
 */
function recompositeCanvases() {
	var main = document.getElementById('maincanvas');
	var tempctx = canvas.temp.getContext('2d');
	var mainctx = main.getContext('2d');

	// Step 1: clear the temp
	canvas.temp.width = canvas.temp.width; // resizing clears

	// Step 2: stamp the draw on the temp (source-over)
	tempctx.drawImage(canvas.draw, 0, 0);

	/* !!!! this way doesn't work on FF:
		// Step 3: stamp the foreground on the temp (!! source-out mode !!)
		tempctx.globalCompositeOperation = 'source-out';
		tempctx.drawImage(image.front.img, 0, 0);

		// Step 4: stamp the background on the display canvas (source-over)
		//mainctx.drawImage(image.back.img, 0, 0);

		// Step 5: stamp the temp on the display canvas (source-over)
		mainctx.drawImage(canvas.temp, 0, 0);
	*/

	// Step 3: stamp the background on the temp (!! source-atop mode !!)
	tempctx.globalCompositeOperation = 'source-atop';
	tempctx.drawImage(image.back.img, 0, 0);

	// Step 4: stamp the foreground on the display canvas (source-over)
	mainctx.drawImage(image.front.img, 0, 0);

	// Step 5: stamp the temp on the display canvas (source-over)
	mainctx.drawImage(canvas.temp, 0, 0);
}

/**
 * Draw a scratch line
 * 
 * @param can the canvas
 * @param x,y the coordinates
 * @param fresh start a new line if true
 */
function scratchLine(can, x, y, fresh) {
	var ctx = can.getContext('2d');
	ctx.lineWidth = 50;
	ctx.lineCap = ctx.lineJoin = 'round';
	ctx.strokeStyle = '#f00'; // can be any opaque color
	if (fresh) {
		ctx.beginPath();
		// this +0.01 hackishly causes Linux Chrome to draw a
		// "zero"-length line (a single point), otherwise it doesn't
		// draw when the mouse is clicked but not moved:
		ctx.moveTo(x+0.01, y);
	}
	ctx.lineTo(x, y);
	ctx.stroke();
}

/**
 * Set up the main canvas and listeners
 */
function setupCanvases() {
	var c = document.getElementById('maincanvas');
	// set the width and height of the main canvas from the first image
	// (assuming both images are the same dimensions)
	c.width = image.back.img.width;
	c.height = image.back.img.height;

	// create the temp and draw canvases, and set their dimensions
	// to the same as the main canvas:
	canvas.temp = document.createElement('canvas');
	canvas.draw = document.createElement('canvas');
	canvas.temp.width = canvas.draw.width = c.width;
	canvas.temp.height = canvas.draw.height = c.height;

	// draw the stuff to start
	recompositeCanvases();

	/**
	 * On mouse down, draw a line starting fresh
	 */
	function mousedown_handler(e) {
		var local = getLocalCoords(c, e);
		mouseDown = true;

		scratchLine(canvas.draw, local.x, local.y, true);
		recompositeCanvases();

		if (e.cancelable) { e.preventDefault(); } 
		return false;
	};

	/**
	 * On mouse move, if mouse down, draw a line
	 *
	 * We do this on the window to smoothly handle mousing outside
	 * the canvas
	 */
	function mousemove_handler(e) {
		if (!mouseDown) { return true; }

		var local = getLocalCoords(c, e);

		scratchLine(canvas.draw, local.x, local.y, false);
		recompositeCanvases();

		if (e.cancelable) { e.preventDefault(); } 
		return false;
	};

	/**
	 * On mouseup.  (Listens on window to catch out-of-canvas events.)
	 */
	function mouseup_handler(e) {
		if (mouseDown) {
			mouseDown = false;
			if (e.cancelable) { e.preventDefault(); } 
			return false;
		}

		return true;
	};

	c.addEventListener('mousedown', mousedown_handler, false);
	c.addEventListener('touchstart', mousedown_handler, false);

	window.addEventListener('mousemove', mousemove_handler, false);
	window.addEventListener('touchmove', mousemove_handler, false);

	window.addEventListener('mouseup', mouseup_handler, false);
	window.addEventListener('touchend', mouseup_handler, false);
}

/**
 * Set up the DOM when loading is complete
 */
function loadingComplete() {
	var loading = document.getElementById('loading');
	var main = document.getElementById('main');

	loading.className = 'hidden';
	main.className = '';
}

/**
 * Handle loading of needed image resources
 */
function loadImages() {
	var loadCount = 0;
	var loadTotal = 0;
	var loadingIndicator;

	function imageLoaded(e) {
		loadCount++;

		if (loadCount >= loadTotal) {
			setupCanvases();
			loadingComplete();
		}
	}

	for (k in image) if (image.hasOwnProperty(k))
		loadTotal++;

	for (k in image) if (image.hasOwnProperty(k)) {
		image[k].img = document.createElement('img'); // image is global
		image[k].img.addEventListener('load', imageLoaded, false);
		image[k].img.src = image[k].url;
	}
}

/**
 * Handle page load
 */
window.addEventListener('load', function() {
	var resetButton = document.getElementById('resetbutton');

	loadImages();

	resetButton.addEventListener('click', function() {
			// clear the draw canvas
			canvas.draw.width = canvas.draw.width;
			recompositeCanvases()

			return false;
		}, false);

}, false);

})();
            
          
!
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