Scratch Reveal with <canvas>

index.html

<figure id="bridgeContainer">
<canvas id="bridge" width="750" height="465"></canvas>
  <figcaption>Downtown Calgary in 2013 and 1943; mouse down or touch on photo to reveal</figcaption>
</figure>

scratch-reveal-with-canvas.markdown

Scratch Reveal with

Mouse down or touch and stroke on canvas to reveal the image underneath. Uses Retina image detection in CSS & JavaScript with image-set and window.devicePixelRatio, image removal with globalCompositeOperation. Full explanatory article on my blog next week. Photographs by KingsdudeDave and davebloggs007

A Pen by Dudley Storey on CodePen.

License.

script.js

var bridge = document.getElementById("bridge"),
bridgeCanvas = bridge.getContext('2d'),
brushRadius = (bridge.width / 100) * 5,
img = new Image();

if (brushRadius < 50) { brushRadius = 50 }

img.onload = function(){  
	bridgeCanvas.drawImage(img, 0, 0, bridge.width, bridge.height);
}
img.loc = 'https://s3-us-west-2.amazonaws.com/s.cdpn.io/4273/';
img.filename = 'calgary-bridge-2013.jpg';
if (window.devicePixelRatio >= 2) {
	var nameParts = img.filename.split('.');
	img.src = img.loc + nameParts[0]+"-2x"+"."+nameParts[1];
} else {
	img.src = img.loc + img.filename;
}

function detectLeftButton(event) {
    if ('buttons' in event) {
        return event.buttons === 1;
    } else if ('which' in event) {
        return event.which === 1;
    } else {
        return event.button === 1;
    }
}

function getBrushPos(xRef, yRef) {
	var bridgeRect = bridge.getBoundingClientRect();
    return {
	  x: Math.floor((xRef-bridgeRect.left)/(bridgeRect.right-bridgeRect.left)*bridge.width),
	  y: Math.floor((yRef-bridgeRect.top)/(bridgeRect.bottom-bridgeRect.top)*bridge.height)
    };
}
      
function drawDot(mouseX,mouseY){
	bridgeCanvas.beginPath();
    bridgeCanvas.arc(mouseX, mouseY, brushRadius, 0, 2*Math.PI, true);
    bridgeCanvas.fillStyle = '#000';
    bridgeCanvas.globalCompositeOperation = "destination-out";
    bridgeCanvas.fill();
}

bridge.addEventListener("mousemove", function(e) {
	var brushPos = getBrushPos(e.clientX, e.clientY);
  var leftBut = detectLeftButton(e);
  if (leftBut == 1) {
		drawDot(brushPos.x, brushPos.y);
  }
}, false);

bridge.addEventListener("touchmove", function(e) {
    e.preventDefault();
    var touch = e.targetTouches[0];
    if (touch) {
    var brushPos = getBrushPos(touch.pageX, touch.pageY);
        drawDot(brushPos.x, brushPos.y);
    }
}, false);

style.scss

body { margin: 0; }

#bridge {
	display: block;
	margin: 0 auto;
  background-image: url('https://s3-us-west-2.amazonaws.com/s.cdpn.io/4273/calgary-bridge-1943.jpg');
	background-image: -webkit-image-set(url('https://s3-us-west-2.amazonaws.com/s.cdpn.io/4273/calgary-bridge-1943.jpg') 1x, url('https://s3-us-west-2.amazonaws.com/s.cdpn.io/4273/calgary-bridge-1943-2x.jpg') 2x );
	background-size: cover;
	width: 100%;
	max-width: 750px;
	height: auto;
  cursor:  crosshair;
  cursor: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/4273/circular-cursor.png) 53 53, crosshair;
}
#bridgeContainer { 
  text-align: center;
  font-family: Avenir, sans-serif;
}
#bridgeContainer figcaption {   
  margin-top: 2rem; 
}

This is awesome, well done!
You get a flash of the hidden image before the front image loads though. I found setting the background-image css property after you draw the front image is a quick-fix for that. There's probably a better way to do it though, maybe using two Image objects.

Im having issues when it comes to mobile, specifically if I scroll at all on a longer page the hit box suddenly shifts. Anyone have this issue and know a fix?

how to fix support touch on the browser mobile ?
https://codepen.io/dudleystorey/pen/yJQxLX

How to detect the % of scratched?