Canvas Eraser

Simple example of drawing in HTML5 canvas

by thillin

HTML

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

CSS

#canvas {
    background-image:url(http://www.fortheyoungdude.com/wp-content/uploads/2011/09/Fresh-Prince-Of-Bel-Air-the-fresh-prince-of-bel-air-13456178-307-230-2.gif);
       
    width: 100%;
    height: 100%;
    position: absolute;
    background-size: cover;
}
#overlay {
    background-image: url(http://i.imgur.com/YdmGHYG.gif);
    background-size: cover;
    width: 100%;
    height: 100%;
    position: absolute;
    pointer-events: none;
    z-index: 9;

}

JavaScript

(function() {
    // Creates a new canvas element and appends it as a child
    // to the parent element, and returns the reference to
    // the newly created canvas element


    function createCanvas(parent, width, height) {
        var canvas = {};
        canvas.node = document.createElement('canvas');
        canvas.context = canvas.node.getContext('2d');
        canvas.node.width = width || 100;
        canvas.node.height = height || 100;
        parent.appendChild(canvas.node);
        return canvas;
    }

    function init(container, width, height, fillColor) {
        var canvas = createCanvas(container, width, height);
        var ctx = canvas.context;
        // define a custom fillCircle method
        ctx.fillCircle = function(x, y, radius, fillColor) {
            this.fillStyle = fillColor;
            this.beginPath();
            this.moveTo(x, y);
            this.arc(x, y, radius, 0, Math.PI * 2, false);
            this.fill();
        };
        ctx.clearTo = function(fillColor) {
            ctx.fillStyle = fillColor;
            ctx.fillRect(0, 0, width, height);
        };
        ctx.clearTo(fillColor || "#ddd");

        // bind mouse events
        canvas.node.onmousemove = function(e) {
            if (!canvas.isDrawing) {
               return;
            }
            var x = e.pageX - this.offsetLeft;
            var y = e.pageY - this.offsetTop;
            var radius = 20; // or whatever
            var fillColor = '#ff0000';
            ctx.globalCompositeOperation = 'destination-out';
            ctx.fillCircle(x, y, radius, fillColor);
        };
        canvas.node.onmousedown = function(e) {
            canvas.isDrawing = true;
        };
        canvas.node.onmouseup = function(e) {
            canvas.isDrawing = false;
        };
    }

    var container = document.getElementById('canvas');
    init(container, 531, 438, '#ddd');

})();