实现刮刮卡刮奖效果的JavaScript，基于HTML5 Canvas，采用原生js编写，不依赖任何类库。
用法
HTML结构

<div id="scratch">
    <div id="card">￥5000000元</div>
</div>

css

#scratch{ position: relative;}
#cover{ position: absolute; top:0; left:0;}
#card{ -webkit-user-select:none; -moz-user-select:none; -ms-user-select:none; user-select:none;}
 #card { height: 100%; font-weight: bold;font-size: 40px;line-height: 100px;text-align: center;background: #FAFAFA;}  
 #scratch {width: 300px;height: 100px;margin: 50px auto 0;border: 1px solid #ccc;}

js

(function(window, document, undefined) {
    'use strict';

    /**
     * Instantiate parameters
     *
     * @constructor
     */
    function LuckyCard(settings, callback) {
        this.cover = null;
        this.ctx = null;
        this.scratchDiv = null;
        this.cardDiv = null;
        this.cHeight = 0;
        this.cWidth = 0;
        this.supportTouch = false;
        this.events = [];
        this.startEventHandler = null;
        this.moveEventHandler = null;
        this.endEventHandler = null;

        this.opt = {
            coverColor: '#C5C5C5',
            coverImg: '',
            ratio: .8,
            callback: null
        };

        this.init(settings, callback);
    };

    function _calcArea(ctx, callback, ratio) {
        var pixels = ctx.getImageData(0, 0, this.cWidth, this.cHeight);
        var transPixels = [];
        _forEach(pixels.data, function(item, i) {
            var pixel = pixels.data[i + 3];
            if (pixel === 0) {
                transPixels.push(pixel);
            }
        });

        if (transPixels.length / pixels.data.length > ratio) {
            callback && typeof callback === 'function' && callback();
        }
    }

    function _forEach(items, callback) {
        return Array.prototype.forEach.call(items, function(item, idx) {
            callback(item, idx);
        });
    }

    function _isCanvasSupported(){
      var elem = document.createElement('canvas');
      return !!(elem.getContext && elem.getContext('2d'));
    }

    /**
     * touchstart/mousedown event handler
     */
    function _startEventHandler(event) {
        event.preventDefault();
        this.moveEventHandler = _moveEventHandler.bind(this);
        this.cover.addEventListener(this.events[1],this.moveEventHandler,false);
        this.endEventHandler = _endEventHandler.bind(this);
        document.addEventListener(this.events[2],this.endEventHandler,false);
    };

    /**
     * touchmove/mousemove event handler
     */
    function _moveEventHandler(event) {
        event.preventDefault();
        var evt = this.supportTouch?event.touches[0]:event;
        var coverPos = this.cover.getBoundingClientRect();
        var pageScrollTop = document.documentElement.scrollTop || document.body.scrollTop;
        var pageScrollLeft = document.documentElement.scrollLeft || document.body.scrollLeft;
        var mouseX = evt.pageX - coverPos.left - pageScrollLeft;
        var mouseY = evt.pageY - coverPos.top - pageScrollTop;

        this.ctx.beginPath();
        this.ctx.fillStyle = '#FFFFFF';
        this.ctx.globalCompositeOperation = "destination-out";
        this.ctx.arc(mouseX, mouseY, 10, 0, 2 * Math.PI);
        this.ctx.fill();
    };

    /**
     * touchend/mouseup event handler
     */
    function _endEventHandler(event) {
        event.preventDefault();
        if (this.opt.callback && typeof this.opt.callback === 'function') _calcArea.call(this,this.ctx, this.opt.callback, this.opt.ratio);
        this.cover.removeEventListener(this.events[1],this.moveEventHandler,false);
        document.removeEventListener(this.events[2],this.endEventHandler,false);
    };

    /**
     * Create Canvas element
     */
    LuckyCard.prototype.createCanvas = function() {
        this.cover = document.createElement('canvas');
        this.cover.id = 'cover';
        this.cover.height = this.cHeight;
        this.cover.width = this.cWidth;
        this.ctx = this.cover.getContext('2d');
        if (this.opt.coverImg) {
            var _this = this;
            var coverImg = new Image();
            coverImg.src = this.opt.coverImg;
            coverImg.onload = function() {
                _this.ctx.drawImage(coverImg, 0, 0, _this.cover.width, _this.cover.height);
            }
        } else {
            this.ctx.fillStyle = this.opt.coverColor;
            this.ctx.fillRect(0, 0, this.cover.width, this.cover.height);
        }
        this.scratchDiv.appendChild(this.cover);
        this.cardDiv.style.opacity = 1;
    }

    /**
     * To detect whether support touch events
     */
    LuckyCard.prototype.eventDetect = function() {
        if('ontouchstart' in window) this.supportTouch = true;
        this.events = this.supportTouch ? ['touchstart', 'touchmove', 'touchend'] : ['mousedown', 'mousemove', 'mouseup'];
        this.addEvent();
    };

    /**
     * Add touchstart/mousedown event listener
     */
    LuckyCard.prototype.addEvent = function() {
        this.startEventHandler = _startEventHandler.bind(this);
        this.cover.addEventListener(this.events[0],this.startEventHandler,false);
    };

    /**
     * Clear pixels of canvas
     */
    LuckyCard.prototype.clearCover = function() {
        this.ctx.clearRect(0, 0, this.cover.width, this.cover.height);
        this.cover.removeEventListener(this.events[0],this.startEventHandler);
        this.cover.removeEventListener(this.events[1],this.moveEventHandler);
        this.cover.removeEventListener(this.events[2],this.endEventHandler);
    };


    /**
     * LuckyCard initializer
     *
     * @param {Object} settings  Settings for LuckyCard
     * @param {function} callback  callback function
     */
    LuckyCard.prototype.init = function(settings, callback) {
        if(!_isCanvasSupported()){
            alert('对不起，当前浏览器不支持Canvas，无法使用本控件！');
            return;
        }
        var _this = this;
        _forEach(arguments, function(item) {
            if (typeof item === "object") {
                for (var k in item) {
                    if (k === 'callback' && typeof item[k] === 'function') {
                        _this.opt.callback = item[k].bind(_this);
                    } else {
                        k in _this.opt && (_this.opt[k] = item[k]);
                    }
                }
            } else if (typeof item === "function") {
                _this.opt.callback = item.bind(_this);
            }
        });
        this.scratchDiv = document.getElementById('scratch');
        this.cardDiv = document.getElementById('card');
        if (!this.scratchDiv || !this.cardDiv) return;
        this.cHeight = this.cardDiv.clientHeight;
        this.cWidth = this.cardDiv.clientWidth;
        this.cardDiv.style.opacity = 0;
        this.createCanvas();
        this.eventDetect();
    };

    /**
     * To generate an instance of object
     *
     * @param {Object} settings  Settings for LuckyCard
     * @param {function} callback  callback function
     */
    LuckyCard.case = function(settings, callback) {
        return new LuckyCard(settings, callback);
    };


    if (typeof define === 'function' && typeof define.amd === 'object' && define.amd) {
        define(function() {
            return LuckyCard;
        });
    } else if (typeof module !== 'undefined' && module.exports) {
        module.exports = LuckyCard.case;
        module.exports.LuckyCard = LuckyCard;
    } else {
        window.LuckyCard = LuckyCard;
    }

})(window, document);

在确保页面相关DOM加载完毕( 如写在页面底部，或document的DOMContentLoaded事件处理函数中 )之后，初始化控件

LuckyCard.case();

 LuckyCard.case({
        ratio: .7
    }, function() {
        //alert('至于你信不信，我反正不信！');
        this.clearCover();
    });

初始化lucky-card控件时，支持传入一个JSON对象和(或)一个回调函数，用于配置控件功能/设置回调函数

LuckyCard.case(settings,callback);
参数settings是一个JSON对象，可选，用于配置控件功能
参数callback是回调函数，可选，也可以写在settings中
基本用法

LuckyCard.case({coverColor:'#CCCCCC',ratio:.6,callback:function(){alert('中奖啦！')}});

刮开层支持使用图片，但图片不能跨域，如果跨域可以考虑将先其转成Data URI，再设置

LuckyCard.case({coverImg:'./demo.jpg'});

callback可作为一个独立的参数存在

LuckyCard.case(function(){
    //清除掉刮开层的所有像素
    this.clearCover();
});