如何使用 HTML5 Canvas 制作水波纹效果

　　今天，我们继续分享 JavaScript 实现的效果例子，这篇文章会介绍使用 JavaScript 实现水波纹效果。水波效果以图片为背景，点击图片任意位置都会触发。有时候，我们使用普通的 Javascript 就可以创建一个很有趣的解决功能。

在线演示源码下载

Step 1. HTML

和以前一样，首先是 HTML 代码：

<!DOCTYPE html>
<html>
    <head>
        <meta charset=utf-8 />
        <title>Water drops effect</title>
        <link rel="stylesheet" href="css/main.css" type="text/css" />
        <script src="js/vector2d.js" type="text/javascript" charset="utf-8"></script>
        <script src="js/waterfall.js" type="text/javascript" charset="utf-8"></script>
    </head>
    <body>
        <div class="example">
            <h3><a href="#">Water drops effect</a></h3>
 
            <canvas id="water">HTML5 compliant browser required</canvas>
            <div id="switcher">
                <img onclick='watereff.changePicture(this.src);' src="data_images/underwater1.jpg" />
                <img onclick='watereff.changePicture(this.src);' src="data_images/underwater2.jpg" />
            </div>
            <div id="fps"></div>
        </div>
    </body>
</html>　

Step 2. CSS

这是用到的 CSS 代码：

body{background:#eee;margin:0;padding:0}
.example{background:#FFF;width:600px;border:1px #000 solid;margin:20px auto;padding:15px;-moz-border-radius: 3px;-webkit-border-radius: 3px}
 
#water {
    width:500px;
    height:400px;
    display: block;
    margin:0px auto;
    cursor:pointer;
}
#switcher {
    text-align:center;
    overflow:hidden;
    margin:15px;
}
#switcher img {
    width:160px;
    height:120px;
}

Step 3. JS

下面是主要的 JavaScript 代码：

100

101

102

103

104

105

106

107

108

109

110

111

112

113

114

115

116

117

118

119

120

121

122

123

124

125

126

127

128

129

130

131

function drop(x, y, damping, shading, refraction, ctx, screenWidth, screenHeight){
    this.x = x;
    this.y = y;
    this.shading = shading;
    this.refraction = refraction;
    this.bufferSize = this.x * this.y;
    this.damping = damping;
    this.background = ctx.getImageData(0, 0, screenWidth, screenHeight).data;
    this.imageData = ctx.getImageData(0, 0, screenWidth, screenHeight);
 
    this.buffer1 = [];
    this.buffer2 = [];
    for (var i = 0; i < this.bufferSize; i++){
        this.buffer1.push(0);
        this.buffer2.push(0);
    }
 
    this.update = function(){
        for (var i = this.x + 1, x = 1; i < this.bufferSize - this.x; i++, x++){
            if ((x < this.x)){
                this.buffer2[i] = ((this.buffer1[i - 1] + this.buffer1[i + 1] + this.buffer1[i - this.x] + this.buffer1[i + this.x]) / 2) - this.buffer2[i];
                this.buffer2[i] *= this.damping;
            } else x = 0;
        }
 
        var temp = this.buffer1;
        this.buffer1 = this.buffer2;
        this.buffer2 = temp;
    }
 
    this.draw = function(ctx){
        var imageDataArray = this.imageData.data;
        for (var i = this.x + 1, index = (this.x + 1) * 4; i < this.bufferSize - (1 + this.x); i++, index += 4){
            var xOffset = ~~(this.buffer1[i - 1] - this.buffer1[i + 1]);
            var yOffset = ~~(this.buffer1[i - this.x] - this.buffer1[i + this.x]);
            var shade = xOffset * this.shading;
            var texture = index + (xOffset * this.refraction  + yOffset * this.refraction * this.x) * 4;
            imageDataArray[index] = this.background[texture] + shade; 
            imageDataArray[index + 1] = this.background[texture + 1] + shade;
            imageDataArray[index + 2] = 50 + this.background[texture + 2] + shade;
        }
        ctx.putImageData(this.imageData, 0, 0);
    }
}
 
var fps = 0;
 
var watereff = {
    // variables
    timeStep : 20,
    refractions : 2,
    shading : 3,
    damping : 0.99,
    screenWidth : 500,
    screenHeight : 400,
    pond : null,
    textureImg : null,
    interval : null,
    backgroundURL : 'data_images/underwater1.jpg',
 
    // initialization
    init : function() {
        var canvas = document.getElementById('water');
        if (canvas.getContext){
 
            // fps countrt
            fps = 0;
            setInterval(function() { 
                document.getElementById('fps').innerHTML = fps / 2 + ' FPS'; 
                fps = 0;
            }, 2000);
 
            canvas.onmousedown = function(e) {
                var mouse = watereff.getMousePosition(e).sub(new vector2d(canvas.offsetLeft, canvas.offsetTop));
                watereff.pond.buffer1[mouse.y * watereff.pond.x + mouse.x ] += 200;
            }
            canvas.onmouseup = function(e) {
                canvas.onmousemove = null;
            }
 
            canvas.width  = this.screenWidth;
            canvas.height = this.screenHeight;
            this.textureImg = new Image(256, 256);
            this.textureImg.src = this.backgroundURL;
            canvas.getContext('2d').drawImage(this.textureImg, 0, 0);
            this.pond = new drop(
                this.screenWidth, 
                this.screenHeight, 
                this.damping,
                this.shading, 
                this.refractions,
                canvas.getContext('2d'),
                this.screenWidth, this.screenHeight
            );
            if (this.interval != null){
                clearInterval(this.interval);
            }
            this.interval = setInterval(watereff.run, this.timeStep);
        }
    },
 
    // change image func
    changePicture : function(url){
        this.backgroundURL = url;
        this.init();
    },
 
    // get mouse position func
    getMousePosition : function(e){
        if (!e){
            var e = window.event;
        } 
        if (e.pageX || e.pageY){
            return new vector2d(e.pageX, e.pageY);
        } else if (e.clientX || e.clientY){
            return new vector2d(e.clientX, e.clientY);
        } 
    },
 
    // loop drawing
    run : function(){
        var ctx = document.getElementById('water').getContext('2d');
        watereff.pond.update();
        watereff.pond.draw(ctx);
        fps++;
    }
}
 
window.onload = function(){
    watereff.init();
}