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最近项目中，ui设计了个水波效果的背景动画，然而并没有gif或svg动画，开始试着用css实现了一下，动画效果并不是很好，网上查了下基本都是用贝赛尔曲线实现，想起以看的各种前波形图，于是想着用三角函数图像初略模拟一下
示例效果如下：
一、绘制sin函数图像
sin函数表达式如下，
y=Asin（wx+φ）+h
y= Asin（wx+φ）+h
y=Asin（wx+φ）+h
其中A表示振幅，ω表示角频率（ω=2π/T,T为函数的周期），φ表示初相，h表示图像向y轴正方向平移的长度；（这里需要注意一下：h在数学学的本来是表示向上平移的，但在canvas中采用的是屏幕坐标系，即左上角为原点，h则表示向下平移）；绘制代码如下：
（1）添加canvas标签
<canvas id="canvas"></canvas>
（2）添加css样式,设置canvas宽高
html,
body {
padding: 0;
margin: 0;
width: 100%;
height: 100%;
}
canvas {
width: 100%;
height: 100%;
}
（3）绘制函数图像
var canvas = document.getElementById("canvas"),
ctx = canvas.getContext('2d'),
width = canvas.width = canvas.offsetWidth,
height = canvas.height = canvas.offsetHeight;
//声明参数
var A=50,
W=1 / 50,
Q=0,
H= height / 2;
//绘图方法
(function draw(){
ctx.clearRect(0, 0, width, height);//清空画布
ctx.beginPath(); //开始路径
ctx.strokeStyle="#000"; //设置线条颜色
ctx.lineWidth = 1; //设置线条宽度
ctx.moveTo(0, height /2); //起始点位置
for (let x = 0; x <= width; x++) {// 绘制x对应y的
var y = A*Math.sin(W*x+Q) +H
ctx.lineTo(x, y)
}
ctx.stroke(); //绘制路径
ctx.closePath(); //闭合路径
})()
这样我们可以得到以下图像：
二、为函数图像添加动画
上面得到的是是一个静态的函数图像，而我们一般见到的的波形图或水波都是随时间连续变化的，这里就要用到上一步中的参数相位φ，（js即代码中的Q） ，我们将φ随时间不断增加或减小，即可得到不同时间的不同图像；使用window.requestAnimationFrame实现帧动画；
修改以上代码为：
var speed=-0.02;
(function draw(){
window.requestAnimationFrame(draw);
ctx.clearRect(0, 0, width, height);
ctx.beginPath();
ctx.strokeStyle="#000";
ctx.lineWidth = 1;
ctx.moveTo(0, height /2);
Q+=speed;
for (let x = 0; x <= width; x++) {
var y = A*Math.sin(W*x+Q) +H;
ctx.lineTo(x, y);
}
ctx.stroke();
ctx.closePath();
})()
效果如下（渣渣截图软件）：
三、绘制完整填充路径
以上路径虽有闭合，但却不满足我们需要填充的部分，直接填充效果如下：
完整填充路径应如图所示：
闭合路径后创建一个渐变颜色，作为填充颜色，代码如下：
var lingrad = ctx.createLinearGradient(0,0,width,0);
lingrad.addColorStop(0, 'rgba(0,186,128,0.8)');
lingrad.addColorStop(1, 'rgba(111,224,195,1)');
(function draw(){
window.requestAnimationFrame(draw);
ctx.clearRect(0, 0, width, height);
ctx.beginPath();
ctx.strokeStyle="#000";
ctx.fillStyle = lingrad;
ctx.lineWidth = 1;
ctx.moveTo(0, height /2);
Q+=speed;
for (let x = 0; x <= width; x++) {
var y = A*Math.sin(W*x+Q) +H;
ctx.lineTo(x, y);
}
ctx.lineTo(width, height);
ctx.lineTo(0, height);
ctx.fill();
ctx.closePath();
})()
效果如下：
四、完善水波动画
1、首先可以对上面波形叠加一个频率更高的波形，使波形无规矩
var s = 0.1*Math.sin(x/150)+1;
var y = A*Math.sin(W*x+Q) +H;
y=y*s;
2、再添加一个相位变化不同的波形，其渐变填充与上一个渐变方向相反使其形成相互重叠的阴影效果；
并设置路径重叠效果globalCompositeOperation；
自己根据效果配置相关参数，调节波形；
完整代码如下：
var canvas = document.getElementById("canvas"),
ctx = canvas.getContext('2d'),
width = canvas.width = canvas.offsetWidth,
height = canvas.height = canvas.offsetHeight;
var A=30,
W=1 /200,
Q=0,
H= height / 2;
var A2=30,
W2=1/300,
Q2=0,
H2= height / 2;
var speed=-0.01;
var speed2=-0.02;
var lingrad = ctx.createLinearGradient(0,0,width,0);
lingrad.addColorStop(0, 'rgba(0,186,128,0.8)');
lingrad.addColorStop(1, 'rgba(111,224,195,1)');
var lingrad2 = ctx.createLinearGradient(0,0,width,0);
lingrad2.addColorStop(0,'rgba(111,224,195,1)');
lingrad2.addColorStop(1, 'rgba(0,186,128,0.8)');
(function draw(){
window.requestAnimationFrame(draw);
ctx.clearRect(0, 0, width, height);
ctx.beginPath();
ctx.fillStyle = lingrad;
ctx.moveTo(0, height /2);
//绘制第一个波形
Q+=speed;
for (let x = 0; x <= width; x++) {
var s = 0.1*Math.sin(x/150)+1;
var y = A*Math.sin(W*x+Q) +H;
y=y*s;
ctx.lineTo(x, y);
}
ctx.lineTo(width, height);
ctx.lineTo(0, height);
ctx.fill();
ctx.closePath()
//设置重叠效果
ctx.globalCompositeOperation = "destination-over"
//绘制第二个波形
ctx.beginPath();
ctx.fillStyle = lingrad2;
Q2+=speed2;
for (let x = 0; x < width; x++) {
var y = A2*Math.sin(x*W2+Q2) +H2;
ctx.lineTo(x, y);
}
ctx.lineTo(width,height);
ctx.lineTo(0,height);
ctx.fill()
ctx.closePath();
})()