知乎登录背景动画效果

摘自:https://www.zhihu.com/question/38407085


<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>canvas</title>
    <style>body{text-align: center;background: #F7FAFC;overflow: hidden;background: #fff;}#canvas{display: inline-block;}</style>
</head>
<body>
<canvas id="canvas"></canvas>
<script>
//定义画布宽高和生成点的个数
var WIDTH = window.innerWidth, HEIGHT = window.innerHeight, POINT = 28;

var canvas = document.getElementById('canvas');
canvas.width = WIDTH,
canvas.height = HEIGHT;
var context = canvas.getContext('2d');
context.strokeStyle = 'rgba(0,0,0,0.02)',
context.strokeWidth = 1,
context.fillStyle = 'rgba(0,0,0,0.05)';
var circleArr = [];

//线条:开始xy坐标,结束xy坐标,线条透明度
function Line (x, y, _x, _y, o) {
this.beginX = x,
this.beginY = y,
this.closeX = _x,
this.closeY = _y,
this.o = o;
}
//点:圆心xy坐标,半径,每帧移动xy的距离
function Circle (x, y, r, moveX, moveY) {
this.x = x,
this.y = y,
this.r = r,
this.moveX = moveX,
this.moveY = moveY;
}
//生成max和min之间的随机数
function num (max, _min) {
var min = arguments[1] || 0;
return Math.floor(Math.random()*(max-min+1)+min);
}
// 绘制原点
function drawCricle (cxt, x, y, r, moveX, moveY) {
var circle = new Circle(x, y, r, moveX, moveY)
        cxt.beginPath()
        cxt.arc(circle.x, circle.y, circle.r, 0, 2*Math.PI)
        cxt.closePath()
        cxt.fill();
return circle;
}
//绘制线条
function drawLine (cxt, x, y, _x, _y, o) {
var line = new Line(x, y, _x, _y, o)
        cxt.beginPath()
        cxt.strokeStyle = 'rgba(0,0,0,'+ o +')'
cxt.moveTo(line.beginX, line.beginY)
        cxt.lineTo(line.closeX, line.closeY)
        cxt.closePath()
        cxt.stroke();

}
//初始化生成原点
function init () {
        circleArr = [];
for (var i = 0; i < POINT; i++) {
            circleArr.push(drawCricle(context, num(WIDTH), num(HEIGHT), num(15, 2), num(10, -10)/40, num(10, -10)/40));
}
draw();
}

//每帧绘制
function draw () {
        context.clearRect(0,0,canvas.width, canvas.height);
for (var i = 0; i < POINT; i++) {
drawCricle(context, circleArr[i].x, circleArr[i].y, circleArr[i].r);
}
for (var i = 0; i < POINT; i++) {
for (var j = 0; j < POINT; j++) {
if (i + j < POINT) {
var A = Math.abs(circleArr[i+j].x - circleArr[i].x),
B = Math.abs(circleArr[i+j].y - circleArr[i].y);
var lineLength = Math.sqrt(A*A + B*B);
var C = 1/lineLength*7-0.009;
var lineOpacity = C > 0.03 ? 0.03 : C;
if (lineOpacity > 0) {
drawLine(context, circleArr[i].x, circleArr[i].y, circleArr[i+j].x, circleArr[i+j].y, lineOpacity);
}
                }
            }
        }
    }

//调用执行
window.onload = function () {
init();
setInterval(function () {
for (var i = 0; i < POINT; i++) {
var cir = circleArr[i];
cir.x += cir.moveX;
cir.y += cir.moveY;
if (cir.x > WIDTH) cir.x = 0;
else if (cir.x < 0) cir.x = WIDTH;
if (cir.y > HEIGHT) cir.y = 0;
else if (cir.y < 0) cir.y = HEIGHT;

}
draw();
}, 16);
}

</script>
</body>
</html>

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