canvas怎样做出黑色背景带特效碎屑烟花

<canvas id="cas" style="
background-color
:rgba(0,5,24,1)" width="1235" height="680">浏览器不支持canvas</canvas><div class="city">
    <img src="city.png" alt=""></div><img src="moon.png" alt="" id="moon" style="
visibility
: hidden;"><div style="display:none">
    <div class="shape">新年快乐</div>
    <div class="shape">阖家欢乐</div>
    <div class="shape">万事如意</div>
    <div class="shape">心想事成</div>
 </div>

css

body { margin: 0; padding: 0; overflow: hidden;
}.city { width: 100%; position: fixed; bottom: 0px; z-index: 100;
}.city img { width: 100%;
}

js

var canvas = document.getElementById("cas");var ocas = document.createElement("canvas");var octx = ocas.getContext("2d");var ctx = canvas.getContext("2d");
ocas.width = canvas.width = window.innerWidth;
ocas.height = canvas.height = window.innerHeight;var bigbooms = [];window.onload = function () {
    initAnimate()
}function initAnimate() {
    drawBg();
    lastTime = new Date();
    animate();
}var lastTime;function animate() {
    ctx.save();
    ctx.fillStyle = "rgba(0,5,24,0.1)";
    ctx.fillRect(0, 0, canvas.width, canvas.height);
    ctx.restore();    var newTime = new Date();    if (newTime - lastTime > 500 + (window.innerHeight - 767) / 2) {        var random = Math.random() * 100 > 2 ? true : false;        var x = getRandom(canvas.width / 5, canvas.width * 4 / 5);        var y = getRandom(50, 200);        if (random) {            var bigboom = new Boom(getRandom(canvas.width / 3, canvas.width * 2 / 3), 2, "#FFF", {                x: x,                y: y
            });
            bigbooms.push(bigboom)
        } else {            var bigboom = new Boom(getRandom(canvas.width / 3, canvas.width * 2 / 3), 2, "#FFF", {                x: canvas.width / 2,                y: 200
            }, document.querySelectorAll(".shape")[parseInt(getRandom(0, document.querySelectorAll(                ".shape").length))]);
            bigbooms.push(bigboom)
        }
        lastTime = newTime;
    }
    stars.foreach(function () {        this.paint();
    })
    drawMoon();
    bigbooms.foreach(function (index) {        var that = this;        if (!this.dead) {            this._move();            this._drawLight();
        } else {            this.booms.foreach(function (index) {                if (!this.dead) {                    this.moveTo(index);
                } else if (index === that.booms.length - 1) {
                    bigbooms[bigbooms.indexOf(that)] = null;
                }
            })
        }
    });
    raf(animate);
}function drawMoon() {    var moon = document.getElementById("moon");    var centerX = canvas.width - 200,
        centerY = 100,
        width = 80;    if (moon.complete) {
        ctx.drawImage(moon, centerX, centerY, width, width)
    } else {
        moon.onload = function () {
            ctx.drawImage(moon, centerX, centerY, width, width)
        }
    }    var index = 0;    for (var i = 0; i < 10; i++) {
        ctx.save();
        ctx.beginPath();
        ctx.arc(centerX + width / 2, centerY + width / 2, width / 2 + index, 0, 2 * Math.PI);
        ctx.fillStyle = "rgba(240,219,120,0.005)";
        index += 2;
        ctx.fill();
        ctx.restore();
    }
}Array.prototype.foreach = function (callback) {    for (var i = 0; i < this.length; i++) {        if (this[i] !== null) callback.apply(this[i], [i])
    }
}var raf = window.requestAnimationFrame || window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame ||    window.oRequestAnimationFrame || window.msRequestAnimationFrame || function (callback) {        window.setTimeout(callback, 1000 / 60);
    };
canvas.onclick = function () {    var x = event.clientX;    var y = event.clientY;    var bigboom = new Boom(getRandom(canvas.width / 3, canvas.width * 2 / 3), 2, "#FFF", {        x: x,        y: y
    });
    bigbooms.push(bigboom)
}var Boom = function (x, r, c, boomArea, shape) {    this.booms = [];    this.x = x;    this.y = (canvas.height + r);    this.r = r;    this.c = c;    this.shape = shape || false;    this.boomArea = boomArea;    this.theta = 0;    this.dead = false;    this.ba = parseInt(getRandom(80, 200));
}
Boom.prototype = {    _paint: function () {
        ctx.save();
        ctx.beginPath();
        ctx.arc(this.x, this.y, this.r, 0, 2 * Math.PI);
        ctx.fillStyle = this.c;
        ctx.fill();
        ctx.restore();
    },    _move: function () {        var dx = this.boomArea.x - this.x,
            dy = this.boomArea.y - this.y;        this.x = this.x + dx * 0.01;        this.y = this.y + dy * 0.01;        if (Math.abs(dx) <= this.ba && Math.abs(dy) <= this.ba) {            if (this.shape) {                this._shapBoom();
            } else this._boom();            this.dead = true;
        } else {            this._paint();
        }
    },    _drawLight: function () {
        ctx.save();
        ctx.fillStyle = "rgba(255,228,150,0.3)";
        ctx.beginPath();
        ctx.arc(this.x, this.y, this.r + 3 * Math.random() + 1, 0, 2 * Math.PI);
        ctx.fill();
        ctx.restore();
    },    _boom: function () {        var fragNum = getRandom(30, 200);        var style = getRandom(0, 10) >= 5 ? 1 : 2;        var color;        if (style === 1) {
            color = {                a: parseInt(getRandom(128, 255)),                b: parseInt(getRandom(128, 255)),                c: parseInt(getRandom(128, 255))
            }
        }        var fanwei = parseInt(getRandom(300, 400));        for (var i = 0; i < fragNum; i++) {            if (style === 2) {
                color = {                    a: parseInt(getRandom(128, 255)),                    b: parseInt(getRandom(128, 255)),                    c: parseInt(getRandom(128, 255))
                }
            }            var a = getRandom(-Math.PI, Math.PI);            var x = getRandom(0, fanwei) * Math.cos(a) + this.x;            var y = getRandom(0, fanwei) * Math.sin(a) + this.y;            var radius = getRandom(0, 2)            var frag = new Frag(this.x, this.y, radius, color, x, y);            this.booms.push(frag);
        }
    },    _shapBoom: function () {        var that = this;
        putValue(ocas, octx, this.shape, 5, function (dots) {            var dx = canvas.width / 2 - that.x;            var dy = canvas.height / 2 - that.y;            for (var i = 0; i < dots.length; i++) {
                color = {                    a: dots[i].a,                    b: dots[i].b,                    c: dots[i].c
                }                var x = dots[i].x;                var y = dots[i].y;                var radius = 1;                var frag = new Frag(that.x, that.y, radius, color, x - dx, y - dy);
                that.booms.push(frag);
            }
        })
    }
}function putValue(canvas, context, ele, dr, callback) {
    context.clearRect(0, 0, canvas.width, canvas.height);    var img = new Image();    if (ele.innerHTML.indexOf("img") >= 0) {
        img.src = ele.
getElementsByTagName
("img")[0].src;
        imgload(img, function () {
            context.drawImage(img, canvas.width / 2 - img.width / 2, canvas.height / 2 - img.width / 2);
            dots = getimgData(canvas, context, dr);
            callback(dots);
        })
    } else {        var text = ele.innerHTML;
        context.save();        var fontSize = 200;
        context.font = fontSize + "px 宋体 bold";
        context.textAlign = "center";
        context.textBaseline = "middle";
        context.fillStyle = "rgba(" + parseInt(getRandom(128, 255)) + "," + parseInt(getRandom(128, 255)) + "," +            parseInt(getRandom(128, 255)) + " , 1)";
        context.fillText(text, canvas.width / 2, canvas.height / 2);
        context.restore();
        dots = getimgData(canvas, context, dr);
        callback(dots);
    }
}function imgload(img, callback) {    if (img.complete) {
        callback.call(img);
    } else {
        img.onload = function () {
            callback.call(this);
        }
    }
}function getimgData(canvas, context, dr) {    var imgData = context.getImageData(0, 0, canvas.width, canvas.height);
    context.clearRect(0, 0, canvas.width, canvas.height);    var dots = [];    for (var x = 0; x < imgData.width; x += dr) {        for (var y = 0; y < imgData.height; y += dr) {            var i = (y * imgData.width + x) * 4;            if (imgData.data[i + 3] > 128) {                var dot = {                    x: x,                    y: y,                    a: imgData.data[i],                    b: imgData.data[i + 1],                    c: imgData.data[i + 2]
                };
                dots.push(dot);
            }
        }
    }    return dots;
}function getRandom(a, b) {    return Math.random() * (b - a) + a;
}var maxRadius = 1,
    stars = [];function drawBg() {    for (var i = 0; i < 100; i++) {        var r = Math.random() * maxRadius;        var x = Math.random() * canvas.width;        var y = Math.random() * 2 * canvas.height - canvas.height;        var star = new Star(x, y, r);
        stars.push(star);
        star.paint()
    }
}var Star = function (x, y, r) {    this.x = x;    this.y = y;    this.r = r;
}
Star.prototype = {    paint: function () {
        ctx.save();
        ctx.beginPath();
        ctx.arc(this.x, this.y, this.r, 0, 2 * Math.PI);
        ctx.fillStyle = "rgba(255,255,255," + this.r + ")";
        ctx.fill();
        ctx.restore();
    }
}var focallength = 250;var Frag = function (centerX, centerY, radius, color, tx, ty) {    this.tx = tx;    this.ty = ty;    this.x = centerX;    this.y = centerY;    this.dead = false;    this.centerX = centerX;    this.centerY = centerY;    this.radius = radius;    this.color = color;
}
Frag.prototype = {    paint: function () {
        ctx.save();
        ctx.beginPath();
        ctx.arc(this.x, this.y, this.radius, 0, 2 * Math.PI);
        ctx.fillStyle = "rgba(" + this.color.a + "," + this.color.b + "," + this.color.c + ",1)";
        ctx.fill()
        ctx.restore();
    },    moveTo: function (index) {        this.ty = this.ty + 0.3;        var dx = this.tx - this.x,
            dy = this.ty - this.y;        this.x = Math.abs(dx) < 0.1 ? this.tx : (this.x + dx * 0.1);        this.y = Math.abs(dy) < 0.1 ? this.ty : (this.y + dy * 0.1);        if (dx === 0 && Math.abs(dy) <= 80) {            this.dead = true;
        }        this.paint();
    }
}

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