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>

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();
 }
}