JavaScript實現煙花綻放動畫效果
先編寫一個煙花綻放的動畫效果。
放煙花時，一個煙花可分為兩個階段：（1）煙花上升到空中；（2）煙花炸開成碎片，炸開的碎片慢慢消散。
為此抽象出兩個對象類：Firework和Particle。其中，Firework用于表示一個煙花對象，Particle用于表示一個煙花炸開后的各碎片。
Firework對象類定義6個屬性：表示煙花上升軌跡中各點的坐標（x,y）、煙花弧狀軌跡的偏轉角度angle、上升階段水平和垂直方向的位移改變量xSpeed和ySpeed、煙花的色彩色相hue。
坐標屬性值y的初始值取畫布的高度，表示煙花從地面上升到空中，其余各屬性的初始值采用隨機數確定。具體定義如下：
function Firework() { this.x = canvas.width/4*(1+3*Math.random()); this.y = canvas.height - 15; this.angle = Math.random() * Math.PI / 4 - Math.PI / 6; this.xSpeed = Math.sin(this.angle) *(6+Math.random()*7); this.ySpeed = -Math.cos(this.angle) *(6+Math.random()*7); this.hue = Math.floor(Math.random() * 360); }
Firework對象類定義3個方法：繪制煙花上升軌跡的方法draw()、煙花上升時坐標改變方法update()和煙花炸開方法explode()。繪制煙花軌跡時，在各點（x,y）處繪制一個寬度為5、高度為15的填充小矩形表示一個軌跡點。煙花上升時，垂直方向速度ySpeed初始值為負的，每次上升時，ySpeed加上一個正值，表示上升在減速，當ySpeed的值大于0時，煙花上升到頂了（不能再上升），就炸開為70個碎片。具體方法的實現見后面的HTML文件內容。
Particle對象類定義8個屬性：表示碎片散開軌跡中各點的坐標（x,y）、碎片弧狀軌跡的偏轉角度angle、散開時水平和垂直方向的位移改變量xSpeed和ySpeed、碎片的色彩色相hue、表示碎片小圓的半徑size、碎片的亮度lightness。
function Particle(x,y,hue) { this.x = x; this.y = y; this.hue = hue; this.lightness = 50; this.size = 15 + Math.random() * 10; this.angle = Math.random() * 2 * Math.PI; this.xSpeed = Math.cos(this.angle) *(1+Math.random() * 6); this.ySpeed = Math.sin(this.angle) *(1+Math.random() * 6); }
Particle對象類定義2個方法：繪制碎片散開軌跡的方法draw()、碎片散開時坐標改變方法update()。碎片散開時逐漸變小（屬性size值減量），當size值小于1時，從碎片數組中刪除該碎片，表示碎片已消亡。
定義兩個數組var fireworks=[];和var particles=[];分別存儲煙花對象和炸開的碎片對象。
模擬動畫的函數loop中，每隔一段時間（用count計數來實現）向fireworks數組中添加一個煙花對象，煙花對象上升到頂炸開后，從fireworks數組中刪除該對象元素，然后向particles數組中添加70個碎片對象。
遍歷兩個數組的各對象，分別調用它們的draw()和update()方法。
編寫的完整HTML文件內容如下。
<html> <head> <title>煙花綻放</title> </head><body><canvas style='border:3px double #996633;background:black;'></canvas><script type='text/javascript'> var canvas=document.getElementById(’myCanvas’); ctx= canvas.getContext(’2d’); var fireworks=[]; var particles=[]; var counter = 0; function Firework() { this.x = canvas.width/4*(1+3*Math.random()); this.y = canvas.height - 15; this.angle = Math.random() * Math.PI / 4 - Math.PI / 6; this.xSpeed = Math.sin(this.angle) *(6+Math.random()*7); this.ySpeed = -Math.cos(this.angle) *(6+Math.random()*7); this.hue = Math.floor(Math.random() * 360); } Firework.prototype.draw= function() { ctx.save(); ctx.translate(this.x, this.y); ctx.rotate(Math.atan2(this.ySpeed, this.xSpeed) + Math.PI / 2); ctx.fillStyle =`hsl(${this.hue}, 100%, 50%)`; ctx.fillRect(0, 0, 5, 15); ctx.restore(); } Firework.prototype.update= function() { this.x = this.x + this.xSpeed; this.y = this.y + this.ySpeed; this.ySpeed += 0.1; } Firework.prototype.explode= function() { for (var i = 0; i < 70; i++) { particles.push(new Particle(this.x, this.y, this.hue)); } } function Particle(x,y,hue) { this.x = x; this.y = y; this.hue = hue; this.lightness = 50; this.size = 15 + Math.random() * 10; this.angle = Math.random() * 2 * Math.PI; this.xSpeed = Math.cos(this.angle) *(1+Math.random() * 6); this.ySpeed = Math.sin(this.angle) *(1+Math.random() * 6); } Particle.prototype.draw= function() { ctx.fillStyle = `hsl(${this.hue}, 100%, ${this.lightness}%)`; ctx.beginPath(); ctx.arc(this.x, this.y, this.size, 0, 2 * Math.PI); ctx.closePath(); ctx.fill(); } Particle.prototype.update= function(index) { this.ySpeed += 0.05; this.size = this.size*0.95; this.x = this.x + this.xSpeed; this.y = this.y + this.ySpeed; if (this.size<1) { particles.splice(index,1); } } function loop() { ctx.fillStyle = 'rgba(0, 0, 0, 0.1)'; ctx.fillRect(0,0,canvas.width,canvas.height); counter++; if (counter==15) { fireworks.push(new Firework()); counter=0; } var i=fireworks.length; while (i--) { fireworks[i].draw(); fireworks[i].update(); if (fireworks[i].ySpeed > 0) { fireworks[i].explode(); fireworks.splice(i, 1); } } var i=particles.length; while (i--) {particles[i].draw(); particles[i].update(i); } requestAnimationFrame(loop); } loop();</script></body> </html>
在瀏覽器中打開包含這段HTML代碼的html文件，可以看到在瀏覽器窗口中呈現出如圖所示的煙花綻放動畫效果。
實現了煙花綻放的效果，我們還可以繼續讓一定區域內的綻放的煙花碎片拼成“Happy New Year”粒子文本。
編寫如下的HTML代碼。
<html> <head> <title>迎新年煙花綻放</title> <style> body { margin: 0; background: black; } canvas { position: absolute; }</style></head><body><canvas id='myCanvas1'></canvas><canvas id='myCanvas2'></canvas><canvas id='myCanvas3'></canvas><script type='text/javascript'> function Particle(x, y, hue) { this.x = x; this.y = y; this.hue = hue; this.lightness = 50; this.size = 15 + Math.random() * 10; this.angle = Math.random() * 2 * Math.PI; this.xSpeed = Math.cos(this.angle) * (1 + Math.random() * 6); this.ySpeed = Math.sin(this.angle) * (1 + Math.random() * 6); this.target = getTarget(); this.timer = 0; } Particle.prototype.draw= function() { ctx2.fillStyle =`hsl(${this.hue}, 100%, ${this.lightness}%)`; ctx2.beginPath(); ctx2.arc(this.x, this.y, this.size, 0, 2 * Math.PI); ctx2.closePath(); ctx2.fill(); } Particle.prototype.update= function(idx) { if (this.target) { var dx = this.target.x - this.x; var dy = this.target.y - this.y; var dist = Math.sqrt(dx * dx + dy * dy); var a = Math.atan2(dy, dx); var tx = Math.cos(a) * 5; var ty = Math.sin(a) * 5; this.size = lerp(this.size, 1.5, 0.05); if (dist < 5) { this.lightness = lerp(this.lightness, 100, 0.01); this.xSpeed = this.ySpeed = 0; this.x = lerp(this.x, this.target.x + fidelity / 2, 0.05); this.y = lerp(this.y, this.target.y + fidelity / 2, 0.05); this.timer += 1; } else if (dist < 10) { this.lightness = lerp(this.lightness, 100, 0.01); this.xSpeed = lerp(this.xSpeed, tx, 0.1); this.ySpeed = lerp(this.ySpeed, ty, 0.1); this.timer += 1; } else { this.xSpeed = lerp(this.xSpeed, tx, 0.02); this.ySpeed = lerp(this.ySpeed, ty, 0.02); } } else { this.ySpeed += 0.05; this.size = this.size*0.95; if (this.size<1) { particles.splice(idx,1); } } this.x = this.x + this.xSpeed; this.y = this.y + this.ySpeed; } function Firework() { this.x = canvas2.width*(1+ 3*Math.random())/4; this.y = canvas2.height - 15; this.angle = Math.random() * Math.PI / 4 - Math.PI / 6; this.xSpeed = Math.sin(this.angle) * (6 + Math.random() * 7); this.ySpeed = -Math.cos(this.angle) * (6 + Math.random() * 7); this.hue = Math.floor(Math.random() * 360); } Firework.prototype.draw= function() { ctx2.save(); ctx2.translate(this.x, this.y); ctx2.rotate(Math.atan2(this.ySpeed, this.xSpeed) + Math.PI / 2); ctx2.fillStyle = `hsl(${this.hue}, 100%, 50%)`; ctx2.fillRect(0, 0, 5, 15); ctx2.restore(); } Firework.prototype.update= function() { this.x = this.x + this.xSpeed; this.y = this.y + this.ySpeed; this.ySpeed += 0.1; } Firework.prototype.explode= function() { for (var i = 0; i < 70; i++) { particles.push(new Particle(this.x, this.y, this.hue)); } } function lerp(a, b, t) { return Math.abs(b - a)> 0.1 ? a + t * (b - a) : b; } function getTarget() { if (targets.length > 0) { var idx = Math.floor(Math.random() * targets.length); var { x, y } = targets[idx]; targets.splice(idx, 1); x += canvas2.width / 2 - textWidth / 2; y += canvas2.height / 2 - fontSize / 2; return { x, y }; } } var canvas1=document.getElementById(’myCanvas1’); ctx1= canvas1.getContext(’2d’); var canvas2=document.getElementById(’myCanvas2’); ctx2= canvas2.getContext(’2d’); var canvas3=document.getElementById(’myCanvas3’); ctx3= canvas3.getContext(’2d’); var fontSize = 200; var fireworks = []; var particles = []; var targets = []; var fidelity = 3; var counter = 0; canvas2.width = canvas3.width = window.innerWidth; canvas2.height = canvas3.height = window.innerHeight; ctx1.fillStyle = ’#000’; var text = ’Happy New Year’; var textWidth = 999999; while (textWidth > window.innerWidth) { ctx1.font = `900 ${fontSize--}px Arial`; textWidth = ctx1.measureText(text).width; } canvas1.width = textWidth; canvas1.height = fontSize * 1.5; ctx1.font = `900 ${fontSize}px Arial`; ctx1.fillText(text, 0, fontSize); var imgData = ctx1.getImageData(0, 0, canvas1.width, canvas1.height); for (var i = 0, max = imgData.data.length; i < max; i += 4) { var alpha = imgData.data[i + 3]; var x = Math.floor(i / 4) % imgData.width; var y = Math.floor(i / 4 / imgData.width); if (alpha && x % fidelity === 0 && y % fidelity === 0) { targets.push({ x, y }); } } ctx3.fillStyle = ’#FFF’; ctx3.shadowColor = ’#FFF’; ctx3.shadowBlur = 25; function loop() { ctx2.fillStyle = 'rgba(0, 0, 0, .1)'; ctx2.fillRect(0, 0, canvas2.width, canvas2.height); counter += 1; if (counter==15) { fireworks.push(new Firework()); counter=0; } var i=fireworks.length; while (i--) { fireworks[i].draw(); fireworks[i].update(); if (fireworks[i].ySpeed > 0) { fireworks[i].explode(); fireworks.splice(i, 1); } } var i=particles.length; while (i--) {particles[i].draw(); particles[i].update(i); if (particles[i].timer >= 100 || particles[i].lightness >= 99) { ctx3.fillRect(particles[i].target.x, particles[i].target.y, fidelity + 1, fidelity + 1); particles.splice(i, 1); } } requestAnimationFrame(loop); } loop();</script></body> </html>
在瀏覽器中打開包含這段HTML代碼的html文件，可以看到在瀏覽器窗口中呈現出如圖所示的煙花綻放迎新年動畫效果。圖2中為了控制圖片的大小，刪除了大量的中間幀，因此和實際運行的效果有所不同。
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