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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，F(xiàn)irework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(zhuǎn)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(shù)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(xiàn)見后面的HTML文件內(nèi)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(zhuǎn)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?。▽傩詓ize值減量），當size值小于1時，從碎片數(shù)組中刪除該碎片，表示碎片已消亡。 定義兩個數(shù)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(shù)loop中，每隔一段時間（用count計數(shù)來實現(xiàn)）向fireworks數(shù)組中添加一個煙花對象，煙花對象上升到頂炸開后，從fireworks數(shù)組中刪除該對象元素，然后向particles數(shù)組中添加70個碎片對象。 遍歷兩個數(shù)組的各對象，分別調(diào)用它們的draw()和update()方法。 編寫的完整HTML文件內(nèi)容如下。 <html> <head> <title>煙花綻放</title> </head> <body> <canvas id="myCanvas" width="800" height="600" 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%, ${this.lightness}%)`; ctx.beginPath(); ctx.arc(this.x, this.y, this.size, 0, 2 * Math.PI); ctx.closePath(); ctx.fill(); } Particle.prototype.update= function(index) { this.ySpeed += 0.05; this.size = this.size*0.95; this.x = this.x + this.xSpeed; this.y = this.y + this.ySpeed; if (this.size<1) { particles.splice(index,1); } } function loop() { ctx.fillStyle = "rgba(0, 0, 0, 0.1)"; ctx.fillRect(0,0,canvas.width,canvas.height); counter++; if (counter==15) { fireworks.push(new Firework()); counter=0; } var i=fireworks.length; while (i--) { fireworks[i].draw(); fireworks[i].update(); if (fireworks[i].ySpeed > 0) { fireworks[i].explode(); fireworks.splice(i, 1); } } var i=particles.length; while (i--) { particles[i].draw(); particles[i].update(i); } requestAnimationFrame(loop); } loop(); </script> </body> </html> 在瀏覽器中打開包含這段HTML代碼的html文件，可以看到在瀏覽器窗口中呈現(xiàn)出如圖1所示的煙花綻放動畫效果。 圖1 煙花綻放 實現(xiàn)了煙花綻放的效果，我們還可以繼續(xù)讓一定區(qū)域內(nèi)的綻放的煙花碎片拼成“Happy New Year”粒子文本。粒子文本的實現(xiàn)方法參見文章“JavaScript動畫實例：粒子文本”（https://www.cnblogs.com/cs-whut/p/13334882.html）。 編寫如下的HTML代碼。 
<html> <head> <title>迎新年煙花綻放</title> <style> body { margin: 0; background: black; } canvas { position: absolute; } </style> </head> <body> <canvas id="myCanvas1"></canvas> <canvas id="myCanvas2"></canvas> <canvas id="myCanvas3"></canvas> <script type="text/javascript"> function Particle(x, y, hue) { this.x = x; this.y = y; this.hue = hue; this.lightness = 50; this.size = 15 + Math.random() * 10; this.angle = Math.random() * 2 * Math.PI; this.xSpeed = Math.cos(this.angle) * (1 + Math.random() * 6); this.ySpeed = Math.sin(this.angle) * (1 + Math.random() * 6); this.target = getTarget(); this.timer = 0; } Particle.prototype.draw= function() { ctx2.fillStyle =`hsl(${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(xiàn)出如圖2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。
圖2 煙花綻放迎新年 |
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