详解canvas粒子系统的构建方法

前面的话

本文将从最基本的imageData对象的理论知识说开去，详细介绍canvas粒子系统的构建

imageData

关于图像数据imageData共有3个方法，包括getImageData()、putImageData()、createImageData()

【getImageData()】

2D上下文可以通过getImageData()取得原始图像数据。这个方法接收4个参数：画面区域的x和y坐标以及该区域的像素宽度和高度

例如，要取得左上角坐标为(10,5)、大小为50*50像素的区域的图像数据，可以使用以下代码：

var imageData = context.getImageData(10,5,50,50);

返回的对象是ImageData的实例，每个ImageData对象有3个属性：width\height\data

1、width：表示imageData对角的宽度

2、height：表示imageData对象的高度

3、data是一个数组，保存着图像中每一个像素的数据。在data数组中，每一个像素用4个元素来保存，分别表示red、green、blue、透明度

[注意]图像中有多少像素，data的长度就等于像素个数乘以4

//第一个像素如下
var data = imageData.data;
var red = data[0];
var green = data[1]; 
var blue = data[2];
var alpha = data[3];

数组中每个元素的值是在0-255之间，能够直接访问到原始图像数据，就能够以各种方式来操作这些数据

[注意]如果要使用getImageData()获取的canvas中包含drawImage()方法，则该方法中的URL不能跨域

【createImageData()】

createImageData(width,height)方法创建新的空白ImageData对象。新对象的默认像素值 transparent black，相当于rgba(0,0,0,0)

var imgData = context.createImageData(100,100);

【putImageData()】

putImageData()方法将图像数据从指定的ImageData对象放回画布上，该方法共有以下参数

imgData：要放回画布的ImageData对象(必须)
x：imageData对象的左上角的x坐标(必须)
y：imageData对象的左上角的y坐标(必须)
dirtyX：在画布上放置图像的水平位置(可选)
dirtyY：在画布上放置图像的垂直位置(可选)
dirtyWidth：在画布上绘制图像所使用的宽度(可选)
dirtyHeight：在画布上绘制图像所使用的高度(可选)

[注意]参数3到7要么都没有，要么都存在

context.putImageData(imgData,0,0);
context.putImageData(imgData,0,0,50,50,200,200);

粒子写入

粒子，指图像数据imageData中的每一个像素点。下面以一个简易实例来说明完全写入与粒子写入

【完全写入】

200*200的canvas1中存在文字'小火柴'，并将canvas1整个作为图像数据写入同样尺寸的canvas2中

<canvas id="drawing1" style="border:1px solid black"></canvas>
<canvas id="drawing2" style="border:1px solid black"></canvas>
<script>
var drawing1 = document.getElementById('drawing1');
var drawing2 = document.getElementById('drawing2');
if(drawing1.getContext){
 var cxt = drawing1.getContext('2d');
 var cxt2 = drawing2.getContext('2d');
 var W = drawing1.width = drawing2.width = 200;
 var H = drawing1.height = drawing2.height = 200;
 var str = '小火柴';
 cxt.textBaseline = 'top';
 var sh = 60;
 cxt.font = sh + 'px 宋体'
 var sw = cxt.measureText(str).width;
 if(sw > W){
  sw = W;
 }
 cxt.fillText(str,(W - sw)/2,(H - sh)/2,W);
 //获取imageData
 var imageData = cxt.getImageData(0,0,W,H); 
 //写入drawing2中 
 cxt2.putImageData(imageData,0,0);
</script>

【粒子写入】

对于完全写入而言，相当于只是简单的复制粘贴，如果要对每个像素点进行精细地控制，则需要使用粒子写入。canvas1中存在着大量的空白区域，只有'小火柴'这三个字的区域是有效的。于是，可以根据图像数据imageData中的透明度对粒子进行筛选，只筛选出透明度大于0的粒子

<canvas id="drawing1" style="border:1px solid black"></canvas>
<canvas id="drawing2" style="border:1px solid black"></canvas>
<script>
var drawing1 = document.getElementById('drawing1');
var drawing2 = document.getElementById('drawing2');
if(drawing1.getContext){
 var cxt = drawing1.getContext('2d');
 var cxt2 = drawing2.getContext('2d');
 var W = drawing1.width = drawing2.width = 200;
 var H = drawing1.height = drawing2.height = 200;
 var str = '小火柴';
 cxt.textBaseline = 'top';
 var sh = 60;
 cxt.font = sh + 'px 宋体'
 var sw = cxt.measureText(str).width;
 if(sw > W){
  sw = W;
 }
 cxt.fillText(str,(W - sw)/2,(H - sh)/2,W);
 //获取imageData
 var imageData = cxt.getImageData(0,0,W,H); 
 //写入drawing2中 
 cxt2.putImageData(setData(imageData),0,0);
 function setData(imageData){
 //从imageData对象中取得粒子，并存储到dots数组中
 var dots = [];
 for(var i = 0; i < W; i++){
  for(var j = 0; j < H ;j++){
  //data值中的红色值
  var k = 4*(i + j*W);
  //data值中的透明度
  if(imageData.data[k+3] > 0){
   //将透明度大于0的data中的红色值保存到dots数组中
   dots.push(k);
  }
  }
 }
 //40000 2336
 console.log(i*j,dots.length);
 //新建一个imageData，并将筛选后的粒子信息保存到新建的imageData中
 var oNewImage = cxt.createImageData(W,H);
 for(var i = 0; i < dots.length; i++){
  oNewImage.data[dots[i]+0] = imageData.data[dots[i]+0];
  oNewImage.data[dots[i]+1] = imageData.data[dots[i]+1];
  oNewImage.data[dots[i]+2] = imageData.data[dots[i]+2];
  oNewImage.data[dots[i]+3] = imageData.data[dots[i]+3];
 }
 return oNewImage;
 }
}
</script>

虽然结果看上去相同，但canvas2只使用了canvas1中40000个粒子中的2336个

粒子筛选

当粒子完全写入时，与canvas复制粘贴的效果相同。而当粒子有所筛选时，则会出现一些奇妙的效果

【按序筛选】

由于取得粒子时，使用的是宽度值*高度值的双重循环，且都以加1的形式递增。如果不是加1，而是加n，则可以实现按序筛选的效果

<canvas id="drawing1" style="border:1px solid black"></canvas>
<canvas id="drawing2" style="border:1px solid black"></canvas>
<p id="con">
 <button>1</button>
 <button>2</button>
 <button>3</button>
 <button>4</button>
 <button>5</button>
</p>
<script>
var oCon = document.getElementById('con');
oCon.onclick = function(e){
 e = e || event;
 var tempN = e.target.innerHTML;
 if(tempN){
 cxt2.clearRect(0,0,W,H);
 cxt2.putImageData(setData(imageData,Number(tempN)),0,0);
 }
}
var drawing1 = document.getElementById('drawing1');
var drawing2 = document.getElementById('drawing2');
if(drawing1.getContext){
 var cxt = drawing1.getContext('2d');
 var cxt2 = drawing2.getContext('2d');
 var W = drawing1.width = drawing2.width = 200;
 var H = drawing1.height = drawing2.height = 200;
 var str = '小火柴';
 cxt.textBaseline = 'top';
 var sh = 60;
 cxt.font = sh + 'px 宋体'
 var sw = cxt.measureText(str).width;
 if(sw > W){
  sw = W;
 }
 cxt.fillText(str,(W - sw)/2,(H - sh)/2,W);
 //获取imageData
 var imageData = cxt.getImageData(0,0,W,H); 
 //写入drawing2中 
 cxt2.putImageData(setData(imageData,1),0,0);
 function setData(imageData,n){
 //从imageData对象中取得粒子，并存储到dots数组中
 var dots = [];
 for(var i = 0; i < W; i+=n){
  for(var j = 0; j < H ;j+=n){
  //data值中的红色值
  var k = 4*(i + j*W);
  //data值中的透明度
  if(imageData.data[k+3] > 0){
   //将透明度大于0的data中的红色值保存到dots数组中
   dots.push(k);
  }
  }
 }
 //新建一个imageData，并将筛选后的粒子信息保存到新建的imageData中
 var oNewImage = cxt.createImageData(W,H);
 for(var i = 0; i < dots.length; i++){
  oNewImage.data[dots[i]+0] = imageData.data[dots[i]+0];
  oNewImage.data[dots[i]+1] = imageData.data[dots[i]+1];
  oNewImage.data[dots[i]+2] = imageData.data[dots[i]+2];
  oNewImage.data[dots[i]+3] = imageData.data[dots[i]+3];
 }
 return oNewImage;
 }
}
</script>

【随机筛选】

除了使用按序筛选，还可以使用随机筛选。 通过双重循环得到的粒子的位置信息，放到dots数组中。通过splice()方法进行筛选，将筛选后的位置信息放到新建的newDots数组中，然后再使用createImageData()，新建一个图像数据对象并返回

<canvas id="drawing1" style="border:1px solid black"></canvas>
<canvas id="drawing2" style="border:1px solid black"></canvas>
<p id="con">
 <button>1000</button>
 <button>2000</button>
 <button>3000</button>
 <button>4000</button>
</p>
<script>
var oCon = document.getElementById('con');
oCon.onclick = function(e){
 e = e || event;
 var tempN = e.target.innerHTML;
 if(tempN){
 cxt2.clearRect(0,0,W,H);
 cxt2.putImageData(setData(imageData,1,Number(tempN)),0,0);
 }
}
var drawing1 = document.getElementById('drawing1');
var drawing2 = document.getElementById('drawing2');
if(drawing1.getContext){
 var cxt = drawing1.getContext('2d');
 var cxt2 = drawing2.getContext('2d');
 var W = drawing1.width = drawing2.width = 200;
 var H = drawing1.height = drawing2.height = 200;
 var str = '小火柴';
 cxt.textBaseline = 'top';
 var sh = 60;
 cxt.font = sh + 'px 宋体'
 var sw = cxt.measureText(str).width;
 if(sw > W){
  sw = W;
 }
 cxt.fillText(str,(W - sw)/2,(H - sh)/2,W);
 //获取imageData
 var imageData = cxt.getImageData(0,0,W,H); 
 //写入drawing2中 
 cxt2.putImageData(setData(imageData,1),0,0);
 function setData(imageData,n,m){
 //从imageData对象中取得粒子，并存储到dots数组中
 var dots = [];
 for(var i = 0; i < W; i+=n){
  for(var j = 0; j < H ;j+=n){
  //data值中的红色值
  var k = 4*(i + j*W);
  //data值中的透明度
  if(imageData.data[k+3] > 0){
   //将透明度大于0的data中的红色值保存到dots数组中
   dots.push(k);
  }
  }
 } 
 //筛选粒子，仅保存m个到newDots数组中。如果不传入m，则不进行筛选
 var newDots = [];
 if(m && (dots.length > m)){
  for(var i = 0; i < m; i++){
  newDots.push(Number(dots.splice(Math.floor(Math.random()*dots.length),1)));
  }
 }else{
  newDots = dots;
 } 
 //新建一个imageData，并将筛选后的粒子信息保存到新建的imageData中
 var oNewImage = cxt.createImageData(W,H);
 for(var i = 0; i < newDots.length; i++){
  oNewImage.data[newDots[i]+0] = imageData.data[newDots[i]+0];
  oNewImage.data[newDots[i]+1] = imageData.data[newDots[i]+1];
  oNewImage.data[newDots[i]+2] = imageData.data[newDots[i]+2];
  oNewImage.data[newDots[i]+3] = imageData.data[newDots[i]+3];
 }
 return oNewImage;
 }
}
</script>

像素显字

下面来使用粒子筛选来实现一个像素显字的效果。像素显字即从不清晰的效果逐步过渡到完全显示

【按序像素显字】

按序像素显字的实现原理非常简单，比如，共有2000个粒子，共10个程度的过渡效果。则使用10个数组，分别保存200,400,600,800,100,1200,1400,1600,1800和2000个粒子。然后使用定时器将其逐步显示出来即可

<canvas id="drawing1" style="border:1px solid black"></canvas>
<button id="btn">开始显字</button>
<script>
var drawing1 = document.getElementById('drawing1');
if(drawing1.getContext){
 var cxt = drawing1.getContext('2d');
 var W = drawing1.width = 200;
 var H = drawing1.height = 200;
 var str = '小火柴';
 cxt.textBaseline = 'top';
 var sh = 60;
 cxt.font = sh + 'px 宋体'
 var sw = cxt.measureText(str).width;
 if(sw > W){
  sw = W;
 }
 cxt.fillText(str,(W - sw)/2,(H - sh)/2,W);
 //获取imageData
 var imageData = cxt.getImageData(0,0,W,H); 
 cxt.clearRect(0,0,W,H);
 //获得10组粒子
 var imageDataArr = [];
 var n = 10;
 var index = 0;
 for(var i = n; i > 0; i--){
 imageDataArr.push(setData(imageData,i));
 }
 var oTimer = null;
 btn.onclick = function(){
 clearTimeout(oTimer);
 showData();
 }
 function showData(){
 oTimer = setTimeout(function(){
  cxt.clearRect(0,0,W,H);
  //写入drawing1中 
  cxt.putImageData(imageDataArr[index++],0,0); 
  //迭代函数  
  showData();  
  if(index == 10){
　　  index = 0;
  clearTimeout(oTimer);
  }  

 },100);  
 } 
 function setData(imageData,n,m){
 //从imageData对象中取得粒子，并存储到dots数组中
 var dots = [];
 for(var i = 0; i < W; i+=n){
  for(var j = 0; j < H ;j+=n){
  //data值中的红色值
  var k = 4*(i + j*W);
  //data值中的透明度
  if(imageData.data[k+3] > 0){
   //将透明度大于0的data中的红色值保存到dots数组中
   dots.push(k);
  }
  }
 } 
 //筛选粒子，仅保存m个到newDots数组中。如果不传入m，则不进行筛选
 var newDots = [];
 if(m && (dots.length > m)){
  for(var i = 0; i < m; i++){
  newDots.push(Number(dots.splice(Math.floor(Math.random()*dots.length),1)));
  }
 }else{
  newDots = dots;
 } 
 //新建一个imageData，并将筛选后的粒子信息保存到新建的imageData中
 var oNewImage = cxt.createImageData(W,H);
 for(var i = 0; i < newDots.length; i++){
  oNewImage.data[newDots[i]+0] = imageData.data[newDots[i]+0];
  oNewImage.data[newDots[i]+1] = imageData.data[newDots[i]+1];
  oNewImage.data[newDots[i]+2] = imageData.data[newDots[i]+2];
  oNewImage.data[newDots[i]+3] = imageData.data[newDots[i]+3];
 }
 return oNewImage;
 }
}
</script>

【随机像素显字】

随机像素显字的原理类似，保存多个不同数量的随机像素的数组即可

<canvas id="drawing1" style="border:1px solid black"></canvas>
<button id="btn">开始显字</button>
<script>
var drawing1 = document.getElementById('drawing1');
if(drawing1.getContext){
 var cxt = drawing1.getContext('2d');
 var W = drawing1.width = 200;
 var H = drawing1.height = 200;
 var str = '小火柴';
 cxt.textBaseline = 'top';
 var sh = 60;
 cxt.font = sh + 'px 宋体'
 var sw = cxt.measureText(str).width;
 if(sw > W){
  sw = W;
 }
 cxt.fillText(str,(W - sw)/2,(H - sh)/2,W);
 //获取imageData
 var imageData = cxt.getImageData(0,0,W,H); 
 cxt.clearRect(0,0,W,H);
 //获得10组粒子
 var imageDataArr = [];
 var n = 10;
 var index = 0;
 for(var i = n; i > 0; i--){
 imageDataArr.push(setData(imageData,1,i));
 }
 var oTimer = null;
 btn.onclick = function(){
 clearTimeout(oTimer);
 showData();
 }
 function showData(){
 oTimer = setTimeout(function(){
  cxt.clearRect(0,0,W,H);
  //写入drawing1中 
  cxt.putImageData(imageDataArr[index++],0,0); 
  //迭代函数  
  showData();  
  if(index == 10){
  clearTimeout(oTimer);
  index = 0;
  }  
 },100);  
 } 
 function setData(imageData,n,m){
 //从imageData对象中取得粒子，并存储到dots数组中
 var dots = [];
 for(var i = 0; i < W; i+=n){
  for(var j = 0; j < H ;j+=n){
  //data值中的红色值
  var k = 4*(i + j*W);
  //data值中的透明度
  if(imageData.data[k+3] > 0){
   //将透明度大于0的data中的红色值保存到dots数组中
   dots.push(k);
  }
  }
 } 
 //筛选粒子，仅保存dots.length/m个到newDots数组中
 var newDots = [];
 var len = Math.floor(dots.length/m);
 for(var i = 0; i < len; i++){
  newDots.push(Number(dots.splice(Math.floor(Math.random()*dots.length),1)));
 }
 //新建一个imageData，并将筛选后的粒子信息保存到新建的imageData中
 var oNewImage = cxt.createImageData(W,H);
 for(var i = 0; i < newDots.length; i++){
  oNewImage.data[newDots[i]+0] = imageData.data[newDots[i]+0];
  oNewImage.data[newDots[i]+1] = imageData.data[newDots[i]+1];
  oNewImage.data[newDots[i]+2] = imageData.data[newDots[i]+2];
  oNewImage.data[newDots[i]+3] = imageData.data[newDots[i]+3];
 }
 return oNewImage;
 }
}
</script>

粒子动画

粒子动画并不是粒子在做动画，而是通过getImageData()方法获得粒子的随机坐标和最终坐标后，通过fillRect()方法绘制的小方块在做运动。使用定时器，不断的绘制坐标变化的小方块，以此来产生运动的效果

【随机位置】

<canvas id="drawing1" style="border:1px solid black"></canvas>
<button id="btn1">开始显字</button>
<button id="btn2">重新混乱</button>
<script>
var drawing1 = document.getElementById('drawing1');
if(drawing1.getContext){
 var cxt = drawing1.getContext('2d');
 var W = drawing1.width = 200;
 var H = drawing1.height = 200;
 var str = '小火柴';
 cxt.textBaseline = 'top';
 var sh = 60;
 cxt.font = sh + 'px 宋体'
 var sw = cxt.measureText(str).width;
 if(sw > W){
  sw = W;
 }
 cxt.fillText(str,(W - sw)/2,(H - sh)/2,W);
 //获取imageData
 var imageData = cxt.getImageData(0,0,W,H); 
 cxt.clearRect(0,0,W,H);
 function setData(imageData,n,m){
 //从imageData对象中取得粒子，并存储到dots数组中
 var dots = [];
 //dots的索引
 var index = 0;
 for(var i = 0; i < W; i+=n){
  for(var j = 0; j < H ;j+=n){
  //data值中的红色值
  var k = 4*(i + j*W);
  //data值中的透明度
  if(imageData.data[k+3] > 0){
   //将透明度大于0的data中的红色值保存到dots数组中
   dots.push(k);
   dots[index++] = {
   'index':index,
   'x':i,
   'y':j,
   'red':k,
   'randomX':Math.random()*W,
   'randomY':Math.random()*H,
   }
  }
  }
 } 
 //筛选粒子，仅保存dots.length/m个到newDots数组中
 var newDots = [];
 var len = Math.floor(dots.length/m);
 for(var i = 0; i < len; i++){
  newDots.push(dots.splice(Math.floor(Math.random()*dots.length),1)[0]);
 }
 return newDots;
 }
 //获得粒子数组
 var dataArr = setData(imageData,1,1);
 var oTimer1 = null;
 var oTimer2 = null;
 btn1.onclick = function(){
 clearTimeout(oTimer1);
 showData(10);
 } 
 btn2.onclick = function(){
 clearTimeout(oTimer2);
 showRandom(10);
 } 
 function showData(n){
 oTimer1 = setTimeout(function(){
  cxt.clearRect(0,0,W,H);
  for(var i = 0; i < dataArr.length; i++){
  var temp = dataArr[i];
  var x0 = temp.randomX;
  var y0 = temp.randomY;
  var disX = temp.x - temp.randomX;
  var disY = temp.y - temp.randomY;
  cxt.fillRect(x0 + disX/n,y0 + disY/n,1,1); 
  } 
  showData(n-1); 
  if(n === 1){
  clearTimeout(oTimer1);
  }  
 },60); 
 } 
 function showRandom(n){
 oTimer2 = setTimeout(function fn(){
  cxt.clearRect(0,0,W,H);
  for(var i = 0; i < dataArr.length; i++){
  var temp = dataArr[i];
  var x0 = temp.x;
  var y0 = temp.y;
  var disX = temp.randomX - temp.x;
  var disY = temp.randomY - temp.y;
  cxt.fillRect(x0 + disX/n,y0 + disY/n,1,1);    
  }  
  showRandom(n-1); 
  if(n === 1){
  clearTimeout(oTimer2);
  }  
 },60); 
 } 
}
</script>

【飘入效果】　

飘入效果与随机显字的原理相似，不再赘述

<canvas id="drawing1" style="border:1px solid black"></canvas>
<button id="btn1">左上角飘入</button>
<script>
var drawing1 = document.getElementById('drawing1');
if(drawing1.getContext){
 var cxt = drawing1.getContext('2d');
 var W = drawing1.width = 200;
 var H = drawing1.height = 200;
 var str = '小火柴';
 cxt.textBaseline = 'top';
 var sh = 60;
 cxt.font = sh + 'px 宋体'
 var sw = cxt.measureText(str).width;
 if(sw > W){
  sw = W;
 }
 cxt.fillText(str,(W - sw)/2,(H - sh)/2,W);
 //获取imageData
 var imageData = cxt.getImageData(0,0,W,H); 
 cxt.clearRect(0,0,W,H);
 function setData(imageData,n,m){
 //从imageData对象中取得粒子，并存储到dots数组中
 var dots = [];
 //dots的索引
 var index = 0;
 for(var i = 0; i < W; i+=n){
  for(var j = 0; j < H ;j+=n){
  //data值中的红色值
  var k = 4*(i + j*W);
  //data值中的透明度
  if(imageData.data[k+3] > 0){
   //将透明度大于0的data中的红色值保存到dots数组中
   dots.push(k);
   dots[index++] = {
   'index':index,
   'x':i,
   'y':j,
   'red':k,
   'randomX':Math.random()*W,
   'randomY':Math.random()*H,
   }
  }
  }
 } 
 //筛选粒子，仅保存dots.length/m个到newDots数组中
 var newDots = [];
 var len = Math.floor(dots.length/m);
 for(var i = 0; i < len; i++){
  newDots.push(dots.splice(Math.floor(Math.random()*dots.length),1)[0]);
 }
 return newDots;
 }
 //获得粒子数组
 var dataArr = setData(imageData,1,1);
 var oTimer1 = null;
 btn1.onclick = function(){
 clearTimeout(oTimer1);
 showData(10);
 } 
 function showData(n){
 oTimer1 = setTimeout(function(){
  cxt.clearRect(0,0,W,H);
  for(var i = 0; i < dataArr.length; i++){
  var temp = dataArr[i];
  var x0 = 0;
  var y0 = 0;
  var disX = temp.x - 0;
  var disY = temp.y - 0;
  cxt.fillRect(x0 + disX/n,y0 + disY/n,1,1); 
  } 
  showData(n-1); 
  if(n === 1){
  clearTimeout(oTimer1);
  }  
 },60); 
 } 
}
</script>

鼠标交互

一般地，粒子的鼠标交互都与isPointInPath(x,y)方法有关

【移入变色】

当鼠标接近粒子时，该粒子变红。实现原理很简单。鼠标移动时，通过isPointInPath(x,y)方法检测，有哪些粒子处于当前指针范围内。如果处于，绘制1像素的红色矩形即可

<canvas id="drawing1" style="border:1px solid black"></canvas>
<script>
var drawing1 = document.getElementById('drawing1');
if(drawing1.getContext){
 var cxt = drawing1.getContext('2d');
 var W = drawing1.width = 200;
 var H = drawing1.height = 200;
 var str = '小火柴';
 cxt.textBaseline = 'top';
 var sh = 60;
 cxt.font = sh + 'px 宋体'
 var sw = cxt.measureText(str).width;
 if(sw > W){
  sw = W;
 }
 cxt.fillText(str,(W - sw)/2,(H - sh)/2,W);
 //获取imageData
 var imageData = cxt.getImageData(0,0,W,H); 
 function setData(imageData,n,m){
 //从imageData对象中取得粒子，并存储到dots数组中
 var dots = [];
 //dots的索引
 var index = 0;
 for(var i = 0; i < W; i+=n){
  for(var j = 0; j < H ;j+=n){
  //data值中的红色值
  var k = 4*(i + j*W);
  //data值中的透明度
  if(imageData.data[k+3] > 0){
   //将透明度大于0的data中的红色值保存到dots数组中
   dots.push(k);
   dots[index++] = {
   'index':index,
   'x':i,
   'y':j,
   'red':k,
   'randomX':Math.random()*W,
   'randomY':Math.random()*H,
   }
  }
  }
 } 
 //筛选粒子，仅保存dots.length/m个到newDots数组中
 var newDots = [];
 var len = Math.floor(dots.length/m);
 for(var i = 0; i < len; i++){
  newDots.push(dots.splice(Math.floor(Math.random()*dots.length),1)[0]);
 }
 return newDots;
 }
 //获得粒子数组
 var dataArr = setData(imageData,1,1); 
 //鼠标移动时，当粒子距离鼠标指针小于10时，则进行相关操作
 drawing1.onmousemove = function(e){
 e = e || event;
 var x = e.clientX - drawing1.getBoundingClientRect().left;
 var y = e.clientY - drawing1.getBoundingClientRect().top;
 cxt.beginPath();
 cxt.arc(x,y,10,0,Math.PI*2);
 for(var i = 0; i < dataArr.length; i++){
  var temp = dataArr[i];
  if(cxt.isPointInPath(temp.x,temp.y)){ 
  cxt.fillStyle = 'red';
  cxt.fillRect(temp.x,temp.y,1,1);
  }  
 } 
 }
}
</script>

【远离鼠标】

鼠标点击时，以鼠标指针为圆心的一定范围内的粒子需要移动到该范围以外。一段时间后，粒子回到原始位置

实现原理并不复杂，使用isPointInPath(x,y)方法即可，如果粒子处于当前路径中，则沿着鼠标指针与粒子坐标组成的直线方向，移动到路径的边缘

<canvas id="drawing1" style="border:1px solid black"></canvas>
<script>
var drawing1 = document.getElementById('drawing1');
if(drawing1.getContext){
 var cxt = drawing1.getContext('2d');
 var W = drawing1.width = 200;
 var H = drawing1.height = 200;
 var str = '小火柴';
 cxt.textBaseline = 'top';
 var sh = 60;
 cxt.font = sh + 'px 宋体'
 var sw = cxt.measureText(str).width;
 if(sw > W){
  sw = W;
 }
 //渲染文字
 cxt.fillText(str,(W - sw)/2,(H - sh)/2,W);
 //获取imageData
 var imageData = cxt.getImageData(0,0,W,H); 
 cxt.clearRect(0,0,W,H);
 function setData(imageData,n,m){
 //从imageData对象中取得粒子，并存储到dots数组中
 var dots = [];
 //dots的索引
 var index = 0;
 for(var i = 0; i < W; i+=n){
  for(var j = 0; j < H ;j+=n){
  //data值中的红色值
  var k = 4*(i + j*W);
  //data值中的透明度
  if(imageData.data[k+3] > 0){
   //将透明度大于0的data中的红色值保存到dots数组中
   dots.push(k);
   dots[index++] = {
   'index':index,
   'x':i,
   'y':j,
   'red':k,
   'randomX':Math.random()*W,
   'randomY':Math.random()*H,
   'mark':false
   }
  }
  }
 } 
 //筛选粒子，仅保存dots.length/m个到newDots数组中
 var newDots = [];
 var len = Math.floor(dots.length/m);
 for(var i = 0; i < len; i++){
  newDots.push(dots.splice(Math.floor(Math.random()*dots.length),1)[0]);
 }
 return newDots;
 }
 //获得粒子数组
 var dataArr = setData(imageData,2,1); 
 //将筛选后的粒子信息保存到新建的imageData中
 var oNewImage = cxt.createImageData(W,H);
 for(var i = 0; i < dataArr.length; i++){
 for(var j = 0; j < 4; j++){
  oNewImage.data[dataArr[i].red+j] = imageData.data[dataArr[i].red+j];
 }
 } 
 //写入canvas中
 cxt.putImageData(oNewImage,0,0);
 //设置鼠标检测半径为r
 var r = 20;
 //鼠标移动时，当粒子距离鼠标指针小于20时，则进行相关操作
 drawing1.onmousedown = function(e){
 e = e || event;
 var x = e.clientX - drawing1.getBoundingClientRect().left;
 var y = e.clientY - drawing1.getBoundingClientRect().top;
 cxt.beginPath();
 cxt.arc(x,y,r,0,Math.PI*2);
 for(var i = 0; i < dataArr.length; i++){
  var temp = dataArr[i];
  if(cxt.isPointInPath(temp.x,temp.y)){ 
  temp.mark = true;
  var angle = Math.atan2((temp.y - y),(temp.x - x));
  temp.endX = x - r*Math.cos(angle);
  temp.endY = y - r*Math.sin(angle);
  var disX = temp.x - temp.endX;
  var disY = temp.y - temp.endY;
  cxt.fillStyle = '#fff';
  cxt.fillRect(temp.x,temp.y,1,1);
  cxt.fillStyle = '#000';
  cxt.fillRect(temp.endX,temp.endY,1,1); 
  dataRecovery(10);
  }else{
  temp.mark = false;
  }  
 }
 var oTimer = null;
 function dataRecovery(n){
  clearTimeout(oTimer);
  oTimer = setTimeout(function(){
  cxt.clearRect(0,0,W,H);
  for(var i = 0; i < dataArr.length; i++){
   var temp = dataArr[i];
   if(temp.mark){
   var x0 = temp.endX;
   var y0 = temp.endY;
   var disX = temp.x - x0;
   var disY = temp.y - y0;
   cxt.fillRect(x0 + disX/n,y0 + disY/n,1,1); 
   }else{
   cxt.fillRect(temp.x,temp.y,1,1);
   }
  } 
  dataRecovery(n-1); 
  if(n === 1){
   clearTimeout(oTimer);
  }  
  },17);
 } 
 } 
}
</script>

综合实例

下面将上面的效果制作为一个可编辑的综合实例

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>Document</title>
</head>
<body>
<canvas id="drawing1" style="border:1px solid black"></canvas>
<br>
<p style="margin-bottom:10px">
 <span>粒子设置：</span>
 <input type="text" id="textValue" value="小火柴的蓝色理想"> 
 <button id="btnSetText">文字设置确认</button>
 <button id="btnchoose2">按序筛选</button>
 <button id="btnchoose3">随机筛选</button>
 <button id="btnchoose1">不筛选</button> 
</p>
<p style="margin-bottom:10px">
 <span>粒子效果：</span>
 <button id="btn1">按序显字</button>
 <button id="btn2">随机显字</button> 
 <button id="btn3">混乱聚合</button>
 <button id="btn4">重新混乱</button>
</p>
<p>
 <span>鼠标效果：</span>
 <span>1、鼠标移到文字上时，文字颜色变红；</span>
 <span>2、鼠标在文字上点击时，粒子远离鼠标指针</span>
</p>
<script>
if(drawing1.getContext){
 var cxt = drawing1.getContext('2d');
 var W = drawing1.width = 300;
 var H = drawing1.height = 200; 
 var imageData;
 var dataArr;
 btnSetText.onclick = function(){
 fnSetText(textValue.value);
 } 
 function fnSetText(str){
 cxt.clearRect(0,0,W,H);
 cxt.textBaseline = 'top';
 var sh = 60;
 cxt.font = sh + 'px 宋体'
 var sw = cxt.measureText(str).width;
 if(sw > W){
  sw = W;
 }
 cxt.fillText(str,(W - sw)/2,(H - sh)/2,W); 
 imageData = cxt.getImageData(0,0,W,H); 
 dataArr = setData(imageData,1,1); 
 }
 fnSetText('小火柴');
 btnchoose1.onclick = function(){
 dataArr = setData(imageData,1,1);
 saveData(dataArr); 
 }
 btnchoose2.onclick = function(){
 dataArr = setData(imageData,2,1);
 saveData(dataArr); 
 }
 btnchoose3.onclick = function(){
 dataArr = setData(imageData,1,2);
 saveData(dataArr); 
 } 
 //筛选粒子
 function setData(imageData,n,m){
 //从imageData对象中取得粒子，并存储到dots数组中
 var dots = [];
 //dots的索引
 var index = 0;
 for(var i = 0; i < W; i+=n){
  for(var j = 0; j < H ;j+=n){
  //data值中的红色值
  var k = 4*(i + j*W);
  //data值中的透明度
  if(imageData.data[k+3] > 0){
   //将透明度大于0的data中的红色值保存到dots数组中
   dots.push(k);
   dots[index++] = {
   'index':index,
   'x':i,
   'y':j,
   'red':k,
   'green':k+1,
   'blue':k+2,
   'randomX':Math.random()*W,
   'randomY':Math.random()*H,
   'mark':false
   }
  }
  }
 } 
 //筛选粒子，仅保存dots.length/m个到newDots数组中
 var newDots = [];
 var len = Math.floor(dots.length/m);
 for(var i = 0; i < len; i++){
  newDots.push(dots.splice(Math.floor(Math.random()*dots.length),1)[0]);
 }
 return newDots;
 }
 function saveData(dataArr){
 //将筛选后的粒子信息保存到新建的imageData中
 var oNewImage = cxt.createImageData(W,H);
 for(var i = 0; i < dataArr.length; i++){
  for(var j = 0; j < 4; j++){
  oNewImage.data[dataArr[i].red+j] = imageData.data[dataArr[i].red+j];
  }
 }
 //写入canvas中
 cxt.putImageData(oNewImage,0,0);  
 }
 //显示粒子
 function showData(arr,oTimer,index,n){
 oTimer = setTimeout(function(){
  cxt.clearRect(0,0,W,H);
  //写入canvas中 
  saveData(arr[index++]); 
  if(index == n){
  clearTimeout(oTimer);
  }else{
  //迭代函数  
  showData(arr,oTimer,index,n);   
  }      
 },60);  
 } 
 //重新混乱
 function showDataToRandom(dataArr,oTimer,n){
 oTimer = setTimeout(function fn(){
  cxt.clearRect(0,0,W,H);
  for(var i = 0; i < dataArr.length; i++){
  var temp = dataArr[i];
  var x0 = temp.x;
  var y0 = temp.y;
  var disX = temp.randomX - temp.x;
  var disY = temp.randomY - temp.y;
  cxt.fillRect(x0 + disX/n,y0 + disY/n,1,1);    
  } 
  n--;
  if(n === 0){
  clearTimeout(oTimer);
  }else{
  showDataToRandom(dataArr,oTimer,n); 
  }    
 },60); 
 } 
 //混乱聚合
 function showRandomToData(dataArr,oTimer,n){
 oTimer = setTimeout(function(){
  cxt.clearRect(0,0,W,H);
  for(var i = 0; i < dataArr.length; i++){
  var temp = dataArr[i];
  var x0 = temp.randomX;
  var y0 = temp.randomY;
  var disX = temp.x - temp.randomX;
  var disY = temp.y - temp.randomY;
  cxt.fillRect(x0 + disX/n,y0 + disY/n,1,1); 
  } 
  n--;
  if(n === 0){
  clearTimeout(oTimer);
  }else{
  showRandomToData(dataArr,oTimer,n); 
  }  
 },60); 
 }
 btn1.onclick = function(){
 btn1.arr = [];
 for(var i = 10; i > 1; i--){
  btn1.arr.push(setData(imageData,i,1));
 }
 showData(btn1.arr,btn1.oTimer,0,9);
 }
 btn2.onclick = function(){
 btn2.arr = [];
 for(var i = 10; i > 0; i--){
  btn2.arr.push(setData(imageData,2,i));
 }
 showData(btn2.arr,btn2.oTimer,0,10);
 } 
 btn3.onclick = function(){
 clearTimeout(btn3.oTimer);
 showRandomToData(dataArr,btn3.oTimer,10);
 }
 btn4.onclick = function(){
 clearTimeout(btn4.oTimer);
 showDataToRandom(dataArr,btn4.oTimer,10);
 } 
 //鼠标移动
 drawing1.onmousemove = function(e){
 e = e || event;
 var x = e.clientX - drawing1.getBoundingClientRect().left;
 var y = e.clientY - drawing1.getBoundingClientRect().top;
 cxt.beginPath();
 cxt.arc(x,y,10,0,Math.PI*2);
 for(var i = 0; i < dataArr.length; i++){
  var temp = dataArr[i];
  if(cxt.isPointInPath(temp.x,temp.y)){ 
  cxt.fillStyle = 'red';
  cxt.fillRect(temp.x,temp.y,1,1);
  }  
 }
 cxt.fillStyle = 'black'; 
 } 
 //鼠标点击
 drawing1.onmousedown = function(e){
 var r = 20;
 e = e || event;
 var x = e.clientX - drawing1.getBoundingClientRect().left;
 var y = e.clientY - drawing1.getBoundingClientRect().top;
 cxt.beginPath();
 cxt.arc(x,y,r,0,Math.PI*2);
 for(var i = 0; i < dataArr.length; i++){
  var temp = dataArr[i];
  if(cxt.isPointInPath(temp.x,temp.y)){ 
  temp.mark = true;
  var angle = Math.atan2((temp.y - y),(temp.x - x));
  temp.endX = x - r*Math.cos(angle);
  temp.endY = y - r*Math.sin(angle);
  var disX = temp.x - temp.endX;
  var disY = temp.y - temp.endY;
  cxt.fillStyle = '#fff';
  cxt.fillRect(temp.x,temp.y,1,1);
  cxt.fillStyle = '#f00';
  cxt.fillRect(temp.endX,temp.endY,1,1); 
  cxt.fillStyle="#000";
  dataRecovery(10);
  }else{
  temp.mark = false;
  }  
 }
 var oTimer = null;
 function dataRecovery(n){
  clearTimeout(oTimer);
  oTimer = setTimeout(function(){
  cxt.clearRect(0,0,W,H);
  for(var i = 0; i < dataArr.length; i++){
   var temp = dataArr[i];
   if(temp.mark){
   var x0 = temp.endX;
   var y0 = temp.endY;
   var disX = temp.x - x0;
   var disY = temp.y - y0;
   cxt.fillRect(x0 + disX/n,y0 + disY/n,1,1); 
   }else{
   cxt.fillRect(temp.x,temp.y,1,1);
   }
  } 
  dataRecovery(n-1); 
  if(n === 1){
   clearTimeout(oTimer);
  }  
  },17);
 } 
 } 
}
</script> 
</body>
</html>

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