国庆节到了,利用JS实现一个生成国庆风头像的小工具 详解实现过程

<div class="wrapper">
    <!-- 选择框 -->
    <div class="main-box">
      <a class="prev" onClick='changeHat()'></a>
      <div class="main-img">
        <div id="content">
          <canvas id='cvs'></canvas>
        </div>
      </div>
      <a class="next" onClick='changeHat()'></a>
    </div>
    <!-- 导出图 -->
    <img id='export-img' alt='国庆专属头像' src='' crossorigin="anonymous"/>
    <!-- 操作按钮 -->
    <div class="operation-btns">
      <a class="upload-btn">
        <input id='upload' type='file' onchange='viewer()' style='opacity: 0;'/>
      </a>
      <a class="export-btn" onClick='exportFunc()'></a>
    </div>
  </div>
 <!-- 模板 -->
  <div style='display: none'>
    <img id='img' src='' alt='' />
    <img class='hide' id='hat0' src='img/1.png' />
    <img class='hide' id='hat1' src='img/2.png' />
    <img class='hide' id='hat2' src='img/3.png' />
    <img class='hide' id='hat3' src='img/4.png' />
    <img class='hide' id='hat4' src='img/5.png' />
    <img class='hide' id='hat5' src='img/6.png' />
    <img class='hide' id='hat6' src='img/7.png' />
  </div>

body,
html {
    min-height: 100%;
    width: 100%;
    user-select: none;
    font-size: 18px;
}

.wrapper {
    width: 100%;
    height: 100%;
    max-width: 620px;
    max-height: 800px ;
    margin: 0 auto;
    background-image: url('../img/bg.png');
    background-repeat: no-repeat;
    background-size: 100% 100%;
    padding-top: 13em;
}

#export-img {
    display:none;
    margin:0 auto;
    width:250px;
    height:250px;
}

.main-box {
    display: flex;
    align-items: center;
    justify-content: center;
}

.main-box .next,
.main-box .prev {
    background-image: url('../img/next.png');
    background-size: contain;
    border-radius: 50%;
    width: 2.275rem;
    height: 2.275rem
}

.main-box .prev {
    transform: rotate(180deg)
}

.main-box .main-img {
    margin: 0 .75rem;
    background: #fff;
    border: .25rem solid #fbe6b5;
    border-radius: .75rem;
    font-size: 0
}

#content {
    border-radius: .5rem;
    position: relative;
    width: 9.5rem;
    height: 9.5rem;
    margin-left: 50%;
    transform: translateX(-50%);
    overflow: hidden
}

.operation-btns {
    display: flex;
    align-items: center;
    justify-content: center;
    flex-direction: column;
    margin-top: .75rem
}

.operation-btns .upload-btn {
    width: 11.6rem;
    height: 3.6rem;
    background-size: 100% 100%;
    background-image: url('../img/upload.png')
}

.operation-btns .export-btn {
    display: none;
    width: 11.6rem;
    height: 3.75rem;
    background-size: 100% 100%;
    background-image: url('../img/export.png')
}

let canvasFabric;   // 画布实例
let hat = "hat0";  // 当前的模板class
let hatInstance;  // 模板图层实例
const screenWidth = document.getElementById("content").scrollHeight;  // 内容框的高度

function viewer() {
  // 获取上传的图片文件
  const file = document.getElementById("upload").files[0];
  // 获取需要展示图片的标签
  const img = document.getElementById("img");
  // 创建文件读取文件对象
  const reader = new FileReader();
  if (file) {
    // 将文件转化为Base64
    reader.readAsDataURL(file);
    // 当文件读取成功之后触发
    reader.onload = () => {
      // 将base64的url赋值给要展示图片的标签
      img.src = reader.result;
      // 图片加载完成触发
      img.onload = () => img2Cvs(img);
    }
  } else {
    img.src = ""
  }
}

npm install fabric --save

<script src="http://cdnjs.cloudflare.com/ajax/libs/fabric.js/2.4.6/fabric.min.js"></script>

function img2Cvs(img) {
   // 获取并展示canvas画布，并将画布的大小设置为图片的大小
    const cvs = document.getElementById("cvs");
    cvs.width = img.width;
    cvs.height = img.height;
    cvs.style.display = "block";
   
   // 创建一个画布，并设置其位置和背景图
    canvasFabric = new fabric.Canvas("cvs", {
      width: screenWidth,
      height: screenWidth,
      backgroundImage: new fabric.Image(img, {
        scaleX: screenWidth / img.width,
        scaleY: screenWidth / img.height
      })
    });
   // 切换模板
    changeHat();
  // 隐藏上传图片按钮，展示下载图片按钮
    document.getElementsByClassName("upload-btn")[0].style.display = "none";
    document.getElementsByClassName("export-btn")[0].style.display = "block";
  }

function changeHat() {
   // 隐藏当前的模板
    document.getElementById(hat).style.display = "none";
   // 获取所有的模板
    const hats = document.getElementsByClassName("hide");
    hat = "hat" + (+hat.replace("hat", "") + 1) % hats.length;
   // 获取当前的模板并展示出来
    const hatImage = document.getElementById(hat);
    hatImage.style.display = "block";
   // 如果当前存在图层，就将其移除
    if (hatInstance) {
      canvasFabric.remove(hatInstance)
    }
   // 将当前的模板添加为图层对象
    hatInstance = new fabric.Image(hatImage, {
      top: 0,
      left: 0,
      scaleX: screenWidth / hatImage.width,
      scaleY: screenWidth / hatImage.height,
      cornerColor: "#0b3a42",
      cornerStrokeColor: "#fff",
      cornerStyle: "circle",
      transparentCorners: false,
      rotatingPointOffset: 30
    });
   // 将图层对象设置为不可拉伸
    hatInstance.setControlVisible({
       mt: false, 
       mb: false, 
       ml: false, 
       mr: false, 
       bl: false, 
       br: false, 
       tl: false, 
       tr: false, 
       mtr: false, 
    })
   // 将图层添加到画布中
    canvasFabric.add(hatInstance);
  }

function exportFunc() {
  // 隐藏选择框、上传下载按钮、canvas画布
  document.getElementsByClassName("main-box")[0].style.display = "none";
  document.getElementsByClassName("operation-btns")[0].style.display = "none";
  document.getElementById("cvs").style.display = "none";

  // 将画布生成URL，并赋值给对应标签进行展示
  const exportImage = document.getElementById("export-img");
  exportImage.style.display = "block";
  exportImage.src = canvasFabric.toDataURL({
    width: screenWidth,
    height: screenWidth
  });
  // 下载生成的图片
  window.confirm("是否下载头像") ? download(exportImage.src, "国庆风头像", 'image/png') : void 0
}