js多张图片合成一张图，canvas(海报图，将二维码和背景图合并) -----vue

思路：vue中图片合并

　　首先准备好要合并的背景图，和请求后得到的二维码，

        canvas画图，将两张背景图和一张二维码用canvas画出来，

        将canvas再转为img

        注意canvas和图片的清晰图和图片的尺寸位置

       开始时canvas是隐藏的，两张背景图时显示的，当canvas画完后再转为img的时候，隐藏canvas和背景图，显示canvas转完的图片（也就是合并后的图片）

       这个适配方式可能有些瑕疵，所以会加了很多设备的判断

代码：

　　html

    

<div class="wap-poster" ref="imageWrapper" id="target" style=" 100%;">
    <canvas id="mycanvas" width="100%" height="100%" v-if="!infactQrCode" style="transform: scale(2);display: none"></canvas>
    <img  v-if="!infactQrCode"  src="../../../assets/imgs/posterbg.jpg" id="bgImg" alt="" style=" 100%;height:100%;">
    <img  v-if="!infactQrCode"  src="../../../assets/imgs/poster0.png" id="poster" alt="" style="position: absolute;">
    <img  v-if="!infactQrCode" id="qrcode" :src="qrCodeImg" alt="" style="position: absolute;top:247px; 140px;height: 140px;display: none">
    <img :src="infactQrCode" alt="" v-if="infactQrCode" style=" 100%;height:100%">
  </div>

      js<script>

  import html2canvas from  'html2canvas'
  export default {
    name: 'Poster',
    data() {
      return {
        qrCodeImg: "",
        infactQrCode:"",
      }
    },
    created(){
　　　　
      //二维码图片后台返回
      this.qrCodeImg="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1568183940&di=49b7187fdb7fee3e30c20583150f09b6&imgtype=jpg&er=1&src=http%3A%2F%2Fdown.admin5.com%2Fuploads%2Fallimg%2F170630%2F2160_170630144656_1.jpg";
    },
    methods: {
      drawImg(){
        let _self=this;
        var canvas = document.getElementById("mycanvas");
        var a = setInterval(() =>{
          // 重复获取
          canvas = document.getElementById("mycanvas");
          if(!canvas){
            return false
          } else {
            clearInterval(a);
            var context = canvas.getContext('2d');
            context.scale(2,2)     //图片清晰度解决办法，缩放2倍后，其他图片的宽高也要比ui设计的宽高乘以2
            var img = new Image();
            img.setAttribute('crossOrigin', 'anonymous');
            let bgUrl=document.getElementById("bgImg").src;
            img.src=bgUrl;
            img.onload = function(){
              if(img.complete){
　　　　　　　　　　//画第一张背景图，图片的宽高撑满整个屏幕　　
                context.drawImage(img,0,0,window.screeWidth,window.screeHeight);
                var img1 = new Image();
                let bgUrl1=document.getElementById("poster");
                img1.src=bgUrl1.src;
                img1.setAttribute('crossOrigin', 'anonymous');
                img1.onload = function(){
　　　　　　　　　　　　//img1为第2张背景图，开始画第2张图，图片的的定位，left为整个屏幕-ui设计图片的宽*图片的适配比例，最后除以2保证图片居中，
　　　　　　　　　　　　//图片的left  top   width  height都是这样计算的，

                  if(img1.complete){
                    var left =  (window.screeWidth-600*window.rateWidth)/2;
                    if(window.screeHeight>=812&&window.screeWidth<768){   //iphonx的计算方式
                      context.drawImage(img1,left,200*window.rateWidth,600*window.rateWidth,890*window.rateHeight);
                    }else{
                      context.drawImage(img1,left,110*window.rateWidth,600*window.rateWidth,1100*window.rateHeight);
                    }

                    var img2 = new Image();
                    img2.src=document.getElementById("qrcode").src;
                    img2.crossOrigin="*";
                    img2.onload = function(){
                      if(img2.complete){
　　　　　　　　　　　　　　//二维码图片的画图left top widht height
                        var left =  (window.screeWidth-290*window.rateWidth)/2;
                        if(window.screeWidth==600&&window.screeHeight==1024){   //ipad 
                          var left =  (window.screeWidth-240*window.rateWidth)/2;
                          context.drawImage(img2,left,460*window.rateHeight,240*window.rateWidth,240*window.rateWidth);
                          var image = new Image();
                          _self.infactQrCode=canvas.toDataURL("image/png");
                          return
                        }
                        if(window.screeWidth>=768&&window.screeHeight>812){   //其他设备
                          var left =  (window.screeWidth-230*window.rateWidth)/2;
                          context.drawImage(img2,left,430*window.rateHeight,230*window.rateWidth,230*window.rateWidth);
                        }else if (window.screeHeight>=812&&window.screeWidth<768){
                          context.drawImage(img2,left,410*window.rateHeight,290*window.rateWidth,290*window.rateWidth);
                        }else{
                          context.drawImage(img2,left,410*window.rateHeight,290*window.rateWidth,290*window.rateWidth);
                        }

　　　　　　　　　　　　　　//以上为2张背景图和一张二维码画合并后图画到canvas的结果，下面为将canvas转为图片的方法
                        var image = new Image();
                        _self.infactQrCode=canvas.toDataURL("image/png");
                      }
                    }
                  }
                }
              }
            }
          }
        },1)
      },
    },
    mounted(){
      this.drawImg();  //主要代码再该方法，将图片转为canvas，再将canvas转为图片，因为canvas是无法长按保存和识别二维码的
      const that = this;
      window.screeWidth=window.innerWidth || document.documentElement.clientWidth || document.body.clientWidth;
      window.screeHeight= window.innerHeight || document.documentElement.clientHeight || document.body.clientHeight;
      var left =  (window.screeWidth-140)/2;
      var qrcode=document.getElementById("qrcode");
      qrcode.style.left=left+'px';
      this.screeWidth=window.screeWidth;
      var canvas = document.getElementById("mycanvas");
      //适配
      window.rateWidth = window.screeWidth/ 750;
      window.rateHeight = window.screeHeight/1334;
      //清晰度解决办法
      canvas.setAttribute("width",window.screeWidth*2);
      canvas.setAttribute("height",window.screeHeight*2);





      let bgUrl1=document.getElementById("poster");
      var left =  (window.screeWidth-600*window.rateWidth)/2;
      if(window.screeHeight>=812&&window.screeWidth<768){
        bgUrl1.style.left=left+"px";
        bgUrl1.style.top=200*window.rateWidth+"px";
        bgUrl1.style.width=600*window.rateWidth+"px";
        bgUrl1.style.height=890*window.rateHeight+"px";
      }else{
        bgUrl1.style.left=left+"px";
        bgUrl1.style.top=110*window.rateWidth+"px";
        bgUrl1.style.width=600*window.rateWidth+"px";
        bgUrl1.style.height=1100*window.rateHeight+"px";
      }

    },
  }
</script>