canvas图片截取并下载保存```html截取该区域 ```

<!DOCTYPE html> 
<html> 
<head lang="en"> 
  <meta charset="UTF-8"> 
  <title>canvas-图像截取</title> 
  <style> 
    canvas{ 
      border: 1px dashed red; 
      float: left; 
      position: relative; 
 
    } 
  </style> 
  </head> 
<body> 
  <div id="cantox" style="position: relative"> 
    <canvas id="oldcanvas" width="500px" height="300px"> 
    </canvas> 
    <div id="cliptox" style="position: absolute;display:none"></div> 
  </div> 
  <button id ="btnclip" style="float: left">截取该区域</button> 
  <canvas id="nowcanvas" width="500px" height="300px" > 
  </canvas> 
 
  <script> 
    //获取div元素 
    var cantox = document.getElementById("cantox"); 
    var cliptox = document.getElementById("cliptox"); 
    var btnclip = document.getElementById("btnclip"); 
    //获取到canvas元素 
    var oldcanvas = document.getElementById("oldcanvas"); 
    var nowcanvas = document.getElementById("nowcanvas"); 
    //获取canvas中的画图环境 
    var oldcontext = oldcanvas.getContext('2d'); 
    var nowcontext = nowcanvas.getContext('2d'); 
 
    var img = new Image(); 
    img.src = "https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1600346292409&di=4f1bb5197032c4777a2864e0a87a24c4&imgtype=0&src=http%3A%2F%2Fa3.att.hudong.com%2F14%2F75%2F01300000164186121366756803686.jpg"; 
 
    //加载图像到canvas画布中 
    window.onload = function (){ 
      oldcontext.drawImage(img,0,0,oldcanvas.width,oldcanvas.height); 
    } 
    var startPoint;//截取图像开始坐标 
    var endPoint;//截图图像的结束坐标 
    var w; //截取图像的宽度 
    var h; //截取图像的高度 
    var flag = false; //用于判断移动事件事物控制 
    //鼠标按下事件 
    cantox.onmousedown = function (e){ 
      flag = true; 
      cliptox.style.display = 'block'; 
      startPoint = windowToCanvas(oldcanvas, e.clientX, e.clientY); 
      cliptox.style.left = startPoint.x+'px'; 
      cliptox.style.top = startPoint.y+'px'; 
    } 
 
    //鼠标移动事件 
    cantox.onmousemove = function (e){ 
      if(flag){ 
        cliptox.style.background = 'rgba(0,0,0,0.5)'; 
        endPoint = windowToCanvas(oldcanvas, e.clientX, e.clientY); 
        w = endPoint.x - startPoint.x; 
        h = endPoint.y - startPoint.y; 
        cliptox.style.width = w +'px'; 
        cliptox.style.height = h+'px'; 
      } 
    } 
    //鼠标释放事件 
    cantox.onmouseup = function (e){ 
      flag = false; 
    } 
 
    //按钮截取事件 
    btnclip.onclick = function (){ 
      imgCut(nowcontext,img,oldcanvas.width,oldcanvas.height,startPoint.x,startPoint.y,w,h); 
    } 
 
    /* 
    * 图像截取函数 
    * context:绘制环境对象 
    * image：图像对象 
    * imgElementW：图像显示的宽度 
    * imgElementH：图像显示的高度 
    * sx:截取图像的开始X坐标 
    * sy:截取图像的开始Y坐标 
    * w:截取图像的宽度 
    * h:截取图像的高度 
    * */ 
    function imgCut(context,image,imgElementW,imgElementH,sx,sy,w,h){ 
      //清理画布，便于重新绘制 
      context.clearRect(0,0,imgElementW,imgElementH); 
      //计算 ：比例 = 原图像/显示图像 
      var ratioW = image.width/imgElementW; 
      var ratioH = image.height/imgElementH; 
      //根据截取图像的所占位置及大小计算出在原图所占的位置及大小 
      //.drawImage(图像对象,原图像截取的起始X坐标,原图像截取的起始Y坐标,原图像截取的宽度,原图像截取的高度， 
      // 绘制图像的起始X坐标,绘制图像的起始Y坐标,绘制图像所需要的宽度,绘制图像所需要的高度); 
        context.drawImage(image,ratioW*sx,ratioH*sy,ratioW*w,ratioH*h,0,0,w,h);
	    console.log(Canvas2Image.saveAsImage(nowcanvas, w, h)); 
	    var data = nowcanvas.toDataURL();//转换成url文件
		//下载图片
        var oA = document.createElement("a");
            oA.download = '';// 设置下载的文件名，默认是'下载'
            oA.href = data;
            document.body.appendChild(oA);
            oA.click();
           oA.remove(); // 下载之后把创建的元素删除
    } 
 
    /* 
     * 坐标转换：将window中的坐标转换到元素盒子中的坐标，并返回(x,y)坐标 
     * element：canvas元素对象 
     * x:鼠标在当前窗口X坐标值 
     * y:鼠标在当前窗口Y坐标值 
     * */ 
    function windowToCanvas(element,x,y){ 
      //获取当前鼠标在window中的坐标值 
      // alert(event.clientX+"-------"+event.clientY); 
      //获取元素的坐标属性 
      var box = element.getBoundingClientRect(); 
      var bx = x - box.left; 
      var by = y - box.top; 
      return {x:bx,y:by}; 
    } 
  </script> 
</body> 
</html>