vue下canvas裁剪图片实例讲解

oMark2['canvas1'] = 'canvasImg2'; // 展示结果canvas id
oMark2['canvas2'] = 'dymCurrImg2'; //画布原始图片canvas id
oMark2['ImgUrl'] = carImgUrl;
oMark2['offsetLeft'] = location[0];
oMark2['offsetTop'] = location[2];
oMark2['offsetWidth'] = location[1] - location[0];
oMark2['offsetHeight'] = location[3] - location[2];

TailoringImg(oMark) {
    // 设置三个canvas 分别为 canvas1 ，canvas2，canvas3
    //  每个canvas的作用 canvas1原始图片画布（页面中隐藏）；
    //  canvas2原图和裁剪结果之间转换；
    //  canvas3裁剪之后的结果展示（页面中展示）；
    let res2 = oMark['ImgUrl'];
    let that = this;
    return new Promise(function (resolve, reject) {
     //图片剪切处理
     var canvas1 = document.getElementById(oMark['canvas2']);
     var canvas3 = document.getElementById(oMark['canvas1']);
     canvas1.height = 1080;
     canvas1.width = 1920;

     canvas3.height = 198;
     canvas3.width = 400;
     var cxt1 = canvas1.getContext("2d"); // getContext() 方法返回一个用于在画布上绘图的环境
     var img = new Image();
     img.crossOrigin = '';
     img.src = res2;
     var canvas2 = document.createElement("canvas"); // 创建虚拟画布环境
     var cxt2 = canvas2.getContext("2d");

     img.Onload= function () {
      // 计算图片缩放比例
      var Rwidth = canvas1.width / img.width;
      var Rheight = canvas1.height / img.height;

      cxt1.drawImage(img, 0, 0, canvas1.width, canvas1.height); // --第一步-- 原图绘制在画布上 drawImage方法在画布上绘制图像、画布或视频。也能够绘制图像的某些部分，以及/或者增加或减少图像的尺寸。
　　　　　　　　// 计算缩放好后的尺寸
      var srcX = oMark.offsetLeft * Rwidth;
      var srcY = oMark.offsetTop * Rheight;
      var sWidth = oMark.offsetWidth * Rwidth;
      var sHeight = oMark.offsetHeight * Rheight;
      var dataImg = cxt1.getImageData(srcX, srcY, sWidth, sHeight); // --第二步-- getImageData() 复制原图画布上指定矩形的像素数据

      canvas2.width = sWidth;
      canvas2.height = sHeight;
      cxt2.putImageData(dataImg, 0, 0, 0, 0, canvas2.width, canvas2.height); // --第三步-- 通过 putImageData() 将原图图像数据放到canvas2画布中
      var img2 = canvas2.toDataURL("image/png"); // --第四步-- toDataURL()将canvas2画布保存为图像

      var cxt3 = canvas3.getContext("2d"); // getContext() 方法返回一个用于在画布上绘图的环境
      var img3 = new Image(); // 创建图像对象
      img3.crossOrigin = ''; // 图像跨域设置
      img3.src = img2;    // 设置图像地址
      img3.Onload= function () { // onload内可以获取图像信息
       cxt3.drawImage(img3, 0, 0, canvas3.width, canvas3.height) // --第五步-- 将canvas2画布生成的图像放在canvas3画布中

      };
      resolve();
     }
    })
   },