dom提高分辨率轉canvas

dom 内容轉成canvas圖檔

依賴庫為html2canvas.js

由于手機的高分辨率，實際dom像素值會很小，解除的圖會發虛。

下面的代碼示範了如果将目前撐滿手機螢幕的dom元素放大2倍的分辨率（ipad為1.2倍）截出圖檔。

// source dom
  var $div = $('#shareImage');
  // target canvas
  var canvas = document.createElement('canvas');
  // 設定大小
  (function () {
    var scale = document.documentElement.clientWidth > 500 ? 1.2 : 2; // ipad or phone
    canvas.width = document.documentElement.clientWidth * scale;
    canvas.height = document.documentElement.clientHeight * scale;
    $div.width(canvas.width);
    $div.height(canvas.height);
    canvas.style.width = canvas.width + 'px';
    canvas.style.height = canvas.height + 'px';
  }());

  // 截圖
  html2canvas($div[0], {useCORS: true, canvas: canvas}).then(function (canvas) {
    console.log(canvas);
    var b64;
    try {
      b64 = canvas.toDataURL("image/png");
    } catch (err) {
      console.log(err);
      alert(err);
    }

    var img = new Image();
    img.src = b64;
    img.style.transform =  'scale(.5)';
    img.style.transformOrigin = 'left top';
    // target dom
    $('#thePng').prepend(img);
  });