选择较低分辨率时，iOS会旋转图像(HTML5输入文件)

function handleFileSelect(evt) {
  var files = evt.target.files;
  var file = files[0];
  
  var reader = new FileReader();
  reader.onload = function(e) {
    var img = new Image();
    img.onload = function() {
      var canvas = document.createElement('canvas');
      var ctx = canvas.getContext('2d');
      
      // 根据图像的方向进行旋转
      switch (getOrientation(file)) {
        case 3:
          canvas.width = img.height;
          canvas.height = img.width;
          ctx.rotate(180 * Math.PI / 180);
          ctx.drawImage(img, -img.width, -img.height);
          break;
        case 6:
          canvas.width = img.height;
          canvas.height = img.width;
          ctx.rotate(90 * Math.PI / 180);
          ctx.drawImage(img, 0, -img.height);
          break;
        case 8:
          canvas.width = img.height;
          canvas.height = img.width;
          ctx.rotate(270 * Math.PI / 180);
          ctx.drawImage(img, -img.width, 0);
          break;
        default:
          canvas.width = img.width;
          canvas.height = img.height;
          ctx.drawImage(img, 0, 0);
      }
      
      // 将旋转后的图像显示在页面上
      document.body.appendChild(canvas);
    };
    img.src = e.target.result;
  };
  reader.readAsDataURL(file);
}

function getOrientation(file) {
  var reader = new FileReader();
  reader.onload = function(e) {
    var view = new DataView(e.target.result);
    if (view.getUint16(0, false) != 0xFFD8) return -2;
    var length = view.byteLength, offset = 2;
    while (offset < length) {
      var marker = view.getUint16(offset, false);
      offset += 2;
      if (marker == 0xFFE1) {
        if (view.getUint32(offset += 2, false) != 0x45786966) return -1;
        var little = view.getUint16(offset += 6, false) == 0x4949;
        offset += view.getUint32(offset + 4, little);
        var tags = view.getUint16(offset, little);
        offset += 2;
        for (var i = 0; i < tags; i++)
          if (view.getUint16(offset + (i * 12), little) == 0x0112)
            return view.getUint16(offset + (i * 12) + 8, little);
      }
      else if ((marker & 0xFF00) != 0xFF00) break;
      else offset += view.getUint16(offset, false);
    }
    return -1;
  };
  reader.readAsArrayBuffer(file.slice(0, 64 * 1024));
}

document.getElementById('file-input').addEventListener('change', handleFileSelect, false);