logseq/src/main/frontend/exif.js

// copied from https://stackoverflow.com/questions/7584794/accessing-jpeg-exif-rotation-data-in-javascript-on-the-client-side

function objectURLToBlob(url, callback) {
  var http = new XMLHttpRequest();
  http.open("GET", url, true);
  http.responseType = "blob";
  http.onload = function(e) {
    if (this.status == 200 || this.status === 0) {
      callback(this.response);
    }
  };
  http.send();
}

export var getEXIFOrientation = function (img, callback) {
  var reader = new FileReader();
  reader.onload = e => {
    var view = new DataView(e.target.result)

    if (view.getUint16(0, false) !== 0xFFD8) {
      return callback(-2)
    }
    var length = view.byteLength
    var offset = 2
    while (offset < length) {
      var marker = view.getUint16(offset, false)
      offset += 2
      if (marker === 0xFFE1) {
        if (view.getUint32(offset += 2, false) !== 0x45786966) {
          return callback(-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) {
            var o = view.getUint16(offset + (i * 12) + 8, little);
            return callback(o)
          }
        }
      } else if ((marker & 0xFF00) !== 0xFF00) {
        break
      } else {
        offset += view.getUint16(offset, false)
      }
    }
    return callback(-1)
  };

  objectURLToBlob(img.src, function (blob) {
    reader.readAsArrayBuffer(blob.slice(0, 65536));
  });
}