JavaScript function reading dimensions of TIFF image. Helpful when you need such functionality in Google Apps Script.

tiff_dimensions.js

/**
 * Materials:
 * How to read only portion of bytes in a file on Google Drive:
 *   https://stackoverflow.com/questions/60181888/google-apps-script-get-range-of-bytes-from-binary-file
 * TIFF File structure: https://www.awaresystems.be/imaging/tiff/faq.html#q3
 */

const TIFF_IFD_FIELD_LENGTH_IN_BYTES = 12;
const NUMBER_OF_BYTES_TO_READ_FROM_FILE = 500000;

// https://www.awaresystems.be/imaging/tiff/tifftags/baseline.html
const TIFF_TAG_IMAGE_WIDTH = 256;
const TIFF_TAG_IMAGE_LENGTH = 257;
const TIFF_TAG_X_RESOLUTION = 282;
const TIFF_TAG_Y_RESOLUTION = 283;
const TIFF_TAG_RESOLUTION_UNIT = 296;

// fileId can be read from sharable URL to Google Drive file
function getTIFFDimensionsFromFileID(fileId) {
  const fileFirstBytes = readFileFirstBytes(fileId, NUMBER_OF_BYTES_TO_READ_FROM_FILE);
  return getTIFFDimenstionsFromBytes(fileFirstBytes);
}

function readFileFirstBytes(fileId, bytesCount) {
  const start = 0;
  const end = bytesCount;
  const url = "https://www.googleapis.com/drive/v3/files/" + fileId + "?alt=media";
  const params = {
    method: "get",
    headers: {
      Authorization: "Bearer " + ScriptApp.getOAuthToken(),
      Range: "bytes=" + start + "-" + end,
    },
  };
  return UrlFetchApp.fetch(url, params).getContent();
}

function getTIFFDimenstionsFromBytes(bytes) {
  const isLittleEndian = bytes[0] == 73 && bytes[1] == 73 ? true : false;
  const buffer = new ArrayBuffer(TIFF_IFD_FIELD_LENGTH_IN_BYTES);
  const view = new DataView(buffer);

  view.setInt8(0, bytes[2]);
  view.setInt8(1, bytes[3]);
  if (view.getInt16(0, isLittleEndian) != 42) { // check magic byte
    console.log('Not a TIFF file.');
    return;  
  }

  // Offset to first Image File Directory (IFD) is encoded in 4-7 bytes
  view.setInt8(0, bytes[4]);
  view.setInt8(1, bytes[5]);
  view.setInt8(2, bytes[6]);
  view.setInt8(3, bytes[7]);

  const firstIFDOffset = view.getInt32(0, isLittleEndian);
  view.setInt8(0, bytes[firstIFDOffset]);
  view.setInt8(1, bytes[firstIFDOffset + 1]);
  const tagsCount = view.getInt16(0, isLittleEndian);

  const imageDimenstions = {
    widthInPixels: -1,
    heightInPixels: -1,
    xResolution: {
      numerator: -1,
      denominator: -1,
    },
    yResolution: {
      numerator: -1,
      denominator: -1,
    },
    resolutionUnit: "inch"
  }

  const firstTagOffset = firstIFDOffset + 2;
  
  for (let i=0; i < tagsCount; i++) {
    readFileDimensionsFromTiffTag(firstTagOffset + 12 * i, bytes, view, isLittleEndian, imageDimenstions);
  }
  return imageDimenstions;
}

function readFileDimensionsFromTiffTag(offset, bytes, view, isLittleEndian, imageDimenstions) {
  for (let i = 0; i < TIFF_IFD_FIELD_LENGTH_IN_BYTES; i++) {
    view.setInt8(i, bytes[offset + i]);
  }
  const tagCode = view.getInt16(0, isLittleEndian);
  const tagDatatype = view.getInt16(2, isLittleEndian);
  let tagData = -1;
  let tagDataNumerator = -1;
  let tagDataDenominator = -1;
  let valueOffset = -1;

  switch (tagDatatype) {
    case 3:
      // read 16 bits, 2 bytes
      tagData = view.getInt16(8, isLittleEndian);
      break;
    case 4:
      // read 32 bits, 4 bytes
      tagData = view.getInt32(8, isLittleEndian);
      break;
    case 5:
      valueOffset = view.getInt32(8, isLittleEndian);
      for (let i = 0; i < 8; i++) {
        view.setInt8(i, bytes[valueOffset + i]);
      }
      tagDataNumerator = view.getInt32(0, isLittleEndian);
      tagDataDenominator = view.getInt32(4, isLittleEndian);
      break;
  }

  switch (tagCode) {
    case TIFF_TAG_IMAGE_WIDTH:
      imageDimenstions.widthInPixels = tagData;
      break;
    case TIFF_TAG_IMAGE_LENGTH:
      imageDimenstions.heightInPixels = tagData;
      break;
    case TIFF_TAG_X_RESOLUTION:
      imageDimenstions.xResolution.numerator = tagDataNumerator;
      imageDimenstions.xResolution.denominator = tagDataDenominator;
      break;
    case TIFF_TAG_Y_RESOLUTION:
      imageDimenstions.yResolution.numerator = tagDataNumerator;
      imageDimenstions.yResolution.denominator = tagDataDenominator;
      break;
    case TIFF_TAG_RESOLUTION_UNIT:
      if (tagData == 1) {
        imageDimenstions.resolutionUnit = "none";
      } else if (tagData == 2) {
        imageDimenstions.resolutionUnit = "inch";
      } else if (tagData == 3) {
        imageDimenstions.resolutionUnit = "cm";
      }
      break;
  }
}