How WiscScan injects OME-XML into an existing TIFF file

InjectOMEXML.cpp

strLen = OMEXMLMetadataString.size();

// Can't just overwrite the TIFFTAG_IMAGEDESCRIPTION since TIFFOPEN will just rewrite the tiff
// Need to do it like the 4d case

CFile myFile;
if(!myFile.Open(newTemp, CFile::modeReadWrite) )
  continue;

strOffset = myFile.GetLength();			

/* read the head of the TIFF file to get the IFH (Image File Header);
* offset to byte 5 to get the 4 byte offset to the first Image File Directory (IFD) structure
* TIFF HEADER: first 8 bytes are 0x49492A00 then the 4 byte offset to the IFH
* 4949 = intel byte ordering; 2A = version; 00 unused???
*/

if (myFile.Seek( 0x4, CFile::begin ) != 4 || myFile.Read(&IFDOffset, 0x4) != 4) {
  TRACE("updateOME4DTS can't read tiff header");		//um, panic?  bad file
  continue;
}				
    
/* walk the (essentially linked list in the file) of IFD structures to find the IMAGEDESCRIPTION
*
* IFD consists of 2 byte directory entry count, then said entry count * 12 bytes of Directory 
* Entry structure, then either 0x00000000 to indicate end or ptr to next IFD
*
* NB: this only walks 1 such IFD structure -- should be OK, IMAGEDESCRIPTION in first set of structs
*/

// Seek to the IFD 
lActual = myFile.Seek(IFDOffset, CFile::begin);

// Read the number of IFD's? Still confused on this one
myFile.Read(&count, 4);
for(int k=0; k<count; k++){

  // The offset to the first directory entry
  LONG nextDirEntryOffset = LONG(IFDOffset) + 2 + 12*k;
  lActual = myFile.Seek(nextDirEntryOffset, CFile::begin);
  myFile.Read(&tag, 2);

  if(tag == 0x010E /* dec 270, TIFFTAG_IMAGEDESCRIPTION */ ){		
    lActual = myFile.Seek(nextDirEntryOffset+4, CFile::begin);
    myFile.Write(&strLen, 4);
    lActual = myFile.Seek(nextDirEntryOffset+8, CFile::begin);
    myFile.Write(&strOffset, 4);
    lActual = myFile.Seek(strOffset, CFile::begin);
    myFile.Write(OMEXMLMetadataString.c_str(), UINT(strLen));

    lActual = myFile.Seek(nextDirEntryOffset+4, CFile::begin);
    myFile.Read(&tmp1, 4);
    lActual = myFile.Seek(nextDirEntryOffset+8, CFile::begin);
    myFile.Read(&tmp2, 4);

    break;
  }
}
myFile.Close();

This code is part of LOCI's WiscScan application; I cannot take credit for the actual implementation.