TIFFCustomDirectory - routines to create a custom directory

#include <tiffio.h>

The following routines create a custom directory and retrieve information about directories in an open TIFF file.

TIFFCreateCustomDirectory(), TIFFCreateEXIFDirectory(), TIFFCreateGPSDirectory() will setup a custom directory or one of the predefined EXIF or GPS directories and set the context of the TIFF-handle tif to that custom directory for functions like TIFFSetField().

TIFFWriteCustomDirectory() will write the contents of the current custom directory to the file and return the offset to that directory in pdiroff. That offset has to be written to the main-IFD:

 /* Go back to the first directory, and add the EXIFIFD pointer. */
TIFFSetDirectory(tif, 0);
TIFFSetField(tif, TIFFTAG_EXIFIFD, pdiroff);

TIFFReadCustomDirectory() will read the custom directory from the arbitrary offset into the infoarray and sets the context of the TIFF-handle tif to that custom directory for functions like TIFFReadField(). The TIFFFieldArray infoarray has to be according the layout of the custom directory. For the predefined EXIF and GPS directories, the relevant TIFFFieldArray definitions are hidden within the functions TIFFReadEXIFDirectory() and TIFFReadGPSDirectory() The code is very similar to TIFFReadDirectory(). The offset to the custom directory diroff has to be read from the relative TIFF tag first.

_TIFFGetExifFields() and _TIFFGetGpsFields() will return a pointer to the libtiff internal definition list of the EXIF and GPS tags, respectively.

Be aware

  • that until a directory is not written to file AND read back, the query functions won’t retrieve the correct information!
  • that the newly created directory will not exist on the file till TIFFWriteDirectory(), TIFFCheckpointDirectory(), TIFFFlush() or TIFFClose() has been called.
  • that TIFFCreateDirectory() and TIFFWriteDirectory() create a new directory, free the *tif structure and set up a new one.
  • that unlike TIFFWriteDirectory(), TIFFCheckpointDirectory() does not free up the directory data structures in memory.
  • that LibTiff does not support custom directory chains (NextIFD pointing to another IFD). NextIFD of custom directories is always set to zero and should be zero when reading.

Unfortunately to create or read custom directories with predefined fields it is necessary to include the private tif_dir.h. However, for EXIF and GPS directories, which have a predefined schema within libtiff, this is not necessary. There are some test programmes that briefly demonstrate the creation and reading of EXIF, GPS and custom directories. See test/custom_dir.c and test/custom_dir_EXIF_231.c

Writing TIFF files with more than one directory (IFD) is not easy because some side effects need to be known.

The main point here is that there can only be one tif structure in the main memory for a file, which can only hold the tags of one directory at a time. It is useless to work with two different tiffOut1, tiffOut2 pointers, because there is only ONE TIFF object (TIFF directory) within the libtiff. If you want to address a second directory in the file, the tags of the current directory must first be saved in the file, otherwise they will be lost (overwritten or mixed). Then the tif structure in the main memory must be tidied up, otherwise the old tags will beincluded in the new directory. This can be done either by creating a new, empty tif structure or by reading in an directory previously saved in the file.

A sequence to handle a second (or third) TIFF directory - in this case the GPS IFD - is as follows:

Create TIFF file
Complete the “normal” metadata
Set the tag for the custom directory with a “dummy” value in order to get the tag reserved. The final value will be inserted lateron. This prevents the main directory from being written to the file again at an additional area, leaving the first memory area unused:
TIFFSetField(tiffOut, TIFFTAG_GPSIFD, dir_offset);
Save current TIFF-directory to file. Otherwise, it will be lost. Remember also the number of the current directory:
dirNum = TIFFCurrentDirectory(tiffOut);
Create the second TIFF-directory (e.g. custom directory) and set its fields. The TIFFFieldArray infoarray has to be specified beforehand somewhere in your private include files. An example is given for the EXIF directory in tif_dirinfo.c
TIFFCreateCustomDirectory(tiffOut, infoarray);        /* for a real custom directory */
/* or alternatively, use GPS or EXIF with pre-defined TIFFFieldArray IFD field structure */
TIFFSetField(tiffOut, GPSTAG_VERSIONID, gpsVersion);  /* set fields of the custom directory */

Be aware that every TIFFCreateDirectory() or TIFFWriteDirectory() apparently frees the *tif structure and sets up a new one!

Write custom directory to file. The offset to that directory in the file is returned in dir_offset.
TIFFWriteCustomDirectory(tiffOut, &dir_offset);
Reload the first directory (i.e. the original TIFF directory). Apparently, this reads the data back from file.
TIFFSetDirectory(tiffOut, dirNum);
Set the correct offset value of the custom directory IFD tag and save that changes to file.
TIFFSetField(tiffOut, TIFFTAG_GPSIFD, dir_offset);

1 is returned when the contents are successfully written to the file. Otherwise, 0 is returned if an error was encountered when writing the directory contents.

All error messages are directed to the TIFFErrorExtR() routine. Likewise, warning messages are directed to the TIFFWarningExtR() routine.

libtiff (3tiff), TIFFCreateDirectory (3tiff), TIFFquery (3tiff), TIFFSetDirectory (3tiff), TIFFWriteDirectory (3tiff)

LibTIFF contributors

1988-2022, LibTIFF contributors

September 8, 2023 4.6