NAME

       TIFFCustomDirectory - routines to create a custom directory

SYNOPSIS

          #include <tiffio.h>

       int TIFFCreateCustomDirectory(TIFF *tif, const TIFFFieldArray *infoarray)

       int TIFFCreateEXIFDirectory(TIFF *tif)

       int TIFFCreateGPSDirectory(TIFF *tif)

       int TIFFWriteCustomDirectory(TIFF *tif, uint64 *pdiroff)

       int TIFFReadCustomDirectory(TIFF *tif, toff_t diroff, const TIFFFieldArray *infoarray)

       int TIFFReadEXIFDirectory(TIFF *tif, toff_t diroff)

       int TIFFReadGPSDirectory(TIFF *tif, toff_t diroff)

       const TIFFFieldArray *_TIFFGetExifFields(void)

       const TIFFFieldArray *_TIFFGetGpsFields(void)

DESCRIPTION

       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.

NOTES

       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

HINTS AND DETAILED INSTRUCTIONS

       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:

       1. Create TIFF file

       2. Complete the "normal" metadata

       3. 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);

       4. Save current TIFF-directory to file. Otherwise, it will be lost.  Remember  also  the  number  of  the
          current directory:

          TIFFWriteDirectory(tiffOut);
          dirNum = TIFFCurrentDirectory(tiffOut);

       5. 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 */
          TIFFCreateGPSDirectory(tiffOut);
          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!

       6. Write custom directory to file. The offset to that directory in the file is returned in dir_offset.

          TIFFWriteCustomDirectory(tiffOut, &dir_offset);

       7. Reload the first directory (i.e. the original TIFF directory).  Apparently, this reads the  data  back
          from file.

          TIFFSetDirectory(tiffOut, dirNum);

       8. Set the correct offset value of the custom directory IFD tag and save that changes to file.

          TIFFSetField(tiffOut, TIFFTAG_GPSIFD, dir_offset);
          TIFFWriteDirectory(tiffOut);

RETURN VALUES

       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.

DIAGNOSTICS

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

SEE ALSO

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

AUTHOR

       LibTIFF contributors

COPYRIGHT

       1988-2024, LibTIFF contributors

4.5                                               Sep 05, 2024                        TIFFCUSTOMDIRECTORY(3tiff)