LibTIFF: Extract all tags from a TIFF image

Question

I'm currently working on a project which requires me to split a TIFF image into a file containing all tags and a file containing all image data and to reconstruct a TIFF image from these files. The only problem is that it seems that LibTIFF provides no easy way to grab all tags from an image. I've tried using TIFFGetTagListCount and then TIFFGetField to retrieve the tag, but this only returns a small subset of the tags. I've started rolling my own version, but I just want to double check and make sure I'm not overlooking something as this seems like a pretty obvious feature that should be included in the library.

Answer 1

addingtags.html in the libtiff documentation contains information on handling non-standard tags (by default they're ignored). Could that be the problem?

Answer 2

You can use tif_dir field of the image. It is a struct with at least the following fields:

• td_customValueCount contains the number of 'custom' tag counts,
• td_customValues is the list of td_customValueCount tag values (indexed from 0), and is of type TIFFTagValue *.

So you should be able to do something like this (in true fashion of online code, this is untested, of course!):

for (i=0; i < tiffimage->tif_dir->td_customValueCount; ++i) {
    const TIFFFieldInfo *info = tiffimage->tif_dir->td_customValues[i].info;
    const char *tagname = info->field_name();
    /* process tag */
}

See TIFFDirectory Struct Reference. Hope that helps.

Answer 3

The tiffdump utility (which ships with libtiff) accomplishes this, but looking at the code, it looks like they're mostly working around the library. They actually call lseek and read to read the tag info.

Answer 4

This may also be useful: http://www.lfd.uci.edu/~gohlke/code/tifffile.py.html

Answer 5

Printing all tags with tifffile.py:

from tifffile import TIFFfile
for page in TIFFfile(path_to_file):
    for tag in page.tags.values():
        print tag.name, tag.code, tag.dtype, tag.count, tag.value

Answer 6

Here is the closes you can get for scanning all tags:

 #include "LibTIFF/tif_dir.h"

    for (int fi = 0, nfi = tif->tif_nfields; nfi > 0; nfi--, fi++) {
        const TIFFFieldInfo* fip = tif->tif_fieldinfo[fi];

        // test if tag value is set
        // (lifted directly form LibTiff _TIFFWriteDirectory)

        if( fip->field_bit == FIELD_CUSTOM ) {
            int ci, is_set = FALSE;

            for( ci = 0; ci < td->td_customValueCount; ci++ )
                is_set |= (td->td_customValues[ci].info == fip);

            if( !is_set )
                continue;
        } else if(!TIFFFieldSet(tif, fip->field_bit))
            continue;

        // else: process the fip->field_tag


    }

Note that you will have to take into account that some tags will appear twice (LONG and SHORT version), but only one of these will have value. The correct type to use can be looked up in the included header (the TIFFDirectory struct).

There also other catches on how to read the tags, but this at least will make you loop over all of them (the standard ones). See tif_dirinfo.c for pointers if you get stuck.