Carrying Photo Metadata from Darktable to Piwigo

Darktable exports photo fields to metadata as follows (as reported by exiv2 pr -pa {file}.jpg):

Darktable title -> Xmp.dc.title
Darktable description -> Xmp.dc.description
Darktable tagging section -> Xmp.dc:subject (as a comma-separated list)

Unfortunately, Piwigo does not read XMP metadata so only the description is available as Exif.Image.ImageDescription. IPTC is required for the title and tags. Darktable does not export IPTC, but can be scripted to copy XMP tags to IPTC. See instructions here: https://discuss.pixls.us/t/name-description-and-keywords-are-not-recognized-by-shutterstock/6726/26

and source code here: https://github.com/chrik5/lua-scripts/blob/exiftool/contrib/exiftool_export.lua

In Darktable, using exiftool on export from above, check enable exiftool, set options to -@ and create file xmp2iptc.txt with the following contents:

-XMP-dc:Subject > IPTC:Keywords
-XMP-dc:Description > IPTC:Caption-Abstract
# -XMP-dc:Title > IPTC:ObjectName

(uncomment out the last line if you want to use the Title in Darktable rather than the filename Piwigo uses by default; and update use_iptc_mapping array below)
Set this file as Argumentsfile.
Then export the files from Darktable.

Using Piwigo plugin LocalFiles Editor add these lines to local config file local/config/config.inc.php:

// use_iptc: Use IPTC data during database synchronization with files
// metadata
$conf['use_iptc'] = true;
// use_iptc_mapping : in which IPTC fields will Piwigo find image 
// information? This setting is used during metadata synchronisation. It 
// associates a piwigo_images column name to a IPTC key 
// See https://exiftool.org/TagNames/IPTC.html 
// and http://piwigo.org/doc/doku.php?id=user_documentation:metadata&s 
$conf['use_iptc_mapping'] = array(   
  'keywords'        => '2#025',
  'comment'         => '2#120'   
  );

From this point on, you’ll only have to remember to activate exiftool on export in Darktable and Piwigo will do the rest.