Questions about Metadata mappings and PNG files

[Dejay Clayton]

Hello, I've enabled "PNG TextualData - Parameters" (PNG.PNG.Image.Parameters) from the PNG section of Exiftool viewer in the digiKam configuration Metadata tab.  When browsing PNG files that have this field, I see the appropriate entries in digiKam's Metadata tab.  Thus, I know that this functionality works in digiKam, and is also populated in the PNG files I'm viewing.

How do I "manage metadata namespaces" in digiKam's advanced configuration tab, so that I can perform Advanced Searches that query this particular field from PNG files?  I've tried adding all sorts of mappings to the "Caption" field metadata to try to accomplish this, including values such as "Exif.PNG.Parameters", "Exif.PNG.PNG.Parameters", "Exif.PNG.Image.Parameters", "Exif.PNG.PNG.Image.Parameters", "Xmp.exif.PNG.Parameters", etc. etc..  Nothing seems to work.

A few questions.  First, where does the list of valid values for metadata mapping come from?  I notice that none of the namespace identifiers in advanced metadata mapping are equivalent to the namespaces within the Exiftool viewer.  E.g. "Tags" -> "Xmp.lr.hierarchicalSubjects" appears in the advanced mapping tab, but "XMP.XMP-lr.Image.HierarchicalSubject" appears in the Exiftool viewer.  Do these identifiers correlate to each other at all?

Second, how can I map the PNG TextualData Parameters to Caption in digiKam, so that I can perform an Advanced Search on that field?

As a final observation, using mailing lists for support seems so stuck in the 1980's.

Regards,
Dejay Clayton