add gt-labelling to OcrdMets API

[bertsky]

Note: In METS, the labels are a flat sequence of gt:state elements with @prop from the above mentioned schema file, one per page.

   <mets:dmdSec ID="DMDGT_0001">
      <mets:mdWrap MDTYPE="OTHER" OTHERMDTYPE="GT">
         <mets:xmlData>
            <gt:gt>
               <gt:state prop="granularity/physical/document-related/word"/>
               <gt:state prop="granularity/physical/document-related/text-line"/>
               <gt:state prop="granularity/physical/document-related/region"/>
               <gt:state prop="data-attributes/document-related/visual/text/font/multi-font/typefaces"/>
               <gt:state prop="data-attributes/document-related/visual/text/font/multi-font/font-sizes"/>
               <gt:state prop="data-attributes/language/mixed"/>
               <gt:state prop="condition/production-related/document-faults/ink-from-facing"/>
               <gt:state prop="condition/wear/additions/informative/annotations"/>
               <gt:state prop="condition/production-related/document-characteristics/low-contrast"/>
               <gt:state prop="condition/acquisition/method-flaws/imaging/uneven-illumination"/>
            </gt:gt>
         </mets:xmlData>
      </mets:mdWrap>
   </mets:dmdSec>

These are then referenced under each physical structMap's page via @DMDID.

IMO in core we first need some additional API to support that. Like (in analogy to pageId):

OcrdMets.get_gt_labelling(self, for_fileIds=None) # returns dict of file ID to label list
OcrdMets.get_gt_labelling_for_file(self, ocrd_file) # returns label list
OcrdMets.set_gt_labelling_for_file(self, labels, ocrd_file) # takes label list
# but also:
OcrdMets.add_file(self, ... labels=None, ...) # add full label list
OcrdMets.find_files(self, ... labels=None, ...) # filter by label list (match any)

What's your opinion, @kba?

Perhaps – instead of parsing this from the METS, we could also see to it that OCR-D mirrors them in the parsed PAGE-XML, i.e. OcrdPage.

For example as:

  <MetadataItem type="imageProperties" name="gt-labelling">
    <Labels externalModel="https://github.com/OCR-D/gt-labelling/blob/master/xsd_schema/OCR-D_GT_schema.xsd" externalId="http://www.ocr-d.de/GT/">
      <Label value="granularity/physical/document-related/word"/>
      <Label value="granularity/physical/document-related/text-line"/>
      <Label value="granularity/physical/document-related/region"/>
      <Label value="data-attributes/document-related/visual/text/font/multi-font/typefaces"/>
      <Label value="data-attributes/document-related/visual/text/font/multi-font/font-sizes"/>
      <Label value="data-attributes/language/mixed"/>
      <Label value="condition/production-related/document-faults/ink-from-facing"/>
      <Label value="condition/wear/additions/informative/annotations"/>
      <Label value="condition/production-related/document-characteristics/low-contrast"/>
      <Label value="condition/acquisition/method-flaws/imaging/uneven-illumination"/>
    </Labels>
  </MetadataItem>

This would make it easier to access the labels from a processor or PAGE viewer.

Originally posted by @bertsky in hnesk/browse-ocrd#36 (comment)

[bertsky]

@tboenig perhaps relevant for gt-guideline-examples etc.