read and write AlternativeImage

[bertsky]

OCR-D processors are required to respect the AlternativeImage annotation of the METS/PAGE pair, cf. spec (ff.). That implies,

• on the page level: do not just read @imageFilename, but prefer a AlternativeImage/@filename if it exists; regardless, write the result as a new PAGE, merely referencing the resulting image as additional AlternativeImage in the PageType and mets:file (in one of the OCR-D-IMG-* fileGrps) in METS

• on the region level: do not just read @imageFilename and cut the respective region from it, but instead:

  1. prefer a AlternativeImage/@filename (for the region) if it exists, or
  2. prefer a AlternativeImage/@filename (for the page) if it exists and cut the region from it, otherwise
  3. use @imageFilename and cut the respective region from it;

  regardless, write the result as a new PAGE, merely referencing the resulting image as additional AlternativeImage in the RegionType and mets:file (in one of the OCR-D-IMG-* fileGrps) in METS

• etc.

[bertsky]

But mind that an effort is currently under way to incorporate a nice API for all that into core. (Because there is a lot more to it, cf. OCR-D/ocrd_tesserocr#33.) So I recommend waiting for the next release of ocrd first.