workspace.save_image_file: set dpi on image.save, #343

[kba]

No description provided.

[bertsky]

We should move this forward.

Looking up any existing image for DPI info (without making sure it's the original image) will work, as long as images only ever enter via this function (or some other DPI-preserving method).

Regarding (in)efficiency, we should immediately take action on the processor side, so the dpi kwarg is used everywhere.

(BTW, if we split this into the opt-in / dpi-kwarg and fallback / find_files part, we will get a window where we can adapt processors without paying the costs already.)

[bertsky]

We must adapt for the generator now, I believe.

Suggested change

	orig_img_file = self.mets.find_files(pageId=page_id, mimetype='//^image')[0]
	orig_img_file = next(self.mets.find_files(pageId=page_id, mimetype='//^image', local_only=True))

[bertsky]

Another idea I had: couldn't we at least try to force PIL.Image objects we ourselves produce (Workspace.image_from_page and image_from_segment) to retain or piggy-back the DPI info? Or even store that info in the workspace instance (e.g. by attaching the OcrdExif to the OcrdFile of the PAGE-XML, which itself could be cached in the OcrdMets; this would require rewriting Processor.input_files, but allow reusing these iterations)?