Extension:CommonsMetadata

CommonsMetadata is an attempt at extracting metadata from Wikimedia Commons pages. It adds some extra information to the imageinfo API, based on templates and categories in the image description.

The extension in its current form is intended to be a temporary solution, eventually replaced by Wikidata on Commons.

Motivation & design choices
See http://lists.wikimedia.org/pipermail/wikitech-l/2013-August/071593.html

The assumptions of this extension are:
 * At some point in the future, wikidata will take over handling metadata at commons. In order to avoid disruptive changes, which will soon need to be changed again, the extension should work with commons metadata as it currently is (so not introducing new parser functions). Hence screen scraping.
 * The content of many of the fields on a commons description page include rich formatting (In particular: Links, italics, bold. In some cases more complex things like embedded images)
 * As a result, extension outputs parsed html (wikitext sucks, plain text doesn't capture the data)
 * Futhermore, the data tends to be formatted for human display, rather than (for example) machine formatted dates. When the date field says something like "circa 1600s", its hard to convert that to a precise date (otoh, many examples can be).
 * To carry that forward, also apply formatting to exif metadata, which is controlled on wiki (For example, commons links the camera name to a wikipedia article)
 * If we can't extract info from the description page, but the file has the author tagged in exif/XMP/iptc metadata, we should use that as a fallback.
 * Ideally such a system would be as commons-inspecific as possible, with the commons and non-commons part separated.
 * Commons description pages have multilingual descriptions. Lots of users probably just want one language.
 * In this implementation, it applies per language conventions to dates and things. Additionally for explicitly multi-lingual fields (description), there is an option to return all, or just a single language. Even in single language mode, some things are still language specific (like the thousands seperator on numbers)

Installation
In a setup where there is a local wiki and a remote image repository, for optimal results CommonsMetadata should be installed on the remote (or both). When installed on the local wiki only, it will still provide some extra information about remotely hosted images, but not as much as it would the other way around.

Usage
Use the imageinfo API, and include  as an image info property specified via.

Example usage:
 * https://commons.wikimedia.org/w/api.php?action=query&prop=imageinfo&format=xml&iiprop=extmetadata&iilimit=10&titles=File%3ACommon%20Kingfisher%20Alcedo%20atthis.jpg

View this example in the API sandbox:
 * https://www.mediawiki.org/wiki/Special:ApiSandbox#action=query&prop=imageinfo&format=xml&iiprop=extmetadata&iilimit=10&titles=File%3ACommon%20Kingfisher%20Alcedo%20atthis.jpg

Returned data
The extension currently provides the following items in the  field of the response (the field names were chosen, where possible, to follow the IPTC-IIM format used in EXIF headers):

Data based on machine-readable data in the Information template:
 * ImageDescription - image description
 * Artist/Credit - authorship information
 * DateTimeOriginal - time of creation
 * ObjectName - title (for a book/painting)

Data based on machine-readable data in the Location template:
 * GPSLatitude - latitude
 * GPSLongitude - longitude

Data based on machine-readable data in the license template: For multi-licensed images these values are currently unreliable.
 * LicenseShortName - short human-readable license name
 * LicenseUrl
 * UsageTerms
 * Copyrighted -  or   (for public domain images)

Other data:
 * CommonsMedadataExtension - this is just a convenient way of testing that the extension is installed
 * License - a best guess at the license of the image (mostly for internal use by MediaViewer, might change; LicenseShortName is probably more reliable)
 * Categories - a -separated list of the categories of the image. (this is mostly broken at the moment)