Copy RDFa metadata (Firefox add-on)

This add-on for Firefox allows RDFa-embedded metadata about images to be copied to the clipboard, and pasted into other applications. It also includes a function for pasting images with metadata into supporting webpages.

http://commonsmachinery.se/labs/ contain more information about plugins and applications that support pasting an image with metadata that has been copied by this add-on.

Site support

RDFa metadata is not very widespread, and when it it present it can often be difficult to know what resource on the page it describes. The technical details section below describes this further.

The addon has been tested against these sites:

Flickr (site-specific support in add-on)
Europeana (standard RDFa)
CC REL by example (standard RDFa)
Commons Machinery labs: Mediagoblin (standard RDFa)

Copy metadata and images

When an image has metadata, it's context menu now has a new menu item: "Copy image with credits". This extract the metadata and puts both the image and the RDF/XML on the clipboard. An application that supports this format of data can paste both the image and the metadata into another document, and use the metadata to e.g. create an automatic credit line.

An application that doesn't use the metadata will still be able to paste the image itself.

The page context menu can also have an additional menu items called "Copy main image with credits". This is available when the addon can determine that a particular image is the "main" image, e.g. on a Flickr photo page. This is mainly useful on some web sites where the image may be difficult to click on to get the context menu.

There are two additional metadata menu items that are mostly useful to developers, so they must be explicitly enabled in the preferences for the addon. When enabled, "Copy image metadata" and "Copy page metadata" can be used to copy all metadata for an image or on the page as a whole as RDF/XML to the clipboard as regular text. It can then be pasted into e.g. a text editor for further study.

Limitations

On MacOSX, this addon only supports copying metadata within the same Firefox instance, e.g. to another tab. The MacOSX clipboard makes it a bit more complicated to put custom data on the clipboard than what Linux/X and Windows does, and thus the clipboard code in Firefox for MacOSX does not support copying the metadata to the global clipboard.

This add-on currently only supports tags. It could be extended to support

The add-on may not find metadata that is added dynamically after the page has been loaded.

Paste images with metadata

The add-on includes experimental support for pasting images with metadata into browser-based editors. The context menu includes "Paste image" when clicked inside an editor area that supports this. (With the current SDK, this option is enabled even when there isn't any image on the clipboard. See note below.)

There's a simple page implementing this in the example directory, and at http://commonsmachinery.se/labs/ you can find an Aloha Editor instance supporting this.

Limitations

On MacOSX, this only works for metadata copied within the same browser instance.

Installing

The add-on can be installed from the Mozilla registry: https://addons.mozilla.org/en-US/firefox/addon/copy-rdfa-metadata/

It is also uploaded to the repository here (may be more recent if a new addon version is yet to be reviewed by the Mozilla addon team): https://github.com/commonsmachinery/copyrdf-addon/raw/master/copyrdf.xpi

Building the add-on yourself

If you build the add-on with the current stock SDK, the "Paste image" menu item will always be available in editor areas even when there is no image on the clipboard. The SDK is being patched to add support to the add-on to only show this item when there is an image on the clipboard.

In the meantime, you can try this by cloning this SDK tree: https://github.com/commonsmachinery/addon-sdk/tree/922558

Then run cfx with --force-use-bundled-sdk.

Technical details

This is experimental code, looking at ways to use metadata and the clipboard. This section documents more precisely how the add-on locates metadata, how it is copied, and how a browser-based editor can support pasting metadata.

Locating metadata

RDF metadata consists of triples, consisting of a subject, a predicate, and an object. Finding the metadata for a given image means that we have to find all the triples whose subject is the image.

Subjects are identified by a URI, which results in various ways to denote the image. The add-on uses the following methods in this order:

The best way is to mark the image with an ID attribute, and address the image with the URI of the page and the ID. If it is the main image of a page, a good name can be #this. That will result in the URI for the subject will be http://example.org/images/34327/#this, which is likely a unique and long-term viable URL, which can be used to locate the source of the image and find the original RDFa metadata on the page.

Secondarily, the image source URI can be the subject, e.g. http://example.org/resources/34327.jpg. This has the drawback that anyone looking up the image based on this URI will only find the image file itself, without the context of a web page where it might be published. This also means that it might not be the image as a work that is found, but just a specific thumbnail or medium resolution file.

A third way is to look for an og:image predicate, whose object is the image source. Then the subject of that predicate is likely about the image, since this predicate indicates that this is the "main" image of the page and should be used to represent it when shown inline in e.g. Facebook or Twitter streams. This is less precise than the two previous methods, but works on sites that are primarily about images.

og:image is also used to find the main image on the page for the "Copy main image with metadata" command.

However, some sites have reasonably good RDFa, but with no good link to the image. Flickr is a good example. It associates the RDFa with the page URI itself. And while it has an og:image predicate, that links to a smaller-size image that isn't present on the standard or the lightbox page. To make the addon work on Flickr, it contains custom code that understands the peculiarities of the DOM tree and the RDFa markup on that site. This will of course be more brittle than the methods above.

Copying metadata

When the addon has found some metadata, the RDF triples are serialised as RDF/XML and put on the clipboard identified by the MIME type application/rdf+xml. The triples include not only the ones directly about the image, but also any triples about subjects that are referred to by the image (e.g. source works).

When the metadata is copied together with an image, a special triple is added to the RDF to help the destination application locate the image subject:

<> <dc:source> <imageSubjectURI>

The empty subject <> is a convention taken from the RDF subset XMP to indicate "the surrounding or associated file". In this context, the image on the clipboard together with the metadata is considered the associated file. This triple thus says what the source is for the image data on the clipboard, and the application can then find out more information by looking at the triples that has the subject <imageSubjectURI>.

When only metadata is copied without the image, this triple is not added as there is no image data to associate it with. Since this will probably mainly be used for investigating/debugging purposes, the metadata is also put on the clipboard as plain strings to make it easy to paste it into a standard text editor.

Paste metadata

To support pasting images into a browser editor with this addon, the editor must add the HTML class x-enable-paste-image to the element that wraps the editor (typically a div). When the image is pasted, an x-onpaste-image custom event is generated with these detail parameters:

image: image data in format "data:MIMETYPE;base64,DATA...". Create an img tag and set src to this string to show it in the page.
rdfxml: serialised RDF/XML associated with the image, or null if there was no metadata.
target: the target element for the paste action

There is a simple page implementing this in the example directory, using https://github.com/linkeddata/rdflib.js to process the incoming RDF/XML.

License

Author(s): Peter Liljenberg peter@commonsmachinery.se Artem Popov artfwo@commonsmachinery.se

Distributed under an GPLv2 license, please see the LICENSE file for details.

Green Turtle

The add-on includes the Green Turtle RDFa.js module from http://code.google.com/p/green-turtle/

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

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rdflib.js

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