Proposal for a formal way of expressing public license information in Exif
- Matt Lee, Technical Lead, Creative Commons
- Rob Myers, Software Developer, Creative Commons
January 2016, version 0.1.
Millions of digital photographs are available under Creative Commons licenses, but there is no commonly used machine readable manner for marking these images.
The Exchangeable image file format (Exif) is a standard that specifies the format of a series of tags used in digital photography.
Creative Commons produces a commonly used series of licenses and deeds that photographers and artists use to express how they'd like their work to be used by others, in the form of a deed. Creative Commons has six current copyright licenses, approximately 30 historical licenses and two deeds for works in the public domain. Every Creative Commons license has a Uniform Resource Indentifer (URI), and many Creative Commons languages are translated into multiple languages, each with their own URI.
Exif provides tags for many aspects of digital photography, include some which have privacy concerns, including the location and serial number of the camera. Other Exif tags include more technical aspects of the image, such as orientation (rotation) of the camera, exposure time and resolution.
Amongst these many tags are fields for Author, Title and
Copyright. Our proposed standard allows photographers and artists to
use these fields to express their usage rights for their work, while
allowing for tools to strip other Exif data for reasons of
privacy. There is also another metadata standard, XMP which has some
historical usage here, but is generally less visible to users. XMP may
be used alongside our suggestions for XMP and tools such as
libexif typically read/write both.
Exif has a “Copyright” field (p.40) that is meant to be an an “interoperability copyright statement including date and rights” ASCII string. The example given (p.41) has a simple human-readable format:
Copyright, Anne Other, 2015. All rights reserved.
But the first example on Wikimedia’s Exif article does not follow this standard:
This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/ or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA.
The Exif "Copyright" field can store both photographer and editor information, separated by an ASCII NULL character. The length and formatting of the data express which of photographer and editor is present.
Exif also has "Artist" and "ImageDescription" fields -- the former is not necessarily the copyright holder, and the latter is meant to be the image title.
Extending Exif fields to include URIs
All Creative Commons licenses require attribution on the resulting works. Our suggestion extends the three useful fields of Exif to include relevant URIs which can be used to enhance the display of an image.
Cityscape at Sunset <http://flickr.com/photos/anne.other/23456789>
Anne Other <http://flickr.com/profile/anne.other>
Cityscape at Sunset by Anne Other. This work is licensed under the Creative Commons Attribution-ShareAlike 4.0 International License. To view a copy of this license, visit <http://creativecommons.org/licenses/by-sa/4.0/>
An application would be able to easily parse the URIs in these fields to extend to markup such as:
<p><a href="http://flickr.com/photos/anne.other/23456789">Cityscape at Sunset</a> by <a href="http://flickr.com/profile/anne.other">Anne Other</a> <a href="http://creativecommons.org/licenses/by-sa/4.0/">Creative Commons Attribution-ShareAlike 4.0 International License</a></p>
Marking digital images in this manner will allow them to exist on their own, distinct from any web pages they may be embedded in. In future, we hope platforms and applications will use this data as the basis of providing automated correct attribution when embedding an image.