Sometimes I download images from The Internet™ for later use. For reference I'd like to store some meta data inside the image itself:
The question is now: Which meta data field should I use for those URLs?
Finding the right property
EXIF
Basic meta data can be stored in the image's EXIF data, but there is no URL field:
$ exiftool -list -EXIF:All|grep -i url $
The Exif 2.3 metadata for XMP document also does not list a single URL field, so exiftool was right.
XMP
The XMP standard is another way to store meta data in files. XMP Specification Part 1 defines multiple fields that one could use:
| Property | Type | Description |
|---|---|---|
| dc:relation | Unordered array of Text | A related resource. |
| dc:source | Text | A related resource from which the described resource is derived. |
Unfortunately there are no "real" URL fields.
IPTC
There is another vocabulary, the IPTC Photo Metadata Standard.
| Property | Type | Description |
|---|---|---|
| Iptc4xmpCore:Source | Text | The name of a person or party who has a role in the content supply chain. |
| Iptc4xmpCore:CreatorContactInfoCiUrlWork | URL, multiple |
The creator's contact information provides all necessary information to get in contact with the creator of this item and comprises a set of sub-properties for proper addressing. The contact information web address part. Multiple addresses can be given, separated by a comma. |
| plus:ImageSupplier | Seq ImageSupplierDetail | Identifies the most recent supplier of the item, who is not necessarily its owner or creator. For identifying the supplier either a well known and/or registered company name or a URL of the company's web site may be used. |
Only Iptc4xmpCore:CreatorContactInfoCiUrlWork sounds like it could be used to identify the web site that linked to the image, but I think it is meant to directly link to the creator's homepage - and not to a random URL that just contains a image tag.
Metadata Working Group
The Metadata Working Group published the Guidelines for Handling Image Metadata spec in 2010, and it contains a tag that actually matches my idea of "URL of website that linked to the image":
| Property | Type | Description |
|---|---|---|
| mwg-coll:CollectionURI | URI | URI describing the collection resource. |
A "collection" in MWG speak is a group of images that this specific image is part of. And a website that links to the image can be seen as such a group.
Conclusion
I'm not satisfied with the available properties I found. But instead of inventing my own namespace with source and website properties, I'll simply use the Dublin Core XMP properties:
| Property | Usage |
|---|---|
|
|
URL of image that was downloaded |
| dc:relation | URL of website that linked to the image |
exiftool
Let's say that I visited http://cweiske.de/bdrem.htm and downloaded the image http://cweiske.de/graphics/bdrem/html.png. Now I want to add the website URL and image URL to its meta data.
Embedding the URLs in the downloaded image is easy with exiftool:
$ wget http://cweiske.de/graphics/bdrem/html.png -O bdrem-html.png $ exiftool -source=http://cweiske.de/graphics/bdrem/html.png\ -relation=http://cweiske.de/bdrem.htm bdrem-html.png Warning: [minor] IPTC:Source exceeds length limit (truncated) 1 image files updated
Despite the warning, the full source URL is stored in the image file. But on JPG files the source is really truncated:
$ exiftool -S -source -relation bdrem-html.jpg Source: http://cweiske.de/graphics/bdrem Relation: http://cweiske.de/bdrem.htm
To work around this issue, we force exiftool to use the XMP source property instead of the IPTC source property:
$ exiftool -XMP:source=http://cweiske.de/graphics/bdrem/html.png\ -XMP:relation=http://cweiske.de/bdrem.htm bdrem-html.jpg 1 image files updated
Extracting the data is also possible:
$ exiftool bdrem-html.png ExifTool Version Number : 9.46 File Name : bdrem-html.png ... MIME Type : image/png Image Width : 463 Image Height : 122 ... Software : Shutter Source : http://cweiske.de/graphics/bdrem/html.png XMP Toolkit : Image::ExifTool 9.46 Relation : http://cweiske.de/bdrem.htm Image Size : 463x122 $ exiftool -S -source -relation bdrem-html.png Source: http://cweiske.de/graphics/bdrem/html.png Relation: http://cweiske.de/bdrem.htm
Browsers
Adding the meta data manually is possible, but it would be best if they were added automatically when saving-by-right-clicking the image in my brower. Unfortunately, no browser supports this.
Update 2020-04: rdf:about
Daniel Aleksandersen suggested that we already have a property that means "URL of the thing we talk about", and that is rdf:about:
[...] goes on to identify the resource the statement is about (the subject of the statement) using the rdf:about attribute to specify the URIref of the subject resource.