Metadata

Introduction

Metadata is structured data about data.

In the computing industry, there are ongoing standardization efforts towards metadata with the goal of promoting industry interoperability and efficiency. Content creators should track these developments and include appropriate metadata in their SVG content which conforms to these various metadata standards as they emerge.

The W3C has a Semantic Web Activity which has been established to serve a leadership role, in both the design of enabling specifications and the open, collaborative development of technologies that support the automation, integration and reuse of data across various applications. The Semantic Web Activity builds upon the earlier W3C Metadata Activity, including the definition of RDF (Resource Description Framework). The set of six specifications for RDF can be found starting with the Resource Description Framework Primer [RDF]

Another activity relevant to most applications of metadata is the Dublin Core [DCORE], which is a set of generally applicable core metadata properties (e.g., Title, Creator/Author, Subject, Description, etc.).

A popular usage of RDF metadata in the SVG community is the inclusion of license terms using the Creative Commons [CC] license framework. The popular open-source SVG authoring tool Inkscape can automatically include this license information, and inclusion of such a license is required by many collaborative Web sites. This indicates the potential and impact of metadata.

Individual industries or individual content creators are free to define their own metadata schema but are encouraged to follow existing metadata standards and use standard metadata schema wherever possible to promote interchange and interoperability. If a particular standard metadata schema does not meet your needs, then it is usually better to define an additional metadata schema in an existing framework such as RDF and to use custom metadata schema in combination with standard metadata schema, rather than totally ignore the standard schema.

SVG provides two mechanisms for adding metadata directly to a document: the 'metadata' element, and several metadata attributes. These different mechanisms may be used independently, or in concert. If both are being used for RDF (that is, RDF and RDFa), then an RDF processor should combine them into the same graph. (Note: metadata attributes should not be used directly on RDF elements.)

The 'metadata' element

Metadata which is included with SVG content should be specified within 'metadata' elements. The contents of the 'metadata' should be elements from other XML namespaces, with these elements from these namespaces expressed in a manner conforming with either the Namespaces in XML 1.0 or Namespaces in XML 1.1 Recommendations [XML-NS10][XML-NS].

Authors should provide a 'metadata' child element to the 'svg' element within a stand-alone SVG document. The 'metadata' child element to an 'svg' element serves the purposes of identifying document-level metadata.

If a 'metadata' element is placed as a child of a non-root element with the intent to apply directly to that element, it is recommended that the author indicate this explicitly in the content of the 'metadata' element, if that content provides a way to do so. For example, when using RDF, the target element should be given an 'id' attribute, and the 'about' attribute of the RDF should reference that 'id'.

A 'metadata' element example

Here is an example of how metadata can be included in an SVG document. The example uses the Creative Commons schema to indicate the usage license of a work of art. (Other XML-compatible metadata languages, including ones not based on RDF, can be used also.)

Extensible metadata attributes

In addition to the 'metadata' element, which allows for the direct inclusion of metadata document fragments from other namespaces, SVG includes several attributes that may be placed on any element, for the use of attribute-based metadata formats. These include the 'class', 'role', 'rel', 'rev', 'about', 'content', 'datatype', 'property', 'resource', and 'typeof' attributes. SVG makes no specific requirements about the values for these attributes, other than their particular value data types, such as a string or a space-separated lists of strings. Other specifications, such as RDFa [RDFA], Microformats patterns [MF], or ARIA ontologies [ARIA], may impose stricter requirements in order to conform to that particular language, which should be expressed as an additional schema for purposes of validation. A few informative examples of such restrictions include:

The 'class' attribute, when used for CSS selector names, allow only specific characters and cannot start with a digit, but allow any other combination of those characters, and has no keyword restrictions; when used with Microformats, only particular keywords will be recognized; for other purposes, neither of these restrictions may apply.
The 'role' attribute, to be used with WAI-ARIA [ARIA], must match one or more values designated as role names in that specification. In this case, WAI-ARIA defines additional processing for the elements bearing these 'role' values, including requirements for the use of further WAI-ARIA-defined attributes with value pertinent to the designated roles. Note that this processing does not interfere with host language processing, but supplements it with regard to the mapping of content to accessibility APIs.
For the 'rel' and 'rev' attributes, both RDFa and Microformats require that the values must contain one of a set of predefined reserved keywords, though these keywords are defined in different ontologies. RDFa enforces the further requirement that each value string must be a CURIE (i.e., a reserved keyword that may or may not be a prefixed string, such as 'cc:license' to indicate a Creative Commons license; [RDFA], section 7), while Microformats does not use CURIEs or namespace-prefixed values. Since SVG requires only that the attribute value is a space-separated list of strings, there is no conflict with either of these formats, and they may be used independently, or in combination provided that the keywords do not clash and that their processors are able to ignore unknown values.
Specific keywords that are part of no formal language or format, such as the 'rel' value 'nofollow' intended for processing by search engines, require only that the keyword be included.

SVG does not mandate or require support for any of these languages, formats, or ontologies, but it includes these metadata attributes to enable their use as the author desires. Note that this specification's description of these complementary formats is intended for illustrative purposes only, and does not impose additional restrictions on those formats.

In order to maintain consistency and simplicity of implementation, and prevent conflict or ambiguity of use, if an author or a non-RDFa format reuses the 'about', 'content', 'datatype', 'property', 'resource', or 'typeof' attributes, it is recommended that this is done in a manner consistent with the RDFa Syntax Processing Rules [RDFA].

Currently, many SVG documents contain RDF metadata that provides a title in the Dublin Core [DCORE] namespace, which is useful for certain types of processing, but which is not treated as a title for the document by many user agents. It is recommended that authoring tools which embed RDF metadata that contains a title also provide at least a document-level 'title' element. The following example shows how the 'property' attribute may be combined with the descriptive elements to supply both a human- and machine-readable title, in the manner of RDFa.