Linking

References

Overview

On the Internet, resources are identified using URLs (Internationalized Resource Identifiers). For example, an SVG file called someDrawing.svg located at http://example.com might have the following URL:

http://example.com/someDrawing.svg

An URL can also address a particular element within an XML document by including an URL fragment identifier as part of the URL. An URL which includes an URL fragment identifier consists of an optional base URL, followed by a "#" character, followed by the URL fragment identifier. For example, the following URL can be used to specify the element whose ID is "Lamppost" within file someDrawing.svg:

http://example.com/someDrawing.svg#Lamppost

URLs and URIs

Internationalized Resource Identifiers (URLs) are a more generalized complement to Uniform Resource Identifiers (URIs). An URL is a sequence of characters from the Universal Character Set [UNICODE]. A URI is constructed from a much more restricted set of characters. All URIs are already conformant URLs. A mapping from URLs to URIs is defined by the URL specification, which means that URLs can be used instead of URIs in XML documents, to identify resources. URLs can be converted to URIs for resolution on a network, if the protocol does not support URLs directly.

Previous versions of SVG, following XLink, defined an URL reference type as a URI or as a sequence of characters which must result in an URL after a particular escaping procedure was applied. The escaping procedure was repeated in the XLink 1.0 specification [XLINK], and in the W3C XML Schema Part 2: Datatypes specification [SCHEMA2]. This copying introduced the possibility of error and divergence, but was done because the URL specification was not yet standardized.

In this specification, the correct term URL is used for this "URI or sequence of characters plus an algorithm" and the escaping method, which turns URLs into URIs, is defined by reference to the URL specification [RFC3987], which has since become an IETF Proposed Standard. Other W3C specifications are expected to be revised over time to remove these duplicate descriptions of the escaping procedure and to refer to URL directly.

Syntactic forms: URL and FuncURL

FuncURL has been removed. This section needs to be rewritten.

URLs are used in the 'animate/xlink:href' attribute. Some attributes allow both URLs and text strings as content. To disambiguate a text string from a relative URL, the functional notation FuncURL is used. This is simply an URL delimited with a functional notation. Note: For historical reasons, the delimiters are "url(" and ")", for compatibility with the CSS specifications. The FuncURL form is used in presentation attributes.

SVG makes extensive use of URL references, both absolute and relative, to other objects. For example, to fill a rectangle with a linear gradient, you first define a 'linearGradient' element and give it an ID, as in:

You then reference the linear gradient as the value of the 'fill' property for the rectangle, as in the following example:

SVG supports two types of URL references:

Processing of URL references

The following rules apply to the processing of URL references:

The following list describes the elements and properties that allow URL references and the valid target types for those references:

The following rules apply to the processing of invalid URL references:

URL reference attributes

URL references are normally specified with an 'href' attribute in the XLink [XLink] namespace. For example, if the prefix of 'xlink' is used for attributes in the XLink namespace, then the attribute is specified as 'xlink:href'. The value of this attribute forms a reference for the desired resource (or secondary resource, if there is a fragment identifier).

The value of the 'href' attribute must be an URL.

If the protocol, such as HTTP, does not support URLs directly, the URL is converted to a URI by the SVG implementation, as described in section 3.1 of the URL specification [RFC3987].

Because it is impractical for any application to check that a value is an URL reference, this specification follows the lead of the URL Specification in this matter and imposes no such conformance testing requirement on SVG applications.

If the URL reference is relative, its absolute version must be computed by the method described in XML Base before use [XML-BASE].

xlink:type = "simple"

Identifies the type of XLink being used. In SVG 1.1, only simple links are available. Links are simple links by default, so the attribute xlink:type="simple" is optional and may be omitted on simple links. Refer to the XML Linking Language (XLink) [XLINK].

Animatable: no.

xlink:role = "<url>"

An optional URL reference that identifies some resource that describes the intended property. The value must be an URL reference as defined in [RFC3987], except that if the URL scheme used is allowed to have absolute and relative forms, the URL portion must be absolute. When no value is supplied, no particular role value shall be inferred. Refer to the XML Linking Language (XLink) [XLINK].

Animatable: no.

xlink:arcrole = "<url>"

An optional URL reference that identifies some resource that describes the intended property. The value must be an URL reference as defined in [RFC3987], except that if the URL scheme used is allowed to have absolute and relative forms, the URL portion must be absolute. When no value is supplied, no particular role value shall be inferred. The arcrole attribute corresponds to the [RDF-PRIMER] notion of a property, where the role can be interpreted as stating that "starting-resource HAS arc-role ending-resource." This contextual role can differ from the meaning of an ending resource when taken outside the context of this particular arc. For example, a resource might generically represent a "person," but in the context of a particular arc it might have the role of "mother" and in the context of a different arc it might have the role of "daughter." Refer to the XML Linking Language (XLink) [XLINK].

Animatable: no.

xlink:title = "<anything>"

The title attribute shall be used to describe the meaning of a link or resource in a human-readable fashion, along the same lines as the role or arcrole attribute. A value is optional; if a value is supplied, it shall contain a string that describes the resource. In general it is preferable to use a 'title' child element rather than a 'title' attribute. The use of this information is highly dependent on the type of processing being done. It may be used, for example, to make titles available to applications used by visually impaired users, or to create a table of links, or to present help text that appears when a user lets a mouse pointer hover over a starting resource. Refer to the XML Linking Language (XLink) [XLINK].

Animatable: no.

xlink:show = "new' | 'replace' | 'embed' | 'other' | 'none'

This attribute is provided for backwards compatibility with SVG 1.1. It provides documentation to XLink-aware processors. In case of a conflict, the target attribute has priority, since it can express a wider range of values. Refer to the XML Linking Language (XLink) [XLINK].

Animatable: no.

xlink:actuate = "onLoad'

This attribute is provided for backwards compatibility with SVG 1.1. It provides documentation to XLink-aware processors. Refer to the XML Linking Language (XLink) [XLINK].

Animatable: no.

In all cases, for compliance with either the "Namespaces in XML 1.0" or the "Namespaces in XML 1.1" Recommendation [XML-NS10][XML-NS], an explicit XLink namespace declaration must be provided whenever one of the above XLink attributes is used within SVG content. One simple way to provide such an XLink namespace declaration is to include an 'xmlns' attribute for the XLink namespace on the 'svg' element for content that uses XLink attributes. For example:


  

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SVG provides an 'a' element, to indicate links (also known as hyperlinks or Web links). The 'a' element may contain any element that its parent may contain, except itself.

SVG uses XLink ([XLink]) for all link definitions. SVG 1.1 only requires that user agents support XLink's notion of simple links. Each simple link associates exactly two resources, one local and one remote, with an arc going from the former to the latter.

A simple link is defined for each separate rendered element contained within the 'a' element; thus, if the 'a' element contains three 'circle' elements, a link is created for each circle. For each rendered element within an 'a' element, the given rendered element is the local resource (the source anchor for the link).

The remote resource (the destination for the link) is defined by an URL specified by the 'xlink:href' attribute on the 'a' element. The remote resource may be any Web resource (e.g., an image, a video clip, a sound bite, a program, another SVG document, an HTML document, an element within the current document, an element within a different document, etc.). By activating these links (by clicking with the mouse, through keyboard input, voice commands, etc.), users may visit these resources.

Example link01 assigns a link to an ellipse.

If the above SVG file is viewed by a user agent that supports both SVG and HTML, then clicking on the ellipse will cause the current window or frame to be replaced by the W3C home page.

Attribute definitions:

Name Value Lacuna value Animatable
xlink:show new | replace (none) no
xlink:actuate onRequest (none) no

These attribute provides documentation to XLink-aware processors.

If target="_blank" then use xlink:show="new" else use 'replace'. In case of a conflict, the target attribute has priority, since it can express a wider range of values.

'xlink:actuate' indicates that an application should traverse from the starting resource to the ending resource only on a post-loading event triggered for the purpose of traversal.

Refer to the XML Linking Language (XLink) [XLINK].

Name Value Lacuna value Animatable
xlink:href <url> (none) yes
The location of the referenced object, expressed as an URL reference.
Name Value Lacuna value Animatable
target _replace | _self | _parent | _top | _blank | <XML-Name> _self yes

This attribute should be used when there are multiple possible targets for the ending resource, such as when the parent document is a multi-frame HTML or XHTML document. This attribute specifies the name or portion of the target window, frame, pane, tab, or other relevant presentation context (e.g., an HTML or XHTML frame, iframe, or object element) into which a document is to be opened when the link is activated:

_replace
The current SVG image is replaced by the linked content in the same rectangular area in the same frame as the current SVG image.
_self
The current SVG image is replaced by the linked content in the same frame as the current SVG image.
_parent
The immediate frameset parent of the SVG image is replaced by the linked content.
_top
The content of the full window or tab, including any frames, is replaced by the linked content
_blank
A new un-named window or tab is requested for the display of the linked content. If this fails, the result is the same as _top
<XML-Name>
Specifies the name of the frame, pane, or other relevant presentation context for display of the linked content. If this already exists, it is re-used, replacing the existing content. If it does not exist, it is created (the same as '_blank', except that it now has a name).

Note: The value '_new' is not a legal value for target (use '_blank').

Linking into SVG content: URL fragments and SVG views

Because SVG content often represents a picture or drawing of something, a common need is to link into a particular view of the document, where a view indicates the initial transformations so as to present a closeup of a particular section of the document.

SVG fragment identifiers

SVG 2 Requirement: Merge the SVG 1.1 SE text and the SVG Tiny 1.2 text on fragment identifiers link traversal and add media fragments.
Resolution: SVG 2 will media fragment identifiers.
Purpose: To align with Media Fragments URI.
Owner: Cyril (ACTION-3442)

To link into a particular view of an SVG document, the URL fragment identifier needs to be a correctly formed SVG fragment identifier. An SVG fragment identifier defines the meaning of the "selector" or "fragment identifier" portion of URLs that locate resources of MIME media type "image/svg+xml".

An SVG fragment identifier can come in the following forms:

  • Shorthand bare name form of addressing (e.g., MyDrawing.svg#MyView). This form of addressing, which allows addressing an SVG element by its ID, is compatible with the fragment addressing mechanism for older versions of HTML.
  • An SVG view specification (e.g., MyDrawing.svg#svgView(viewBox(0,200,1000,1000))). This form of addressing specifies the desired view of the document (e.g., the region of the document to view, the initial zoom level) completely within the SVG fragment specification. The contents of the SVG view specification are the five parameter specifications, viewBox(...), preserveAspectRatio(...), transform(...), zoomAndPan(...) and viewTarget(...), whose parameters have the same meaning as the corresponding attributes on a 'view' element, or, in the case of transform(...), the same meaning as the corresponding attribute has on a 'g' element).
  • Basic media fragments identifiers of type spatial or temporal (e.g., MyDrawing.svg#xywh=0,200,1000,1000 or MyAnimation.svg#t=10)) as defined in [Media Fragments URI 1.0 (basic)] and possibly combined using the & sign (e.g. MyDrawing.svg#xywh=0,200,1000,1000&t=10).

    This implies that it is not possible anymore in SVG 2.0 to define id for elements starting with 'xywh=' or 't='.

An SVG fragment identifier is defined as follows:

SVGFragmentIdentifier ::= BareName *( "&" timesegment ) |
                          SVGViewSpec *( "&" timesegment ) | 
                          spacesegment *( "&" timesegment ) |
                          timesegment *( "&" spacesegment )  
                
BareName ::= XML_Name
SVGViewSpec ::= 'svgView(' SVGViewAttributes ')'
SVGViewAttributes ::= SVGViewAttribute |
                      SVGViewAttribute ';' SVGViewAttributes
                      
SVGViewAttribute ::= viewBoxSpec |
                     preserveAspectRatioSpec |
                     transformSpec |
                     zoomAndPanSpec |
                     viewTargetSpec
viewBoxSpec ::= 'viewBox(' ViewBoxParams ')'
preserveAspectRatioSpec = 'preserveAspectRatio(' AspectParams ')'
transformSpec ::= 'transform(' TransformParams ')'
zoomAndPanSpec ::= 'zoomAndPan(' ZoomAndPanParams ')'
viewTargetSpec ::= 'viewTarget(' ViewTargetParams ')'

where:

  • ViewBoxParams corresponds to the parameter values for the 'viewBox' attribute on the 'view' element. For example, viewBox(0,0,200,200).
  • AspectParams corresponds to the parameter values for the 'preserveAspectRatio' attribute on the 'view' element. For example, preserveAspectRatio(xMidYMid).
  • TransformParams corresponds to the parameter values for the 'transform' property that is available on many elements. For example, transform(scale(5)).
  • ZoomAndPanParams corresponds to the parameter values for the 'zoomAndPan' attribute on the 'view' element. For example, zoomAndPan(magnify).
  • ViewTargetParams corresponds to the parameter values for the 'view/viewTarget' attribute on the 'view' element. For example, viewTarget(MyElementID).

Spaces are not allowed in fragment specifications; thus, commas are used to separate numeric values within an SVG view specification (e.g., #svgView(viewBox(0,0,200,200))) and semicolons are used to separate attributes (e.g., #svgView(viewBox(0,0,200,200);preserveAspectRatio(none))).

Semicolons used to separate 'SVGViewAttribute' in SVG fragments may be url-escaped (as %3B); this is useful when animating a (semi-colon separated) list of URLs because otherwise the semicolon would be interpreted as a list separator.

The five types of SVGViewAttribute may occur in any order, but each type may only occur at most one time in a correctly formed SVGViewSpec.

When a source document performs a link into an SVG document, for example via an HTML anchor element ([HTML4], section 12.2; i.e., <a href=...> element in HTML) or an XLink specification [XLINK], then the SVG fragment identifier specifies the initial view into the SVG document, as follows:

  • If no SVG fragment identifier is provided (e.g, the specified URL did not contain a "#" character, such as MyDrawing.svg), then the initial view into the SVG document is established using the view specification attributes (i.e., 'viewBox', etc.) on the outermost svg element.
  • If the SVG fragment identifier addresses a space segment (e.g., MyDrawing.svg#xywh=0,0,100,100),then the initial view into the SVG document is established using the view specification attributes on the outermost svg element where the 'viewBox' is overriden by the x, y, width and height values provided by the fragment identifier.

    Do we allow the "pixel:" or "percent:" part of the spatial identifier? If yes, how do they map onto SVG user units?

  • If the SVG fragment identifier addresses a time segment (e.g., MyDrawing.svg#t=10),then the initial view into the SVG document is established as if no fragment identifier was provided. The rendering of the SVG Document shall be as if the setCurrentTime method on the SVG Document element had been called with the begin time value from the fragment identifier. Additionally, if an end time value is provided in the fragment identifier, the effect is equivalent to calling the pauseAnimations method on the SVG Document when the document time reaches the end time of the fragment identifier.
  • If the SVG fragment identifier addresses a 'view' element within an SVG document (e.g., MyDrawing.svg#MyView or MyDrawing.svg#xpointer(id('MyView'))) then the closest ancestor 'svg' element is displayed in the viewport. Any view specification attributes included on the given 'view' element override the corresponding view specification attributes on the closest ancestor 'svg' element.
  • If the SVG fragment identifier addresses specific SVG view (e.g., MyDrawing.svg#svgView(viewBox(0,200,1000,1000))), then the document fragment defined by the closest ancestor 'svg' element is displayed in the viewport using the SVG view specification provided by the SVG fragment identifier.
  • If the SVG fragment identifier addresses any element other than a 'view' element, then the document defined by the closest ancestor 'svg' element is displayed in the viewport using the view specification attributes on that 'svg' element.
  • If the SVG fragment identifier addresses a combination of the above non-time related identifiers with a time-related identifier (ie. a timesegment), the behavior of each identifier is applied.

Predefined views: the 'view' element

The 'view' element is defined as follows:

Attribute definitions:

Name Value Lacuna value Animatable
viewTarget <XML-Name> (none) no
Indicates the target object associated with the view.

Highlighting views

It is helpful to users if the target element(s) are highlighted. The visual styling of this highlight should be decided by the document author, because the SVG user agent has no way to determine what changes would make the elements more visible.

The CSS :target selector ([SELECTORS], section 6.2.2) may be used in a stylesheet to provide alternate styling for elements which are the target of links. For example:

DOM interfaces

Interface SVGAElement

The SVGAElement interface corresponds to the 'a' element.

interface SVGAElement : SVGGraphicsElement {
  readonly attribute SVGAnimatedString target;
};

SVGAElement implements SVGURIReference;
Attributes:
target (readonly SVGAnimatedString)
Corresponds to attribute 'target' on the given 'a' element.

Interface SVGViewElement

The SVGViewElement interface corresponds to the 'view' element.

interface SVGViewElement : SVGElement {
  readonly attribute SVGStringList viewTarget;
};

SVGViewElement implements SVGFitToViewBox;
SVGViewElement implements SVGZoomAndPan;
Attributes:
viewTarget (readonly SVGStringList)
Corresponds to attribute 'viewTarget' on the given 'view' element. A list of DOMString values which contain the names listed in the 'viewTarget' attribute. Each of the DOMString values can be associated with the corresponding element using the getElementById() method call.