19 July, 2001

17 Linking



17.1 Links out of SVG content: the 'a' element

SVG provides an 'a' element, analogous to HTML's 'a' element, to indicate links (also known as hyperlinks or Web links). SVG uses XLink ([XLink]) for all link definitions.

SVG 1.0 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 a URI 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.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20010719//EN" 
<svg width="5cm" height="3cm" viewBox="0 0 5 3">
  <desc>Example link01 - a link on an ellipse
  <rect x=".01" y=".01" width="4.98" height="2.98" 
        fill="none" stroke="blue"  stroke-width=".03"/>
  <a xlink:href="http://www.w3.org">
    <ellipse cx="2.5" cy="1.5" rx="2" ry="1"
             fill="red" />
Example link01
Example link01 - a link on an ellipse

View this example as SVG (SVG-enabled browsers only)

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.

<!ENTITY % aExt "" >
<!ELEMENT a       (#PCDATA|desc|title|metadata|defs|
                   %ceExt;%aExt;)* >
  xmlns:xlink CDATA #FIXED "http://www.w3.org/1999/xlink"
  xlink:type (simple) #FIXED "simple" 
  xlink:role %URI; #IMPLIED
  xlink:arcrole %URI; #IMPLIED
  xlink:title CDATA #IMPLIED
  xlink:show (new|replace) 'replace'
  xlink:actuate (onRequest) #FIXED 'onRequest'
  xlink:href %URI; #REQUIRED
  externalResourcesRequired %Boolean; #IMPLIED
  class %ClassList; #IMPLIED
  style %StyleSheet; #IMPLIED
  transform %TransformList; #IMPLIED
  target %LinkTarget; #IMPLIED >

Attribute definitions:

xmlns [:prefix] = "resource-name"
Standard XML attribute for identifying an XML namespace. This attribute makes the XLink [XLink] namespace available to the current element. Refer to the "Namespaces in XML" Recommendation [XML-NS].
Animatable: no.
xlink:type = 'simple'
(See generic description of xlink:type attribute.)
xlink:role = '<uri>'
(See generic description of xlink:role attribute.)
xlink:arcrole = '<uri>'
(See generic description of xlink:arcrole attribute.)
xlink:title = '<string>'
(See generic description of xlink:title attribute.)
xlink:show = 'new | replace'
Indicates whether, upon activation of the link, traversing to the ending resource should load it in a new window, frame, pane, or other relevant presentation context or load it in the same window, frame, pane, or other relevant presentation context in which the starting resource was loaded. Refer to the "XML Linking Language (XLink)" [XLink].
Animatable: no.
xlink:actuate = 'onRequest'
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].
Animatable: no.
xlink:href = "<uri>"
The location of the referenced object, expressed as a URI reference. Refer to the "XML Linking Language (XLink)" [XLink].
Animatable: yes.
target = "<frame-target>"
This attribute has applicability 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 of the target location (e.g., an HTML or XHTML frame) into which a document is to be opened when the link is activated. For more information on targets, refer to the appropriate HTML or XHTML specifications.
Animatable: yes.
Attributes defined elsewhere:
%stdAttrs; %langSpaceAttrs;, class, transform, %graphicsElementEvents;, %testAttrs;, externalResourcesRequired style, %PresentationAttributes-All;.

17.2 Linking into SVG content: URI fragments and SVG views

17.2.1 Introduction: URI fragments and SVG views

On the Internet, resources are identified using URIs (Uniform Resource Identifiers) [URI]. For example, an SVG file called MyDrawing.svg located at http://example.com might have the following URI:


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


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.

17.2.2 SVG fragment identifiers

To link into a particular view of an SVG document, the URI 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 URIs that locate resources of MIME media type "image/svg+xml".

An SVG fragment identifier can come in three forms:

An SVG fragment identifier is defined as follows:

SVGFragmentIdentifier ::= BareName |
                          XPointerIDRef |
BareName ::= XML_Name

SVGViewSpec ::= 'svgView(' SVGViewAttributes ')'

SVGViewAttributes ::= SVGViewAttribute |
                      SVGViewAttribute ';' SVGViewAttributes
SVGViewAttribute ::= viewBoxSpec |
                     preserveAspectRatioSpec |
                     transformSpec |
                     zoomAndPanSpec |

viewBoxSpec ::= 'viewBox(' ViewBoxParams ')'

preserveAspectRatioSpec = 'preserveAspectRatio(' AspectParams ')'

transformSpec ::= 'transform(' TransformParams ')'

zoomAndPanSpec ::= 'zoomAndPan(' ZoomAndPanParams ')'

viewTargetSpec ::= 'viewTarget(' ViewTargetParams ')'


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))).

When a source document performs a link into an SVG document via an HTML [HTML4] anchor element (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:

17.2.3 Predefined views: the 'view' element

The 'view' element is defined as follows:

<!ENTITY % viewExt "" >
<!ELEMENT view (%descTitleMetadata;%viewExt;) >
<!ATTLIST view
  externalResourcesRequired %Boolean; #IMPLIED
  viewBox %ViewBoxSpec; #IMPLIED
  preserveAspectRatio %PreserveAspectRatioSpec; 'xMidYMid meet'
  zoomAndPan (disable | magnify) 'magnify' 
  viewTarget CDATA #IMPLIED >

Attribute definitions:

viewTarget = "XML_Name [XML_NAME]*"
Indicates the target object associated with the view. If provided, then the target element(s) will be highlighted.
Animatable: no.

Attributes defined elsewhere:
%stdAttrs;, viewBox, preserveAspectRatio, zoomAndPan externalResourcesRequired.

17.3 DOM interfaces

The following interfaces are defined below: SVGAElement, SVGViewElement.

Interface SVGAElement

The SVGAElement interface corresponds to the 'a' element.

IDL Definition
interface SVGAElement : 
                events::EventTarget { 

  readonly attribute SVGAnimatedString target;

readonly SVGAnimatedString target
Corresponds to attribute target on the given 'a' element.

Interface SVGViewElement

The SVGViewElement interface corresponds to the 'view' element.

IDL Definition
interface SVGViewElement : 
                SVGZoomAndPan { 

  readonly attribute SVGStringList viewTarget;

readonly SVGStringList viewTarget
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.