Chapter 16: Linking

Contents

• • 16.1. References
    • 16.1.1. Overview
    • 16.1.2. URLs and URIs
    • 16.1.3. Syntactic forms: URL and FuncURL
    • 16.1.4. Processing of URL references
    • 16.1.5. URL reference attributes
  • 16.2. Links out of SVG content: the ‘a’ element
  • 16.3. Linking into SVG content: URL fragments and SVG views
    • 16.3.1. SVG fragment identifiers
    • 16.3.2. Predefined views: the ‘view’ element
    • 16.3.3. Highlighting views
  • 16.4. DOM interfaces
    • 16.4.1. Interface SVGAElement
    • 16.4.2. Interface SVGViewElement

16.1. References

16.1.1. 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

16.1.2. 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.

16.1.3. Syntactic forms: URL and FuncURL

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

URLs are used in the ‘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:

<linearGradient id="MyGradient">...</linearGradient>

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

<rect fill="url(#MyGradient)"/>

SVG supports two types of URL references:

• local URL references, where the URL reference does not contain an <absoluteURL> or <relativeURL> and thus only contains a fragment identifier (i.e., #<elementID>)
• non-local URL references, where the URL reference does contain an <absoluteURL> or <relativeURL>

16.1.4. Processing of URL references

The following rules apply to the processing of URL references:

• URL references to nodes that do not exist shall be treated as invalid references.
• URL references to elements which are inappropriate targets for the given reference shall be treated as invalid references (see list below for appropriate targets). For example, the ‘clip-path’ property can only refer to ‘clipPath’ elements. The property setting clip-path:url(#MyElement) is an invalid reference if the referenced element is not a ‘clipPath’.
• URL references that directly or indirectly reference themselves are treated as invalid circular references.

If the above rules are meant to be definitions of the terms invalid reference and invalid circular reference then we should mark them up as such, and make them linkable. Should they perhaps also be moved to the intro chapter?

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

• the ‘a’ element can reference any local or non-local resource
• the ‘animate’ element (see Identifying the target element for an animation for reference rules)
• the ‘animateMotion’ element (see Identifying the target element for an animation for reference rules)
• the ‘animateTransform’ element (see Identifying the target element for an animation for reference rules)
• the ‘clip-path’ property must reference a ‘clipPath’ element
• the ‘cursor’ element must reference a resource that can provide an image for the cursor graphic
• the ‘cursor’ property must reference a resource that can provide an image for the cursor graphic
• the ‘feImage’ element must reference any local or non-local resource
• the ‘fill’ property (see Specifying paint for reference rules)
• the ‘filter’ element must reference a ‘filter’ element
• the ‘filter’ property must reference a ‘filter’ element
• the ‘image’ element must reference any local or non-local resource
• the ‘linearGradient’ element must reference a ‘linearGradient’ or ‘radialGradient’ element
• the ‘marker’, ‘marker-start’, ‘marker-mid’ and ‘marker-end’ properties must reference a ‘marker’ element.
• the ‘mask’ property must reference a ‘mask’ element
• the ‘pattern’ element must reference a ‘pattern’ element
• the ‘radialGradient’ element must reference a ‘linearGradient’ or ‘radialGradient’ element
• the ‘script’ element must reference an external resource that provides the script content
• the ‘stroke’ property (see Specifying paint for reference rules)
• the ‘textPath’ element must reference a ‘path’ element
• the ‘set’ element (see Identifying the target element for an animation for reference rules)
• the ‘use’ element can reference any local or non-local resource

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

• An invalid local URL reference (i.e., an invalid references to a node within the current document) represents an error (see Error processing), apart from the ‘xlink:href’ attribute on the ‘a’ element and the properties that allow for backup values in the case where the URL reference is invalid (see ‘fill’ and ‘stroke’).
• An invalid circular URL reference represents an error (see Error processing).

16.1.5. 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].

Need an attribute definition table.

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:

<svg xmlns:xlink="http://www.w3.org/1999/xlink" ...>
  <image xlink:href="foo.png" .../>
</svg>

16.2. Links out of SVG content: the ‘a’ element

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.

<?xml version="1.0" standalone="no"?>
<svg width="5cm" height="3cm" viewBox="0 0 5 3" version="1.1"
     xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
  <desc>Example link01 - a link on an ellipse
  </desc>
  <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" />
  </a>
</svg>

Example link01

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.

Attribute definitions:

Name	Value	Lacuna value	Animatable
xlink:href	URL [URL]	(none)	yes

The location of the referenced object, expressed as an URL reference.

Name	Value	Lacuna value	Animatable
target	_self | _parent | _top | _blank | <XML-Name>	_self	yes

Should the remaining differences with the "valid browsing context name or keyword" definition from HTML be harmonized? Specifically: HTML keywords are case-insensitive; HTML allows any string that doesn't start with "_" as a custom name, (requiring an XML name is more restrictive).

This attribute should be used when there are multiple possible targets for the ending resource, such as when the parent document is embedded within an HTML or XHTML document, or is viewed with a tabbed browser. This attribute specifies the name of the browsing context (e.g., a browser tab or an SVG, HTML, or XHTML iframe or object element) into which a document is to be opened when the link is activated:

_self

The current SVG image is replaced by the linked content in the same browsing context as the current SVG image.

_parent

The immediate parent browsing context of the SVG image is replaced by the linked content, if it exists and can be securely accessed from this document.

_top

The content of the full active window or tab is replaced by the linked content, if it exists and can be securely accessed from this document

_blank

A new un-named window or tab is requested for the display of the linked content, if this document can securely do so. If the user agent does not support multiple windows/tabs, the result is the same as _top.

<XML-Name>

Specifies the name of the browsing context (tab, inline frame, object, etc.) for display of the linked content. If a context with this name already exists, and can be securely accessed from this document, 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). The name must be a valid XML Name [XML11], and should not start with an underscore (U+005F LOW LINE character), to meet the requirements of a valid browsing context name from HTML.

The normative definitions for browsing contexts and security restrictions on navigation actions between browsing contexts is HTML 5 [HTML], specifically the chapter on loading web pages.

Previous versions of SVG defined the special target value '_replace'. It was never well implemented, and the distinction between '_replace' and '_self' has been made redundant by changes in the HTML definition of browsing contexts. Use '_self' to replace the current SVG document.

The value '_new' is not a legal value for target. Use '_blank' to open a document in a new tab/window.

16.3. 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.

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 ')'

16.3.2. Predefined views: the ‘view’ element

The ‘view’ element is defined as follows:

Attribute definitions:

Name	Value	Lacuna value	Animatable
viewTarget	(see below)	(none)	no

Indicates the target object associated with the view. This attribute must be any value other than the empty string.

Define the expected effect of declaring a 'viewTarget', here or in an svgView target fragment.

16.3.3. 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:

<style type="text/css">
#foo:target {filter: url(#glow)}
/* when the element with id foo is linked to, use a glow filter */

.bar :target {stroke: green; fill-opacity: 0.5}
/* when any descendants of elements with class bar are linked
   to, make the fill partly transparent and use a green stroke */

:target {stroke: red }
/* for everything else, just use a red stroke */ 
</style>

16.4. DOM interfaces

16.4.1. 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.

16.4.2. 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.