14 Linking

14.1 References

14.1.1 Overview

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

http://example.com/someDrawing.svg

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

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

Altering the xlink:href attribute

14.1.2 IRIs and URIs

Internationalized Resource Identifiers (IRIs) are a more generalized complement to Uniform Resource Identifiers (URIs). An IRI is a sequence of characters from the Universal Character Set [Unicode40]. A URI is constructed from a much more restricted set of characters. All URIs are already conformant IRIs. A mapping from IRIs to URIs is defined by the IRI specification, which means that IRIs can be used instead of URIs in XML documents, to identify resources. IRIs can be converted to URIs for resolution on a network, if the protocol does not support IRIs directly.

Previous versions of SVG, following XLink, defined a [IRI Reference] type as a URI or as a sequence of characters which must result in a URI reference after a particular escaping procedure was applied. The escaping procedure was repeated in the XLink 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 IRI specification was not yet standardized.

In this specification, the correct term IRI is used for this URI or sequence of characters plus an algorithm' and the escaping method is defined by reference to the IRI specification, 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 IRI directly.

14.1.3 Syntactic forms: XMLRI and funcXMLRI

An XML resource identifier is a character string meant to be used as an IRI reference or URI reference. An XML resource identifier (XMLRI) may contain characters that, according to [RFC 3987] and [RFC 3986], must be escaped before the string can be used to retrieve the referenced resource. For details, see [XLink version 1.1]. XMLRI are used in the xlink:href attribute.

Some attributes allow both IRIs and text strings as content. To disambiguate a text string from a relative IRI, the functional notation FuncXMLRIis used. This is simply an XMLRI delimited with a functional notation. For historical reasons, the delimiters are "url(" and ")". The FuncXMLRI form is used in presentation attributes and navigation attributes.

SVG makes extensive use of IRI 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 xml: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)"/>
    

14.1.4 Reference Restrictions

Some of the elements using IRI references have restrictions on them. Which parts of IRI references that are allowed on each element is listed in the table below.

The following possibilities exist in SVG. A referencing element supports one or more of these possibilities.

The referencing attribute can:

• A: Reference a fragment within the current SVG document fragment (e.g. "#someelement").
• B: Reference an SVG document fragment, either within the current document (e.g. "#someelement") or an external document (e.g. "afile.svg#anelement").
• C: Reference an SVG document (e.g. "afile.svg").
• D: Reference a media resource other than SVG, with or without the use fragments (e.g. "someimage.jpg" or "somecontainer#fragment"). Where applicable, the table shows the supported Media Types.
• E: Use data: IRIs (e.g. "data:image/jpeg;base64,/9j..."). Note that data: (if XML) produces a different tree, but the data is already loaded as it is part of the IRI itself

An IRI reference not complying to the restrictions in the table below is an invalid IRI reference.

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

• A circular IRI reference shall represent an error (see Error processing).
• When attribute externalResourcesRequired has been set to true on the referencing element or one of its ancestors, then an unresolved external IRI reference (i.e., a resource that cannot be located) shall represent an error (see Error processing).

It is recommended that, wherever possible, referenced elements be defined inside of a 'defs' element. Among the elements that are always referenced are 'linearGradient' and 'radialGradient'. Defining these elements inside of a 'defs' element promotes understandability of the SVG content and thus promotes accessibility.

14.1.5 IRI reference attributes

IRI 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 be 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 XML Resource Identifier.

If the protocol, such as HTTP, does not support IRIs directly, the IRI is converted to a URI by the SVG implementation, as described in section 3.1 of the IRI specification.

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

If the IRI reference is relative, its absolute version must be computed by the method of [XML-Base] before use.

Additional XLink attributes can be specified that provide supplemental information regarding the referenced resource.

    <define name='svg.XLinkBase.attr' combine='interleave'>
      <optional>
        <attribute name='xlink:type' svg:animatable='true' svg:inheritable='false'>
          <value>simple</value>
        </attribute>
      </optional>
      <optional>
        <attribute name='xlink:role' svg:animatable='false' svg:inheritable='false'>
          <ref name='XMLRI.datatype'/>
        </attribute>
      </optional>
      <optional>
        <attribute name='xlink:arcrole' svg:animatable='false' svg:inheritable='false'>
          <ref name='XMLRI.datatype'/>
        </attribute>
      </optional>
      <optional>
        <attribute name='xlink:title' svg:animatable='false' svg:inheritable='false'><text/></attribute>
      </optional>
    </define>

    <define name='svg.XLinkHrefRequired.attr' combine='interleave'>
      <optional>
        <attribute name='xlink:href' svg:animatable='true' svg:inheritable='false'>
          <ref name='XMLRI.datatype'/>
        </attribute>
      </optional>
    </define>

    <define name='svg.XLinkBaseRequired.attr' combine='interleave'>
      <ref name='svg.XLinkBase.attr'/>
      <ref name='svg.XLinkHrefRequired.attr'/>
    </define>


    <define name='svg.XLinkActuateOnLoad.attr' combine='interleave'>
      <optional>
        <attribute name='xlink:actuate' svg:animatable='false' svg:inheritable='false'>
          <value>onLoad</value>
        </attribute>
      </optional>
    </define>

    <define name='svg.XLinkShowOther.attr' combine='interleave'>
      <optional>
        <attribute name='xlink:show' svg:animatable='false' svg:inheritable='false'>
          <value>other</value>
        </attribute>
      </optional>
    </define>


    <define name='svg.XLinkEmbed.attr' combine='interleave'>
      <optional>
        <attribute name='xlink:show' svg:animatable='false' svg:inheritable='false'>
          <value>embed</value>
        </attribute>
      </optional>
      <ref name='svg.XLinkActuateOnLoad.attr'/>
      <ref name='svg.XLinkBaseRequired.attr'/>
    </define>


    <define name='svg.XLinkRequired.attr' combine='interleave'>
      <ref name='svg.XLinkShowOther.attr'/>
      <ref name='svg.XLinkActuateOnLoad.attr'/>
      <ref name='svg.XLinkBaseRequired.attr'/>
    </define>


    <define name='svg.XLinkReplace.attr' combine='interleave'>
      <optional>
        <attribute name='xlink:show' svg:animatable='false' svg:inheritable='false'>
          <choice>
            <value>new</value>
            <value>replace</value>
          </choice>
        </attribute>
      </optional>
      <optional>
        <attribute name='xlink:actuate' svg:animatable='false' svg:inheritable='false'>
          <value>onRequest</value>
        </attribute>
      </optional>
      <ref name='svg.XLinkBaseRequired.attr'/>
    </define>

    <define name='svg.XLink.attr' combine='interleave'>
      <optional>
        <ref name='svg.XLinkHrefRequired.attr'/>
      </optional>
      <ref name='svg.XLinkShowOther.attr'/>
      <ref name='svg.XLinkActuateOnLoad.attr'/>
      <ref name='svg.XLinkBase.attr'/>
    </define>
      

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.

Example: XLink namespace declaration

<?xml version="1.0"?>
<svg xmlns:xlink="http://www.w3.org/1999/xlink"
  xmlns="http://www.w3.org/2000/svg" version="1.2" baseProfile="tiny">
  <desc>Declaring the XLink namespace, as well as the SVG one</desc>
  <image xlink:href="foo.png"/>
</svg>
    

Example: use and animation

The two files below are the referenced files in the use and animation examples further down.

<?xml version="1.0" encoding="UTF-8"?>
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" 
     version="1.2" baseProfile="tiny" 
     xml:id="animationRef" width="150" height="50" viewBox="0 0 150 50" fill="inherit">  

    <rect xml:id="aMovingRect" width="50" height="50" rx="5" ry="5" fill="inherit" stroke-width="3" stroke="black">
         <animateTransform attributeName="transform" type="translate" values="0,0;0,100" begin="0" dur="2" fill="freeze"/>
    </rect>
</svg>
    

<?xml version="1.0" encoding="UTF-8"?>
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" 
     version="1.2" baseProfile="tiny" 
     xml:id="animationRef" width="150" height="50" viewBox="0 0 150 50" fill="inherit">  

    <rect xml:id="aMovingRect" width="50" height="50" rx="5" ry="5" fill="rgb(255,28,141)" stroke-width="3" stroke="black">
         <animateTransform attributeName="transform" type="translate" values="0,0;0,100" begin="0" dur="2" fill="freeze"/>
    </rect>
</svg>
    

The following example illustrates how to reference SVG content from the animation element. Different 'fill' values are used to show the way properties are inherited on content referenced from the animation element.

<?xml version="1.0" encoding="UTF-8"?>
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" 
     version="1.2" baseProfile="tiny" width="100%" height="100%" viewBox="0 0 580 400">  

    <g fill="rgb(157,0,79)">
        <animation x="20" xlink:href="referencedRect.svg"/>
        <animation x="100"  xlink:href="referencedRect2.svg"/>
        <animation begin="1" x="180" viewport-fill="rgb(255,28,141)" xlink:href="referencedRect.svg"/>
    </g>
    
</svg>    

The image below shows the correct rendering of the animation example above. The arrows indicates the animation. The grayed rectangles shows the initial state (i.e. time=0), the colored rectangles shows the final state (animations are completed).

The following example illustrates the different ways SVG content can be referenced from a use element. Different 'fill' values are used to show the way properties are inherited on content referenced from the use element.

<?xml version="1.0" encoding="UTF-8"?>
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" 
     version="1.2" baseProfile="tiny" width="100%" height="100%" viewBox="0 0 580 400">  
    <defs>
        <g fill="green">
            <rect xml:id="aMovingRect" width="50" height="50" rx="5" ry="5" fill="inherit" stroke-width="3" stroke="black">
                <animateTransform attributeName="transform" type="translate" values="0,0;0,100" begin="0" dur="2" fill="freeze"/>
            </rect>
        </g>
    </defs>
    
    <g fill="rgb(157,0,79)">
        <use x="20" xlink:href="#aMovingRect"/>
            
        <use x="100" fill="rgb(255,28,141)" xlink:href="#aMovingRect"/>
    
        <use x="180" xlink:href="referencedRect.svg#aMovingRect"/>

        <use x="260" fill="rgb(255,28,141)" xlink:href="referencedRect.svg#aMovingRect"/>
    </g>

</svg>    

The image below shows the correct rendering of the use example above. The arrows indicates the animation. The grayed rectangles shows the initial state (i.e. time=0), the colored rectangles shows the final state (animations are completed).

14.1.6 Externally referenced documents

In this section, the term "primary document" refers to a rootmost SVG document or SVG document fragment. If an SVG document is loaded directly by a web browser (e.g., the browser views file foo.svg), then it is the primary document. If an SVG document as a whole is referenced for inclusion by a parent document, such as using the HTML object or SVG animation elements, then that document itself shall also be a primary document. If an SVG document fragment is embedded inline within a non-SVG document, then the svg element shall define the root element for a subtree which acts as a primary document. In other words, it is the rootmost svg element for an SVG document fragment.

Implementations are free to optimize for the case of large resource library loaded into multiple primary documents, but logically each primary document shall represent its own separate, self-contained document instance. In particular, the conceptual processing model is that events (e.g., the document load event) are fired; scripts are executed in normal fashion and resource documents are modifiable by scripts; coordinate systems transformations are applied; timelines are initiated and animations begin; etc. For example, if document A.svg includes an animation element which refers to B.svg, then both are primary documents, and both shall represent separate, self-contained documents with their own scripting contexts and animation timelines.

The term "resource document" refers to a complete, self-contained SVG document which has at least one of its elements referenced as a resource by a primary document. For example, suppose document A.svg is loaded into a browser for viewing, and this document refers to a path element within B.svg via an IRI reference on a use element. In this case, A.svg is a primary document and B.svg is a resource document.

Each primary document must have an associated dictionary that maps all IRIs for resource documents it references; initially it is populated only with the primary document itself. Each resource or subresource loaded directly or indirectly must be resolved through that dictionary with resource documents downloaded as needed.

The conceptual model is that each resource document is loaded only once; if the same resource document is referenced multiple times directly or indirectly by the same primary document, that resource document is only retrieved and processed one time.

The conceptual processing model for resource documents is that the document is processed as a complete and separate SVG document instance. The only difference between a resource document and a primary document is that the primary document is rendered directly to the canvas, whereas all resource documents are conceptually rendered to an invisible (offscreen) canvas. In particular, the conceptual processing model is that events (e.g., the document load event) are fired; scripts are executed in normal fashion and resource documents are modifiable by scripts; coordinate systems transformations are applied; stylesheets are applied and the CSS cascade is run; timelines are initiated and animations begin; sXBL transformations are applied (not relevant to SVGT1.2); etc.

Because of HTTP redirects, the same source document might be retrieved from multiple different source IRIs. The rule for SVG is that documents are considered to be unique based on string comparisons of the full IRI after resolving relative IRIs into absolute IRIs and after taking into account HTTP redirects (i.e., use the post-redirect IRI instead of the original source IRI).

14.2 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 Tiny 1.2 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 IRI 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, etc.). By activating these links (by clicking with the mouse, through keyboard input, voice commands, etc.), users may traverse hyperlinks to these resources.

Example link01 assigns a link to an ellipse.

Example: 17_01.svg

<?xml version="1.0"?>
<svg width="5cm" height="3cm" viewBox="0 0 5 3" version="1.2" baseProfile="tiny"
     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>
    

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.

The element definition schema and content model for 'a' are not defined here. It is defined in all the places it can occur.

    <define name='a.AT' combine='interleave'>
      <ref name='svg.Core.attr'/>
      <ref name='svg.Conditional.attr'/>
      <ref name='svg.Properties.attr'/>
      <ref name='svg.FocusHighlight.attr'/>
      <ref name='svg.External.attr'/>
      <ref name='svg.Focus.attr'/>
      <ref name='svg.Transform.attr'/>
      <ref name='svg.XLinkReplace.attr'/>
      <optional>
        <attribute name='target' svg:animatable='true' svg:inheritable='false'>
          <choice>
            <value>_replace</value>
            <value>_self</value>
            <value>_parent</value>
            <value>_top</value>
            <value>_blank</value>
            <ref name='XML-Name.datatype'/>
          </choice>
        </attribute>
      </optional>
    </define>
      

14.3 Linking into SVG content: IRI fragments and SVG views

14.3.1 Introduction: IRI 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.

14.3.2 SVG fragment identifiers

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

An SVG fragment identifier can come in two forms:

1. Shorthand bare name form of addressing (e.g., someDrawing.svg#someView). 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 and the shorthand bare name formulation in "XPointer Framework" [XPTRFW].
2. SVG view specification (e.g., someDrawing.svg#svgView(transform(scale(2))). 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 is transform(...) whose parameters have the same meaning as the corresponding attribute has on a 'g' element).

An SVG fragment identifier is defined as follows:

SVGFragmentIdentifier ::= BareName |
                          SVGViewSpec

BareName ::= NCName
SVGViewSpec ::= 'svgView(' SVGViewAttributes ')'
SVGViewAttributes ::= SVGViewAttribute |
                      SVGViewAttribute ';' SVGViewAttributes

SVGViewAttribute ::= transformSpec
transformSpec ::= 'transform(' TransformList ')'

where:

• NCName conforms to the Namespaces in XML production for 'NCName'
• TransformList corresponds to the TransformList value. For example, transform(scale(5)).

An SVG fragment identifier must match the specified grammar. Spaces shall not be allowed in fragment specifications and commas must be used to separate numeric values within an SVG view specification.

Note: since fragment identifiers are stripped from IRIs before resolution, there is no need to escape any characters in fragments that are outside the repertoire of US-ASCII.

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

• If no SVG fragment identifier is provided (i.e., the specified IRI did not contain a "#" character, such as someDrawing.svg), then the initial view into the SVG document shall be established using the view specification attributes (i.e., viewBox, etc.) on the rootmost svg element.
• If the SVG fragment identifier addresses specific SVG view (e.g., linking-svgView-102-t.svg#svgView(transform(rotate(30, 150, 150)))), 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 (e.g., #rectId or someDrawing.svg#rectId) and the element indicated by the fragment identifier is found, then the current translation of the SVG document's coordinate system shall be adjusted such that the centerpoint of the decorated bounding box of the identified element is positioned in the center of the viewport. If the element's decorated bounding box is too large to fit within the current viewport, and the zoomAndPan attribute of the rootmost svg element is not set to 'disable', then the viewport shall not only reposition but also have the current scale expanded to accommodate the entire width and height of the element's decorated bounding box. By contrast, if the bounding box of the target element is smaller than the viewport, the viewport shall remain at the preestablished values (i.e., it will not automatically zoom in on the element). If the specified element is not found, or does not have a decorated bounding box, then the current translate and current scale are not changed from the established values. Regardless of changes to the current translation or scale of the viewport, the current rotation of the current coordinate system shall be preserved (that is, the centerpoint of the target decorated bounding box shall be the centerpoint of the rotation, with a constant rotation angle), and the existing aspect ratio shall not be altered. In the case of traversal from an external link, the viewport shall be established by the values specified in the rootmost svg element, and in the case of an internal link, the initial viewport shall additionally be adjusted by any previous zooming operations (e.g. previously navigated links, user zooming, script alterations of the current coordinate system, etc.) such that any translation or scaling that happens as a result of the traversal shall use the existing coordinate system as a starting state. If the element is not found, or does not have a decorated bounding box, then the viewport does not move or zoom. In all cases of traversal, the view shall be established instantly, with no animated panning or other enhanced transition toward the target element. The viewbox shall not be continually animated to match the animations of a target element's decorated bounding box. Future specifications may allow more customizable behavior for traversal through another mechanism.
• If the SVG fragment identifier addresses any element and the element is not found, the initial view into the SVG document shall be established using the view specification attributes (i.e., viewBox, etc.) on the rootmost svg element, as if no fragment had been specified.

Note: in SVG Tiny, only a single 'svg' element is allowed. Thus, the closest ancestor 'svg' element and the rootmost 'svg' element are the same. This is not true in other profiles of SVG, where the distinction becomes significant.