20 Private Data

20.1 Introduction

SVG is defined to be a validating XML grammar. This means that elements not defined in the SVG (e.g., elements defined in a different namespace) are only allowed in particular designated sections of the SVG grammar.

The only places in the SVG grammar where arbitrary elements and/or character data are allowed are:

The <style> element (??? need link) within a <defs> element (??? need link), which is where CSS styles can be defined
The <script> element (??? need link) within a <defs> element (??? need link), which is where scripts (e.g., ECMAScript) can be defined
The <althtml> element (??? need link) which provides the HTML (usually maybe in the form of images) which should be used when viewing the file in a browser which doesn't yet support SVG
The <private> element which is described here. A <private> element also can occur within a <defs> element (??? need link).

The <private> element is meant for elements and character data which are not defined as a part of the formal SVG grammar. Common uses for <private> are:

A place where authoring applications can put their own private data to serve their own "roundtripping" purposes (i.e., the ability to write, then read a file without loss of higher-level information)
A place to define supplemental data for extensibility. For example, suppose you have an extrusion extension which takes any 2D graphics and extrudes it in three dimensions. When applying the extrusion extension, you probably will need to set some parameters. The <private> element is the expected place to establish the parameters for the extension.

For example, a business graphics authoring application might want to include some private data within the <defs> section when it writes a SVG file so that it could properly reassemble the chart (a pie chart in this case) upon reading it back in:

<g>
  <defs>
    <private xmlns:myapp="http://mycompany/mapapp" >
      <myapp:piechart title="Sales by Region">
        <piece label="Northern Region" value="1.23"/>
        <piece label="Eastern Region" value="2.53"/>
        <piece label="Southern Region" value="3.89"/>
        <piece label="Western Region" value="2.04"/>
        <!-- Other private data goes here -->
      </myapp:piechart>
    </private>
  </defs>
  <!-- In here would be the actual graphics elements which
       draw the pie chart -->
</g>