20 Private Data

20.1 Introduction

SVG allows inclusion of elements from foreign namespaces anywhere with the SVG document tree. In general, the SVG user agent will ignore any unknown elements except to include them within the DOM. (The notable exception is described under Embedding Foreign Object Types.)

SVG's ability to include foreign namespaces can be used for the following purposes:

• Application-specific information so that authoring applications can include model-level data in the SVG document to serve their "roundtripping" purposes (i.e., the ability to write, then read a file without loss of higher-level information).
• 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 parameters can be included in the SVG document by inserting elements from an extrusion extension namespace.

To illustrate, a business graphics authoring application might want to include some private data within an SVG document so that it could properly reassemble the chart (a pie chart in this case) upon reading it back in:

<?xml version="1.0" standalone="yes"?>
<svg width="4in" height="3in"
     xmlns = 'http://www.w3.org/Graphics/SVG/svg-19990625.dtd'>
  <defs>
    <myapp:piechart xmlns:myapp="http://mycompany/mapapp"
                    title="Sales by Region">
      <myapp:pieslice label="Northern Region" value="1.23"/>
      <myapp:pieslice label="Eastern Region" value="2.53"/>
      <myapp:pieslice label="Southern Region" value="3.89"/>
      <myapp:pieslice label="Western Region" value="2.04"/>
      <!-- Other private data goes here -->
    </myapp:piechart>
  </defs>
  <desc>This chart includes private data in another namespace
  </desc>
  <!-- In here would be the actual graphics elements which
       draw the pie chart -->
</svg>

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