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<!-- author Thomas.DeWeese@kodak.com                                        -->
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<!DOCTYPE document PUBLIC "-//APACHE//DTD Documentation V2.0//EN" "http://forrest.apache.org/dtd/document-v20.dtd">
<document>
  <header>
    <title>Extending Batik</title>
  </header>

  <body>
    <p>
      This page provides an overview of the built in extension mechanisms of
      Batik.  As an open source project, people can of course make any
      extension they feel is interesting, however Batik has been
      designed with several forms of extension in mind.
    </p>

    <p>
      In general, extensions are added through the Service
      Provider Interface mechanism as described in the 
      <a href="http://java.sun.com/j2se/1.4/docs/guide/jar/jar.html#Service Provider">jar
        file documentation</a>.  This allows for the extension of Batik
      simply by adding a new jar file(s) to the class path, and thus no
      modification of the Batik source is required!
    </p>
    <note>
      If you feel that the Batik team has overlooked an
      important area for extension please let your feelings be
      known on the mailing lists.
    </note>

    <section id="customXMLTags">
      <title>Custom XML elements</title>
      <p>
        First one must ask what it means to support custom 
        XML elements?  There are three basic options Batik considers:
      </p>
      <dl>
        <dt>Having your elements appear in the DOM tree</dt>
        <dd>
          <p>
            As long as your custom elements are well formed
            XML they will appear in the SVG DOM tree.  When rendering Batik
            will skip branches of the tree that use elements it doesn’t know
            about (so even if standard SVG elements are child nodes they will
            not be displayed).  Note that you must make use of XML namespaces
            for your personal elements even if you are not planning on
            validating the XML.
          </p>
          <p>
            This can be useful if you want to add extra pieces of
            data into the standard SVG drawing.  These might be
            annotations, or other application specific data.  In
            general this wouldn’t be particularly useful with
            squiggle (the SVG browser) or the rasterizer, but
            might be very useful if you were writing a custom
            browser, rasterizer, or pre/post processing tools.
          </p>
        </dd>

        <dt>Adding functionality to your custom element DOM objects</dt>
        <dd>
          <p>
            If you need your elements to use a custom element subclass
            in the DOM tree (for behavioral or performance reasons) then you
            need to provide an <a href="#domExtension">extension to the Batik
              DOM</a>.
          </p>
          <p>
            Doing this gives you the opportunity to override the standard
            methods on DOM elements, or to provide additional methods to
            your DOM elements.  For example, you may wish to add specialized
            get and set methods for attributes on your custom elements, so
            that they can be manipulated more easily than just using the
            string-based <code>getAttribute</code> and
            <code>setAttribute</code> methods provided by DOM Core.
          </p>
        </dd>
  
        <dt>Having your custom elements be rendered</dt>
        <dd>
          <p>
            Probably the most common reason to develop custom elements is to
            add new rendering primitives to the SVG language.  In this case
            you must provide an <a href="#bridgeExtension">extension to the
              Batik bridge</a>.  The bridge extension is resposible
            for constructing the class(es) that will handle the rendering of
            the new primitive in Batik.
          </p>
          <p>
            In most cases it will also be necessary to write a DOM
            extension to make the element behave like other SVG elements
            (most notably for support of styling).
          </p>
        </dd>
      </dl>

      <section id="domExtension">
        <title>Writing a Batik DOM extension</title>
        <p>
          The ability to extend the elements used in the SVG DOM tree allows
          users to provide implementations for nodes that can be used in
          place of Batik’s default implementation of a node.  This may be
          done for a variety of reasons, but is most commonly done to
          extend the behavior of standard node calls (such as
          to include styling in attribute lookup), or to
          implement the DOM interface for an element.
        </p>
        <p>
          The key class for building the DOM tree is
          <a class="class" href="../javadoc/org/apache/batik/dom/ExtensibleSVGDOMImplementation.html">org.apache.batik.dom.ExtensibleSVGDOMImplementation</a>.
          When an instance of this class is constructed it searches for
          instances of the
          <a class="class" href="../javadoc/org/apache/batik/dom/svg/DomExtension.html">org.apache.batik.dom.svg.DomExtension</a>
          Service Provider Interface.  It then calls the
          <code>registerTags</code> method, passing itself as the only
          parameter.  This method typically would typically call
          <code>registerCustomElementFactory</code> for each element that
          it wishes to handle.
        </p>
        <p>
          With Batik the most likely reason to extend a node is to provide
          proper CSS styling of the node attributes.  To this end Batik
          provides a class you can extend:
          <a class="class" href="../javadoc/org/apache/batik/extension/PrefixableStylableExtensionElement.html">org.apache.batik.extension.PrefixableStylableExtensionElement</a>.
          If you derive a new DOM class from this class you are only
          required to implement three methods: <code>getLocalName</code>,
          <code>getNamespaceURI</code>, and <code>newNode</code> 
          (plus constructors).  If all you want is proper style support
          (commonly the case) then you are done implementing your element
          at this point.
        </p>

        <p>
          The distribution comes with a number of examples:
        </p>
        <ul>
          <li><code>org.apache.batik.extension.svg.BatikStarElement</code></li>
          <li><code>org.apache.batik.extension.svg.BatikRegularPolygonElement</code></li>
          <li><code>org.apache.batik.extension.svg.BatikHistogramNormalizationElement</code></li>
          <li><code>org.apache.batik.extension.svg.SolidColorElement</code></li>
          <li><code>org.apache.batik.extension.svg.ColorSwitchElement</code></li>
        </ul>
        <p>
          Included with these examples is 
          <code>org.apache.batik.extension.svg.BatikDomExtension</code>,
          which is the required instance of <code>DomExtension</code>
          used to register the elements with the
          <code>ExtensibleSVGDOMImplementation</code>.
        </p>
        <p>
          If your new element requires new “presentation attributes”
          (XML attributes that can be modified through CSS, or, depending 
          on your viewpoint, the other way around&#x2014;CSS properties that 
          can be specified using XML attributes), you will also need to
          extend the CSS engine.  This can be done by registering a
          custom CSS value factory.  Both of the color examples do
          this (see <code>BatikDomExtension</code>).
        </p>
      </section>

      <section id="bridgeExtension">
        <title>Writing a Batik bridge extension</title>
        <p>
          Before you write a bridge extension it may be useful to
          understand what role the bridge package plays in
          Batik. The bridge package is responsible for creating and
          maintaining elements in the Graphics Vector Toolkit (GVT)
          tree based on the corresponding element in the SVG DOM. This
          is done because, for a variety of reasons, the SVG DOM is not
          well suited for rendering, thus the GVT tree is used for all
          rendering and transcoding operations.
        </p>
        <p>
          The key class for managing this link is the
          <a class="class" href="../javadoc/org/apache/batik/bridge/BridgeContext.html">BridgeContext</a>.
          This class maintains an association between a element name
          with namespace and a particular bridge instance that will
          handle it. The work of constructing the proper entity or
          entities in the GVT tree is then deferred to the
          <a class="class" href="../javadoc/org/apache/batik/bridge/Bridge.html">Bridge</a>
          registered for a particular element. If no bridge is
          regiestered nothing is done.
        </p>
        <p>
          New associations can be added by implementors of the
          <a class="class" href="../javadoc/org/apache/batik/bridge/BridgeExtension.html">BridgeExtension</a>
          Service Provider Interface. This interface has a number
          of methods that provide information about the particular
          extension being registered (including contact information,
          and the list of implemented extensions). It also has a
          <code>registerTags</code> method which is responsible for
          registering the bridge instances with a
          <code>BridgeContext</code>. All the built-in
          bridges are bundled together with a
          <code>BridgeExtension</code> (the
          <a class="class" href="../javadoc/org/apache/batik/bridge/SVGBridgeExtension.html">org.apache.batik.bridge.SVGBridgeExtension</a>
          class), as are the example extensions
          (<a class="class" href="../javadoc/org/apache/batik/extension/svg/BatikBridgeExtension.html">org.apache.batik.extension.svg</a>),
          so these are both good places to start.
        </p>
        <p>
          The <code>Bridge</code> interface itself is very simple. 
          It only includes methods to get the namespace and local 
          name of the element the bridge is responsible for.  This
          interface is then extended for each of the major
          concepts present in SVG:
        </p>
        <dl>
          <dt><a href="#graphicsNodeBridge">GraphicsNodeBridge</a></dt>
          <dd>
            <p>
              These are probably the most common SVG elements, as
              they represent graphic elements in the “visible” SVG tree.
              These are the elements most other bridges modify in some way
              (by clipping, masking, filtering, etc).
            </p>
            <p>
              <em>Example SVG elements:</em>
              <code>svg</code>, <code>g</code>, <code>path</code>,
              <code>rect</code>.
            </p>
            <p>
              <em>Example extension bridges:</em>
              <code>BatikRegularPolygonElementBridge</code>,
              <code>BatikStarElementBridge</code>.
            </p>
          </dd>
          <dt>FilterBridge</dt> 
          <dd>
            <p>
              This handles the SVG <code>filter</code> element.  If you
              wanted to implement a new element that could be
              referenced from the <code>filter</code> attribute on an SVG
              graphics node then you would need to subclass this
              bridge. However, adding new types of filters to the
              existing SVG <code>filter</code> element is accomplished via the
              FilterPrimitiveBridge.
            </p>
            <p>
              <em>Example SVG element:</em> <code>filter</code>
            </p>
          </dd>
          <dt><a href="#filterPrimitiveBridge">FilterPrimitiveBridge</a></dt> 
          <dd>
            <p>
              This constructs an element in the filter chain
              applied to an SVG graphics node.
            </p>
            <p>
              <em>Example SVG elements:</em>
              <code>feBlend</code>, <code>feCompose</code>,
              <code>feGaussianBlur</code>.
            </p>
            <p>
              <em>Example extension bridge:</em>
              <code>BatikHistogramNormalizationElementBridge</code>
            </p>
          </dd>
          <dt><a href="#paintBridge">PaintBridge</a></dt>
          <dd>
            <p>
              This constructs a Java
              <a class="class" href="http://java.sun.com/j2se/1.5.0/docs/api/java/awt/Paint.html">Paint</a>
              object to be used in filling or stroking graphic elements.
            </p>
            <p>
              <em>Example SVG elements:</em>
              <code>gradient</code>, <code>pattern</code>.
            </p>
            <p>
              <em>Example extension bridge:</em>
              <code>ColorSwitchBridge</code>.
            </p>
          </dd>
          <dt>ClipBridge</dt>
          <dd>
            <p>
              This constructs a
              <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/renderable/ClipRable.html">ClipRable</a>
              to apply to a graphics node.  This provides a path that data
              is clipped to.
            </p>
            <p>
              <em>Example SVG element:</em> <code>clipPath</code>.
            </p>
          </dd>
          <dt>MarkerBridge</dt> 
          <dd>
            <p>
              This constructs a
              <a class="class" href="../javadoc/org/apache/batik/gvt/Marker.html">Marker</a>
              for annotating the path of a graphics node.
            </p>
            <p>
              <em>Example SVG element:</em> <code>marker</code>.
            </p>
          </dd>
          <dt>MaskBridge</dt>
          <dd>
            <p>
              This constructs a mask filter to apply to a graphics
              node.  Mask filters typically modify the alpha channel of the
              graphics node output to make portions fully or partially
              transparent that wouldn’t be otherwise.
            </p>
            <p>
              <em>Example SVG element:</em> <code>mask</code>.
            </p>
          </dd>
        </dl>

        <p>
          Extension writers are free to work with any of the
          above bridges, however the three most common are
          likely to be the <code>GraphicsNodeBridge</code>, the
          <code>FilterPrimitiveBridge</code>, and the 
          <code>PaintBridge</code> (each of which has example
          extensions available for inspection).  Each of these interfaces
          has several very useful subclasses that handle much of the
          common behavior among elements.
        </p>

        <p>
          In some simple cases it is possible to provide only an
          extension to the bridge and achieve your desired effect,
          however in most cases you will find that for your
          element to behave like a normal SVG element (for
          example, to support styling) you will need to provide
          a DOM extension as well.
        </p>

        <section id="graphicsNodeBridge">
          <title>GraphicsNodeBridge</title>
          <p>
            The graphics node bridge is oriented around
            constructing a new
            <a class="class" href="../javadoc/org/apache/batik/gvt/GraphicsNode.html">GraphicsNode</a>
            in the GVT tree. The <code>GraphicsNode</code> is the basic
            object that makes up the GVT tree.  Each
            <code>GraphicsNode</code> has a <code>paint</code> method that
            is responsible for painting the object (including considering
            clipping, masking, filtering, and opacity for the
            node).
          </p>
          <p> 
            If you want to you can implement the
            <a class="class" href="../javadoc/org/apache/batik/bridge/GraphicsNodeBridge.html">GraphicsNodeBridge</a>
            interface directly, or you can subclass the
            <a class="class" href="../javadoc/org/apache/batik/bridge/AbstractGraphicsNodeBridge.html">AbstractGraphicsNodeBridge</a>
            class.  This gives you the most flexibility since you can
            construct your new subclass of <code>GraphicsNode</code>, where
            you can implement the paint method to do essentially anything
            you want.  This is quite involved, however, and the steps
            necessary to create a full <code>GraphicsNodeBridge</code>
            are not detailed here.
          </p>
          <p>
            However, if you just want to generate a custom
            filled or stroked shape the easiest way is to
            subclass one of the following two classes.  In this
            case you are essentially only responsible for
            constructing a standard Java
            <a class="class" href="http://java.sun.com/j2se/1.5.0/docs/api/java/awt/Shape.html">Shape</a>
            object to describe the desired area to operate on:
          </p>
          <dl>
            <dt>SVGShapeElementBridge</dt>
            <dd>
              <p>
                Subclasses of this class only need to implement
                <code>buildShape</code>, <code>getNamespaceURI</code>,
                and <code>getLocalName</code>.  <code>buildShape</code>
                generally constructs a <code>Shape</code> object and sets it
                on the provided <code>shapeNode</code> object, however it
                may adjust other features of the given shape node.
              </p>
            </dd>
            <dt>SVGDecoratedShapeElementBridge</dt>
            <dd>
              <p>
                This is very similar to <code>SVGShapeElementBridge</code>,
                except that it also handles the standard marker properties.
                Markers will be placed at the end of each segment of
                the path that describes the shape.
              </p>
            </dd>
          </dl>
          <p>
            If you decide that you need to implement a new subclass
            of <code>GraphicsNode</code> it is strongly suggested that
            you extend
            <a class="class" href="../javadoc/org/apache/batik/gvt/AbstractGraphicsNode.html">AbstractGraphicsNode</a>,
            as this class does much of the work to behave like other
            rendered elements in SVG (like clipping, filtering and
            masking).  In this case you implement the
            <code>primitivePaint</code> method instead of the
            <code>paint</code> method.
          </p>
        </section>

        <section id="filterPrimitiveBridge">
          <title>FilterPrimitiveBridge</title>
          <p>
            The
            <a class="class" href="../javadoc/org/apache/batik/bridge/FilterPrimitiveBridge.html">FilterPrimitiveBridge</a>
            is concerned with the construction of individual elements of the
            filter chain.  Unlike graphics nodes, which generally just
            draw new objects on top of the destination, filters take
            existing image data and modify it to apply effects.
          </p>
          <p>
            This part of GVT rendering is based on the Java2D
            <a class="class" href="http://java.sun.com/j2se/1.5.0/docs/api/java/awt/image/renderable/RenderableImage.html">java.awt.image.renderable.RenderableImage</a>
            and <a class="class" href="http://java.sun.com/j2se/1.5.0/docs/api/java/awt/image/renderable/RenderedImage.html">java.awt.image.RenderedImage</a>
            interfaces. These provide a convenient framework to handle
            image processing (an inherently resolution dependent
            operation) in the resolution independent system defined by
            SVG.
          </p>
          <p>
            The <code>org.apache.batik.ext.awt.image</code> package
            hierarchy contains a large set of generally useful extensions
            to the core JDK classes and methods, that help to implement
            SVG-related graphics operations.
          </p>
          <p>
            Note that the <code>FilterPrimitiveBridge</code> is invoked 
            once for each reference to the <code>filter</code> element that
            the filter primitive is part of.  So if a filter effect is used
            a half dozen times the <code>createFilter</code> method will be
            called a half dozen times, even though the element may only
            appear once in the file. This means that it is safe for the
            filters returned to be “fixed” for a particular
            <code>GraphicsNode</code> being filtered.
          </p>
          <p>
            You will notice that Batik uses extended versions of
            the standard <code>RenderableImage</code> and <code>RenderedImage</code> 
            interfaces to provide additional information about
            surrounding requirements for operations as well as a few
            convenience methods.  These interfaces are called:
            <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/renderable/Filter.html">org.apache.batik.ext.awt.image.renderable.Filter</a>
            and
            <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/rendered/CacheableRed.html">org.apache.batik.ext.awt.image.rendered.CacheableRed</a>.
            Batik contains simple wrapper classes that can take the default
            JDK <code>RenderableImage</code> and <code>RenderedImage</code>
            interfaces.  Within the codebase the naming convention “Red” for
            classes implementing <code>RenderedImage</code> and “Rable” for classes 
            implementing <code>RenderableImage</code> is commonly
            used (“Red” is to be pronounced like the color, and
            “Rable” is to be pronounced like “horrible” with a
            silent “h”).
          </p>
          <p>
            The <code>FilterPrimitiveBridge</code> has only
            one method, <code>createFilter</code>, that must 
            construct an instance of <code>Filter</code> to perform 
            the required operation.  This is still a fairly complex task 
            given the general need to support accessing the various
            standard sources of image data.  To this end there is
            a provided subclass,
            <a class="class" href="../javadoc/org/apache/batik/bridge/AbstractSVGFilterPrimitiveElementBridge.html">AbstractSVGFilterPrimitiveElementBridge</a>,
            that provides convenience methods to handle many common tasks.
          </p>
          <p>
            Generally the bulk of the work in writing a filter
            extension is the writing of the concrete <code>Filter</code>
            class, not tying it into the GVT tree.  Batik does contain
            several base classes that make this processes a bit
            easier:
            <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/renderable/AbstractRable.html">org.apache.batik.ext.awt.image.renderable.AbstractRable</a>,
            <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/rendered/AbstractRed.html">org.apache.batik.ext.awt.image.rendered.AbstractRed</a>,
            and
            <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/rendered/AbstractTiledRed.html">org.apache.batik.ext.awt.image.rendered.AbstractTiledRed</a>,
            <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/rendered/TiledRed.html">TiledRed</a>
            ties into the Batik tile cache (use this with caution as it is a
            complex area of the Batik code).
          </p>
          <p>
            The <code>org.apache.batik.ext.awt.image.rendered</code>
            and <code>org.apache.batik.ext.awt.image.renderable</code>
            packages contain quite a number of fairly general examples
            covering most common cases, please refer to them for more detail.
          </p>
        </section>

        <section id="paintBridge">
          <title>PaintBridge</title>
          <p>
            The
            <a class="class" href="../javadoc/org/apache/batik/bridge/PaintBridge.html">PaintBridge</a>
            constructs an instance of <code>java.awt.Paint</code> to be used
            to fill or stroke shapes/text (part of the paint server
            architecture of SVG).
          </p>
          <p>
            Like the filter primitive bridge, the <code>PaintBridge</code> 
            is invoked for each reference to the paint.  This makes
            it possible to customize the <code>Paint</code> returned for the
            particular element to be painted.
          </p>
          <p>
            This is how gradients and patterns are implemented
            in Batik, so it is possible to construct rather
            complex paint effects through this mechanism.
          </p>
          <p>
            For paints you are mostly on your own, because unlike
            the other cases there aren’t any really generally useful
            base classes to derive off, the closest is the
            <a class="class" href="../javadoc/org/apache/batik/bridge/AbstractSVGGradientElementBridge.html">AbstractSVGGradientElementBridge</a>,
            which is used to handle most of the radial and linear gradient
            attributes.  
          </p>
          <p>
            The existing gradient paint implementations are in the
            <code>org.apache.batik.ext.awt</code>, and the pattern
            implementation is in <code>org.apache.batik.gvt</code> since it
            requires access to GVT internals.
          </p>
        </section>
      </section>
    </section>

    <section id="imageTagFormats">
      <title>New image file formats</title>
      <p> 
        When Batik encounters an <code>image</code> element and it
        determines the element does not reference an SVG file, it defers
        the loading of the referenced image to
        <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/spi/ImageTagRegistry.html">org.apache.batik.ext.awt.image.spi.ImageTagRegistry</a>. 
        This class maintains a list of
        <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/spi/RegistryEntry.html">RegistryEntry</a>s,
        generally one for each format.
      </p>
      <p>
        Since the formats supported natively by Batik are also
        implemented through this mechanism. The 
        <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/codec/jpeg/JPEGRegistryEntry.html">JPEGRegistryEntry</a>
        and
        <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/codec/png/PNGRegistryEntry.html">PNGRegistryEntry</a>
        classes should be used as good references for extensions.
      </p>
      <section>
        <title>RegistryEntry</title>
        <p>
          There are currently two flavors of <code>RegistryEntry</code>:
        </p>
        <dl>
          <dt>URLRegistryEntry</dt>
          <dd>
            <p>
              A
              <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/spi/URLRegistryEntry.html">URLRegistryEntry</a>
              takes a
              <a class="class" href="../javadoc/org/apache/batik/util/ParsedURL.html">ParsedURL</a>
              and tries to decide if the URL is intended for it.  This group
              of entries is mostly intended to handle alternate network
              protocols.  It can also be useful for interfacing with
              libraries that want a URL instead of a stream.
            </p>
          </dd>
          <dt>StreamRegistryEntry</dt>
          <dd>
            <p>
              A
              <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/spi/StreamRegistryEntry.html">StreamRegistryEntry</a>
              works with a markable
              <a class="class" href="http://java.sun.com/j2se/1.5.0/docs/api/java/io/InputStream.html">InputStream</a>. 
              This is the preferred form of registry entry as it generally
              avoids opening a potentially expensive connection multiple
              times, instead it opens the stream once and relies on mark and
              reset to allow entries to check the stream.
            </p>
          </dd>
        </dl>
      </section>

      <section>
        <title>Helper classes</title>
        <p> 
          There exists quite a number of classes to assist in
          implementing a <code>RegistryEntry</code>.  It is strongly
          recommended that you review these classes and make use
          of them where appropriate.  They will likely save you
          time and improve the integration with Batik.
        </p>
        <dl>
          <dt>MagicNumberRegistryEntry</dt>
          <dd>
            <p>
              <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/spi/MagicNumberRegistryEntry.html">MagicNumberRegistryEntry</a>
              is an abstract class that can handle the
              <code>isCompatibleStream</code> method for formats that 
              make use of “magic numbers.”  Magic numbers are a well 
              known sequence of bytes at a well known offset in the
              file, that are commonly used to identify image file formats.
            </p>
          </dd>
          <dt>RedRable</dt>
          <dd>
            <p>
              <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/renderable/RedRable.html">RedRable</a>
              takes any <code>java.awt.image.RenderedImage</code> and wraps it
              into a <code>Filter</code> (Batik’s subinterface of
              <code>RenderableImage</code>).  This is very useful for
              <em>single resolution</em> file formats.  
            </p>
          </dd>
          <dt>DeferRable</dt>
          <dd>
            <p>
              <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/renderable/DeferRable.html">DeferRable</a>
              allows one to load the image in a background
              thread, rather than hold up the construction of the
              GVT tree while reading the image (useful since
              reading the image is generally I/O bound, so it
              makes a good background task).  This is used by most of
              the current image readers.
            </p>
          </dd>
          <dt>AbstractRable</dt>
          <dd>
            <p>
              <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/renderable/AbstractRable.html">AbstractRable</a>
              is an abstract base class that makes it relatively
              easy to implement the Filter interface.
            </p>
          </dd>
          <dt>AbstractRed</dt>
          <dd>
            <p>
              <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/rendered/AbstractRed.html">AbstractRed</a>
              is an abstract base class that makes it relatively
              easy to implement the
              <a class="class" href="../javadoc/org/apache/batik/ext/awt/image/rendered/CacheableRed.html">CacheableRed</a>
              interface (Batik's subclass of <code>RenderedImage</code>).
            </p>
          </dd>
        </dl>
      </section>
    </section>

    <section id="urlProtocols">
      <title>New URL protocols</title>
      <p> 
        For a variety of reasons (not the least of which is the
        heavy use of the <code>data:</code> URL protocol in SVG), several
        parts of Batik use a
        <a class="class" href="../javadoc/org/apache/batik/util/ParsedURL.html">ParsedURL</a>
        instead of the JDK’s
        <a class="class" href="http://java.sun.com/j2se/1.5.0/docs/api/java/net/URL.html">java.net.URL</a>
        class.
      </p>
      <p>
        <code>ParsedURL</code> offers a few advantages over the JDK’s 
        <code>URL</code> class. First, it is designed to make minimal use of
        exceptions, so it is possible to use it to parse a malformed 
        URL and get “the good parts”.  Second, it is extensible, so 
        support for new URL protocols can be added, even those that 
        change the normal parsing rules for URLs (such as our friend the 
        <code>data:</code> protocol).  Third, it can automatically check a
        when a stream can be opened for common compression types and decode
        them for you (this behavior can also be bypassed if needed).
      </p>
      <p>
        The service class is
        <a class="class" href="../javadoc/org/apache/batik/util/ParsedURLProtocolHandler.html">org.apache.batik.util.ParsedURLProtocolHandler</a>.
        This interface consists of three methods: one returns the protocol
        to be handled, one is for parsing an absolute URL string and
        one is for parsing relative URL strings. Both the parsing
        methods return an object of type
        <a class="class" href="../javadoc/org/apache/batik/util/ParsedURLData.html">ParsedURLData</a>
        (the instance may of course be a subclass of
        <code>ParsedURLData</code>).
      </p>
      <p>
        The <code>ParsedURLData</code> class holds all the data and 
        implements all the stream handling commands for the 
        <code>ParsedURL</code> class. This allows
        <code>ParsedURLProtocolHandler</code>s to return custom subclasses
        for particular protocols.
      </p>
    </section>

    <section id="interpreters">
      <title>Additional script interpreters</title>

      <p>
        While conforming SVG browsers need support only ECMAScript as a scripting
        language, Batik can support any scripting language given the right glue
        to connect it to the rest of the system.
      </p>
      <p>
        Script interpreters are also handled via the Service Provider Interface,
        The interface that needs to be implemented to expose a new interpreter
        to Batik is
        <a class="class" href="../javadoc/org/apache/batik/script/InterpreterFactory.html">InterpreterFactory</a>.
        This class has two methods: <code>getMimeType</code>, which returns
        a string that specifies what script type this intepreter handles
        (specifically, what the <code>type</code> attribute of
        <code>script</code> elements must be for them to be handled by this
        intepreter), and <code>createInterpreter</code>, which creates an
        instance of the
        <a class="class" href="../javadoc/org/apache/batik/script/Interpreter.html">Interpreter</a>
        interface.
      </p>
      <p>
        Batik comes with implementations of <code>Interpreter</code> and
        <code>IntepreterFactory</code> to support TCL and Python script in
        SVG documents, if the Jacl and Jython distributions are installed,
        respectively.  See the classes in the
        <code>org.apache.batik.script.jacl</code> and
        <code>org.apache.batik.script.jython</code> packages to guidance
        on how to implement the interpreter interfaces, and the
        <a href="site:install">installation notes</a> on what jar files are
        needed for TCL and Python support.
      </p>
    </section>
  </body>
</document>
