Getting an SVG Bounding Box out of Batik

I was using CorelDRAW to create some GIFs using a script, but CorelDRAW 11 has a lousy GIF exporter, so I couldn't really get the output that I wanted. So, I thought I would export my CorelDRAW data to SVG, and use the Batik SVG libraries to export the data I wanted to GIF (using the GIF exporter I described in the previous post).

I did encounter a problem though in that CorelDRAW defined a page size on the SVG document, and the Batik transcoders always exported the entire page (instead of just the parts of the page that contained my image). If I removed the page information from the SVG document, then Batik would just export an image with a default size. What I needed was a way to get a bounding box on my image, which I could then pass to Batik as the export size.

When I searched on Google, all the information that I could find about creating bounding boxes involved creating UI objects to render into, which triggers Batik to calculate bounding box information. This seemed a little bit overly heavy to me, but by looking at how the Batik transcoders render their images, I was able to just reuse their rendering code to trigger a bounding box calculation without actually rendering to a UI.

The first step is to remove the page sizing information from the SVG document (otherwise, Batik will just use the page size as the bounding box). Just run this XSLT filter over it:


<?xml:namespace prefix = xsl />
<xsl:stylesheet version="1.0" svgns="http://www.w3.org/2000/svg" xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml">

<xsl:template match="@*node()">
<xsl:copy>
<xsl:apply-templates select="@*node()">
</xsl:copy>
</xsl:template>

<!-- Remove width, height, and viewBox attributes (substitute different things in later on when we've calculated a bounding box) -->
<xsl:template match="svgns:svg/@height"></xsl:template>
<xsl:template match="svgns:svg/@width"></xsl:template>
<xsl:template match="svgns:svg/@viewBox"></xsl:template>

</xsl:stylesheet>

Then, you can take the resulting SVG code and feed it into Batik to calculate a bounding box like so:

String parser = XMLResourceDescriptor.getXMLParserClassName();
SAXSVGDocumentFactory f = new SAXSVGDocumentFactory(parser);
SVGDocument doc = (SVGDocument)f.createDocument(svg);
GVTBuilder builder = new GVTBuilder();
BridgeContext ctx;
ctx = new BridgeContext(new UserAgentAdapter());
GraphicsNode gvtRoot = builder.build(ctx, doc);
return gvtRoot.getSensitiveBounds();

You can then use the returned information to set the "viewBox," "height," and "width" attributes on the svg tag of the SVG file. When you then run the transformed SVG file through Batik, Batik will export the image with the bounding box set tightly around the contents.

-Ming