Secrets for embedding SVG images in PDF documents

Nhovestone reports don’t look good when zooming, because the charts are bitmap images. While trying to render them as vector images, a few interesting things did appear.

(Most of them relate to FOP-1.0 and Batik-1.7 which are the latest versions at this time.)

There was a bug (fixed in HEAD) preventing from embedding SVG images in PDF documents. Novelang resolves the image path (in n:vector-image/n:resource-location) as relative to the content directory. But most of times the stylesheet doesn’t reside right under the content root, so FOP (the PDF renderer) couldn’t find the SVG file. Now XSL stylesheets receive a $content-directory parameter to solve this.

There are several manners to embed an SVG file into a PDF. In order to keep the vector graphics (means: avoiding an ugly conversion to a bitmap) FOP needs a rather twisted declaration of the form:

  <svg:svg width="123px" height="456px" >
    <svg:image width="123px" height="456px" xlink:href="file:/my/image.svg" />

The documentation explains that width and the height must appear two times, with exactly the same values.

It’s important to note that an SVG image must always have width and height. Moreover, they must be in pixel unit. Pixel unit is bad, as it introduces a dependency to document’s resolution. But units other than pixel don’t work.

Knowing this, the graphs started to look really good. Unfortunately, the grid becomes bigger when zooming in. In theory, the solution is to set line with to a zoom-independant value with vector-effect="non-scaling-stroke" property (thanks to StackOverflow for that hint). Unfortunately, this is SVG 1.2. Batik doesn’t support this yet and some archived mails show this is somewhat tricky to implement.

Conclusion: while SVG embedding in PDF is far from perfect, it’s much better than rasterized images that seemed to be FOP’s approach. In order to get best vector images we'll have to wait for non-scaling stroke feature in Batik, and maybe some FOP upgrade in order to propagate this feature to generated PDF.

1 comment:

dave said...

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