Matthew Petroff

I have now merged the second phase of my Google Summer of Code project, unit support improvements, into the Inkscape trunk. Back in April when I proposed this project, I thought refactoring Inkscape’s unit support would be the easier part of the project and improving the unit support would be the more difficult part. In retrospect, the opposite was true. To refactor Inkscape’s unit support, I had to learn the details of all the different unit handling systems and become familiar with the rest of the code they interacted with. Inkscape contains over 500k lines of code, and learning the code base was the most time consuming part of this project. By the time I finished phase one, I was familiar with the unit support code and the code that used it, so extending unit support was fairly straightforward. The most difficult part of phase two was tracking down and fixing transform bugs the were manifested during document unit changes.

Highlights of phase two include:

• Use of real world units for page sizes
• Use of real world document units via a viewBox
• Refactor of expression evaluator and addition of exponent operator to it
• Addition of expression evaluator support to toolbars

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I have now implemented pages sizes using real world units as well as document units using a viewBox as described in my previous post. Although it is mostly working, there are a few known bugs when changing the document unit, namely connectors and 3d boxes do not scale properly. I have also worked on fixing regressions introduced by the phase one unit refactor.

Besides document-wide units, it is also useful to have object specific units for convenience. As real world units are handled as scaling factors of the document unit by the SVG specification, they are only recommended for the document size, not individual objects, and their values are counterintuitive when a viewBox is used to specify document-wide units. Therefore, I’ve decided to implement object specific units using an inkscape:unit attribute instead, which will also allow for units not defined in the SVG specification, e.g. feet. The manner in which object sizes and positions are written to a SVG file will not change; the attribute will only be used to remember which unit should be displayed when an object is selected in the Inkscape UI.
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Since my last update, my branch for phase one of my Google Summer of Code project, unit refactoring, has been merged into the Inkscape trunk. I have now created a new branch for phase two, improvement of unit support. In order to improve unit support, one must first start with how units are defined in the SVG specification. In SVG, there are two types of coordinates, those defined in user space and those defined in real world units. If no unit is specified, it is assumed to be in user space units. By defining the document size in real world units and applying an equivalent viewBox attribute, one can define the user space unit to be a real world unit, e.g. millimeters.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="74mm" height="52mm" version="1.1"
     viewBox="0 0 74 52" xmlns="http://www.w3.org/2000/svg">
  <!-- The dimensions for this rectange are given in user space, but
       due to the viewBox attribute having the same values as the
       document size specified in millimeters, the user units are
       millimeters. -->
  <rect x="10" y="5" width="30" height="20"></rect>
</svg>

As the width and height of the document are 74mm and 52mm respectively and the viewBox stretches the user space width from 0 to 74 and the user space height from 0 to 52, the user space unit is equivalent to millimeters.
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When I began working on phase one of my Google Summer of Code project, Inkscape’s unit handling code was scattered between multiple implementations with different files using different, incompatible implementations. I since consolidated this code into one implementation, residing in util/units.*, eliminating SPUnit, SPMetric, and unit-constants.h.

I also started to replace the SVGLength class but then changed my mind about doing so as the class is specific to the SVG specification, not general units. For example, common units, e.g. feet, are not part of the specification. I still may make it a child class of Util::Unit during phase two, depending on how I end up implementing things, but will leave it alone for now.

With these changes, I have consolidated Inkscape’s unit handling code, completing phase one of my Google Summer of Code project. Although there are only some minor user-facing changes, mostly bug fixes, the internal changes will facilitate the addition of user-facing changes both in phase two and beyond. As an example, pixels per inch are no longer hard-coded, so I plan on adding an option both in Inkscape preferences and document properties to configure the relationship between pixels and physical units early on in phase two. To begin phase two, I plan on hand-crafting SVG files with various types of unit configurations and then implementing support for them starting with a default document-wide unit.