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

The unit support refactor during phase one made these improvements possible and will ease the development of future improvements. With the exception of object specific unit support, I finished everything in my proposal. In working on the project, I decided, due to the way units are defined in the SVG specification, that there’s no good way to store object specific units. This leaves having an Inkscape specific attribute that only affects the UI, but I’m not even sure if this behavior is desirable.

Two other ideas I picked up along the way that I would have liked to implement but didn’t get around to are an option to flip the y-axis in Inkscape and a configurable pixel to real world unit ratio. Implementing the former would involve some extensive refactoring due to the parts of Inkscape that are tied directly to the current document coordinate system. To say the latter isn’t implemented is only partially true. With the completion of phase one, all of Inkscape’s unit conversions are defined in units.xml. By editing this file, one can change the conversion factors Inkscape uses. For example, here is a units.xml file using a 10px/mm ratio. The preceding file uses millimeters as the primary unit of length, unlike the current units.xml file that uses pixels. Therefore, the preceding file allows one to change the pixel to real world unit ratio by changing one line. An option could be added to the Inkscape preferences dialog to override this line, allowing one to easily set one’s preferred ratio, something I might add in the future.

I would like to thank my mentor, Johan Engelen, for his support and encouragement throughout this project, as well as the Inkscape community and Google for giving me this wonderful opportunity. It has been a good summer, and I hope to remain involved in the project for the foreseeable future.

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.

Choosing the appropriate unit to display is trivial when only one object is selected or multiple objects are selected with the same unit—display the object’s or objects’ unit. Things become more complicated, however, when objects with different units are selected. Either the unit selector can be left to keep its current unit, the document unit can be displayed, or the most common unit can be displayed, chosen arbitrarily when two or more units are equally common. I think leaving the unit selector alone is the most intuitive solution but am open to suggestions. When objects are deselected, the unit selector will return to its previous unit, unless the unit was manually changed while the objects were selected in which case it would retain its unit.

<?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"/>
</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.

The other method for representing units is using real world units.

<?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"
     xmlns="http://www.w3.org/2000/svg">
  <!-- The dimensions for this rectange are given in real
       world units. -->
  <rect x="10mm" y="5mm" width="30mm" height="20mm"/>
</svg>

The dimensions of the rectangle are specifically specified in millimeters, giving the same result as the first example.

Unit handling becomes more troublesome when the two are combined, however.

<?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 real world
       units when the user space units are equivalent to real
       world units. -->
  <rect x="10mm" y="5mm" width="30mm" height="20mm"/>
</svg>

While one might expect this example to be the same as the previous two, it is not due to the way units are handled in SVG. When no unit is specified, the unit is implicitly a px, e.g. 10 and 10px are equivalent. Real world units are then defined in the specification as an exact number of pixels, e.g. 1mm would be 3.543307px and therefore 3.543307 user units at 90 pixels per inch resolution. Hence, the last example is scaled by a factor of 3.543307. Also, if any sort of scaling transformations are applied, the size of real world units are scaled by an equivalent amount.

For implementation of document-wide default units, I plan on using the method illustrated in the first example, defining the page size in real world units and applying an appropriate viewBox attribute in addition to the current inkscape:document-units attribute. When other units are specified for individual objects, the unit will be specified on the dimensions, after first applying the appropriate transformation for user space units. Although this method, as well as transformations, will result in a similar loss of precision as the current method of specifying everything in px, I don’t see a better way and the appropriate unit will still be shown each time the object is selected.

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.