Why SVG?

Or, in other words, why not another vector graphics format, like DXF, DWG, PDF or Adobe Illustrator’s?

  • SVG is an open standard defined by the W3C
    • all modern browsers support directly visualising SVG
    • lots of vector graphics software support editing SVG
  • SVG is XML, and this brings several advantages like
    • ability to do manual editing with a text editor
    • availability of advanced processing tools
    • ease of translation to other formats, both vector and raster
  • SVG is part of official recommendations and guidelines:
    • it is one of the two preferred formats for vector graphics at the Archaeology Data Service (together with DWG)
    • it is the only recommended format for vector graphics in the MINERVA EC Guidelines for Digital Cultural Content Creation Programmes
    • unfortunately the Deutsches Archäologisches Institut still doesn’t mention SVG in their IT-Leitfaden (recommendations)

In this chapter, we are first going to look at some other popular formats, with their limitations, and then we will examine why SVG is the best available choice for publishing pottery on the Web.

Other Formats: Why Not ... ?

There are several possible digital formats for pottery drawings. Some are commonly used, some are not, but all have in common one thing: they are almost never used for publication (with the exception of PDF), where print or raster images are the current standard.

An example of raster images used to disseminate line drawings of archaeological pottery is Roman Amphorae: a digital resource (University of Southampton 2005), where a typological collection is presented and for every type of amphora one or more profiles are available as JPEG images. In this section we discuss why most formats are not suitable for Web publishing.

DWG

DWG is a proprietary format developed by Autodesk Inc., and it is the native format of the AutoCAD family of software programs. AutoCAD is often used for digitising pottery drawings, for several reasons. First, it is used for other common tasks of archaeological routine, such as site plans. Second, it has a comprehensive set of drawing tools. Third, AutoCAD guarantees good-quality printed graphics. But DWG is not a good choice. The first main problem with DWG is that most often it is used, but not chosen. Rather, it just happens to be the default format of a popular software. The second problem is that DWG is proprietary. Every version of AutoCAD introduces a new version of the format, incompatible with earlier versions. There is no open source software capable of reading and writing all DWG versions. No web browser has or is likely to have in the future support for DWG. As a consequence, DWG is not suitable for being published on the Web.

DXF

DXF is an interoperable version of DWG. Its specification is open (even though it remains property of Autodesk Inc.) and there are several open source libraries and programs capable of reading and writing DXF. However, DXF files tend to be very large in size compared to the corresponding DWG files, and have the same complex framework of incompatible versions.

PDF

PDF is an open standard, registered with ISO and part of lots of recommendations as format for archiving documents of many different types. PDF is a vector format, that is, vector graphics features (lines, polygons, text) can be scaled retaining their quality. However, objects contained in PDF are almost completely unstructured: if three lines form a triangle, this is not relevant in PDF, and therefore there is no way to extract the information “this file contains a triangle”. For this kind of purpose, pottery drawings are no different than triangles. PDF is suitable for archiving because it is easy to create but generally not easy to edit and PDF documents are usually intended to stay as they are. Images can be included in PDF also as raster images (with JPEG compression), so using PDF for vector drawings is an ambiguous recommendation. Moreover, PDF on the Web is an ambiguous presence: despite being very widespread, it is not natively supported by web browsers (unless using additional plug-ins) and it is treated as a downloadable file. PDF files containing pottery drawings are for example the ones published by the FOLD&R journal.

EPS/AI

EPS and AI are two variants developed by Adobe Systems Inc. for their Illustrator software. They share almost all problems of PDF, and they are not native Web formats. To make things more complicated, recent versions of AI are proprietary and not interoperable.

GML

GML is an XML language and an open standard of the Open Geospatial Consortium. GML can be used to describe any spatial feature, as commonly seen in GIS. Because all vector spatial features are also geometric features, it is possible to represent a vector graphics object in GML. GML has two significant limitations. First, it is not a vector graphics format, and normal image viewing programs are not able to deal with it. Second, it is deeply rooted in the geospatial domain, and therefore all data is assumed to be in a geospatial coordinate reference system (CRS). If no CRS is specified, WGS84 geographic coordinates are assumed, and most importantly there is no straightforward way of marking a GML file as having a local, non-geographic coordinate system.

SVG: strengths and limitations

Among the reasons why SVG is important in the specific case of pottery drawings, some are significant for the purpose of sustainability.

First of all, SVG can be used for disseminating high-quality vector graphics on the Web instead of raster images: in other words, this means that drawings will have a quality equal or better than a 1:1 print, and it will be also possible to perform manual or automated editing (e.g. based on personal taste, or requirements from publishers, colours and hatchings can be easily changed accordingly).

Secondly, SVG works across most vector graphics programs, and has a rich community focused around the format and not the software. This gives a higher sustainability to archives of SVG drawings: should one program become obsolete or unmaintained, the migration path would be comparatively easy, because no conversion of format would be needed. Finally, dissemination on the Web is a good way to preserve data, thanks to the existing mirroring services, and to the inevitable downloads that will happen.

SVG is not without problems though, and it should be clear that SVG is far from perfect as a native format to create and archive pottery drawings, while remaining the format of choice for Web publishing.

The three main issues we could identify testing an archive of several hundreds of drawings are:

  • a lack of native support for units (i.e. expressing the vector data in real units and not in pixels) can be worked around only with complex procedures, based on the <viewPort> element. To date, there is no straightforward way to deal with this issue, even though it could be addressed developing one or more plug-ins for the most used software programs;
  • all programs have specific ways of dealing with image layers, that are not part of the native SVG 1.1 specification, but generally this is done through the <g> (group) element. This is not a severe limitation, in that single objects can be assigned an id attribute, that is unique in each file. This possibility is discussed below in the advanced workflows section;
  • except for basic elements such as <title>, metadata can be expressed in different ways. While Inkscape uses Dublin Core and Creative Commons vocabularies, Adobe Illustrator follows the XMP standard. Both ways are acceptable and “standard” in their own way, but this discrepancy introduces another layer of complexity, making it sub- optimal to use different programs onto the same archive.

Finally, even though it is not strictly a limitation, SVG is a file format, so the expected structure of an archive is one file per drawing, as with other image formats. Collating more than one drawing in the same file is discouraged, even though this is common when preparing illustrations for print. In theory, nothing prevents saving serialised SVG data in a database [JYY2004], to retain the relational structure often used in archaeological information systems.

[JYY2004]W. Jianting, L. Yan and J. Youguang 2004 “A Design and Implementation of Spatial Database Based on XML-SVG”, in SVG Open 2004. 3rd Annual Conference on Scalable Vector Graphics. http://www.svgopen.org/2004/papers/ADesignandImplementationofSpatialDatabasebasedonxmlsvg/