A small tool has been developed to investigate and demonstrate Migration on Request as an approach to digital preservation, and to identify any challenges associated with implementing this form of migration.

Vector graphic files define images in terms of their components. For example, a stick man could be formed from a circle and a few straight lines. Each part is generally termed an 'object' or 'element'. A vector graphic file would store such a picture in terms of each element's co-ordinates and attributes (such as fill colour, line colour, line thickness etc).

Vector graphics are a good genre of file to create a testbed Migration on Request tool for. There is a good balance between the complexity of the formats and the amount of development time needed to implement tool.

Here is a discussion of what has been achieved so far.

The vector graphic Migration on Request tool

Two input modules are implemented:

• Acorn Draw
  

  This module supports Draw's path, group and text elements. Text areas and transformed objects are not yet supported. The former would require the tool to format and lay out columns of text - this would be difficult without detailed knowledge of the fonts being used. The latter would be relatively simple to implement and this may be done in later versions of the tool.

• Windows Meta File
  

  The common graphical shapes are supported by the WMF input module. This module does not preserve the structure of the original file completely. This is because the 'create-select-delete object' method of applying styles does not transfer to the hierarchical group arrangement of the intermediate format. This is not seen as a major problem since the development of vector graphic files show a trend towards the hierarchical structure, and the method used in Windows Metafiles is not really found elsewhere. The structure is represented as closely as possible by using groups of elements with inherited effects.

An SVG input module has not been implemented yet as this would require a lot of development work which is not feasible for a small test case like this. The tool is being written to investigate the Migration on Request approach, rather than provide a complete solution for converting vector graphic files, so it was decided not to spend a lot of resources developing the input module. (Read more).

Three output modules are implemented:

• Acorn Draw
  

  This produces a fairly accurate rendition of the data in the intermediate structure. There may be errors or estimations about the positioning of element bounding boxes, but most (if not all) Draw-based applications are unaffected by this.

• Windows Meta File
  

  When imported into a document, the image may have additional white space around it (to the left and below). This is due to the position of the image on the page in the original file. In Word you may have to use the 'Edit picture' function and select the 'resize boundary' option before the image will fill its frame.

• SVG
  

  All elements from the intermediate structure can be output by this module.