Significant properties

As with other file types, when archiving vector images it is essential that the core significant properties of the file are identified and maintained within the chosen format. The JISC report The Significant Properties of Vector Images (Coyne et al 2007) more than adequately details the properties that should be identified and maintained when archiving vector images and these will only be briefly outlined here.

As would be expected, the significant properties of a vector image revolve around the geometry of the image, the vector objects and their relationships, and the conventions (colours, line types and weights) used within the file and maintaining the appearance of these throughout file migrations and archiving. The following elements are those that should be monitored during the archiving of vector files:

• Points, paths (open or closed) and objects (e.g. text, referenced or embedded objects)
• Basic shapes / Primitives (e.g. rectangle, circle, ellipse, etc.)
• Image structure (i.e. the grouping and layering of objects)
• Clipping and masking
• Transformations and Coordinate Systems (if present, though these are mostly present in GIS and CAD vector files)

In addition to ensuring that these specific vector elements remain physically present within a file in the same format, there are a number of obvious properties that can be applied to these. Where standardised conventions exist and have been followed (such as specific meanings associated with certain line colours or dash types) it is essential that these properties and formatting is maintained in archived files. Such properties include:

• Line width, cap and join types
• Miter limit
• Dash pattern and offset
• Colour
• Opacity
• Rendering
• Interior definition
• Gradient and gradient smoothness
• Pattern
• Text attributes such as font size, weight or style.

Interestingly, the JISC report has proposed a potential hierarchy of significant properties which may be used to assess which properties to prioritise if a file does not easily transfer to a preservation format (Coyne et al 2007, 41).

Unfortunately there is no simple, objective way to compare between original files and migrated versions other than visually assessing that all elements are rendered in the correct way. When migrating a file to a new format, aside from the obvious visible elements, it is also important to check for hidden information such as invisible objects or layers and to assess the relevance or suitability of these for archiving. Linked data, such as embedded raster files or external fonts, may also be present in the original image and the appearance of such data should be maintained through any file migration. Successfully preserving a vector file is largely dependant on the nature of original, its complexity and whether it contains pure vector data or incorporates raster data as well. Such mixed files may require the separation of data types into suitable preservation formats.

File formats

Preservation Format	Requirements
.svg	As an open xml-based standard, SVG is the preferred format for the long-term storage and preservation of vector images.
.pdf/a	Although suitable for preserving vector graphics, pdf/a is recommended as a ‘last resort’ format as, although the format retains the vector nature of the file, it does reduces the reuse potential to simple ‘view only’.