XObjects / Form XObjects
A Form XObject represents the possibility to combine a number of PDF objects in the form of a container.
The main purpose for creating XObjects is to describe a container of objects once in the PDF and then use it multiple times in the PDF file, mainly to save storage space. Typical examples are:
- Column title of a multi-page magazine: the column title does not change over a certain distance in the magazine and can therefore be stored only once and referenced multiple times.
- Impositions: Print Items that are imposed multiple times in an Imposition are often stored as XObjects by Imposition programs and referenced for multiple use, mainly to save storage space.
- OPI Workflows: In OPI Workflows - which are rather outdated - XObjects are used to display low-resolution preview images and then replace them with the high-resolution images in the output.
The following arguments can be seen as an advantage for the use of XObjects:
- Storage space: by referencing the contents of a container, storage space can be effectively saved very quickly, since the contents of the container only need to be stored once.
- Container: XObjects form a kind of group of objects in a PDF file. As long as the group is not resolved the group can be treated as one object and thus objects can be quickly repositioned or deleted.
- Check: When running a Preflight, the contents of an XObject can be checked only once and the result can be applied to the referenced containers. This saves a lot of time in data checking!
The only disadvantage of Form XObjects is that the contents of the container can (currently) not be edited. To edit the container, all referenced containers must also be resolved, which means that all the advantages mentioned can very quickly mutate into disadvantages.