VDP jobs in the DFE are created as PDF/VT files. PDF/VT files are standard PDF files with extensions that allow recurring elements of the job to be tagged in a well-defined, standard way. These tags provide hints to the RIP, which can use this information for more efficient processing; it knows that such tagged content will be re-used, and so it only needs to RIP it once.
However, just because a PDF file is PDF/VT does not automatically mean it will RIP faster. Care needs to be taken in the way the PDF/VT file is made to make sure that the hints can really lead to faster processing. This document outlines some rules of thumb to follow in order to get the best performance out of Imaging Engine for DFE.
Use PDF/VT hints wisely
In the PDF file, PDF/VT extensions allow recurring elements of the job to be tagged to give a hint to the RIP that such content will be re-used and only needs to be processed once. In a well-constructed PDF/VT file this can lead to significant RIP speed improvements. However it is possible to construct a PDF/VT file poorly so that RIP times are as bad as (or even worse than) standard PDF.
Although it is clearly more efficient to RIP recurring objects only once, doing this does come with some overheads inside Imaging Engine. For instance, the recurring object has to be re-united with the variable content on the final printed page, which takes extra processing.
The size of the cache is finite, so if non-recurring objects are cached too, then it’s likely that the cache will fill up and subsequent recurring objects will then not be cached.
Only tag an object for re-use if it really represents recurring (static) content that appears on each page.
Keep the size of the recurring object to a minimum
When Imaging Engine encounters a recurring object for the first time it processes the object and stores the result in memory, ready to be re-used in subsequent pages. This cache memory is shared among all the workers of a distributed RIP and is of fixed size. Once the cache fills up, if further recurring objects are encountered, they can no longer be cached but have to be fully processed each time they are encountered.
The dimension of the recurring object directly affects the size of the cached object in memory. This determines how much of the cache the object will take up, but it can also affect how much processing is needed to merge the object back with the variable content. For maximum efficiency, it's best to set the bounding box of the recurring group to the minimum enclosing rectangle.
Set the bounding box of the recurring content to the minimum that encloses the content.
Use PDF transparency with care
Recurring content is only really recurring if it is the same each time it is used. That means that transparency in VDP jobs has to be used with care when performance is a concern. For instance if the recurring content uses PDF transparency features, and it overlaps some variable content underneath, it will have to be rendered separately each time as variable data to obtain the correct final output.
Similarly, if variable content using transparency features is put on top of recurring content, the recurring content will be rendered again with the variable data to obtain the correct final output.
The following diagrams show which combinations of objects work best for VDP jobs.
If the recurring content makes use of PDF transparencies, prefer to put it in the background rather than on top of variable content.
The same problem can occur if one recurring object is on top of another recurring object. To avoid this, if possible put all recurring content into a single transparency group and make that composite group the recurring object.
If there are overlapping recurring objects on each page, especially if they use PDF transparency features, group them together into a single reusable group.
Use Overprint with care
In order to get the best results for colour management, Imaging Engine needs to know the full colour value of each separation at each pixel of the page. To get that information, overprint must be processed internally as if it is a form of PDF transparency. Effectively it means that, when considering performance, overprint must be used with the same care as PDF transparency.
Treat overprint as another form of PDF transparency.