[xep-support] Image access when generating PDF

[David Tolpin]

> Just curious: Why would XEP care if the graphic has been modified?
> What is the time critical problem that XEP is addressing for it
> to check if a graphic file has been modified in the middle of
> processing?
>

Hi Earl,

in the currently shipped version of XEP there is a separate subsystem  
responsible for retrieval (and caching) of remote resources.

The subsystem optimizes retrieval of images in a way similar to what  
browsers do to improve performance. In particular, if a resource retrieved  
via HTTP, then a cached copy is used unless the validity time is expired  
and the document is modified since the previous retrieval. Local files do  
not require this optimization because local access is fast.

This design helped to solve performance problems in the past but is not  
the most appropriate one for the purposes of XSL formatting. A better  
approach would be to just retrieve remote images once per document, and  
leverage optimizations spanning a number of documents to general purpose  
resource proxies.

However, the current solution works well enough and provides predictable  
and maintainable behaviour. Instead of patching the current legacy code,  
RenderX is  full speed into development of a new generation of the engine  
and tool suite, and the new code shares approximately 0% with the old code  
base.

There is a semi-official wiki about the effort: http://www.renderx.net/  
(accessible as the "developer's zone" from the main site,  
http://www.renderx.com/). An online formatter demo (a real formatter with  
a few impressive examples) is a demonstration that the thing approaches  
the release: http://power.renderx.com/cgi-bin/webdemo.cgi .

David Tolpin
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