JTC1/SC34/WG4 Liaison Report: Open Packaging Conventions (OPC) Relative References

["Dennis E. Hamilton" <dennis.hamilton@acm.org>]

On June 17-20, 2013, I attended sessions of SC34 WG4 (OOXML).  My particular attention was on MCE and on OPC.  This report covers what I found interesting in the OPC discussion.

OPC is specified in IS 29500-2:2011.  The Open Packaging Conventions are intended to be independently usable, and indeed they are.  Microsoft uses them with the XML Paper Specification (XPS) and in other applications, including Visual Studio software-development artifacts.  There are also third-party uses of OPC.  The Document Container File Core committee draft appears to be amenable to employing with OPC at some point.

How IRIs are used for relative cross-references came up for lengthy discussion.  This is because EPUB 3 container format, ODF 1.2 Packages, and OPC all have provisions for relative-reference cross-references.  

It happens that relative IRIs to other parts of the same package are resolved the same in ODF 1.2 Packages and OPC.  It was not clear that differences seen in how XPS makes relative references and how OOXML implementations, such as DOCX, were compatible.  We needed a speed course in [RFC 3987].

What is noticed that some parts in OPC implementations make relative references in the manner they are usually seen in ODF.  In addition, it happens that RFC 3987 allows references of forms such as "/content.xml" (i.e., in an XLINK HREF) to be taken as relative to the root of the package.  That's because of the implicit base for references from parts within the package.  In OPC applications, sometimes this is done from parts that are at deeper levels, and sometimes others go up-level with HREFs such as "../../content.xml".  But either works. 

MY OBSERVATION

This should also be true in ODF 1.2 Packages, but the relevant section in ODF 1.2 Part 3 is very difficult to decode.  If it's not true because of some sort of mangling of the ODF 1.2 text, it should be made so.  The peculiar case for ODF is that it is possible to relative reference out of the package, so that something like "/.." is a reference to the folder, if any, that the package is contained in, and it is not possible to "come back in."  In effect, passing up through "/" changes to a new base.

 - Dennis

PS: EPUB is being presented to JTC1/SC34 although it is not clear what the precise arrangement is.  There is some keen interest about it at SC34.