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From Joerg Ehrlich <jehrl...@adobe.com>
Subject RE: Metadata situation and XMP support in Tika
Date Fri, 13 Apr 2012 12:32:18 GMT

Looking at the current constants defined for the Metadata map, the interfaces do not follow
a common pattern.
They are organized in interfaces for specific namespaces like DublinCore or XMPDM, there are
interfaces for specific standards like IPTC or CreativeCommons and there are interfaces for
a specific functional purpose like Geographic or MSOffice. There are also namespace interfaces
that mix properties from different namespaces, e.g. TIFF. 
Overall a clear separation of responsibility and semantic is not always ensured. 

I would propose to reorganize and rename the interfaces in two groups: First in namespaces
and second in standards which simply contain lists with the properties they use from the namespace
The reason is that only those two concepts have unambiguous and clearly defined semantics
where each client knows what to do with it.
Properties which are currently not connected to a namespace (like the properties from MSOffice
interface) would also be moved to a namespace.

Old property definitions should be kept intact, of course, for existing clients, but that
is independent of the internal interface organization.

For example the IPTC standard uses properties from six different namespaces (dc, photoshop,
plus, iptc_core, iptc_ext, xmp_rights), but not all of the properties that are defined in
those namespaces. 
I think it would make sense to have in this case six interfaces for the namespaces which contain
all properties from those namespaces. And one interface for IPTC itself which contains just
lists of the properties they use from the namespaces. The IPTC interface already has those
lists, I would just remove all the property definitions from it.
A mapping of EXIF properties to XMP for example, which is defined to use five namespaces (exif,
exifEX, tiff, xmp and dc), can then also reuse the properties defined in the namespace interfaces.
The functional interface "Geographic" I would rename for example to "W3C_Geographic" or that
like as it clearly defines the semantic which is bound to the W3C vocabulary, which is different
then what is meant by the mapping to the EXIF namespace defined by CIPA.
In case of the MSOffice metadata this could either be mapped to properties defined in Open
Document standard [1] or the Microsoft OOXML one [2].

A parser can then map properties to those namespaces it sees fit or several at the same time
and the client can then decide which semantic (i.e. properties) it would like to use.


[1] http://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-part1.html
[2] http://www.ecma-international.org/publications/standards/Ecma-376.htm  (in part 2)

Jörg Ehrlich | Computer Scientist | XMP Technology | Adobe Systems | joerg.ehrlich@adobe.com
| work: +49(40)306360

-----Original Message-----
From: Mattmann, Chris A (388J) [mailto:chris.a.mattmann@jpl.nasa.gov] 
Sent: Donnerstag, 5. April 2012 16:21
To: dev@tika.apache.org
Subject: Re: Metadata situation and XMP support in Tika

Hi Jörg,

Great summary! I would be in favor of option #2 as well, with the caveat that if we take it
slow, I think there might be a way to not really have as much of a client/API impact, using
deprecations and other techniques as you suggested.

Looking forward to your participation!


On Apr 5, 2012, at 5:58 AM, Joerg Ehrlich wrote:

> Hi everyone,
> I am an engineer in the XMP/Metadata team at Adobe and we would like to leverage Tika
in current projects for metadata extraction (and mimetype detection).
> Our current systems primarily use the XMP data model to manage and interact with metadata.
> As far as I can see, the support for the XMP data model and also for standard metadata
schemas/namespaces (like IPTC, Exif, etc.) in Tika is pretty suboptimal as of today.
> But instead of wrapping Tika in own layers of code in our systems, we feel that it would
be more useful to contribute to the project instead going forward.
> I have had a deeper look in Tika and how to improve the metadata/XMP output of it.
> I saw that you have a bug for XMP already (TIKA-756), which I would probably use to submit
any patches related to that.
> But I am currently unsure what the best approach would be to do the mapping to XMP and
I would like to hear your opinion on it before starting any work.
> Let me quickly summarize if I have understood the basic metadata concept correctly:
> 1.       Each parser fills a Metadata map which is a simple key-value list where values
can also be multi-values
> 2.       Mostly the keys for the Metadata map are taken from fixed lists which are defined
as interfaces in the Metadata class
> 3.       Those keys are usually Property objects, where the Property class also serves
as a static list which registers every property that is created in the Metadata interfaces.
This Property class resembles the XMP data model to some extend but does not store e.g. any
hierarchical information. And it leaves every client the choice to store property names with
prefixes or not.
> 4.       Any metadata outputter just iterates over the Metadata map and could query the
Property list for additional information.
> 5.       In case of the XMP outputter (XMPContentHandler) only those properties are outputted
which are stored with a prefix in the Property list.
> I see two potential ways to improve the situation:
> 1.       Have a fixed mapping table for each mime type which would be used in XMPContentHandler
to map from the Metadata map to the XMP data model. Such mapping tables would be pretty ugly
as each parser produces different metadata maps and there is no consistent way to handle them.
This option would be least invasive for other clients of Tika but would also be a real hack
and would not really improve the metadata situation in Tika in general.
> 2.       Try to improve the Key interface lists of Metadata class and adjust all parsers
accordingly. This could be done by adding new keys with prefixes and keeping/deprecating the
existing ones to not disturb existing clients. Similar to what is proposed for the DublinCore
namespace in TIKA-859 and TIKA-842.
> This would be more invasive but would offer the opportunity to really improve the metadata
situation. I already saw a couple of places in the code that clearly break existing standards.
But there are also examples where mapping might have to be done to different properties at
the same time: If you look at the mapping of GPS data from Exif, this is currently mapped
to W3C vocabulary in Tika. But in XMP this mapping is defined otherwise [1] by CIPA (the EXIF
standardization committee). So probably both mappings have to be supported.
> I personally would prefer option two. What do you think?
> Looking forward to working with you guys.
> Regards
> Jörg
> [1] 
> http://www.cipa.jp/english/hyoujunka/kikaku/cipa_e_kikaku_list.html

Chris Mattmann, Ph.D.
Senior Computer Scientist
NASA Jet Propulsion Laboratory Pasadena, CA 91109 USA
Office: 171-266B, Mailstop: 171-246
Email: chris.a.mattmann@nasa.gov
WWW:   http://sunset.usc.edu/~mattmann/
Adjunct Assistant Professor, Computer Science Department University of Southern California,
Los Angeles, CA 90089 USA

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