Bug #1116

Issues with namespace 'video'

Added by Robin Mills almost 2 years ago. Updated 3 months ago.

Status:ClosedStart date:08 Sep 2015
Priority:NormalDue date:
Assignee:Andreas Huggel% Done:


Category:xmpEstimated time:4.00 hours
Target version:0.26


This issue relates to the namespace 'video' and probably also applies to 'audio'

It is listed as available:

$ exiv2 -vV -g video
exiv2 0.25 001900 (64 bit build)
1) Cannot use it "out of the box"
$ curl --silent -O http://dev.exiv2.org/attachments/download/805/DSC_7154.jpg
$ exiv2 -M"Xmp.video.title movie" DSC_7154.jpg 
-M option 1: Invalid command `Xmp.video.title'
exiv2: Error parsing -M option arguments
Usage: exiv2 [ options ] [ action ] file ...

Manipulate the Exif metadata of images.
It can be used if registered:
$ exiv2 -M"reg video http://www.video" -M"set Xmp.video.title movie" DSC_7154.jpg 
$ exiv2 -px DSC_7154.jpg 
Xmp.xmp.Rating                               XmpText     1  0
Xmp.xmp.ModifyDate                           XmpText    25  2015-07-16T20:25:28+01:00
Xmp.video.title                              XmpText     5  movie
Xmp.dc.description                           LangAlt     1  lang="x-default" Classic View
2 Throws an exception in exiv2json:
$ bin/exiv2json -x DSC_7154.jpg 
Caught Exiv2 exception 'XMP Toolkit error 9: Fatal namespace map problem'

Related issues

Related to Exiv2 - Feature #640: method is missing in Exiv2 to get list of known XMP names... Closed 18 Jul 2009

Associated revisions

Revision 3933
Added by Robin Mills almost 2 years ago

#1116. Fix submitted.

Revision 3981
Added by Andreas Huggel almost 2 years ago

#1116: Reverted changes to BasicIo::BasicIo() introduced in r3933

Revision 3982
Added by Andreas Huggel almost 2 years ago

#1116: Register missing namespaces (audio, video), changed xapG to xmpG in accordance with the XMP specs.


#1 Updated by Alan Pater almost 2 years ago

Several random thoughts:

Does the 'video' namespace only apply to video files?

Is video support enabled in the build?

Is video support read only?
If that is the case, then trying to set a Xmp.video property should not work.

#2 Updated by Robin Mills almost 2 years ago

  • % Done changed from 0 to 70
  • Estimated time set to 4.00

Fix submitted: r3933.

The command/message in Fault 1 in the issue report is incorrect. It should be:

$ exiv2 -M"set Xmp.video.title movie" DSC_7154.jpg
-M option 1: Invalid key `Xmp.video.title'
exiv2: Error parsing -M option arguments
Usage: exiv2 [ options ] [ action ] file ...

Manipulate the Exif metadata of images.
662 rmills@rmillsmbp:~/gnu/exiv2/trunk $ 

Fault 1 (Invalid key Xmp.video.title) is caused by libexiv2 namespaces not being registered with XMPsdk.

Exiv2 has no library initialization function. If we had this, we could register namespaces uniquely known to libexiv2 with XMPsdk. I've hijacked BasicIo::BasicIo to call XmpProperties::registeredNamespaces() to force this. This imposes overhead on every Io object.

Fault 2 (exiv2json throwing exception XMP Toolkit error 9: Fatal namespace map problem) has two causes.

a) Failure to register libexiv2 namespace (as Fault 1).
However even explicitly calling XmpProperties::registeredNamespaces() from exiv2json.cpp/main() was insufficient to fix this.
b) Duplicated namespace.
We were registering the namespace 'video' with two URIs http://www.video and http://www.video/

#3 Updated by Andreas Huggel almost 2 years ago

Exiv2 has no library initialization function. If we had this, we could register namespaces uniquely known to libexiv2 with XMPsdk. I've hijacked BasicIo::BasicIo to call XmpProperties::registeredNamespaces() to force this. This imposes overhead on every Io object.

We do, and it does exactly that; register namespaces in XMPSDK: Exiv2::XmpParser::initialize. There is also a comment in properties.cpp that says new namespaces need to be registered with XMP-SDK there:

    extern const XmpNsInfo xmpNsInfo[] = {
        // Schemas   -   NOTE: Schemas which the XMP-SDK doesn't know must be registered in XmpParser::initialize - Todo: Automate this
        { "http://purl.org/dc/elements/1.1/",             "dc",             xmpDcInfo,        N_("Dublin Core schema")                        },

When I last looked (in connection with #640 the other day), this was simply one of the namespaces missing there. I wanted to fix that: those namespaces that Exiv2 knows and XMP-SDK doesn't just need to be registered there (I think), but those that XMP-SDK has and Exiv2 doesn't, will require a bit more thought and work. For the current implementation of XmpProperties::registerNamespaces(), registering the missing namespaces in XMP-SDK in XmpParser::initialize() should suffice.

See also http://dev.exiv2.org/projects/exiv2/wiki/Thread_safety

#4 Updated by Robin Mills almost 2 years ago

  • Status changed from Assigned to Feedback
  • Assignee changed from Robin Mills to Andreas Huggel
  • % Done changed from 70 to 80

Thanks, Andreas. I'm pushing this to you for comment (status 'Feedback').

I'm not happy with calling XmpParser::initialize() from main(). Do we have to also in-line the CriticalSection stuff? This is overly complicated and obscure.

Can we add a new API Exiv2::libraryInitialize() function and I'll call this from exiv2json.cpp/main()?

I will restore BasicIo::BasicIo() to the empty in-line function in include/exiv2/basicio.hpp: BasicIo::BasicIo{}

Exiv2::libraryInitialize() should call a new API Exiv2::XmpProperties::initialize()

XmpProperties::initialize() can call XmpParser::initialize(), because he's a friend class. And he should include the following code, which can then be removed from XmpProperty::registeredNamespaces()

for (unsigned int i = 0; i < EXV_COUNTOF(xmpNsInfo); ++i) {

#5 Updated by Andreas Huggel almost 2 years ago

The idea of looping through xmpNsInfo[] to register the namespaces in the XMP parser automatically is good, I like that. But let's take this step by step. The real issue is that Exiv2 and XMPSDK are not in sync, let me fix that first: Exiv2 and XMPSDK must be in agreement of what are the known namespaces. That will make sure we don't hit namespace-related obscure errors and will make XmpProperties::registeredNamespaces() trivial.

As for the API:

You do not need to call XmpParser::initialize() from your exiv2json.cpp main(). Exiv2 will take care of that. Only multithreaded apps have to be careful. They need to follow the simple guidelines in the Thread-safety Wiki article, then they are fine. For now, we need XmpParser::initialize() in the new function to list the registered namespaces, because it uses the XMP parser.

We currently only have XmpParser::initialize() because that is all we need: the XMP parser (XMPSDK) needs to be initialized. Exif and IPTC do not need any initialization code, phew! Generally, an initialization function in a C++ library is bad. It should be avoided at all costs. That's why there is no function to initialize the library. I think we should not add one, unless we really really have to, and I hope we never will.

Let's try hard to keep the API as stable and as small as possible. (API: Consider any code change in the include/exiv2/* files an API change.) Every time we modify or delete an API, we create an incompatible release. Every API we do not add, we will not have to modify or delete later. We have hardly made changes since the last release, but the upcoming release is already binary incompatible.
Exiv2 is a library to deal with image metadata. We should provide only a very specific and consistent API related to this task and resist the temptation to add kitchen-drawer all-purpose interfaces, as convenient they may seem. Every application has its own utilities and every developer has her own preferences.
The API is already quite overwhelming as it is. Imagine you're new to Exiv2 and look at our class list. Where do you start? I've had comments "Exiv2 only deals with image metadata, why do you need 100 classes for that?" (97 at the moment, and only counting those in the public API.) The question is valid and difficult to answer, especially if you consider how many Exiv2 classes you actually need in an average application.

#6 Updated by Alan Pater almost 2 years ago

When adding previously unknown XMP namespaces, I've been listing them in xmp.cpp


Do these changes mean that those entries are not longer needed?

#7 Updated by Robin Mills almost 2 years ago

  • Status changed from Feedback to Assigned

Thank Alan and Andreas for your comments.

The calls you make in XmpParser::initialize() register those namespace directly with XMPsdk using SXMPMeta::RegisterNamespace().

The for loop above is currently in properties.cpp/XmpProperties::registeredNamespaces() which calls xmp.cpp/Exiv2::XmpParser::registerNs() which calls SXMPMeta::RegisterNamespace().

I'm sorry to have misunderstood the initialization code. I was under the impression that we did not have a library initialization function. And while I understand your point of view that you don't wish to see a function such as Exiv2::libraryInitialize(), we none-the-less have to sync those disparate sources of namespaces. And if we don't push this responsibility on the user, we should assume it in BasicIo::BasicIo() - or some such gateway to libexiv2. I'll leave you to resolve this. I set this to 70% complete with the intention of revisiting BasicIo::BasicIo() to install a lower overhead synchronisation function (possibly XmpParser::initialize()).

I'm going to leave you to fix this as you think best. You may begin by rolling back the changes to BasicIo.{cpp|hpp} in r3933. The changes in r3933 to properties.cpp concerning the declaration of 'video' and 'audio' are essential to fix fault 2 above.

To help a new user get quickly started with libexiv2, I believe our sample applications are of great importance. I've discussed giving the man pages a makeover for this very purpose. #1061. You are both aware that I endeavour to help users on the Forum. We're providing a rich mix of solid code-base and build, api documentation, good man-pages, simple to understand samples and on-line support.

#8 Updated by Robin Mills 3 months ago

  • Status changed from Assigned to Closed
  • % Done changed from 80 to 100

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