Accessing XMP data in MP4 files?

[Numpsy]

Hi,
I see that the roadmap in the readme file says

Support for Broadcast wave metadata : aXML and XMP

but are there any thoughts on reading or writing XMP data in MP4 files?

I had a really quick go at reading the data in Numpsy@8887dee and it seems to work ok (though that test only works for a single UUID field in the file).
I haven't made any attempt at writing data into mp4 files yet.

[Zeugma440]

Hi there,

That part of the roadmap is aimed at WAV files, but I can consider starting with MP4 if you need it, sure.
Could you please e-mail me a sample file with XMP data to play with?

Thanks in advance~

PS : I see you're using a wide array of features. What are you building? 😄

[Numpsy]

If ATL and the C++ XMP SDK can read it then it's ok for me, I just don't know if there are any other tool sthat might be more strict about it

[Zeugma440]

I suggest we keep it that way for now. If anyone finds a case where the xpacket wrapping is mandatory, I'll investigate further.

[Numpsy]

A question about the writing format for namespaces (I think this might actually be hitting a bug in the C# version of XmpCore / MetadataExtractor and I'll query over there), but for the record:
If I have an M4A file with the initial XMP header of

<x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="XMP Core 5.6.0">
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
      <rdf:Description rdf:about=""
            xmlns:xmp="http://ns.adobe.com/xap/1.0/"
            xmlns:xmpDM="http://ns.adobe.com/xmp/1.0/DynamicMedia/"
            xmlns:tmi="http://titus.com/tmi/1.0/">

and I modify the XMP with 5.21, then trhe result is this, with the namespaces grouped into the top node:

<x:xmpmeta xmlns:x="adobe:ns:meta/" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:xmp="http://ns.adobe.com/xap/1.0/" xmlns:xmpDM="http://ns.adobe.com/xmp/1.0/DynamicMedia/"
    xmlns:tmi="http://titus.com/tmi/1.0/" x:xmptk="XMP Core 5.6.0">
    <rdf:RDF>
        <rdf:Description>

If I read the second one with the C++ Adobe XMP SDK then it reads the inner data as expected, but with MetdataExtractor (or the .NET port of XmpCore directly) it throws at https://github.com/drewnoakes/xmp-core-dotnet/blob/0f59b87a8e28fa9648856d30737ab8c38098195f/XmpCore/Impl/ParseRdf.cs#L108 and says the data is invalid.

As it happens, section C.2.2 (pages 37-38) of the Adobe specification at https://github.com/adobe/XMP-Toolkit-SDK/blob/main/docs/XMPSpecificationPart1.pdf states that the rdf:RDF node can't have attributes other than namespaces and the official C++ parser code at https://github.com/adobe/XMP-Toolkit-SDK/blob/5f6fe44f19842ae43dedc3da3fd5d5dafb29e501/XMPCore/source/ParseRDF.cpp#L743 throws if it does, so I think from that that the data written by ATL is ok and it's the validation in xmp-core-dotnet that's wrong (not 100% certain though)

[Zeugma440]

First thing first, I'm not a LINQ veteran by any means, but I have trouble understanding how

if ( ! xmlNode.attrs.empty() )

could be ported to

if (!rdfRdfNode.Attributes().Any())

so yeah, I also have doubts concerning the implementation (that also seems to violate the specs, btw).

I get that, from System.xml's point of view, a namespace is an Attribute, which prevents from writing exactly the same code as in C++.
One way of fixing that could be something like (not sure that compiles, but you get the idea)

if (rdfRdfNode.Attributes().Any(!a.Name.StartsWith("xmlns:"))

How about that?

If the maintainer isn't happy with that, we could always force ATL to write the rdf namespace at the level of the rdf:RDF node to make it work. That won't be a problem on my side.

Please let me know~

[Numpsy]

According to drewnoakes/metadata-extractor-dotnet#418 (comment) the C# code was derived from the Java version (which has since been abandoned by Adobe), so it sounds like it's just tripped a rather long standing bug inherited from that, and hopefully we can get that fixed.

From the spec document, I wondered if something like

if (rdfRdfNode.Attributes().Any(attr => attr.IsNamespaceDeclaration == false))

would be more like it

[Zeugma440]

XMP support for WAV and M4A/MP4 has been released with today's v5.21