Empty XMP content when using only_xmp to open video file

[wsyxbcl]

This should be an issue of the C++ SDK (kXMPFiles_OpenOnlyXMP), but I'm using the Rust crate to reproduce it, so I'm reporting it here as well. I'm unsure whether this behavior is a bug or expected (It’s also possible that the video file I'm working with is non-standard. I can provide a test file if you'd like), so I would appreciate your thoughts.

Expected Behaviour

OpenFileOptions::default().only_xmp() returns xmp

Actual Behaviour

OpenFileOptions::default().only_xmp() returns xmp with empty content (when reading mp4 video files)

Reproduce Scenario

The input media is a mp4 video file, with only_xmp:

let mut media_xmp = XmpFile::new()?;
    if media_xmp
        .open_file(media.clone(), OpenFileOptions::default().only_xmp())
        .is_ok()
    {
        if let Some(xmp) = media_xmp.xmp() {
            xmp_string = xmp.to_string_with_options(
                ToStringOptions::default().set_newline("\n".to_string()),
            )?;
        println!("XMP: {}", xmp_string)
    }

Output

<?xpacket begin="" id="W5M0MpCehiHzreSzNTczkc9d"?>
<x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="XMP Core 6.0.0">
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
      <rdf:Description rdf:about=""/>
   </rdf:RDF>
</x:xmpmeta>
                                                                                                    
                                                                                                    
                                                                                                    
                                                                                                    
                                                                                                    
                                                                                                    
                                                                                                    
                                                                                                    
                                                                                                    
                                                                                                    
                                                                                                    
                                                                                                    
                                                                                                    
                                                                                                    
                                                                                                    
                                                                                                    
                                                                                                    
                                                                                                    
                                                                                                    
                                                                                                    
                           
<?xpacket end="w"?>

without only_xmp:

let mut media_xmp = XmpFile::new()?;
    if media_xmp
        .open_file(media.clone(), OpenFileOptions::default().only_xmp())
        .is_ok()
    {
        if let Some(xmp) = media_xmp.xmp() {
            xmp_string = xmp.to_string_with_options(
                ToStringOptions::default().set_newline("\n".to_string()),
            )?;
        println!("XMP: {}", xmp_string)
    }

Output

  XMP: <?xpacket begin="" id="W5M0MpCehiHzreSzNTczkc9d"?>
  <x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="XMP Core 6.0.0">
     <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/">
           <xmp:CreateDate>2024-06-02T13:11:36Z</xmp:CreateDate>
           <xmp:ModifyDate>2024-06-02T13:11:36Z</xmp:ModifyDate>
           <xmpDM:duration rdf:parseType="Resource">
              <xmpDM:value>20187</xmpDM:value>
              <xmpDM:scale>1/1000</xmpDM:scale>
           </xmpDM:duration>
        </rdf:Description>
     </rdf:RDF>
  </x:xmpmeta>
                                                                                                      
                                                                                                      
                                                                                                      
                                                                                                      
                                                                                                      
                                                                                                      
                                                                                                      
                                                                                                      
                                                                                                      
                                                                                                      
                                                                                                      
                                                                                                      
                                                                                                      
                                                                                                      
                                                                                                      
                                                                                                      
                                                                                                      
                                                                                                      
                                                                                                      
                                                                                                      
                             
  <?xpacket end="w"?>

[wsyxbcl]

adobe/XMP-Toolkit-SDK#97