What Is the Minimum XML for XMP Metadata?

[markolbert]

I'm writing an app to edit keywords in image files. I'm using Magick.NET to handle PNG files, and storing the keywords in the XMP subject node.

I want to be able to handle situations where a PNG file doesn't yet have any XMP metadata. Through experimentation (i.e., deleting stuff from the XML document and watching what happens :)) it looks like the minimum XML document for XMP metadata looks like this:

<x:xmpmeta xmlns:x="adobe:ns:meta/" >
  <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
    <rdf:Description xmlns:xmp="http://ns.adobe.com/xap/1.0/" xmlns:dc="http://purl.org/dc/elements/1.1/">
      <dc:subject>
        <rdf:Bag>
          <rdf:li>Farm</rdf:li>
          <rdf:li>house</rdf:li>
        </rdf:Bag>
      </dc:subject>
    </rdf:Description>
  </rdf:RDF>
</x:xmpmeta>

Does this seem reasonable? Or am I missing something important?

I've checked a PNG file where I use this structure against exiftool, and it correctly extracts the keywords I defined (it also doesn't flag any issues).

However, I just noticed that Adobe Bridge cannot "see" the keywords. So, I'll have to experiment with adding some stuff back in to see if I can make the keywords visible to Bridge.

[dlemstra]

You might also need to add an xpacket tag as shown here: https://en.wikipedia.org/wiki/Extensible_Metadata_Platform. We are also doing that in the pdf encoder of the ImageMagick library.

[markolbert]

Thanx for the quick reply. However, when I write the XML including the packet tag -- in fact, when I simply "echo back" the XML obtained from ToXDocument(), but with new keywords -- Bridge still can't see the keywords. Yet exiftool shows they're there.

Digging a little deeper into the metadata with exiftool, I noticed the "source" XML is stored in an iTxt chunk, while the revised XML -- the values written by Magick.NET when it saves the file -- are stored in a zTxt chunk.

Is there a way to force Magick.NET to store the XML data in an iTxt chunk? I don't know if that would solve the problem -- I'm grasping at straws here :) -- but it would at least remove one of the differences between the source and revised PNG file.

[dlemstra]

You can disable zTxt by setting the ExcludeChunks of the PngWriteDefines to PngChunkFlags.zTXt. This should write your data to an iTxt chunk instead.

[markolbert]

Thanx! I'll give it a try.