Writing XMP Metadata in jpeg (with PHP) - Using Single or Multiple rdf:Description blocks

Question

About RDF

It appears that what Photoshop is doing is reading a valid, well formed, RDF/XML serialization of some data, and then displaying it back to the user in UI in another valid, well-formed, RDF/XML serialization that happens to follow some additional conventions.

RDF is a graph-based data representation. The fundamental piece of knowledge in RDF is the triple, also called a statement. Each triple has a subject, a predicate, and an object. Subjects, predicates, and objects may all be IRI references; subjects and objects can also be blank nodes, and objects may also be literals (e.g., a string). RDF/XML is one particular serialization of RDF. The RDF/XML snippet:

<rdf:Description rdf:about="" xmlns:photoshop="http://ns.adobe.com/photoshop/1.0/">
  <photoshop:Instructions>OOOInstructions</photoshop:Instructions>
  <photoshop:Headline>OOOHeadline</photoshop:Headline>
  <photoshop:CaptionWriter>OOO </photoshop:CaptionWriter>
</rdf:Description>

contains three triples:

<this-document> <http://ns.adobe.com/photoshop/1.0/Instructions> "OOOInstructions"
<this-document> <http://ns.adobe.com/photoshop/1.0/Headline> "OOOHeadline"
<this-document> <http://ns.adobe.com/photoshop/1.0/CaptionWriter> "OOO "

where <this-document> is the result of resolving the reference "" (the value of the rdf:about attribute. (Page 21 of the XMP documentation says that the value of the rdf:about attribute may be an empty string …, which means that the XMP is physically local to the resource being described. Applications must rely on knowledge of the file format to correctly associate the XMP with the resource".)

Doing

<rdf:Description rdf:about=""
    xmlns:Iptc4xmpCore="http://iptc.org/std/Iptc4xmpCore/1.0/xmlns/">
  <Iptc4xmpCore:IntellectualGenre/>
</rdf:Description>

<rdf:Description rdf:about=""
    xmlns:photoshop="http://ns.adobe.com/photoshop/1.0/">
  <photoshop:Instructions>OOOInstructions</photoshop:Instructions>
  <photoshop:Headline>OOOHeadline</photoshop:Headline>
  <photoshop:CaptionWriter>OOO </photoshop:CaptionWriter>
</rdf:Description>

is exactly the same as doing

<rdf:Description rdf:about=""
    xmlns:Iptc4xmpCore="http://iptc.org/std/Iptc4xmpCore/1.0/xmlns/"
    xmlns:photoshop="http://ns.adobe.com/photoshop/1.0/">
  <Iptc4xmpCore:IntellectualGenre/>
  <photoshop:Instructions>OOOInstructions</photoshop:Instructions>
  <photoshop:Headline>OOOHeadline</photoshop:Headline>
  <photoshop:CaptionWriter>OOO </photoshop:CaptionWriter>
</rdf:Description>

They serialize the same set of triples. Neither is invalid or incorrect. It's just a matter of which you prefer. Other variations are possible as well. For instance, in some cases you can use element attributes to indicate property values. The triple:

<this-document> <http://ns.adobe.com/photoshop/1.0/Instructions> "OOOInstructions"

can be seralized using elements, as described in Section 2.2 Node Elements and Property Elements of the RDF/XML recommendation:

<rdf:Description rdf:about="" xmlns:photoshop="http://ns.adobe.com/photoshop/1.0/">
  <photoshop:Instructions>OOOInstructions</photoshop:Instructions>
</rdf:Description>

or using attributes to indicate the property value, as described in Section 2.5 Property Attributes of the same document:

<rdf:Description rdf:about="" xmlns:photoshop="http://ns.adobe.com/photoshop/1.0/"
    photoshop:Instructions="OOOInstructions">
</rdf:Description>

So, as to your second question:

Why should I spend the time to format my output to the RDF specs when it works nicely all jumbled together in a single rdf:Description?

If the output is supposed to be in RDF, you should make it valid RDF. Whether it's in a particular aesthetically pleasing format is an entirely different question. It's relatively easy to translate between the two of these, and I expect that what Photoshop is doing is reading a blob of RDF as it should (i.e., not depending on any particular structure of the XML serialization, since that's not always the same (e.g., you shouldn't try to manipulate RDF with XPath)) and then formatting that data for the user in a way that it considers nice, namely, the convention that you mentioned.

If you're not already, I very strongly suggest that you use an RDF library in PHP to construct the metadata graph, and not try to construct the RDF/XML serialization by hand.

About XMP in RDF

Note: this is an update based on the documentation. According to the documentation, page 19, XMP only supports a subset of RDF, so it is still a meaningful question about whether the RDF above and in the question, though suitable as RDF, is suitable as XMP. However, also from page 19:

The sections below describe the high-level structure of XMP data in an XMP Packet:

The outermost element is optionally an x:xmpmeta element

It contains a single rdf:RDF element

which in turn contains one or more rdf:Description elements

each of which contains one or more XMP Properties.

Page 20 contains some elaboration about the rdf:Description elements (emphasis added):

The rdf:RDF element can contain one or more rdf:Description elements. … By convention, all properties from a given schema, and only that schema, are listed within a single rdf:Description element. (This is not a requirement, just a means to improve readability.)

The part with added emphasis is what we need in order to conclude that both forms we've seen above are acceptable. It's probably easier to just create one big blob, and consider yourself lucky if some other tool splits it into the conventional form for you.