Namespaces in IMSC

This article covers the subject of XML namespaces, giving you enough information to recognise their usage in IMSC, and be able to use it effectively.

What are XML namespaces?

Namespaces are basically the mechanism that you use in XML to differentiate different families of markup (which may have features with the same name), and allow them to be used in the same document.

To help you understand what we mean by this, let's use a real world example — human family names. There are many people in the world called Mary. One way to tell them apart is by their full names — there can be a Mary Smith and a Mary Jones.

In XML you can also give elements and attributes a "family name", which is their namespace. Namespaces define what family an XML vocabulary belongs to, and generally consist of an identifying string of characters. The <p> element is available in both HTML and IMSC, so perhaps you could use the namespace html to specify HTML's <p>, and imsc to specify IMSC's <p>?

As with many things, it's not that simple. There might be another XML vocabulary named IMSC and it may not be related to subtitles. This is the same with Mary Smith — There are many Mary Smiths in the world, so more information is needed to tell them apart — their birthdays, hair color, address, etc.

So normally you use longer strings as namespace names. A URL is a very popular form of namespace. It is easy to remember and can also point to further information about that XML vocabulary.

If you use the namespace http://www.w3.org/ns/ttml it is quite safe to assume that you are referring to elements from the IMSC vocabulary.

Setting namespaces in documents

So how do you express in an IMSC document that the <p> element belongs to the namespace http://www.w3.org/ns/ttml? You need to include the namespace in the document. The simple way to do this could be to include it in each element and attribute that comes from that namespace. You set the namespace of an element by specifying the namespace identifier inside its xmlns attribute:

<tt xmlns="http://www.w3.org/ns/ttml" xml:lang="en">
  <body xmlns="http://www.w3.org/ns/ttml">
    <div xmlns="http://www.w3.org/ns/ttml">
      <p xmlns="http://www.w3.org/ns/ttml">Hello world</p>
    </div>
  </body>
</tt>

But this is not very efficient. Imagine a document with hundreds of subtitles. This would be very verbose.

Default namespaces

Fortunately, you don't need to do the above — instead you can just use a default namespace. If you set the attribute xmlns on the  document's root element to the value http://www.w3.org/ns/ttml, all elements nested inside the root will inherit this namespace — they will all have that namespace too.

<tt xmlns="http://www.w3.org/ns/ttml" xml:lang="en">
  <body>
    <div>
      <p>Hello world</p>
    </div>
  </body>
</tt>

In this example the elements <tt>, <body>, <div> and <p> have all the namespace http://www.w3.org/ns/ttml.

Because nearly all XML elements you need in an IMSC document are in the namespace http://www.w3.org/ns/ttml, this makes life a lot easier. If you want to use an element from another vocabulary inside an IMSC document, you can still overwrite the default namespace.

<tt xmlns="http://www.w3.org/ns/ttml" xml:lang="en">
  <head>
   <metadata>
     <documentPublisher xmlns="urn:ebu:tt:metadata">MDN</documentPublisher>
   </metadata>
  </head>
  <body>
    <div>
      <p>Hello world</p>
    </div>
  </body>
</tt>

The element <documentPublisher> comes from the EBU Part M metadata vocabulary. The elements in this vocabulary have the namespace urn:ebu:tt:metadata. By setting the xmlns attribute on the element <documentPublisher> to urn:ebu:tt:metadata, the namespace http://www.w3.org/ns/ttml gets overwritten. Now the <documentPublisher> element and all its descendents have the namespace urn:ebu:tt:metadata.

A better way to overwrite a default namespace is by using prefixes.

<tt xmlns="http://www.w3.org/ns/ttml" xml:lang="en"
 xmlns:ebuttm="urn:ebu:tt:metadata">
  <head>
   <metadata>
     <ebuttm:documentPublisher>MDN</ebuttm:documentPublisher>  
   </metadata>
  </head>
  <body>
    <div>
      <p>Hello world</p>
    </div>
  </body>
</tt>

We explain in the following section how XML namespace prefixes work.

Namespaced attributes

We've looked at elements, but how can we specify the namespace of IMSC attributes, and without being too verbose? Contrary to XML elements there is no default namespace for attributes.

In addition, IMSC attributes are contained in more than one namespace. Let's explain further — in IMSC there are different categories of attributes, styling attributes for example. The different categories have different namespaces. For example, all styling attributes have the namespace http://www.w3.org/ns/ttml#styling.  

As for XML elements, it would be too verbose to always write the complete namespace for each attribute, e.g. color_http://www.w3.org/ns/ttml#styling="yellow".

Luckily XML has the concept of prefixes. A prefix can be thought of as a "shortcut" for a namespace. For example, we can define an attribute namespace on the root element:

<tt xmlns="http://www.w3.org/ns/ttml" xml:lang="en" 
 xmlns:tts="http://www.w3.org/ns/ttml#styling"/>

By defining xmlns:tts="http://www.w3.org/ns/ttml#styling on the <tt> element you "bind" the prefix tts to the styling namespace. Subsequently, whenever you prefix an attribute (or element) with tts (plus a colon) it is given the namespace http://www.w3.org/ns/ttml#styling. This way you can write the prefix throughout your document, not the whole namespace each time.

<tt xmlns="http://www.w3.org/ns/ttml" xml:lang="en"
 xmlns:tts="http://www.w3.org/ns/ttml#styling" >
  <body>
    <div>
      <p tts:color="yellow" tts:fontSize="120%">
        Hello world
      </p>
     <p tts:color="white" tts:fontSize="120%">
        Hi!
     </p> 
    </div>
  </body>
</tt>

Much more readable, isn't it?

Note: The namespace/prefix match is only a document-wide agreement. Theoretically you can use another prefix than tts to bind the styling namespace. It is completely legal to define xmlns:foo="http://www.w3.org/ns/ttml#styling" and then write <p foo:color="yellow">. But it makes your IMSC document much more readable if you use the official prefixes listed in namespace section of the IMSC standard.