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RE: XML-Schema Requirements for SMIL

From: Cohen, Aaron M <aaron.m.cohen@intel.com>
Date: Mon, 15 May 2000 15:31:05 -0700
Message-ID: <D5E932F578EBD111AC3F00A0C96B1E6F0626AA35@orsmsx31.jf.intel.com>
To: "'Dan Connolly'" <connolly@w3.org>, www-xml-schema-comments@w3.org
Cc: "'Patrick Schmitz'" <pschmitz@microsoft.com>
Dan:
Thanks for taking a look at this. I'll take a closer look at your SMIL
Animation Schema later this week when I have a chance.

Meanwhile, I have a question. You integrated the SMIL timing attributes with
the "P" element by making your own version of xhtml.xsd that allowed
including qualified attribute names from other schemas. I'm not sure that
this is a total solution.

However, in my understanding of our discussion of namespaces, I got the
impression that redefining elements to include new content and/or attributes
and then "putting" those elements in a "new" namespace was okay. So it seems
that you can define hybrid languages, it's just that they don't have a
"lineage" reflected in a namespace structure, or in an XML Schema structure
(because the attributes on an element can only be defined in one place).

This will lead to integrations that need to be done very differently in
DTD's vs. XML Schemas. What I mean is that in some cases, the schema will
have to repeat a lot of stuff defined in a "base" schema, while the DTD will
not. The case in point is XHTML+SMIL, which it seems, will have to repeat
the definitions of all of the XHTML-based elements, add the timing
attributes (which can be done nicely in attribute groups in the smil
modules), and declare them in a new namespace. So while there will be very
tight semantics coupling between XHTML and XHTML+SMIL, this coupling will
not be reflected in either the namespace or the schema for XHTML+SMIL.

About the best that I think could be done is if the XHTML-WG creates
"proto-HTML" types that can be used in the derived languages to define the
actual elements.

I think that you can do something like this for timed <a> link's (correct me
if I am wrong):

In the XHTML.xsd:

<xsd:complexType content="mixed" name="aType">
	<xsd:attribute name="href" type="xsd:uriReference"/>
	...other linking attribute declarations
</xsd:complexType>

In the SMIL.xsd:
<xsd:attributeGroup name="smilTimingAttrs" />
	<xsd:attribute name="begin" type="xsd:string"/>
	<xsd:attribute name="end" type="xsd:string"/>
	...other timing attribute declarations
</xsd:attributeGroup>

And then in the XHTML+SMIL.xsd:
<xsd:element name="a" type="xhtml:aType">
	<xsd:attributeGroup ref="smilTimingAttrs"/>
</xsd:element> 

Of course, the means to do this has to be set up in XHTML.xsd and SMIL.xsd.
I can see us doing this kind of thing  for SMIL, but of course the other
stuff is up to XHTML, SVG, and the other XML-based languages using XML
Schemas.

-Aaron

> -----Original Message-----
> From: Dan Connolly [mailto:connolly@w3.org]
> Sent: Friday, May 12, 2000 3:18 PM
> To: www-xml-schema-comments@w3.org; aaron.m.cohen@intel.com
> Subject: Re: XML-Schema Requirements for SMIL
> 
> 
> > From: Cohen, Aaron M (aaron.m.cohen@intel.com)
> > Date: Mon, Apr 03 2000
> 
> Sorry for the delay in responding... better late than never, I hope...
> 
> > Dear XML Schema folks:
> > 
> > It has recently been brought to my attention that the 
> current draft of
> > XML-Schemas does not provide a mechanism for incrementally 
> building up an
> > attribute set on an element, perhaps from several separate modules
> > containing attributes,
> 
> I believe the attribute group mechanism provides this.
> http://www.w3.org/TR/2000/WD-xmlschema-1-20000407/#Attribute_G
> roup_Definition
> 
> > and also for adding attributes to elements already
> > declared.
> 
> I presume you're talking about this idiom...
> 
> "6.1. Defining additional attributes
> [...]
>  This works because XML permits the definition or
>         extension of the attribute list for an element at any 
> point in a
> DTD. "
> --
> http://www.w3.org/TR/2000/WD-xhtml-building-20000105/developin
> g.html#s_dev_attrs
> 
> The Schema spec provides an analog of this idiom; if you
> want your element declarations to allow other attributes this way,
> just include
> 	<anyAttribute namespace="##any" processContents="strict"/>
> 
> cf
> http://www.w3.org/TR/2000/WD-xmlschema-1-20000407/#element-any
> Attribute
> 
> 
> I would actually recommend ##other rather than ##any; requiring
> "mixed in attributes" to be declared as coming from another
> namespace seens more appropriate than allowing unqalified
> attributes or attributes from the same namespace.
> 
> 
> > This is essential for modularizing SMIL with XML Schemas, 
> and is something
> > that can (and has) already be done with DTD's.
> 
> Well... it can be done with a specific set of conventions 
> layered on top
> of DTDs, using parameter entities. It cannot be done for arbitrary
> combinations of DTDs. And the XHTML modularization mechanisms are
> susceptible to name collisions between modules.
> 
> cf
> http://www.w3.org/TR/2000/WD-xhtml-building-20000105/developin
> g.html#s_dev_attrs
> 
> > Maybe my information is
> > incorrect, or my understanding is faulty, so I am sending 
> this to make you
> > aware of our needs, and asking for guidance in applying XML 
> Schemas in this
> > manner. If my information and understanding are correct, then please
> > consider this a requirement for the SMIL modules to have 
> associated XML
> > Schemas. I am under the impression that XHTML has very 
> similar needs, and my
> > impression is based in part on my exchanges with some of 
> the XHTML folks.
> > 
> > A concrete example will probably make our needs clear.
> 
> Yes, thanks.
> 
> > So I'll discuss our
> > modularization needs in the context of a vastly simplified 
> view of SMIL.
> > 
> > SMIL timing is a set of reusable modules that can be used 
> to incorporate
> > timing relationships into XML languages.
> 
> I started drafting a schema for SMIL animation; see:
> 
> http://www.w3.org/XML/2000/04schema-hacking/smil-animation.xsd
> revision 1.1
> date: 2000/05/04 19:35:09
> 
> aka
> http://www.w3.org/XML/2000/04schema-hacking/smil-animation.xsd.txt
> 
> > This language could be "SMIL", or
> > "XHTML", or something else. We provide time containers, 
> which are grouping
> > elements that contain other elements and provide semantics 
> for the timing
> > relationships between the grouped elements. We also provide 
> attributes that
> > are added to elements to allow authors to specify explicit timing
> > relationships.
> > 
> > For example, SMIL includes the parallel time container, 
> <par>, which allows
> > for several things to be played at once. To play an audio 
> and video clip and
> > the same time (assuming that the language, like SMIL 1.0,  
> includes elements
> > to express playing each of these):
> > 
> > <par>
> > <video .../>
> > <audio .../>
> > </par>
> > 
> > Note that the elements could be from some other language, 
> such as XHTML.
> > Here's how two paragraphs of text might be displayed at the 
> same time:
> > <par>
> > <p>First Paragraph.</p>
> > <p>Second Paragraph.</p>
> > </par>
> > 
> > This becomes much more powerful when the elements that are 
> timed can have
> > their own timing-specific attributes. For this discussion, 
> we'll only have
> > two. 'begin' tells when to start displaying something, and 
> 'dur' specifies
> > the duration, or how long to display it. Building off the 
> XHTML example:
> > <par>
> > <p begin="0s" dur="1s">First Paragraph.</p>
> > <p begin="2s" dur="1s">Second Paragraph.</p>
> > </par>
> > 
> > This displays the first paragraph immediately for 1 second, 
> then there is
> > one second where nothing is displayed, and then at time=2s, 
> the second
> > paragraph is displayed for 1 second.
> > 
> > This admittedly very simple example gets to the root of 
> what we need to be
> > able to accomplish with XML Schemas. The <p> element is 
> already defined in a
> > module by XHTML. For a language designer to be able to 
> incorporate or
> > combine timing with elements already defined, we need to be 
> able to extend
> > the attribute set of an element defined in a different module.
> 
> Or, alternatively, you need the definition of the P element to allow
> attributes declared elsewhere (as noted above, this is the default
> in XML 1.0)
> 
> I mocked up this example...
> 
> http://www.w3.org/XML/2000/04schema-hacking/h+s.html
> 
> =========
> <html
>  xmlns="http://www.w3.org/1999/xhtml"
>  xmlns:xsi='http://www.w3.org/1999/XMLSchema-instance'
>  xmlns:t='http://www.w3.org/2000/TR/smil-animation10'
>  xsi:schemaLocation="http://www.w3.org/1999/xhtml html.xsd
>          http://www.w3.org/2000/TR/smil-animation10 smil-animation.xsd
>                 "
>   >
> <head><title>Example of mixing HTML and SMIL timing</title></head>
> <body><h1>Some stuff</h1>
> 
> <!-- example from 
> http://www.w3.org/TR/NOTE-HTMLplusTIME -->
> 
> <p t:begin="1">
>          This is a paragraph of text that appears after one second
>      </p>
>      <p t:begin="2">
>          This is a paragraph of text that appears after two seconds
>      </p>
>      <p t:begin="3">
>          This is a paragraph of text that appears after three seconds
>      </p>
> 
> </body>
> </html>
> =========
> 
> (The schemaLocation gobbledygook is only necessary until schemas
> for XHTML and SMIL are available by dereferencing
> their namespace identifiers)
> 
> then in html.xsd:
> (i.e.  http://www.w3.org/XML/2000/04schema-hacking/html.xsd)
> 
>  <element name='p'>
>   <complexType content='mixed'>
>    <anyAttribute namespace="##other" processContents="strict"/>
>     [...]
>   </complexType>
>  </element>
> 
> and in smil-animation.xsd :
> 
>    <attribute name='begin' type='string'/>
>    <attribute name='dur' type='string'/>
>    <attribute name='end' type='string'/>
>    <attribute name='restart'>
>     <simpleType base='string'>
>      <enumeration value='always'/>
>      <enumeration value='never'/>
>      <enumeration value='whenNotActive'/>
>     </simpleType>
>    </attribute>
>    <attribute name='repeatCount' type='string'/>
>    <attribute name='repeatDur' type='string'/>
>    <attribute name='fill'>
>     <simpleType base='string'>
>      <enumeration value='remove'/>
>      <enumeration value='freeze'/>
>     </simpleType>
>    </attribute>
> 
> You can check the results using
> 	http://cgi.w3.org/cgi-bin/xmlschema-check
> i.e.
> http://cgi.w3.org/cgi-bin/xmlschema-check?docAddrs=http%3A%2F%
> 2Fwww.w3.org%2FXML%2F2000%2F04schema-hacking%2Fh%2Bs.html
> 
> 
> I have used qualified names for the "mixed in" attributes, per
> this principle:
> 
> 	"The syntax must unambiguously associate an identifier in
> 	a document with the related schema without requiring inspection
> 	of that or another schema."
> 
> 	-- 
> http://www.w3.org/TR/1998/NOTE-webarch-extlang-19980210#Ambiguity
> 
> The schema spec allows you to violate that principle by using
> <anyAttribute namespace="##any"/> on the P element
> declaration, and and attributeForm="unqualified" in the smil-animation
> schema, but I don't recommend that.
> 
> > For this
> > particular example, the language designer needs an XML 
> Schema method of
> > adding the begin and dur attributes defined in a SMIL XML 
> Schema module to
> > the <p> element defined in an XHTML Schema module.
> 
> I believe I have demonstrated this above.
> 
> 
> > It is not sufficient to
> > just be able to declare the members of an attribute set 
> from several places
> > when the element is initially defined.
> > 
> > The same thing goes for the content model of the elements. It may be
> > necessary to extend the permissible child element set of a 
> module beyond how
> > it was initially defined. SMIL Boston is planning on using 
> "levels" of
> > modules. As the functionality goes up in higher levels, we 
> add some elements
> > to the set of allowed children. As above, we'll also add 
> new attributes to
> > existing elements in order to make them more powerful in 
> the higher level
> > modules.
> > 
> > Furthermore, we will use the above process to define our 
> next version of the
> > SMIL Language, and also to define a language/profile that 
> combines XHTML and
> > SMIL timing structure (as well as some other aspects of 
> SMIL), and to define
> > a baseline "SMIL-Basic". So you can see that the use case 
> that we are asking
> > for is a key element in SMIL being a successful and 
> reusable technology. The
> > DTD experts in the group are comfortable that we can 
> produce DTD's that
> > reflect this structure, and we would also like to have a 
> set XML Schemas
> > before SMIL becomes a Recommendation. To this end, we need 
> confirmation that
> > it can be done with XML Schemas, and guidance in how to proceed.
> 
> Does the explanation above provide enough confirmation and guidance?
> 
> > I've been
> > told to expect a note on modularization using XML Schemas. 
> Will our use
> > cases be covered in that note?
> 
> I'm not sure.
> 
> > Please feel free to contact me with any questions you may have about
> > anything that I have said here.
> > 
> > Thank you,
> > Aaron Cohen, Intel
> > Chair of the SYMM Working Group
> 
> 
> -- 
> Dan Connolly, W3C http://www.w3.org/People/Connolly/
> 
Received on Monday, 15 May 2000 18:31:12 UTC

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