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MPEG-7 Problem Issues and Queries

From: Jane Hunter <jane@dstc.edu.au>
Date: Mon, 01 Jan 2001 21:19:17 +1000
Message-Id: <200101011118.f01BIsH26490@piglet.dstc.edu.au>
To: www-xml-schema-comments@w3.org
Cc: mpeg7-ddl@darmstadt.gmd.de
Below is a list of queries raised by the MPEG-7 community whilst developing 
MPEG-7 schemas. On a lot of the issues we're fairly sure of the answer already 
but would be highly appreciative of definite clarification.

regards (and best wishes for 2001),

Jane Hunter
-----------------------------------------------
1. Global attributes

When referring to a global attribute whilst declaring a local attribute, 
can you modify its "use" attribute value and expect this to overwrite the 
original value?

Ex:

<attribute name="myAtt" type="integer"/>

<complexType>
   <!-- can we expect the default value, 0, is specified
        in the following local attribute? -->
    <attribute ref="myAtt" use="default" value="0"/>
</complexType>


--------------------------------------------------------------------
2. Root Elements

Is there a way to specify the root element amongst global elements in an
instance document ?

-------------------------------------------------------------------
3. Union and IDREF

Each IDREF must match a corresponding ID within the XML instance document in 
which it occurs. Can we expect that this rule also applies when IDREF is used 
within a <union>? According to XML Schema definition, it seems not:

  [Definition:]  Union datatypes are those whose *value spaces* and
  *lexical spaces* are the union of the value spaces and lexical 
  spaces of two or more other datatypes. 

Ex:
<simpleType name="referenceType">
  <union memberTypes="IDREF uriReference mpeg7:xPathType"/>
</simpleType>

------------------------------------------------------------------
4. Union and attributes

In Sec 2.5.1.3 of XML Schema Part 2, an example is given as:

  <xsd:element name='size'>
    <xsd:simpleType>
      <xsd:union>
        <xsd:simpleType>
          <xsd:restriction base='integer'/>
        </xsd:simpleType>
        <xsd:simpleType>
          <xsd:restriction base='string'/>
        </xsd:simpleType>
      </xsd:union>
    </xsd:simpleType>
  </xsd:element>

A possible instance is:

  <size>1</size>
  <size>large</size>
  <size xsi:type='xsd:string'>1</size>

What happens in the last instance when "size" is also declared as an
attribute? There seems no way to specify the type using xsi:type 
when the size is also an attribute.

------------------------
5. Is it possible to both extend and restrict a locally defined 
   datatype simultaneously?

Ex:

<element name="LinearMatrix">
  <complexType>
    <simpleContent>
      <restriction base="mpeg7:IntegerMatrixType">
        <minInclusive value="0"/>
        <maxInclusive value="65535"/>
      </restriction>

      <attribute name="sizes" use="fixed" value="3 3"/>
        <!-- IMPOSSIBLE both extension and restriction -->

    </simpleContent>
  </complexType>
</element>

------------------------------------------------------

6. Is there any way to constrain the length of a locally defined 
   list?

Ex:

<element name="ColorValueIndex">
  <simpleType>
    <list itemType="mpeg7:unsigned12"/>
        <!-- <length value="3"/> POSSIBLE ?? -->
  </simpleType>
</element>

<simpleType>
   <list itemType="mpeg7:unsigned8"/>
       <!-- <minLength value="32"/> IMPOSSIBLE -->
       <!-- <maxLength value="256"/> IMPOSSIBLE -->
   <!-- </list> -->
</simpleType>

______________________________________________________

7. When deriving a complexType by restriction on a complexType which 
was itself derived by extension, must you repeat the content models of 
all of the earlier base types in the type hierarchy - or do you only 
need to specify the extended components?

This is a repetition of an earlier question posted on Dec 18 which has yet to 
be answered.

-------------------------------------------------------

8. Is it possible to derive an abstract complexType from a non-abstract
complexType?

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9. Using derivation by restriction, can we change the content model of a group 
from choice[0,n] to sequence?

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Received on Monday, 1 January 2001 06:19:24 GMT

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