[x3d-public] Statement permitted placements

Fri Jul 14 08:45:24 PDT 2017

Hi,

We were recently discussing the permitted placement of the following
statements:

ROUTE

IMPORT

EXPORT

PROTODECLARE

EXTERNPROTODECLARE

The original discussion centred on the draft JSON encoding. However, I have
now been looking at this in the XML and Classic VRML encodings.

Let's take a look at what is permitted by these encodings, and also compare
what the abstract standard says.

Let's take the Classic VRML encoding first, i.e. ISO/IEC 19776-2 V3.3. The
most comprehensive description is the grammar in Annex A.2.

Reference:
http://www.web3d.org/documents/specifications/19776-2/V3.3/Part02/grammar.ht
ml#General

A non-terminal "statement" is defined as:

nodeStatement | 
importStatement |
exportStatement |

protoStatement | 

routeStatement ; 

The non-terminal "nodeStatement" can be followed down to "node", "nodeBody",
and "nodeBodyElement". This last non-terminal has the definition:

initializeOnlyId fieldValue | 
inputOutputId fieldValue |

initializeOnlyId IS initializeOnlyId | 

inputOnlyId IS inputOnlyId | 

outputOnlyId IS outputOnlyId | 

inputOutputId IS inputOutputId | 

routeStatement | 

protoStatement ; 

Notice the lack or importStatement and exportStatement.

This has been pointed out previously. In fact, Mantis issue 698 covers this
very point.

Reference: http://www.web3d.org/member-only/mantis/view.php?id=698

Now let's take the XML encoding, ISO/IEC 19776-1 V3.3. Let's look at an
example node, say Group, defined at 6.2.81.

Reference:
http://www.web3d.org/documents/specifications/19776-1/V3.3/Part01/EncodingOf
Nodes.html#Group

Notice that this Node has a content model denoted by "ChildContentModel".
The description of this is given in 6.3.3 ChildContentModel

Reference:
http://www.web3d.org/documents/specifications/19776-1/V3.3/Part01/EncodingOf
Nodes.html#ChildContentModel

The text states: "ChildContentModel is the child-node content model
corresponding to X3DChildNode, combining all profiles. ChildContentModel can
contain most nodes, other Grouping nodes, Prototype declarations and
ProtoInstances in any order and any combination.". Note that there is no
mention of ROUTES, or IMPORT/EXPORT.

We should also note the text in 6.3.5 ChildContentModelSceneGraphStructure.

Reference:
http://www.web3d.org/documents/specifications/19776-1/V3.3/Part01/EncodingOf
Nodes.html#ChildContentModelSceneGraphStructure

This states "Child-node content model corresponding to SceneGraphStructure
elements, which are not specific X3D nodes. These are ProtoDeclare,
ExternProtoDeclare, IMPORT, EXPORT or ROUTE statements.". This content model
covers all the statements of interest. Where is it used? This content model
is not listed in any  node definition. In fact, it is not mentioned anywhere
else in the standard.

Turning to the XML schema V3.3 now.

Reference: http://www.web3d.org/specifications/x3d-3.3.xsd

Looking at the "Group" node, we see that the content model is derived from
X3DGroupingNode. Looking at the definition for X3DGroupingNode, the
following definition can be extracted:

       <xs:complexContent>

              <xs:extension base="X3DChildNode">

                     <xs:sequence minOccurs="0" maxOccurs="unbounded">

                           <xs:annotation>

<xs:documentation>children</xs:documentation>

                           </xs:annotation>

                           <xs:group ref="ChildContentModel" minOccurs="0"
maxOccurs="unbounded"/>

                           <xs:group
ref="ChildContentModelSceneGraphStructure" minOccurs="0"
maxOccurs="unbounded"/>

                     </xs:sequence>

                     <xs:attribute name="bboxCenter" type="SFVec3f"
default="0 0 0"/>

                     <xs:attribute name="bboxSize"
type="boundingBoxSizeType" default="-1 -1 -1"/>

              </xs:extension>

       </xs:complexContent>

So any node derived from X3DGroupingNode will be passed as valid of it has
any of the statements we are considering. Where else if this content model
used? The XML schema has six instances of this term, as follows:

1.	In the definition of X3DGroupingNode
2.	In the definition of the content model group
ChildContentModelSceneGraphStructure
3.	In the definition of the content model group
GroupingNodeChildContentModel
4.	In the definition of the content model group
SceneGraphFragmentWithPrototypeDeclarationsContentModel
5.	In the definition of the StaticGroup node
6.	In the definition of the GeoLOD node

The GroupingNodeChildContentModel is only used in the definition of the
Scene element.

So, we see that in the XML schema these statements are only permitted in
grouping nodes.

Finally, let us turn to the abstract standard ISO/IEC 19775-1 V3.3. What
does it tell us about the placements of statements. There is no general text
covering all the statements, so we need to look at each one in turn. We will
start with ROUTEs and Prototypes, 7.2.5.7 ROUTE statement, 7.2.5.8 PROTO
statement, and 7.2.5.9 EXTERNPROTO statement.

Reference:
http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components
/core.html#ROUTEStatement

Reference:
http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components
/core.html#PROTOStatement

Reference:
http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components
/core.html#EXTERNPROTOStatement

All three of these clauses have the words "xxxxxx statements may appear
anywhere in the file"

Now look at 9.2.5 IMPORT statement, and 9.2.6 EXPORT statement.

Reference:
http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components
/networking.html#IMPORTStatement

Reference:
http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components
/networking.html#EXPORTStatement

Neither of these clauses has any comment on the placement within the file.

So, what do we conclude?

A.	ROUTEs and prototypes are permitted anywhere.
B.	IMPORT and EXPORT need a statement on placement in the abstract
standard

If we assume that A) is correct, then:

C.	In the XML encoding the content models need to be amended to permit
placement of these statements anywhere
D.	In the Classic VRML encoding the grammar needs to be amended

Of course, we might want to restrict the usage, in which case these needs
proper definition in the abstract standard, and propagation to the
encodings.

All the best,

Roy

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