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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: The Power of Groves: The VRML View
Because it is always hard to get started from an existing language
and its meta-definitions, then map to another meta-language or
framework of definitions, I am presenting an abbreviated version
of the VRML language as described in Late Night VRML 2.0 with
Java (Couch, Ballreich, Roehl and Brown). Typically, many users
start with such books and descriptions, so this a fair challenge
and a convenient source for a condensed description. That is
why the following is in quotations although it is not a direct
quote. If we need more precision, we can use the VRML97
specification online. Some of my inline comments are in
[NOTE ...].
"VRML has two data structures: nodes and fields. A node is the
equivalent of a Java class. A field is almost the equivalent of a
variable and a method in one.
Nodes collect a series of fields that define the properties of that
node. A node is a standalone data structure and a file can declare
a single node. A file cannot contain a single field. Node names
must being each word with a capital letter and cannot contain
underscores (eg, IndexedFaceSet).
VRML defines 54 nodes categorized into nine groups by function;
Grouping nodes, Special Groups, Common nodes, Sensor nodes, Geometry
nodes, Geometric property nodes, Appearance, Interpolator nodes, and
Bindable nodes.
Fields are typed and named. Types are of two kinds: single (SF- eg,
SFFloat) as in a single variable and multi(MF - eg, MFFloat) as in an
array of singles (note:
not having arrays makes using Xschemas nasty and a candidate for
rejection). Field
names start with a lower case letter and use uppercase for each word
thereafter (eg, cycleTime). Fields are semantically named for the
access type:
field (no external access)
eventIn (writeOnly)
eventOut (readOnly)
exposedField(read and write)
To pass events among fields, the nodes must be named (DEFd see below)
and the declaration of the connection is via the ROUTE keyword between
fields of the same type. ROUTE statements name the node.field from and
node.field to.
ROUTE fromNode.eventOutField TO toNode.eventInField
The logical structure of the VRML is called a scene graph. It refers
to the relationships of the visible scene.
Syntax: The scope of a node definition is denoted by matching curly
brackets (same as Java). Nodes are nodeType, open curly bracket,
fields,
closed curly bracket. VRML files must begin with a VRML header of the
form
filetype, version, encoding. Therefore:
#VRML V2.0 utf8
Field declaration is the field name followed by its value. Vectors are
whitespace separated numbers. Eg
Material { diffuseColor 0 0.2 0.1 }
Multivalued fields enclose values with square brackets and use commas or
whitespace as separators.
Grouping nodes contain other nodes in the children field. [NOTE:
a nasty thing, we have to use the children wrapper. Note, fields
can be of type MFNode.]
Each node can contain other nodes in a parent child relationship.
[NOTE: A DTD for this which is precise and restricts illegal
combinations has content models which are very very big and
hard to read.]
Nodes can be reused by associating a name with a node. The syntax
is to use the DEF keyword followed by the name. When used elsewhere, a
USE keyword followed by the name DEFd earlier is provided. So
Transform { translation -1 0 0
children Shape {
geometry DEF thisBox Box {
size 2 1 3 }
}
}
Transform { translation 2 0 0
children Shape {
geometry USE thisBox
}
}
Names are scoped to the file and can only be referenced
within the file. An inlined file is a closed scope. A
prototyped node is a closed scope. A prototype (usually
called simply proto) is a customized node that can be
added to a scene. It is a black box and is treated like
a standard node in the scene. Proto nodes are the extensibility
feature of VRML. It is a canned object that once declared,
can be used to create multiple independent copies. A
DEFd node which is USEd is not independent. If any affective
field property value is changed in the original, it changes
in all the copies. For example, if a Shape node has a Material
value whose color is red, and red is changed to brown, all
USEd copies change to brown. If a proto is changed, only
the local copy changes.
The declaration rules for protos are different. A proto
has a name, field definition, and implementation."
We'll stop here for now. This is probably enough to get
started. Protos are interesting beasties because the enable
inline extensibility such that one can create a proto for
a new kind of shape (say a torus) that is not defined as a
primitive in VRML, then use it as if it were one. Further,
because scripts can be part of the implementation, it is
an object that behaves in response to routed events, or
it can just be a behavior container for wrapping other
nodes.
Protos and Externprotos can be referred to externally
so you can use them to build parts libraries and in fact,
that is precisely how the VRML community shares a lot of
functional code through the VRML Proto Repository.
Len Bullard
Intergraph Public Safety
clbullar@i...
http://fly.hiwaay.net/~cbullard/lensongs.ram
Ekam sat.h, Vipraah bahudhaa vadanti.
Daamyata. Datta. Dayadhvam.h
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