[20]
These benefits are functions of the properties of emodels, namely,
procedural specification - a program defines the model
parameterization - aspects of the model can be tied to changeable values
replication - multiple copies of the model can be made
precision - model parts can interact in mathematically precise ways
continuous representation - equations define shapes, rather than point lists
compression - the data format is space-efficient
composition - complex models can be built from simpler models
simulation - the ability to implement particle systems, behavior simulation, etc.
lighting - lights, surface materials and atmospheric effects described in the model
sound effects - sounds described in the model
Emodels are compact, self-contained, reusable, and customizable. Modularity of this sort
is ideal for any constructive process, be it animation, programming, or game production.
If emodels could be created with VRML as the representation language, 3D game designers
could take advantage of all the properties of emodels listed above. In a significant step
towards the goal of simplifying game design, emodel libraries could be established to
promote the sharing (or selling) of complex models. These models could be props,
characters, behaviors, tools, etc., ready for use in a game.
VRML
as an emodel representation language
The VRML97 specification suggests support for scripting languages. Language choice is
left to the implementers of the browser software, but is typically ECMAscript and / or
Java. When implemented with a scripting language, VRML can be used to create models
that exhibit all of the properties of emodels listed above.
Encapsulation in VRML is achieved through the PROTO and EXTERNPROTO structures.
A PROTO takes an arbitrary collection of nodes and bundles them into a single node
with a single interface for sending and receiving data. Recall that nodes are the atomic
unit in VRML and can represent shapes, colors, lights, textures, sounds, motions,
transformations, etc. The PROTO structure is simply a wrapper node that allows other
nodes to hide inside of it. This can be thought of as analogous to a stereo component
supplying input and ouput jacks for audio, while hiding the circuitry of its audio processing
electronics inside a box.
The EXTERNPROTO structure is a PROTO that resides in a separate (external) file. This
allows complete encapsulation and extensive reuse. A single EXTERNPROTO file can
be referenced by any number of different VRML models, simply by including its parameter
interface and a link to its implementation.
VRML models can therefore be emodels when they are defined as EXTERNPROTOs.
Their parameter interface allows the user to customize the model to meet specific needs.
VRML emodels can themselves be composed of other emodels. This recursive definition
allows progressively more complex objects to be built from basic components.
To illustrate the applicability of VRML to emodel theory, an example VRML emodel has
been created: RocketClock, which is similar to AlarmClock presented by May. The model
exhibits several high level controls such as clock color, setting of the time and alarm, alarm
volume, and exaggeration of ringing motion. Furthermore, RocketClock makes use of
another emodel, ClockMechanism, to drive the rotations of the hands around the face. The
ClockMechanism behavior emodel could easily be reused in another clock model, which
might have completely different geometry.
The RocketClock example is presented on page four, and its implementation is given in
Appendix C.
03
[17], [63]
[16]
[18], [64]
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