Declarative Specification of Ambiance in VRML Landscapes

1
Declarative Specification of Ambiance in VRML
Landscapes

• Vincent Jolivet, Dimitri Plemenos, Patrick
  Poulingeas.
• MSI Laboratory.
• University of Limoges (France).

2
Declarative Specification of Ambiance in VRML
Landscapes

• Introduction
• Declarative Modelling
• Managing declarative specifications with fuzzy
  subsets theory
• Application to the foggy aspect of a scene
• Application to natural lighting
• Conclusion

3
An overview of VRML

• VRML has now become a standard for creating
  virtual worlds integrated to Internet.
• VRML offers to the designer high-level objects
  and manages all the rendering process.
• VRML is an open text-based format which allows a
  processing by script languages.

4
VRML tools

• There are 2 types of tools for a VRML designer
• Geometric modellers (for the static aspect of a
  virtual world).
• Behavorial modellers (for the dynamic aspect of a
  virtual world).
• Our goal is to present an improvement of
• geometric modellers when the virtual world is
• a landscape.
• A perspective similar to the aim of
• Declarative Modelling

5
Declarative Modelling

• The aim of declarative modelling is to produce
  several prototypes of scenes corresponding to a
  high-level and vague specification.
• There are 2 kinds of declarative modellers
• General purpose declarative modellers for any
  kind of scenes.
• Dedicated declarative modellers specialized in a
  domain (This work belongs to this class).

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Phases of Declarative Modelling

1. Description phase (where the designer describes
   her wishes in a natural and declarative
   manner).
2. Scene generation phase (where an algorithm
   generates all the scenes satisfying the
   description).
3. Scene understanding phase (where the solutions
   found are shown to the user with a classification
   method).

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Managing Properties

• Declarative specifications use properties.
• Properties have an inherent imprecision.
• Words like big or tall do not correspond
  to a precise numerical interval.
• Fuzzy subsets theory can take into account this
  imprecision due to natural language.
• This theory can also build new properties with
  modifiers and a few basic properties.

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Two kinds of properties

• Simple properties (The weather is foggy). These
  properties are qualitative.
• Parametric properties (The level of snow is 2
  meters).
• These properties introduces a numerical value.

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Fuzzy Representation of Properties

• Simple and parametric properties are
  represented by trapezoidal membership functions
  in our usage of fuzzy-subsets theory.

• a,b is the kernel of the membership function.
• a-a,bß is the support of the membership
  function.

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Generate Numeric Values for a Property

1. A threshold a is fixed.
2. The a-support set of the membership function
   associated to the property is computed ? An
   interval.
3. A discretization of the computed interval gives
   several values (? several solutions)

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Modifiers

• Modifiers are operators that act on simple
  properties to create a new property.
• Modifiers are
• Extremely little
• Very little
• Rather little
• ? (? No modification of the simple property)
• Rather
• Very
• Extremely

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Action of a Modifier on a Simple Property
The simple property weak is plotted in red.
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Fuzzy Operators

• Fuzzy operators act both on simple and parametric
  properties to create a new property.
• Fuzzy operators are
• Exactly
• Really
• ? (? No modification of the property)
• Neighboring
• More or less
• Vaguely

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Uncertainty in Descriptions

• Previous descriptions were imprecise.
• A new kind of descriptions has been introduced
  descriptions with uncertainty.
• Example of such descriptions It is rather
  certain that the level of snow is 2 meters.
• The word rather acts as an uncertainty operator
  which can increase or decrease the uncertainty of
  the description.

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Uncertainty operators

• Uncertainty operators can be used with simple and
  parametric properties to create a new property.
• The following set of uncertainty operators
• have been chosen
• ? (means that the description is certain)
• Rather
• Enough
• Little
• Very little

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Action of an uncertainty Operator on a Property
There is a reduction of the size of the kernel of
the membership function (of the basic property)
and a contraction of this function.
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Foggy Scenes in VRML

• VRML allows the designer to change the foggy
  aspect of a scene with a Fog node.
• The Fog node has a visibility field which
  corresponds to the visibility limit for the
  Avatar.

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Properties for foggy Aspect

• 2 kinds of properties are proposed to the
• designer
• A simple property like The weather is foggy.
• A parametric property like The visibility limit
  is 50 meters.

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Examples of Solutions found
Action of an operator on a property
The visibility limit is 70 meters
The visibility limit is exactly 70 meters
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Examples of Solutions found
Several solutions are computed for the same
description
The visibility limit is 70 meters.
First scene found for the description
Last scene found for the description
21
Natural Lighting in a Scene

• VRML proposes a node called DirectionalLight for
  a scene illumination.
• This node introduces a directional light in the
  VRML landscapes.
• This directional light will correspond to the
  light of the sun.

22
The lived Time

• To express the natural lighting in a declarative
  manner, the concept of
• lived time (D. Siret) has been chosen.
• In a geocentric model, the possible sun positions
  are computed from descriptions such as
• The end of the afternoon
• Beginning of the day in Winter
• The end of the morning in the middle of Summer.

23
Determination of the Sun Position
A geocentric model (Figure taken from a work of
D. Siret).
24
Intervals corresponding to the lived Time
Figure taken from a work of D. Siret.
25
Example of natural Lighting
One scene corresponding to the description
Midday.
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Example of natural Lighting
One scene corresponding to the description
The middle of the afternoon.
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Drawbacks of the Method

• The drawbacks come from VRML limitations.
• Absence of shadows (damages the realism of sun
  lighting).
• Lighting included in textures becomes
  inconsistent when the sun position changes.
• Impossibility to introduce complex physical
  parameters.

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Future Researches

• In order to overcome the previous drawbacks
• Lighting in textures can be recovered and changed
  with an inverse rendering process.
• VRML viewers like Contact of Blaxxun have
  interesting extensions of VRML standard (e.g.
  Particle and MultiTexture nodes)
• The emergence of X3D would give more
  possibilities than VRML.

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Conclusion

• A framework to manage declarative descriptions
  has been introduced.
• This framework is based on fuzzy subset theory.
• An application to ambiance in VRML landscapes has
  been presented.
• This framework can be reused for handling
  ambiance in more generic scenes.