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).

6. Phases of Declarative Modelling • Description phase (where the designer describes her wishes in a “natural” and declarative manner). • Scene generation phase (where an algorithm generates all the scenes satisfying the description). • Scene understanding phase (where the solutions found are shown to the user with a classification method).

7. 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.

8. 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.

9. 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-α,b+β] is the support of the membership function.

10. Generate Numeric Values for a Property • A threshold α is fixed. • The α-support set of the membership function associated to the property is computed  An interval. • A discretization of the computed interval gives several values ( several solutions)

11. 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

12. Action of a Modifier on a Simple Property The simple property “weak” is plotted in red.

13. 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

14. Uncertainty in Descriptions • Previous descriptions were imprecise. • A new kind of descriptions has been introduced: descriptions with uncertainty. • Example of such descriptions: “It is rathercertain 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.

15. 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

16. 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.

17. 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.

18. 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”.

19. Examples of Solutions found Action of an operator on a property The visibility limit is 70 meters The visibility limit is exactly 70 meters

20. 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”.

26. Example of natural Lighting One scene corresponding to the description: “The middle of the afternoon”.

27. 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.

28. 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.

29. 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.