SurveyComputer animation: from avatars to unrestricted autonomous actors (A survey on replication and modelling mechanisms)
Introduction
Computer animation technologies allow users to generate, control, and interact with life-like representations in virtual worlds. Such worlds may be 2D, 3D, real-time 3D, or real-time 3D shared with other participants, and the actors can be given the form of humans, animals, or animate objects.
Increases in computational power and control methods enable the creation of 3D virtual characters even for real-time interactive applications. Developments and advances in networking and virtual reality let multiple participants share virtual worlds and interact with applications or each other. Artificial intelligence techniques give computer-generated characters a life of their own and let them interact with other characters in virtual worlds.
In the more basic approach the synthetic actor can be seen as a remotely controlled puppet engaged in fictitious interactions with a simple virtual world. As synthetic actors are placed in simulated worlds of growing complexity, an obvious requirement that comes about is to make them perform in these worlds. Advances in computer animation techniques have spurred increasing levels of realism and virtual characters closely mimic physical reality and typically draw upon results from physics, biology, and the cognitive sciences. Maybe they will soon be indistinguishable from real characters.
The aims of this paper are to propose a new classification of synthetic actors and to go through the different motion and behaviour techniques used to animate them. In Section 2, we propose the classification and introduce the techniques. Section 3 deals with motion and behaviour replication, Section 4 with motion modelling, Section 5 with behaviour modelling and finally, some reflections are presented. Some Internet addresses of relevant research work groups and other interesting related links can be found in the Appendix.
Section snippets
Classifying synthetic actors
Several methods exist for classifying animation systems and synthetic actors [1], [2], [3]. In this paper we use the nature of the information required for controlling actors’ motion and behaviour to propose a new classification of synthetic actors. Two primary mechanisms exist to specify motion and behaviour for a synthetic computer-generated character:
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Replication mechanisms that reproduce the motion or behaviour of a real actor.
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Modelling mechanisms where the motion and behaviour are based on
Key-framing
In the animation systems based on keyframing the animator specifies the system's kinematic by means of giving the parameter values in the key-frames. In-betweenings are calculated by the computer applying an interpolation law that can be linear, constrained to a mobile point [4], based on splines [5], [6], [7], or on quaternions [8], [9], [10]. In these schemes, control over animation is total. Nevertheless, if the number of parameters is considerable, motion specification becomes a tedious
Direct and inverse kinematics
Direct and inverse kinematics are used to develop algorithms for generating complex motion, such as human walking and different animal movements. These systems are based on biomechanical and biological studies. Instead of specifying positions, the animator specifies parameters that condense the essence of the motion and allow its individualization. In inverse kinematics, if the position of the final effector (open chains) is given, the calculation of the intermediate links is automatic [21].
Behaviour modelling
A clear difference between physically based modelling and behaviour-based modelling has to be made, although sometimes both techniques are complementary. Physically based modelling, as it has been stated in the previous section, emphasizes on realistic aspects like elasticity, deformation or collision, as can be sampled in hair or clothes modelling. Behaviour-based modelling covers those internal aspects, some not yet well developed, like personality, social differences, perception or reaction.
Reflections
The content of the present survey presents significant and important developments of computer animation and depicts a dynamic area of research where a number of impressive results have already been achieved and successfully carried over to commercial applications.
Application areas include production animation, interactive computer games, interactive digital television, multimedia products, simulations, computer-supported collaborative work, agents (personalizes information), shared virtual
Acknowledgements
We would like to thank Federico Corriente for his interest and help with English. This work has been partially financed by the Spanish “Comision Interministerial de Ciencia y Tecnologı́a”, contract number TIC-980973-C03-02.
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