A Comparison of Design Strategies for 3D Human Motions
Summary (3 min read)
1 Introduction
- In this paper the authors compare the design strategies for 3D human motion.
- The authors can order the various methodologies along a scale beginning at high realism for the motion design process in a production context, then middle realism for the interactive process provided by a wide range of motion modeling systems and finally low realism for real-time applications as simulators, virtual reality and games.
- Then, in the second part the authors focus on the wide range of systems providing interactive response time basically for design purpose or for some of them dedicated to human factors evaluation.
- The objectives of the ESPRIT project HUMANOID 2 are recalled in that context.
2 Motion Design in Film Production Context
- What is required in movie productions is much more than the mere physically-based realism ; it is rather a high believability conveying the intention of the motion and the emotional state of the character.
- Such practical knowledge can now guide the 3D animators [2][3] in bringing to "life" cartoon-like or toy-like characters as recently demonstrated with the movie "Toy Story" [4].
- Apart from that masterpiece which involved an important team (110 persons at Pixar [4]), this type of work is limited to short pieces for cost reasons.
- At this moment, most of them are special effects, commercials and, more and more, some sophisticated games [5].
- The logical requirement on the software tools is to ask both for the highest realism and the greatest freedom of design in order to edit and improve any detail of the motion.
2.1 Physics alone does not bring “Life”
- The major commercial systems for 3D human animation, as Alias-Wavefront-TDI and Softimage, propose various degrees of motion realism from the standard Keyframe techniques, Inverse Kinematics, Direct and Inverse Dynamics to the option of live recording.
- This is achieved with the large set of tools manipulating keyframes [4].
- Such requirements discard language-based interfaces in this context [8][9].
- This explain why the Performance Animation approach has been widely adopted in the film production context [14][5].
2.2 Live recording techniques are still too “superficial”
- Most of the Performance Animation systems dedicated to the recording of human body motion belong to two groups depending on the sensing technologies they rely on, either optical or magnetic.
- Both allow the real-time registration of the human motion, practically speaking from around 20Hz to 100Hz for magnetic, and from 50Hz to 200Hz for optical.
- Locating the anatomical frame at each joint of interest by applying a rigid transformation from the technical frame (or magnetic sensor frame) deriving the anatomical angles from the relative orientation between anatomical frames belonging to successive segments at a given joint.
- There is also a need to improve motion editing methods in order to enforce the cartesian constraints lost in the acquisition process while retaining the initial motion dynamics [20] [12].
3 The interactive simulation environment
- Apart from the wide range of commercial systems providing interactive response time for the purpose of animation design, the authors can consider here the systems dedicated to human factors evaluation, ergonomics, human behaviors studies and autonomous agent simulations.
- An equally important property of this technique is the ability to combine it with goal-oriented motions (defined with Inverse Kinematics) in a hierarchical fashion [7].
- Some interesting generalization of the transition management between multiple postures have be proposed to define simple behaviors that can also be used in real-time applications [32].
- This branch has begun with simple flock of birds and animal herds behaviors and now turns to simulate believable human behaviors in complex environments [22].
4 The real-time simulation environment
- The real-time simulation environment fully integrate the end-user within the animation loop in order to drive strategic simulation parameters.
- In that context, only very small system lag is acceptable in response to user input.
- Only simple characters can be animated in such a way due to the limited number of measured DOFs (even if more than one puppeteer are coordinating their performance, one usually animates the body while the other one animates the face).
- Even in that limited context it can be desirable to automatically alter the displayed hand posture in order to reflect the virtual hand interaction with the virtual objects [37].
5 The HUMANOID environment
- The HUMANOID environment is dedicated to the development of multimedia, VR and CAD applications involving virtual humans [26].
- The displacement is realized as an automatic secondary behavior of the Inverse Kinematics solution [7].
- It is achieved, for each step of the end effector behavior, in a two stages process : construction of each sensor’s kinematic jacobian which is inverted and multiplied to the correction displacement to get the correction posture variation of the character.
- The gravity and collision avoidance can be added thus automatically producing realistic postures as a background behavior.
- The second issue the authors want to highlight here is the interest of specifying the body motion from keyframed motion assigned to grasped objects.
6 Conclusions
- The authors have reviewed the various strategies used to animate 3D human characters by grossly classifying them in three classes of compromise between the requirements of motion realism and real-time interaction.
- On the other hand, some class of viewers can accept the imperfection of the displayed motion, whatever the production tool, if it allows a greater interaction.
- The design strategies for 3D human motions have been the object of important researches since the beginning of the 80s.
- Now and for the years to come, the considerable interest which has supported them is still raising as the computing power only begins to allow convenient handling of these classes of problems.
- Moreover, significant advances are soon to emerge in the direction of autonomous agents reacting to each other and to their environment.
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Citations
121 citations
Cites background from "A Comparison of Design Strategies f..."
...The design time is greatly reduced compared to pure keyframe design even if a second stage of motion refinement is to be planned after the capture [7]....
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...Design of human motion is a complex task and an active research area [7]....
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References
51 citations
"A Comparison of Design Strategies f..." refers background or methods in this paper
...Some interesting generalization of the transition management between multiple postures have be proposed to define simple behaviors that can also be used in real-time applications [32]....
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...• the use of Inverse Kinematics [34], functional models [35] and posture-based behavioral automata [32] (Cf....
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51 citations
"A Comparison of Design Strategies f..." refers methods in this paper
...The same remark is globally valid for other classical functional models as grasping [30] or general goal-oriented motion with IK [31] and general balance control with Inverse Kinetics [25] [7]....
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...They ignore Optimization techniques and Inverse Kinetics (see details in section 3)....
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...Modeling the perception of balance is also very useful for motion design as developed later [9] [25]....
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...Conversely, Inverse Kinetics evaluates the exact influence of each joint on the displacement of the total center of mass [25]....
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42 citations
"A Comparison of Design Strategies f..." refers background or methods in this paper
...In some highly sophisticated real-time environment the system can integrate a real-time performance animation system to either simulate a virtual character interacting with the end user (interactive TV or real-time production environment [14]), or to simulate the virtual body of the operator in the virtual environment [34], or to have bi-directional interaction between operator and virtual character in the virtual world [36]....
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...This explain why the Performance Animation approach has been widely adopted in the film production context [14][5]....
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...An extensive discussion about their relative merits can be found in [14]; we just recall here the major facts : • both approaches place the sensors on the external surface of a human performer • the magnetic technology provides the position and orientation of sensors while the optical technology provides the 3D position of reflective markers....
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...The use of digital glove for real-time production of character animation is called digital puppetry for two reasons : • the interaction metaphor is close to puppetry as the movement is measured on a articulated structure (the hand) rather different from the controlled one (the character) thus requiring some adjustment on the part of the performer [14]....
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38 citations
"A Comparison of Design Strategies f..." refers background or methods in this paper
...• the sum of the correction posture variations due to all the sensors is projected on the null space of the end effector behavior thus not disturbing its realization [7]....
[...]
...The displacement is realized as an automatic secondary behavior of the Inverse Kinematics solution [7]....
[...]
...The same remark is globally valid for other classical functional models as grasping [30] or general goal-oriented motion with IK [31] and general balance control with Inverse Kinetics [25] [7]....
[...]
...An equally important property of this technique is the ability to combine it with goal-oriented motions (defined with Inverse Kinematics) in a hierarchical fashion [7]....
[...]
...Owned to the hierarchical nature of its solution [7], we can integrate secondary behaviors which significantly improve the realism of the resulting motion....
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32 citations