Real Time Animation of Virtual Humans: A Trade-off Between Naturalness and Control

Virtual humans are employed in many interactive applications using 3D virtual environments, including (serious) games. The motion of such virtual humans should look realistic (or ‘natural’) and allow interaction with the surroundings and other (virtual) humans. Current animation techniques differ in the trade-off they offer between motion naturalness and the control that can be exerted over the motion. We show mechanisms to parametrize, combine (on different body parts) and concatenate motions generated by different animation techniques. We discuss several aspects of motion naturalness and show how it can be evaluated. We conclude by showing the promise of combinations of different animation paradigms to enhance both naturalness and control

A Compact Representation of Drawing Movements with Sequences of Parabolic Primitives

Some studies suggest that complex arm movements in humans and monkeys may optimize several objective functions, while others claim that arm movements satisfy geometric constraints and are composed of elementary components. However, the ability to unify different constraints has remained an open question. The criterion for a maximally smooth (minimizing jerk) motion is satisfied for parabolic trajectories having constant equi-affine speed, which thus comply with the geometric constraint known as the two-thirds power law. Here we empirically test the hypothesis that parabolic segments provide a compact representation of spontaneous drawing movements. Monkey scribblings performed during a period of practice were recorded. Practiced hand paths could be approximated well by relatively long parabolic segments. Following practice, the orientations and spatial locations of the fitted parabolic segments could be drawn from only 2–4 clusters, and there was less discrepancy between the fitted parabolic segments and the executed paths. This enabled us to show that well-practiced spontaneous scribbling movements can be represented as sequences (“words”) of a small number of elementary parabolic primitives (“letters”). A movement primitive can be defined as a movement entity that cannot be intentionally stopped before its completion. We found that in a well-trained monkey a movement was usually decelerated after receiving a reward, but it stopped only after the completion of a sequence composed of several parabolic segments. Piece-wise parabolic segments can be generated by applying affine geometric transformations to a single parabolic template. Thus, complex movements might be constructed by applying sequences of suitable geometric transformations to a few templates. Our findings therefore suggest that the motor system aims at achieving more parsimonious internal representations through practice, that parabolas serve as geometric primitives and that non-Euclidean variables are employed in internal movement representations (due to the special role of parabolas in equi-affine geometry)

Top-down and bottom-up processes during observation: Implications for motor learning

Neurophysiological and behavioural research has linked observational practice to a 2 mirroring mechanism encompassing the action-observation network (AON). Although the 3 original findings indicate that biological stimuli alone activate the AON, recent evidence 4 has shown sensitivity to non-biological stimuli. Thus, the AON is suggested to be 5 influenced by interacting bottom-up and top-down processes. In this review, we describe 6 the multi-functional properties of the AON, and discuss the implications for observational 7 practice and subsequent motor learning

Control theoretic models of pointing

This article presents an empirical comparison of four models from manual control theory on their ability to model targeting behaviour by human users using a mouse: McRuer’s Crossover, Costello’s Surge, second-order lag (2OL), and the Bang-bang model. Such dynamic models are generative, estimating not only movement time, but also pointer position, velocity, and acceleration on a moment-to-moment basis. We describe an experimental framework for acquiring pointing actions and automatically fitting the parameters of mathematical models to the empirical data. We present the use of time-series, phase space, and Hooke plot visualisations of the experimental data, to gain insight into human pointing dynamics. We find that the identified control models can generate a range of dynamic behaviours that captures aspects of human pointing behaviour to varying degrees. Conditions with a low index of difficulty (ID) showed poorer fit because their unconstrained nature leads naturally to more behavioural variability. We report on characteristics of human surge behaviour (the initial, ballistic sub-movement) in pointing, as well as differences in a number of controller performance measures, including overshoot, settling time, peak time, and rise time. We describe trade-offs among the models. We conclude that control theory offers a promising complement to Fitts’ law based approaches in HCI, with models providing representations and predictions of human pointing dynamics, which can improve our understanding of pointing and inform design

Top-Down Attentional Processes Modulate the Coding of Atypical Biological Motion Kinematics in the Absence of Motor Signals

The acquisition of sensorimotor parameters that control goal-directed motor behaviors occurs by observing another person in the absence of efferent and afferent motor signals. This is observational practice. During such observation, biological motion properties associated with the observed person are coded into a representation that controls motor learning. Understanding the underlying processes, specifically associated with coding biological motion, has theoretical and practical significance. Here, we examined the following questions. Experiment 1: Are the underlying velocity characteristics associated with observed biological motion kinematics imitated? Experiment 2: Is attention involved in imitating biological motion kinematics? Experiment 3: Can selective attention modulate how biological motion kinematics are imitated/represented? To this end, participants practiced by observing a model performing a movement sequence that contained typical or atypical biological motion kinematics. The differences in kinematics were designed to dissociate the movement constraints of the task and the anatomical constraints of the observer. This way, we examined whether novel motor behaviors are acquired by adopting prototypical movements or coding biological motion. The kinematic analyses indicated the timing and spatial position of peak velocity were represented. Using a dual-task protocol, we attenuated the coding of biological motion kinematics (Experiment 2) and augmented coding using a selective attention protocol (Experiment 3). Findings indicated that velocity characteristics of biological motion kinematics are coded during observational practice, most likely through bottom-up sensorimotor processes. By modulating motion coding using 2 attentional protocols, we showed that bottom-up processes are influenced by input modulation, which is consistent with top-down control during observational practice

Investigating the Informational Nature of a Modeled Visual Demonstration.

This study investigated the informational nature of a modeled visual demonstration of slalom-ski type movements performed on a ski simulator. Hypotheses exist suggesting that a model may convey information primarily about movement coordination (Newell, 1985), or movement form (Whiting, 1988), but there is no empirical evidence that this information is used by the learner so that skill acquisition is facilitated. To investigate this information question, three experiments were conducted that replicated and extended a study by Whiting, Bijlard, and den Brinker (1987) by analyzing movement kinematics of subjects in addition to movement outcome. In the first experiment, the expert model\u27s performance was analyzed. The second and third experiment investigated the acquisition of slalom-ski type movements for groups that observed the expert model on all 5 days, groups that observed the model only on day 1, and groups that learned the skill under discovery learning conditions. Results of movement outcome variables platform amplitude and frequency revealed that observing a model was advantageous over discovery learning. Analysis of movement kinematics suggested that the expert model may have conveyed information about the relative motion of torso and limbs, or movement coordination, that facilitated the acquisition of the slalom-ski type movements. Results further suggested that the coordination information the model may have conveyed was used early in learning, and that observing a model during later stages of learning was of no further benefit

On the effects of action on visual perception & How new movement types are learned

Perception can induce effects on action. Perception of others´ actions can thus influence our own actions. However, research on how action can shape perception is sparse. In this thesis, the effects of (i) motor learning and (ii) simple movements, can influence visual perception. Furthermore, the issue of motor learning is addressed; how can new movement types be learned and do active and passively guided motor training lead to differences in successful acquisition of a new movement

Movement Intermittency in Social Coordination

Coordination of movements in humans has been extensively studied at a macroscopic level, such as the pacing of movements, particularly in tasks of interpersonal and bimanual coordination. However, by examining the fine structure of movement, another form of rhythmicity becomes apparent at a microscopic level. Movement is never completely smooth, but rather is organized into smaller units known as submovements, which appear as recurrent speed breaks occurring at faster timescales (2-3 Hz). These submovements may reflect intermittent feedback-based motor adjustments. To better understand the relationship between submovements in different coordination contexts, we characterized the timing of submovements emission in a series of rhythmic motor coordination task by asking participants to coordinate their index fingers either in-phase or anti-phase with themselves or with a real/virtual partner. In Study 1, we analysed the temporal relationship between submovements emitted by both hands of a single participant during a bimanual coordination task. We also manipulated the availability of visual feedback to understand its impact on the emission of submovements, which are believed to reflect a vision based movement correction mechanism. In Study 2, we explored the dynamics of submovements during interpersonal coordination, and thus with the goal of moving beyond their temporal emission in single individuals. In Study 3, we combined interpersonal and bimanual coordination into a single task by asking participants to coordinate with each other using both their hands. In Study 4, we tested the validity of our results on mutual adaptation of submovements during interpersonal coordination by replacing one member of the pair with an unresponsive virtual partner. Finally, in Study 5, building on the ease of transferability of the previous task to clinical settings, we investigated the pattern of submovements emission in individuals with Parkinson's disease and cerebellar disorders to identify potentially new diagnostic markers and gain novel insights into the neural substrates underlying movement intermittency. Overall, our results suggest that the mechanism responsible for the organization of movement into submovements is at least partly shared across different effectors, such as the two hands, and might be modulated by the availability and usability of visual and proprioceptive feedback. Moreover, the identification of different temporal patterns of submovements emission leads us to conclude that the mechanisms controlling submovements production are highly flexible and tunable depending on the coordinative context. Submovements control can thus provide valuable insights into the low-level motor control mechanisms involved in achieving intra- and interpersonal motor coordination. Finally, submovement-level control may serve as a novel objective marker of individual and social motor coordination capabilities that may be selectively impaired in some neurological and psychiatric conditions.La coordinazione dei movimenti negli esseri umani è stata ampiamente studiata a livello macroscopico, ad es. il ritmo dei movimenti, in particolare in compiti di coordinazione interpersonale e bimanuale. Tuttavia, esaminando la struttura fine del movimento, un'altra forma di ritmicità appare evidente a livello microscopico. Il movimento non è mai completamente fluido, ma è organizzato in unità più piccole note come sottomovimenti, che si manifestano come interruzioni di velocità ricorrenti su una scala temporale più veloce (2-3 Hz). Questi sottomovimenti possono riflettere aggiustamenti motori intermittenti basati sul feedback. Per comprendere meglio la relazione tra i sottomovimenti in contesti di coordinazione diversi, abbiamo caratterizzato i pattern di emissione temporale dei sottomovimenti in una serie di compiti di coordinazione motoria ritmica, chiedendo ai partecipanti di coordinare i loro indici in-fase o in anti-fase con se stessi o con un partner reale/virtuale. Nello Studio 1, abbiamo analizzato la relazione temporale tra i sottomovimenti emessi da entrambe le mani di un singolo partecipante durante un compito di coordinazione bimanuale. Abbiamo anche manipolato la disponibilità del feedback visivo per comprendere il suo impatto sull'emissione dei sottomovimenti, che si ritiene riflettano un meccanismo di correzione dei movimenti basato sulla visione. Nello Studio 2, abbiamo esplorato la dinamica dei sottomovimenti durante la coordinazione interpersonale, con l’obiettivo di indagare i loro pattern di emissione temporale in coppie di individui. Nello Studio 3, abbiamo combinato la coordinazione interpersonale e bimanuale in un unico compito, chiedendo ai partecipanti di coordinarsi reciprocamente utilizzando entrambe le mani. Nello Studio 4, abbiamo testato la validità dei nostri risultati sull'adattamento reciproco dei sottomovimenti durante la coordinazione interpersonale sostituendo uno dei membri della coppia con un partner virtuale non reattivo. Infine, nello Studio 5, considerata la facile trasferibilità del compito precedente in contesti clinici, abbiamo indagato il modello di emissione dei sottomovimenti in individui con malattia di Parkinson e disturbi cerebellari per identificare potenziali nuovi marker diagnostici e acquisire nuove informazioni sui substrati neurali alla base dell'intermittenza del movimento. Complessivamente, i nostri risultati suggeriscono che il meccanismo responsabile dell'organizzazione del movimento in sottomovimenti è almeno in parte condiviso tra differenti effettori, come le due mani, e potrebbe essere modulato dalla disponibilità e utilizzabilità del feedback visivo e propriocettivo. Inoltre, l'identificazione di diversi modelli temporali di emissione dei sottomovimenti ci porta a concludere che i meccanismi che controllano la produzione dei sottomovimenti sono altamente flessibili e adattabili in base al contesto coordinativo. Il controllo dei sottomovimenti può quindi fornire preziose informazioni sui meccanismi di controllo motorio di basso livello coinvolti nel raggiungimento della coordinazione motoria intra- e interpersonale. Infine, il controllo motorio a livello dei sottomovimenti potrebbe fungere da nuovo marker oggettivo delle capacità individuali e sociali di coordinazione motoria che potrebbero essere selettivamente compromesse in alcune condizioni neurologiche e psichiatriche

The Expression of Gender in Synthetic Actors: Modeling and Motion Control Over Invariant Perceptual Cues Leading to Gender Recognition

A perception based strategy for communicating the gender of computer animated characters is evaluated. Motivated by the idea that effective character animation involves the expression of character traits through motion, this study builds upon previous work in the areas of computer animation and ecological psychology in an effort to more fully characterize the dynamic information which leads to the perception of gender. Information specifying the masculinity or femininity of a walking figure is considered in relation to the range across which the indexes may be exaggerated and applied to objects not normally considered male or female