357,933 research outputs found
Robust 3D Action Recognition through Sampling Local Appearances and Global Distributions
3D action recognition has broad applications in human-computer interaction
and intelligent surveillance. However, recognizing similar actions remains
challenging since previous literature fails to capture motion and shape cues
effectively from noisy depth data. In this paper, we propose a novel two-layer
Bag-of-Visual-Words (BoVW) model, which suppresses the noise disturbances and
jointly encodes both motion and shape cues. First, background clutter is
removed by a background modeling method that is designed for depth data. Then,
motion and shape cues are jointly used to generate robust and distinctive
spatial-temporal interest points (STIPs): motion-based STIPs and shape-based
STIPs. In the first layer of our model, a multi-scale 3D local steering kernel
(M3DLSK) descriptor is proposed to describe local appearances of cuboids around
motion-based STIPs. In the second layer, a spatial-temporal vector (STV)
descriptor is proposed to describe the spatial-temporal distributions of
shape-based STIPs. Using the Bag-of-Visual-Words (BoVW) model, motion and shape
cues are combined to form a fused action representation. Our model performs
favorably compared with common STIP detection and description methods. Thorough
experiments verify that our model is effective in distinguishing similar
actions and robust to background clutter, partial occlusions and pepper noise
Gravitaxis of asymmetric self-propelled colloidal particles
Many motile microorganisms adjust their swimming motion relative to the
gravitational field and thus counteract sedimentation to the ground. This
gravitactic behavior is often the result of an inhomogeneous mass distribution
which aligns the microorganism similar to a buoy. However, it has been
suggested that gravitaxis can also result from a geometric fore-rear asymmetry,
typical for many self-propelling organisms. Despite several attempts, no
conclusive evidence for such an asymmetry-induced gravitactic motion exists.
Here, we study the motion of asymmetric self-propelled colloidal particles
which have a homogeneous mass density and a well-defined shape. In experiments
and by theoretical modeling we demonstrate that a shape anisotropy alone is
sufficient to induce gravitactic motion with either preferential upward or
downward swimming. In addition, also trochoid-like trajectories transversal to
the direction of gravity are observed.Comment: 9 pages, 5 figures, 1 tabl
Teleological computer graphics modeling
Summary form only give. Teleological modeling, a developing approach for creating abstractions and mathematical representations of physically realistic time-dependent objects, is described. In this approach, geometric constraint-properties, mechanical properties of objects, the parameters representing an object, and the control of the object are incorporated into a single conceptual framework. A teleological model incorporates time-dependent goals of behavior of purpose as the primary abstraction and representation of what the object is. A teleological implementation takes a geometrically incomplete specification of the motion, position, and shape of an object, and produces a geometrically complete description of the object's shape and behavior as a function of time. Teleological modeling techniques may be suitable for consideration in computer vision algorithms by extending the current notions about how to make mathematical representations of objects. Teleological descriptions can produce compact representations for many of the physically derivable quantities controlling the shapes, combining-operations, and constraints which govern the formation and motion of objects
Empiric Models of the Earth's Free Core Nutation
Free core nutation (FCN) is the main factor that limits the accuracy of the
modeling of the motion of Earth's rotational axis in the celestial coordinate
system. Several FCN models have been proposed. A comparative analysis is made
of the known models including the model proposed by the author. The use of the
FCN model is shown to substantially increase the accuracy of the modeling of
Earth's rotation. Furthermore, the FCN component extracted from the observed
motion of Earth's rotational axis is an important source for the study of the
shape and rotation of the Earth's core. A comparison of different FCN models
has shown that the proposed model is better than other models if used to
extract the geophysical signal (the amplitude and phase of FCN) from
observational data.Comment: 8 pages, 3 figures; minor update of the journal published versio
Real-Time Hand Tracking Using a Sum of Anisotropic Gaussians Model
Real-time marker-less hand tracking is of increasing importance in
human-computer interaction. Robust and accurate tracking of arbitrary hand
motion is a challenging problem due to the many degrees of freedom, frequent
self-occlusions, fast motions, and uniform skin color. In this paper, we
propose a new approach that tracks the full skeleton motion of the hand from
multiple RGB cameras in real-time. The main contributions include a new
generative tracking method which employs an implicit hand shape representation
based on Sum of Anisotropic Gaussians (SAG), and a pose fitting energy that is
smooth and analytically differentiable making fast gradient based pose
optimization possible. This shape representation, together with a full
perspective projection model, enables more accurate hand modeling than a
related baseline method from literature. Our method achieves better accuracy
than previous methods and runs at 25 fps. We show these improvements both
qualitatively and quantitatively on publicly available datasets.Comment: 8 pages, Accepted version of paper published at 3DV 201
On the crescentic shape of barchan dune
Aeolian sand dunes originate from wind flow and sand bed interactions.
According to wind properties and sand availability, they can adopt different
shapes, ranging from huge motion-less star dunes to small and mobile barchan
dunes. The latter are crescentic and emerge under a unidirectional wind, with a
low sand supply. Here, a 3d model for barchan based on existing 2d model is
proposed. After describing the intrinsic issues of 3d modeling, we show that
the deflection of reptating particules due to the shape of the dune leads to a
lateral sand flux deflection, which takes the mathematical form of a non-linear
diffusive process. This simple and physically meaningful coupling method is
used to understand the shape of barchan dunes.Comment: 8 pages, 9 figures, submitted to Eur. Phys. J. B v2 : major changes
in grammar and in presentatio
Physics-Based Modeling of Nonrigid Objects for Vision and Graphics (Dissertation)
This thesis develops a physics-based framework for 3D shape and nonrigid motion modeling for computer vision and computer graphics. In computer vision it addresses the problems of complex 3D shape representation, shape reconstruction, quantitative model extraction from biomedical data for analysis and visualization, shape estimation, and motion tracking. In computer graphics it demonstrates the generative power of our framework to synthesize constrained shapes, nonrigid object motions and object interactions for the purposes of computer animation.
Our framework is based on the use of a new class of dynamically deformable primitives which allow the combination of global and local deformations. It incorporates physical constraints to compose articulated models from deformable primitives and provides force-based techniques for fitting such models to sparse, noise-corrupted 2D and 3D visual data. The framework leads to shape and nonrigid motion estimators that exploit dynamically deformable models to track moving 3D objects from time-varying observations.
We develop models with global deformation parameters which represent the salient shape features of natural parts, and local deformation parameters which capture shape details. In the context of computer graphics, these models represent the physics-based marriage of the parameterized and free-form modeling paradigms. An important benefit of their global/local descriptive power in the context of computer vision is that it can potentially satisfy the often conflicting requirements of shape reconstruction and shape recognition.
The Lagrange equations of motion that govern our models, augmented by constraints, make them responsive to externally applied forces derived from input data or applied by the user. This system of differential equations is discretized using finite element methods and simulated through time using standard numerical techniques. We employ these equations to formulate a shape and nonrigid motion estimator. The estimator is a continuous extended Kalman filter that recursively transforms the discrepancy between the sensory data and the estimated model state into generalized forces. These adjust the translational, rotational, and deformational degrees of freedom such that the model evolves in a consistent fashion with the noisy data.
We demonstrate the interactive time performance of our techniques in a series of experiments in computer vision, graphics, and visualization
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