Siqi Nie

A generative restricted Boltzmann machine based method for high-dimensional motion data modeling

Extended RBM to model spatio-temporal patterns among high-dimensional motion data.Generative approach to perform classification using RBM, for both binary and multi-class classification.High classification accuracy in two computer vision applications: facial expression recognition and human action recognition. Many computer vision applications involve modeling complex spatio-temporal patterns in high-dimensional motion data. Recently, restricted Boltzmann machines (RBMs) have been widely used to capture and represent spatial patterns in a single image or temporal patterns in several time slices. To model global dynamics and local spatial interactions, we propose to theoretically extend the conventional RBMs by introducing another term in the energy function to explicitly model the local spatial interactions in the input data. A learning method is then proposed to perform efficient learning for the proposed model. We further introduce a new method for multi-class classification that can effectively estimate the infeasible partition functions of different RBMs such that RBM is treated as a generative model for classification purpose. The improved RBM model is evaluated on two computer vision applications: facial expression recognition and human action recognition. Experimental results on benchmark databases demonstrate the effectiveness of the proposed algorithm.

Capturing Global and Local Dynamics for Human Action Recognition

Human action analysis has achieved great success especially with the recent development of advanced sensors and algorithms that can effectively track the body joints. Temporal motion of body joints carries crucial information about human actions. However, current dynamic models typically assume stationary local transition and therefore are limited to local dynamics. In contrast, we propose a novel human action recognition algorithm that is able to capture both global and local dynamics of joint trajectories by combining a Gaussian-Binary restricted Boltzmann machine (GB-RBM) with a hidden Markov model (HMM). We present a method to use RBM as a generative model for multi-class classification. Experimental results on benchmark datasets demonstrate the capability of the proposed method in exploiting the dynamic information at different levels.

Learning Bounded Tree-Width Bayesian Networks via Sampling

Learning Bayesian networks with bounded tree-width has attracted much attention recently, because low tree-width allows exact inference to be performed efficiently. Some existing methods [12, 14] tackle the problem by using k-trees to learn the optimal Bayesian network with tree-width up to k. In this paper, we propose a sampling method to efficiently find representative k-trees by introducing an Informative score function to characterize the quality of a k-tree. The proposed algorithm can efficiently learn a Bayesian network with tree-width at most k. Experiment results indicate that our approach is comparable with exact methods, but is much more computationally efficient.

Data-Free Prior Model for Upper Body Pose Estimation and Tracking

Video based human body pose estimation seeks to estimate the human body pose from an image or a video sequence, which captures a person exhibiting some activities. To handle noise and occlusion, a pose prior model is often constructed and is subsequently combined with the pose estimated from the image data to achieve a more robust body pose tracking. Various body prior models have been proposed. Most of them are data-driven, typically learned from 3D motion capture data. In addition to being expensive and time-consuming to collect, these data-based prior models cannot generalize well to activities and subjects not present in the motion capture data. To alleviate this problem, we propose to learn the prior model from anatomic, biomechanics, and physical constraints, rather than from the motion capture data. For this, we propose methods that can effectively capture different types of constraints and systematically encode them into the prior model. Experiments on benchmark data sets show the proposed prior model, compared with data-based prior models, achieves comparable performance for body motions that are present in the training data. It, however, significantly outperforms the data-based prior models in generalization to different body motions and to different subjects.

Efficient learning of Bayesian networks with bounded tree-width

Learning Bayesian networks with bounded tree-width has attracted much attention recently, because low tree-width allows exact inference to be performed efficiently. Some existing methods 24,29 tackle the problem by using k-trees to learn the optimal Bayesian network with tree-width up to k. Finding the best k-tree, however, is computationally intractable. In this paper, we propose a sampling method to efficiently find representative k-trees by introducing an informative score function to characterize the quality of a k-tree. To further improve the quality of the k-trees, we propose a probabilistic hill climbing approach that locally refines the sampled k-trees. The proposed algorithm can efficiently learn a quality Bayesian network with tree-width at most k. Experimental results demonstrate that our approach is more computationally efficient than the exact methods with comparable accuracy, and outperforms most existing approximate methods. This work presents a novel method for BN structure learning with bounded tree-width.The algorithm is based on a fast bijection between k-trees and the Dandelion codes.The Distance Preferable Sampling is designed to effectively cover the space of k-trees.The probabilistic hill climbing algorithm is used to obtain k-trees of high quality.Extensive experiments indicate the efficiency and effectiveness of the proposed algorithm.

An information theoretic feature selection framework based on integer programming

We propose a general framework for information theoretic feature selection based on the integer programming. Filter feature selection methods usually rely on a greedy forward or backward selection heuristic to find a satisfactory set of features, as the exact search is a combinatorial problem. We formulate the existing filter information theoretic criteria into an integer programming problem, and by using objective functions, we can represent many different existing scoring criteria. The integer programming framework can be solved efficiently by the existing solvers. We demonstrate the superior performance of the integer programming formulation over its corresponding criterion empirically.

Latent regression Bayesian network for data representation

Restricted Boltzmann machines (RBMs) are widely used for data representation and feature learning in various machine learning tasks. The undirected structure of an RBM allows inference to be performed efficiently, because the latent variables are dependent on each other given the visible variables. However, we believe the correlations among latent variables are crucial for faithful data representation. Driven by this idea, we propose a counterpart of RBMs, namely latent regression Bayesian networks (LRBNs), which has a directed structure. One major difficulty of learning LRBNs is the intractable inference. To address this problem, we propose an inference method based on the conditional pseudo-likelihood that preserves the dependencies among the latent variables. For learning, we propose to employ the hard Expectation Maximization (EM) algorithm, which avoids the intractability of the traditional EM by max-out instead of sum-out to compute the data likelihood. Qualitative and quantitative evaluations of our model against state-of-the-art models and algorithms on benchmark data sets demonstrate the effectiveness of the proposed algorithm in data representation and reconstruction.

Feature Learning Using Bayesian Linear Regression Model

Data representation plays a key role in many machine learning tasks. Specific domain knowledge can help design some features, but it often needs a long time to handcraft them. On the other hand, unsupervised learning can automatically learn a good representation of either labeled or unlabeled data. Currently one of the dominant approaches is the restricted Boltzmann machine (RBM). In this paper, we investigate an alternative approach for feature learning, which is based on Bayesian linear regression model. This model can also be denoted as Factor analysis, which is a statistical method for modeling the covariance structure of high dimensional data, but has not been used for feature learning. We will compare the proposed framework with RBM on different kinds of computer vision applications. Experiment results on different datasets are reported to demonstrate the effectiveness of the proposed feature learning framework.

Asynchronous linear optical sampling for monitoring impairments in multilevel signal modulation format generation

We propose a general method to monitor the impairments in high speed multilevel signal modulation format generation induced by improper setting and time-drifting of modulator bias offset based on signal constellation diagram. Using asynchronous linear optical sampling technique, we can obtain both the amplitude and phase information of the signal under test with a high time resolution and data speed transparency. We successfully recovered the constellation diagrams of 10Gbaud quadrature phase shift keying (QPSK) after a series proposed signal processing algorithms. Several common types of constellation distortion were observed with the manual tuning of the modulator bias. It proved the effectiveness of the proposed method.