Bhaskara Marthi

NeurIPS Conference 2016 Conference Paper

Generative Shape Models: Joint Text Recognition and Segmentation with Very Little Training Data

• Xinghua Lou
• Ken Kansky
• Wolfgang Lehrach
• CC Laan
• Bhaskara Marthi
• D. Phoenix
• Dileep George

We demonstrate that a generative model for object shapes can achieve state of the art results on challenging scene text recognition tasks, and with orders of magnitude fewer training images than required for competing discriminative methods. In addition to transcribing text from challenging images, our method performs fine-grained instance segmentation of characters. We show that our model is more robust to both affine transformations and non-affine deformations compared to previous approaches.

ICRA Conference 2014 Conference Paper

Unsupervised discovery of object classes with a mobile robot

• Julian Mason
• Bhaskara Marthi
• Ronald Parr

Object detection and recognition are fundamental capabilities for a mobile robot. Objects are a powerful representation for a variety of tasks including mobile manipulation and inventory tracking. As a result, object-based world representations have seen a great deal of research interest in the last several years. However, these systems usually assume that object recognition is well-solved: they require that accurate recognition be available for every object they might encounter. Despite steady advances, object recognition remains a difficult, open problem. Existing object recognition algorithms rely on high-resolution three-dimensional object models or on extensive hand-labeled training data. The sheer variety of objects that occur in natural environments makes manually training a recognizer for every possible object infeasible. In this work, we present a robotic system for unsupervised object and class discovery, in which objects are first discovered, and then grouped into classes in an unsupervised fashion. At each step, we approach the problem as one of robotics, not disembodied computer vision. On a very large robotic dataset, we discover object classes with 98. 7% precision while achieving 71. 8% recall. The scale and quality of these results demonstrate the merit of our approach, and prove the practicality of long-term large-scale object discovery. To our knowledge, no other authors have investigated robotic object discovery at this scale, making direct quantitative comparison impossible. We make our implementation and ground-truth labelings available, and evaluate our technique on a very large dataset. As a result, this work is a baseline against which future work can be compared.

IROS Conference 2012 Conference Paper

An object-based semantic world model for long-term change detection and semantic querying

• Julian Mason
• Bhaskara Marthi

Recent years have seen rising interest in robotic mapping algorithms that operate at the level of objects, rather than two- or three-dimensional occupancy. Such “semantic maps” permit higher-level reasoning than occupancy maps, and are useful for any application that involves dealing with objects, including grasping, change detection, and object search. We describe and experimentally verify such a system aboard a mobile robot equipped with a Microsoft Kinect RGB-D sensor. Our representation is object-based, and makes uniquely weak assumptions about the quality of the perceptual data available; in particular, we perform no explicit object recognition. This allows our system to operate in large, dynamic, and uncon-strained environments, where modeling every object that occurs (or might occur) is impractical. Our dataset, which is publicly available, consists of 67 autonomous runs of our robot over a six-week period in a roughly 1600m 2 office environment. We demonstrate two applications built on our system: semantic querying and change detection.

IROS Conference 2012 Conference Paper

Object disappearance for object discovery

• Julian Mason
• Bhaskara Marthi
• Ronald Parr

A useful capability for a mobile robot is the ability to recognize the objects in its environment that move and change (as distinct from background objects, which are largely stationary). This ability can improve the accuracy and reliability of localization and mapping, enhance the ability of the robot to interact with its environment, and facilitate applications such as inventory management and theft detection. Rather than viewing this task as a difficult application of object recognition methods from computer vision, this work is in line with a recent trend in the community towards unsupervised object discovery and tracking that exploits the fundamentally temporal nature of the data acquired by a robot. Unlike earlier approaches, which relied heavily upon computationally intensive techniques from mapping and computer vision, our approach combines visual features and RGB-D data in a simple and effective way to segment objects from robot sensory data. We then use a Dirichlet process to cluster and recognize objects. The performance of our approach is demonstrated in several test domains.

ICRA Conference 2011 Conference Paper

Cart pushing with a mobile manipulation system: Towards navigation with moveable objects

• Jonathan Scholz
• Sachin Chitta
• Bhaskara Marthi
• Maxim Likhachev

Robust navigation in cluttered environments has been well addressed for mobile robotic platforms, but the problem of navigating with a moveable object like a cart has not been widely examined. In this work, we present a planning and control approach to navigation of a humanoid robot while pushing a cart. We show how immediate information about the environment can be integrated into this approach to achieve safer navigation in the presence of dynamic obstacles. We demonstrate the robustness of our approach through long-running experiments with the PR2 mobile manipulation robot in a typical indoor office environment, where the robot faced narrow and high-traffic passageways with very limited clearance.

ICRA Conference 2011 Conference Paper

Navigation in hybrid metric-topological maps

• Kurt Konolige
• Eitan Marder-Eppstein
• Bhaskara Marthi

We present an approach for navigation in hybrid maps consisting of a topological graph overlaid with local occupancy grids. The topological graph is built on top of a graph SLAM system, which can be efficiently optimized even for very large environments. The novel feature of our system is that it navigates locally using local metric maps, while the overall plan is formed on the topological graph. Unlike many current SLAM methods, we never reconstruct a full occupancy grid of the environment for localization or path planning. We show that our method generates near-optimal plans, and deals gracefully with changes to the map.

ICRA Conference 2010 Conference Paper

Autonomous door opening and plugging in with a personal robot

• Wim Meeussen
• Melonee Wise
• Stuart Glaser
• Sachin Chitta
• Conor McGann
• Patrick Mihelich
• Eitan Marder-Eppstein
• Marius Muja

We describe an autonomous robotic system capable of navigating through an office environment, opening doors along the way, and plugging itself into electrical outlets to recharge as needed. We demonstrate through extensive experimentation that our robot executes these tasks reliably, without requiring any modification to the environment. We present robust detection algorithms for doors, door handles, and electrical plugs and sockets, combining vision and laser sensors. We show how to overcome the unavoidable shortcoming of perception by integrating compliant control into manipulation motions. We present a visual-differencing approach to high-precision plug-insertion that avoids the need for high-precision hand-eye calibration.

ICAPS Conference 2010 Conference Paper

Combined Task and Motion Planning for Mobile Manipulation

• Jason Andrew Wolfe
• Bhaskara Marthi
• Stuart Russell 0001

We present a hierarchical planning system and its application to robotic manipulation. The novel features of the system are: 1) it finds high-quality kinematic solutions to task-level problems; 2) it takes advantage of subtask-specific irrelevance information, reusing optimal solutions to state-abstracted subproblems across the search space. We briefly describe how the system handles uncertainty during plan execution, and present results on discrete problems as well as pick-and-place tasks for a mobile robot.

ICAPS Conference 2008 Conference Paper

Angelic Hierarchical Planning: Optimal and Online Algorithms

• Bhaskara Marthi
• Stuart Russell 0001
• Jason Andrew Wolfe

High-level actions (HLAs) are essential tools for coping with the large search spaces and long decision horizons encountered in real-world decision making. In a recent paper, we proposed an "angelic" semantics for HLAs that supports proofs that a high-level plan will (or will not) achieve a goal, without first reducing the plan to primitive action sequences. This paper extends the angelic semantics with cost information to support proofs that a high-level plan is (or is not) optimal. We describe the Angelic Hierarchical A* algorithm, which generates provably optimal plans, and show its advantages over alternative algorithms. We also present the Angelic Hierarchical Learning Real-Time A* algorithm for situated agents, one of the first algorithms to do hierarchical lookahead in an online setting. Since high-level plans are much shorter, this algorithm can look much farther ahead than previous algorithms (and thus choose much better actions) for a given amount of computational effort.

ICAPS Conference 2007 Conference Paper

Angelic Semantics for High-Level Actions

• Bhaskara Marthi
• Stuart Russell 0001
• Jason Andrew Wolfe

High-level actions (HLAs) lie at the heart of hierarchical planning. Typically, an HLA admits multiple refinements into primitive action sequences. Correct descriptions of the effects of HLAs may be essential to their effective use, yet the literature is mostly silent. We propose an angelic semantics for HLAs, the key concept of which is the set of states reachable by some refinement of a high-level plan, representing uncertainty that will ultimately be resolved in the planning agent's own best interest. We describe upper and lower approximations to these reachable sets, and show that the resulting definition of a high-level solution automatically satisfies the upward and downward refinement properties. We define a STRIPS-like notation for such descriptions. A sound and complete hierarchical planning algorithm is given and its computational benefits are demonstrated.

ICML Conference 2007 Conference Paper

Automatic shaping and decomposition of reward functions

• Bhaskara Marthi

UAI Conference 2006 Conference Paper

A Compact, Hierarchical Q-function Decomposition

• Bhaskara Marthi
• Stuart Russell 0001
• David Andre

Previous work in hierarchical reinforcement learning has faced a dilemma: either ignore the values of different possible exit states from a subroutine, thereby risking suboptimal behavior, or represent those values explicitly thereby incurring a possibly large representation cost because exit values refer to nonlocal aspects of the world (i.e., all subsequent rewards). This paper shows that, in many cases, one can avoid both of these problems. The solution is based on recursively decomposing the exit value function in terms of Q-functions at higher levels of the hierarchy. This leads to an intuitively appealing runtime architecture in which a parent subroutine passes to its child a value function on the exit states and the child reasons about how its choices affect the exit value. We also identify structural conditions on the value function and transition distributions that allow much more concise representations of exit state distributions, leading to further state abstraction. In essence, the only variables whose exit values need be considered are those that the parent cares about and the child affects. We demonstrate the utility of our algorithms on a series of increasingly complex environments.

IJCAI Conference 2005 Conference Paper

BLOG: Probabilistic Models with Unknown Objects

• Brian Milch
• Bhaskara Marthi
• Stuart Russell
• David Sontag
• Daniel L. Ong
• Andrey

This paper introduces and illustrates BLOG, a formal language for defining probability models over worlds with unknown objects and identity uncertainty. BLOG unifies and extends several existing approaches. Subject to certain acyclicity constraints, every BLOG model specifies a unique probability distribution over first-order model structures that can contain varying and unbounded numbers of objects. Furthermore, complete inference algorithms exist for a large fragment of the language. We also introduce a probabilistic form of Skolemization for handling evidence.

AAAI Conference 2005 Short Paper

Concurrent Hierarchical Reinforcement Learning

• Bhaskara Marthi

We describe concurrent ALisp, a language that extends hierarchical reinforcement learning techniques to multi-effector domains.

IJCAI Conference 2005 Conference Paper

Concurrent Hierarchical Reinforcement Learning

• Bhaskara Marthi
• Stuart Russell
• David Latham
• Carlos

We consider applying hierarchical reinforcement learning techniques to problems in which an agent has several effectors to control simultaneously. We argue that the kind of prior knowledge one typically has about such problems is best expressed using a multithreaded partial program, and present concurrent ALisp, a language for specifying such partial programs. We describe algorithms for learning and acting with concurrent ALisp that can be efficient even when there are exponentially many joint choices at each decision point. Finally, we show results of applying these methods to a complex computer game domain.

UAI Conference 2002 Conference Paper

Decayed MCMC Filtering

• Bhaskara Marthi
• Hanna Pasula
• Stuart Russell 0001
• Yuval Peres

Filtering---estimating the state of a partially observable Markov process from a sequence of observations---is one of the most widely studied problems in control theory, AI, and computational statistics. Exact computation of the posterior distribution is generally intractable for large discrete systems and for nonlinear continuous systems, so a good deal of effort has gone into developing robust approximation algorithms. This paper describes a simple stochastic approximation algorithm for filtering called {em decayed MCMC}. The algorithm applies Markov chain Monte Carlo sampling to the space of state trajectories using a proposal distribution that favours flips of more recent state variables. The formal analysis of the algorithm involves a generalization of standard coupling arguments for MCMC convergence. We prove that for any ergodic underlying Markov process, the convergence time of decayed MCMC with inverse-polynomial decay remains bounded as the length of the observation sequence grows. We show experimentally that decayed MCMC is at least competitive with other approximation algorithms such as particle filtering.

NeurIPS Conference 2002 Conference Paper

Identity Uncertainty and Citation Matching

• Hanna Pasula
• Bhaskara Marthi
• Brian Milch
• Stuart Russell
• Ilya Shpitser

Identity uncertainty is a pervasive problem in real-world data analysis. It arises whenever objects are not labeled with unique identifiers or when those identifiers may not be perceived perfectly. In such cases, two ob- servations may or may not correspond to the same object. In this paper, we consider the problem in the context of citation matching—the prob- lem of deciding which citations correspond to the same publication. Our approach is based on the use of a relational probability model to define a generative model for the domain, including models of author and title corruption and a probabilistic citation grammar. Identity uncertainty is handled by extending standard models to incorporate probabilities over the possible mappings between terms in the language and objects in the domain. Inference is based on Markov chain Monte Carlo, augmented with specific methods for generating efficient proposals when the domain contains many objects. Results on several citation data sets show that the method outperforms current algorithms for citation matching. The declarative, relational nature of the model also means that our algorithm can determine object characteristics such as author names by combining multiple citations of multiple papers.