Randal C. Nelson

Obstacle avoidance using flow field divergence

The use of certain measures of flow field divergence is investigated as a qualitative cue for obstacle avoidance during visual navigation. It is shown that a quantity termed the directional divergence of the 2-D motion field can be used as a reliable indicator of the presence of obstacles in the visual field of an observer undergoing generalized rotational and translational motion. The necessary measurements can be robustly obtained from real image sequences. Experimental results are presented showing that the system responds as expected to divergence in real-world image sequences, and the use of the system to navigate between obstacles is demonstrated. >

Detection and Recognition of Periodic, Nonrigid Motion

The recognition of nonrigid motion, particularly that arising from human movement (and by extension from the locomotory activity of animals) has typically made use of high-level parametric models representing the various body parts (legs, arms, trunk, head etc.) and their connections to each other. Such model-based recognition has been successful in some cases; however, the methods are often difficult to apply to real-world scenes, and are severely limited in their generalizability. The first problem arises from the difficulty of acquiring and tracking the requisite model parts, usually specific joints such as knees, elbows or ankles. This generally requires some prior high-level understanding and segmentation of the scene, or initialization by a human operator. The second problem, with generalization, is due to the fact that the human model is not much good for dogs or birds, and for each new type of motion, a new model must be hand-crafted. In this paper, we show that the recognition of human or animal locomotion, and, in fact, any repetitive activity can be done using low-level, non-parametric representations. Such an approach has the advantage that the same underlying representation is used for all examples, and no individual tailoring of models or prior scene understanding is required. We show in particular, that repetitive motion is such a strong cue, that the moving actor can be segmented, normalized spatially and temporally, and recognized by matching against a spatio-temporal template of motion features. We have implemented a real-time system that can recognize and classify repetitive motion activities in normal gray-scale image sequences. Results on a number of real-world sequences are described.

Experimental evaluation of uncalibrated visual servoing for precision manipulation

We present an experimental evaluation of adaptive and non-adaptive visual servoing in 3, 6 and 12 degrees of freedom (DOF), comparing it to traditional joint feedback control. While the purpose of experiments in most other work has been to show that the particular algorithm presented indeed also works in practice, we do not focus on the algorithm but rather on properties important to visual servoing in general. Our main results are: positioning of a 6 axis PUMA 762 arm is up to 5 times more precise under visual control than under joint control; positioning of a Utah/MIT dextrous hand is better under visual control than under joint control by a factor of 2; and a trust-region-based adaptive visual feedback controller is very robust. For m tracked visual features the algorithm can successfully estimate online the m/spl times/3 (m/spl ges/3) image Jacobian (J) without any prior information, while carrying out a 3 DOF manipulation task. For 6 and higher DOF manipulation, a rough initial estimate of J is beneficial. We also verified that redundant visual information is valuable. Errors due to imprecise tracking and goal specification were reduced as the number of visual features, m, was increased. Furthermore highly redundant systems allow us to detect outliers in the feature vector and deal with partial occlusion.

Low level recognition of human motion (or how to get your man without finding his body parts)

The recognition of human movements such as walking, running or climbing has been approached previously by tracking a number of feature points and either classifying the trajectories directly or matching them with a high-level model of the movement. A major difficulty with these methods is acquiring and trading the requisite feature points, which are generally specific joints such as knees or angles. This requires previous recognition and/or part segmentation of the actor. We show that the recognition of walking or any repetitive motion activity can be accomplished on the basis of bottom up processing, which does not require the prior identification of specific parts, or classification of the actor. In particular, we demonstrate that repetitive motion is such a strong cue, that the moving actor can be segmented, normalized spatially and temporally, and recognized by matching against a spatiotemporal template of motion features. We have implemented a real-time system that can recognize and classify repetitive motion activities in normal gray-scale image sequences.<<ETX>>

Qualitative recognition of motion using temporal texture

Abstract We describe a method of visual motion recognition applicable to a range of naturally occurring motions that are characterized by spatial and temporal uniformity. The underlying motivation is the observation that, for objects that typically move, it is frequently easier to identify them when they are moving than when they are stationary. Specifically, we show that certain statistical spatial and temporal features that can be derived from approximations to the motion field have invariant properties, and can be used to classify regional activities such as windblown trees, ripples on water, or chaotic fluid flow, that are characterized by complex, nonrigid motion. We refer to the technique as temporal texture analysis in analogy to the techniques developed to classify grayscale textures. This recognition approach contrasts with the reconstructive approach that has typified most prior work on motion. We demonstrate the technique on a number of real-world image sequences containing complex movement. The work has practical application in monitoring and surveillance, and as a component of a sophisticated visual system.

Qualitative detection of motion by a moving observer

Two complementary methods for the detection of moving objects by a moving observer are described. The first is based on the fact that, in a rigid environment, the projected velocity at any point in the image is constrained to lie on a 1-D locus in velocity space whose parameters depend only on the observer motion. If the observer motion is known, an independently moving object can, in principle, be detected because its projected velocity is unlikely to fall on this locus. We show how this principle can be adapted to use partial information about the motion field and observer motion that can be rapidly computed from real image sequences. The second method utilizes the fact that the apparent motion of a fixed point due to smooth observer motion changes slowly, while the apparent motion of many moving objects such as animals or maneuvering vehicles may change rapidly. The motion field at a given time can thus be used to place constraints on the future motion field which, if violated, indicate the presence of an autonomously maneuvering object. In both cases, the qualitative nature of the constraints allows the methods to be used with the inexact motion information typically available from real-image sequences. Implementations of the methods that run in real time on a parallel pipelined image processing system are described.

A consistent hierarchical representation for vector data

A consistent hierarchical data structure for the representation of vector data is presented. It makes use of a concept termed a line segment fragment to prevent data degradation under splitting or clipping of vector primitives. This means that the insertion and subsequent deletion (and vice versa) of a vector leaves the data unchanged. Vectors are represented exactly and not as digital approximations. The data is dynamically organized by use of simple probabilistic splitting and merging rules. The use of the structure for implementing a geographic information system is described. Algorithms for constructing and manipulating the structure are provided. Results of empirical tests comparing the structure to other representations in the literature are given.

A cubist approach to object recognition

We describe an appearance-based object recognition system using a keyed, multi-level contest representation reminiscent of certain aspects of cubist art. Specifically, we utilize distinctive intermediate-level features in this case automatically extracted 2-D boundary fragments, as keys, which are then verified within a local contest, and assembled within a loose global contest to evoke an overall percept. This system demonstrates good recognition of a variety of 3-D shapes, ranging from sports cars and fighter planes to snakes and lizards with full orthographic invariance. We report the results of large-scale tests, involving over 2000 separate test images, that evaluate performance with increasing number of items in the database, in the presence of clutter, background change, and occlusion, and also the results of some generic classification experiments where the system is tested on objects never previously seen or modeled. To our knowledge, the results we report are the best in the literature for full-sphere tests of general shapes with occlusion and clutter resistance.

A Perceptual Grouping Hierarchy for Appearance-Based 3D Object Recognition

In this paper we consider the problem of 3D object recognition and the role that perceptual grouping processes must play. In particular, we argue that reliance on a single level of perceptual grouping is inadequate, since it is responsible for the specific weaknesses of several well-known recognition techniques. Instead, recognition must use a hierarchy of perceptual grouping processes. We describe an appearance-based system that uses four distinct levels of perceptual grouping to represent 3D objects in a form that allows not only recognition, but reasoning about 3D manipulation of a sort that has been supported in the past only by 3D geometric models. The results of the algorithms have been previously reported, and the main contribution of this paper is the development of the perceptual organization hierarchy.

QUILT: a geographic information system based on quadtrees†

This paper describes QUILT, a prototype geographic information system (GIS) that uses the quadtree data structure as the underlying representation for cartographic data. While QUILT contains many features typically available in a GIS, its primary purpose is to serve as a testbed for the design and testing of new data structures and algorithms for use in computer cartography. Quadtree variants for region, point and line data are implemented using the linear quadtree, organized on disk by a B-tree. QUILT provides a simple attribute attachment system which associates non-spatial data with geographic objects. The user views QUILT as an augmented LISP environment. QUILTs geographic functions include conversion of rasters to and from quadtrees; subset operations to select specified geographic objects; map editing, display, windowing, intersection and union operations; polygon expansion; and computation of geographic object properties such as the centroid, area, perimeter and bounding rectangle for sets of geog...