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3D human body model acquisition from multiple views



In Proceedings of the Fifth International Conference on Computer Vision, pages 618–623, Boston, MA, June 20-23, 1995.

3D Human Body Model Acquisition from Multiple Views
Ioann

is A. Kakadiaris and Dimitri Metaxas
General Robotics and Active Sensory Perception (GRASP) Laboratory Department of Computer and Information Science University of Pennsylvania, Philadelphia, PA, 19104 email: ioannisk@grip.cis.upenn.edu, dnm@central.cis.upenn.edu
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Abstract
We present a novel motion-based approach for the part determination and shape estimation of a human’s body parts. The novelty of the technique is that neither a prior model of the human body is employed nor prior body part segmentation is assumed. We present a Human Body Part Identi?cation Strategy (HBPIS) that recovers all the body parts of a moving human based on the spatiotemporal analysis of its deforming silhouette. We formalize the process of simultaneous part determination, and 2D shape estimation by employing the Supervisory Control Theory of Discrete Event Systems. In addition, in order to acquire the 3D shape of the body parts, we present a new algorithm which selectively integrates the (segmented by the HBPIS) apparent contours, from three mutually orthogonal views. The effectiveness of the approach is demonstrated through a series of experiments, where a subject performs a set of movements according to a protocol that reveals the structure of the human body.

1

Introduction

Computer vision has begun to play an increasingly important role in applications like anthropometry, human factors design, ergonomics, teleconferencing, virtual reality and performance measurement of both athletes and patients with psychomotor disabilities. All of these areas require identi?cation of the parts of a human body and estimation of their shape and motion parameters. The main dif?culties in developing algorithms for human shape and motion analysis stem from the complex 3D non-rigid motions of humans and the occlusion among body parts. To recover the degrees of freedom associated with the shape and motion of a moving human body, most of the existing approaches introduce simpli?cations by either using a model-based approach [10, 4, 1, 15, 14] or employing assumptions on the kinds of motions they can analyze [16, 2, 17, 3, 7, 8]. Most of these techniques use non-deformable models that can only approximate the human body (e.g., generalized cylinders) and cannot adapt to different body sizes. To overcome this limitation other
This work has been supported in part by the following grants: ARO Grant DAAL03-89-C-0031PRI, ARPA Grants N00014-92-J-1647 and DAAH-049510067, NSF Grants CISE/CDA-88-22719, MIP94-20397, IRI93-09917 and MIP94-20393.

researchers assume prior segmentation of the image data into parts [12] and then ?t deformable models that can adapt to data from humans with different anthropometric dimensions [11, 9]. Thus, the process of segmentation and the process of shape and motion estimation are decoupled, leading to possible inaccuracies and lack of robustness. Moreover, no technique exists to automatically acquire a concise 2D model of the human body and its parts using vision sensors. Finally, the very dif?cult problem of 3D shape and motion estimation has not been addressed so far due to the dif?culty in integrating multiple viewpoints and dealing with occlusions between the body parts. Our long term goal is to provide an integrated framework for tracking the body parts of a human moving in 3D which copes with occlusion between the body parts. For some tasks (e.g., determining if a person is moving towards or away from you) information about the movement of the centroid of the silhouette is adequate. For other tasks though (e.g., virtual reality), detailed motion information for the body parts is required. Since humans have widely varied anthropometric dimensions the ?rst part of the tracking process should be the acquisition of the subject’s model. This paper makes three contributions to the topic above. The ?rst contribution is the development of an algorithm that ?rst ?ts only one model to the image data, but then it segments it to the different body parts as the person moves. The second contribution is a new approach to modeling and controlling the processes in HBPIS. The Supervisory Control Theory of Discrete Event Systems [13] allows us to encapsulate both the discrete and the continuous aspects of the ?tting and segmentation processes. Estimating the 3D shape of the body parts, for any human, is the third contribution of this paper.

2

2D Human body model acquisition

As a human moves and attains new postures its apparent contour changes dynamically and large protrusions emerge as the result of the motion of the limbs (?gure II(1-3)). To represent large protrusions or concavities and their shape evolution in a compact and intuitive way, we introduce a new shape representation based on the parametric composition of primitives. Using this representation, we can describe compactly the shape of the union (in the case of protrusions) or intersection (in the case of concavities) of two primitives. For simplicity, we formulate the theory of 1

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composition in 2D, to represent the shape of the boundary of the union of primitives. Let 0 and 1 be two 2D parametric primitives, positioned in space so that 1 (intersecting primitive) intersects 0 (root primitive) at points and (?gure I(a-c)). The shape of the composed primitive ( : IR2 ) can be de?ned in terms of the parameters of the de?ning primitives 0 and 1 as follows (see [5] for more details):
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The Part Decomposition Criterion C and the related algorithm have been described in [6]. In the following, we present the other criteria and the related algorithms in more detail. HBPIS - Step 3a: As a subject moves and attains new postures the apparent contour changes dynamically and large protrusions are emerging as the result of the motion of the limbs (?gure II(1-3)). If there is no hole (by hole we denote the existence of a closed contour within the apparent contour) present within the apparent contour and there is a signi?cant deformation of the apparent contour, we represent the protrusions as the result of composition of two primitives.
Parametric Composition Invocation Criterion: Signal the need for parametric composition of primitives if no hole is evolv- p0 , ing within the apparent contour and p0 init where is an a priori de?ned constant, and p0 , p0 init represent the current and the initial shapes (w.r.t. the modelcentered reference frame) of the model m0 of the apparent contour. Algorithm: Parametric Composition Step 1: Determine the interval 0 of the material coordinate of the model m0 in which maximum variation of the shape over time is detected. Step 2: Perform an eigenvector analysis on the data points that correspond to the area 0 to approximate the parameters of a new deformable model m1 which can ?t these data points. Step 3: Construct a composed parametric primitive m with m0 and m1 as de?ning primitives. by replacing the model m0 with the Step 4: Update the list composed model m.

HBPIS - Step 3d: The models of the updated list are continuously ?tted to the time-varying data. If the parameters of a composed deformable model indicate that its de?ning primitives are moving with respect to one another, then this signals the presence of two distinct parts.
Part Decomposition Criterion A: If the generalized coordinates a composed model m (whose de?ning primitives are the models m0 and m1 ) satisfy the relation ?q ?q init ?q ?q init 0 1 , where 0 and 1 are two a priori de?ned constants, ?q q1 q0 and ?q q 1 q 0 represent the relative translation and orientation of the model-centered coordinate systems of m0 and m1 , then decompose m. Algorithm: Part Decomposition A Step 1: Construct two new models n0 and n1 using the parameters of the de?ning models of the composed model m. Step 2: Update and , by replacing m with n0 and n1 .

HBPIS - Step 3b: The visual event of a hole evolving within the apparent contour which is being tracked indicates that parts which were initially occluded are gradually becoming visible. During this process though, the regions of the initially occluded part that become visible are not contiguous, thus the appearance of the hole. Therefore, 2

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Human Body Part Identi?cation Strategy (HBPIS) Step 1: Initially, assume that the subject’s body consists of a single (with one entry part. Create a list of deformable models initially) that will be used to model the subject’s body parts. In addition, create a graph with one node. The nodes of the graph denote the body parts recovered by the algorithm. The edges of the graph denote which parts are connected by joints. Step 2: If not all the frames of the motion sequence have been processed ?t the models of the list to the image data using the physics-based shape and motion estimation framework [9] and execute steps 3 and 4. Otherwise, output and . Step 3: For each model in , determine: a: if the Parametric Composition Invocation Criterion is satis?ed (e.g., this criterion is satis?ed when the subject lifts her arm towards the horizontal position, as the apparent contour of the arm protrudes from the one of the torso). b: if the Part Decomposition Criterion B is satis?ed (e.g., this criterion is satis?ed during the movement of the legs). c: if the Part Decomposition Criterion C is satis?ed (e.g., this criterion is satis?ed when the subject bends her elbow). For each composed model in , determine: d: if the Part Decomposition Criterion A is satis?ed (e.g., this criterion is satis?ed during the later stages of the movement of the arm with respect to the torso). Step 4: For the protocol of movements movA, for each model in at most one of the criteria speci?ed in step 3 will be satis?ed. Depending on the outcome, determine which of the following algorithms to invoke. For each model in If 3a is satis?ed, invoke the Parametric Composition algorithm. If 3b is satis?ed, invoke the Part Decomposition B algorithm. If 3c is satis?ed, invoke the Part Decomposition C algorithm. For each composed model in

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Since humans have widely varied anthropometric dimensions the ?rst part of the tracking process should be the acquisition of the subject’s body model. Our goal is to automatically segment body apparent contours of moving humans and estimate their shape without assuming a prior model of the human body or body part segmentation. To achieve this goal we perform a controlled experiment. We request that the individual under observation performs a set of movements (protocol movA in [5]) that reveals the structure of the human body and ensures that all the major parts of the human body become visible. The result of processing the image data from this set of movements will be a 2D model of the subject’s body including its parts and their shape.

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when a hole is evolving within the apparent contour whose shape has changed considerably, we do not invoke the parametric composition algorithm. We monitor the evolution of the hole, and when the hole ceases to exist (and the shape of the model exhibits large deformation, the Part Decomposition B algorithm is invoked.
Part Decomposition Criterion B: If an evolving contour within the apparent contour ceases to exist and p0 - p0 , where is an a priori de?ned init constant, and p0 , p0 init represent the current and the initial shapes (w.r.t. the model-centered reference frame) of the model m0 of the apparent contour, then invoke the Part Decomposition B algorithm. Algorithm: Part Decomposition B Step 1: Determine the interval 0 of the material coordinate of the model m0 , in which maximum variation of the shape over time is detected. Step 2: Determine the interval 1 , 1 0 , for which consistent change of the orientation of the ?nite elements within the area 0 of the deformable model is detected. Step 3: Perform an eigenvector analysis on the data points that correspond to the area 1 , to approximate the parameters of a new deformable model m1 which can ?t these data points. Step 4: Update the list by replacing the model m0 with the models m0 init and m1 .

For step 2 of the HBPIS, we ?t the models included in list to the image data, using our weighted-force assignment algorithm [6] which allows the overlap between models and therefore, the instant rotation center of a part can be determined (see [5]).

2.1

Implementation of the HBPIS

3

3D Shape Estimation by Integrating Multiple Views

In this section we present how to combine 2D information from three mutually orthogonal views (?gure II(30)) in order to estimate the 3D shape of a subject’s body parts. Due to the complexity of the shape of the human body and occlusion among the body parts, the subject is requested to perform a different set of movements (protocol movB 3

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In this section we describe the modeling and the control of the processes involved in the system implementing the HBPIS, using the Discrete Event Systems theory. A discrete event system is a dynamical system in which changes in the state occur at discrete instances of time. The Supervisory Control Theory of Discrete Event Systems [13] allows us to encapsulate both the discrete and the continuous aspects of the ?tting and the segmentation processes. In addition, it allows us to investigate the behavior of the participating processes which operate concurrently, to formulate in a compact and elegant way the basic concepts and algorithms for our problem, and to decompose the overall design into components modeled by Finite State Machines (FSM). In terms of the DES theory, there are three components to the system that implements HBPIS: the plant, the supervisor and the observer(s) (Fig. I(d)). The set of all deformable models that are being ?tted to the data at each time instant constitutes the plant. The observers are processes that monitor the quantitative changes to the shape of the modeling primitives and send messages to the supervisor. The supervisor is the process that controls the behavior of the system, handles the complex interactions

between the ?tting processes and invokes the appropriate part composition or decomposition algorithms depending on the input from the observers. Therefore, the observers provide feedback to the supervisor regarding the state of the plant. Each participating process of the system is modeled as a nondeterministic ?nite state machine. The states of this ?nite state machine correspond to a particular discretization of the evolution of the task over time. Transitions between these states are caused by discrete events. Some of these operation), events can be expressed in the form (guard where guard is a boolean valued expression. If the value of the guard is true, the operation (e.g., send/receive message) is executed. The rest of the events are expressed in the form (guard), where the value of guard controls the transition between states. By ? a communication operation is denoted, in which a process receives a message or ! also denotes a comcommand from channel . munication operation in which a process sends a message or command through channel . In the implementation of the HBPIS, for each of the deformable models used at every time instant to ?t the data, there is a corresponding ?tting process whose design is shown in ?gures I(e,f). These processes run in parallel and are synchronized by a global clock. In addition, they receive commands from the supervisor and send messages back. The states of each ?tting process correspond to a discretization of the ?tting task based on which degrees of freedom of the model are used (e.g., global and/or local parameters). For each ?tting process there is an observer process (?gures I(g,j)) that monitors the parameters of the deformable model in order to infer the state of the deformable model. Our goal is to be able to drive a deformable model from any state (the error state and the events associated with the error state have been omitted for clarity) in the set 0 1 2 3 4 5 , back to state 0. This is the desirable state since if all the ?tted deformable models are in this state, then the structure of the data has been captured. This is because in that state there is no signi?cant change from the initial shape and no evolving holes. To drive each deformable model to the desirable state 0, we design a supervisor. The function of the supervisor (?gures I(h,i)) is to select the appropriate action depending on the state of the observer processes and enable, disable or force controllable events in order to ensure correct behavior of the plant.
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in [5]) that incrementally reveals the structure of the body. Our approach is to ?rst build a single 3D model of the human standing, and then to incrementally re?ne it by extracting the 3D models of the different parts as they become visible to the different views. At each stage of the algorithm the data from a speci?c movement are processed. The stages of the algorithm are summarized in ?gure I(k). For each movement, the apparent contours from each active view are ?tted using the techniques described in the previous section. Therefore, for each active view there is a set of 2D deformable models that ?t the corresponding data. Due to occlusion, the number of deformable models in each of the views may not be the same. Consequently, depending on the type of movement a new part or parts may be detected in some of the 2D apparent contours. The new part is either a previously unseen body part or a subpart of a part whose 3D model has already been recovered. In the ?rst case, the 2D models of the corresponding apparent contours of a part are integrated using the algorithm described below (Fig. II(42)). In the second case, the model of the apparent contour of the subpart is intersected with the model for the 3D shape of the part to obtain two new 3D shapes, the 3D shape of the sub-part and the 3D shape of the rest of the part. Due to the fact that more than one parts may be simultaneously determined in 2D (e.g., left and right arms), the extracted spatial location of the part in each of the two views is employed to determine the apparent contour correspondence. It should be noted also that the 3D shape of the new part is obtained at the end of the appropriate set of movements. This is a desirable property, since the shape ?tting is done in 2D and the 3D shape estimation is done only once. In the following, the algorithm for the integration of the 2D models of two apparent contours is described. The input to the algorithm is the 2D models of the apparent contours of the part as observed from two mutually orthogonal views, and the spatial relation between the views. A 3D deformable model is initialized and the nodes that lie on its meridians whose plane is parallel to the planes of the apparent contours are ?tted to the nodes of the 2D models. Local deformations are employed to capture the exact shape of the 2D models, while the rest of the 3D shape is interpolated. Due to the fact that the local deformations may be large in magnitude and to avoid shape discontinuities between the ?tted nodes and the rest of the shape, a thin-plate deformation energy is imposed during the ?tting process. Therefore, the deformable model ?ts accurately the 2D models, while its shape in between the apparent contours (in the absence of any data) varies smoothly.

were carried out on real image sequences in order to evaluate the effectiveness of the proposed approach in general and of the speci?c algorithms in particular. The ?rst experiment was designed to evaluate the performances of the Part Composition Invocation Criterion, and of the Part Decomposition A algorithm. In this experiment, the subject lifted her left arm, until the arm reached the horizontal position (?gures II(1-3)). In ?gure II(5) notice the deformation of the initial shape of the model (?gure II(4)), which is quanti?ed in ?gure II(6). The parametric composition invocation criterion was satis?ed and therefore the Parametric Composition algorithm was invoked. The de?ning primitives of the composed deformable model are shown in ?gure II(7). In a later frame, the Part Decomposition Criterion A was satis?ed and the Part Decomposition A algorithm was invoked in order to recover the underlying parts. Figure II(8) shows the recovered models for the parts, and their ?tting to the data in ?gure II(3). The second experiment was designed to demonstrate the Part Decomposition criterion B and Part Decomposition B algorithm. In this experiment, the subject extended her left leg the front until she reached a comfortable position (?gures II(9-12)). While ?tting the data in ?gure II(11,15) the Part Decomposition Criterion B was satis?ed and therefore the Part Decomposition B algorithm was invoked. The two models recovered are depicted in ?gure II(16), and their ?tting to the data in shown in ?gure II(12). The purpose of the third experiment was to demonstrate the results of the Human Body Part Identi?cation Strategy when the subject moves according to the protocol MovA. Figures II(18-21,24-29) depict some of the models extracted during the motion. Finally, the fourth experiment was designed to demonstrate the feasibility of estimating the 3D shape of body parts by selectively integrating 2D outlines from multiple views (?gure II(30)). Figure II(31) shows the model recovered for the left arm, while ?gures II(32-35) show the models for the hand, lower arm and upper arm and their layout. Figure II(36) shows the recovered model of the torso and ?gures II(37-41) the corresponding parts of the left leg.

5

Conclusion

4

Experimental Results

This paper presented a novel integrated approach to the 3D shape model acquisition of a subject’s body parts from multiple views. The extraction of a subject’s model can be considered as the ?rst part of a tracking process. Our long term goal is to provide an integrated framework for tracking the body parts of a human moving in 3D which copes with occlusion between body parts. The experiments that were carried out have proven that it is possible to determine the parts of a human body from an image sequence and estimate their shape in 3D.

This section presents selected results from a series of experiments (a detailed description is offered in [5]) that 4

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Supervisor State Descriptions 0: 1: Wait state. Execute Parametric Composition algorithm with input the deformable model i for which the corresponding observer has send the message 1. Execute Part Decomposition A algorithm with input the deformable model i for which the corresponding observer has send the message 5. Execute Part Decomposition B algorithm with input the deformable model i for which the corresponding observer has send the message 3. Execute Part Decomposition C algorithm with input the deformable model i for which the corresponding observer has send the message 4. Perform bookkeeping associated with the arrival of new data. Send command to all participating processes that new data have arrived, (Ci ? ). Event Descriptions O0 ? 1 O1 ? 1 On ? 1 Part Composition algorithm has been completed. O0 ? 5 O1 ? 5 On ? 5 Part Decomposition A algorithm has been completed. O0 ? 3 O1 ? 3 On ? 3 Part Decomposition B algorithm has been completed. O0 ? 4 O1 ? 4 On ? 4 Part Decomposition C algorithm has been completed. New data have arrived. Messages have been sent to all the participating ?tting processes and bookkeeping has been completed. (i)

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Figure I: An example of composition of two superellipsoids: (a) depicts x1 1 which intersects x0 0 at points and , (b) depicts their composition x , (c) depicts the composition function ; 0 65 0 77 10 . (d-j) explain the FSM of the components of the implementation of HBPIS. (k) 3D shape estimation (IG stands for the Integration algorithm and IN for the Intersection algorithm). 5
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Observer process State Descriptions 0: In this state the following conditions are true: a) there is no hole evolving within the apparent contour of the deformable model and b) the Part Composition Invocation Criterion and the Part Decomposition Criterion X (X = A, B and C) are not satis?ed. 1: Part Composition Invocation Criterion is satis?ed. 2: Part Composition Invocation Criterion is satis?ed but there is a hole evolving within the apparent contour. 3: Part Decomposition Criterion B is satis?ed. 4: Part Decomposition Criterion C is satis?ed. 5: The deformable model is a composed deformable model and the Part Decomposition Criterion A is satis?ed. Event Descriptions 0 : Hole is evolving within the apparent contour. - p0 init 1 : p0 2 : Part Decomposition Criterion C has been satis?ed. 3 : The deformable model is a composed deformable model and the Part Decomposition Criterion A has been satis?ed. Messages :( 1 3 4 5 ) Report the state identi?er to the supervisor. (j)

3D Parts Acquired Head Arms Hands Upper and lower arms Legs and torso Left thigh, left lower leg and left foot Right thigh, right lower leg and right foot

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References
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