The present invention provides for a method for tracking a number of objects or object parts in image sequences, comprising following a Bayesianlike approach to object tracking, computing, at each time a new image is available, a probability distribution over all possible target configurations for that time, said Bayesianlike approach to object tracking comprising the following steps:
 Prediction step: a probability distribution is computed for the previous image, at time (t1), is propagated to the new image at time (t) according to a probabilistic model of target dynamics, obtaining a predicted distribution at time (t) ;
 Update step: the predicted distribution at time (t) is then aligned with the evidence contained in the new image at time (t) according to a probabilistic model of visual likelihood.
6542621  20030401  
6674877  20040106 
1. Method for tracking a number of objects or object parts in image sequences, comprising following a Bayesianlike approach to object tracking, computing, at each time a new image is available, a probability distribution over target configurations for that time, said Bayesianlike approach to object tracking comprising the following steps:  Prediction step: a probability distribution is computed for the previous image, at time (t1), is propagated to the new image at time (t) according to a probabilistic model of target dynamics, obtaining a predicted, or prior, distribution at time (t) ;  Update step: the predicted distribution at time (t) is then aligned with the evidence contained in the new image at time (t) according to a probabilistic model of visual likelihood, obaining a posterior distribution at time (t); characterized in that in said Update step said method comprises aligning said predicted distribution at time (t) of an hypothetic target configuration associated with a new frame by performing image analysis in the following way:  the image portion in which the target under consideration is expected to be visible under the current configuration is identified, by using a shape rendering function, namely a userdefined shape model of the target that is rendered onto the image for the specific configuration under analysis ;  a probability value is assigned to each pixel of the identified image portion, which is computed as the probability of being occluded by another target, the probability being derived from other targets' prior distributions and their shape model;  a degree of dissimilarity is computed, in a userdefined way, between visual features extracted from the identified image portion and a corresponding characterization, or appearance model, of the target, the importance, or weight, of the different pixels in this calculation being thereby calculated by said probability of being occluded;  to said value of degree of dissimilarity, a further dissimilarity term per each other tracked target is added, each such other targetterm being computed in form of the expectation, under its prior distribution, of the dissimilarity values calculated on those configurations that map closer to the camera (or to any other shooting or picture recording device used) than the one of the target under analysis, the target currently under consideration being neglected;  the distribution value assigned to the hypothesized configuration of the target under analysis is finally calculated by multiplying the prior value with the negative exponential of the overall dissimilarity score as obtained according to the previous two items.
2. Method according to claim 1, characterized in that said distribution value assigned to the hypothesized configuration of the target under analysis is calculated recursively according to the following expression:
where:

where:
 L^{k}(z_{t} w^{k}), called foreground term, is said degree of dissimilarity between visual features extracted from the identified image region and a corresponding characterization, or appearance model, of the target;

3. Method according to claim 2, characterized in that said foreground term L^{k}(z_{t}w^{k}) calculates a dissimilarity score between the image part where target k is expected to be visible, and its appearance model by the following steps:
 the image region identified by the shape model for the hypothesis
4. Method according to claim 2, characterized in that said background term
5. Method according to any of claims from 2 to 4, characterized in that said pixel weights and weight maps w^{k},w^{mk} are scalar images computed by attributing to each pixel of z a probability value in the range [0,1], the probability value being interpreted as the importance of that pixel when computing the likelihood score for target k.
6. Method according to claim 5, characterized in that said pixel weights and weight maps w^{k},w^{mk} are computed for each pixel u as follows:
where
7. Apparatus for tracking a number of objects or object parts in image sequences comprising means for the implementation of the method as in any of the preceding claims.
8. Computer program comprising computer program code means adapted to perform all the steps of claims 1 to 6, when said program is run on a computer.
9. A computer readable medium having a program recorded thereon, said computer readable medium comprising computer program code means adapted to perform all the steps of claims 1 to 6, when said program is run on a computer.
The present invention relates to a method and apparatus for tracking a number of objects or object parts in image sequences.
As known, video tracking is the process of locating a moving object (or several ones) in time using a camera (or several ones). An algorithm analyses the video frames and outputs the location, optionally in real time.
Visual tracking of multiple moving targets is a challenging problem. Independent tracking of individual bodies is a simple solution but fails in the presence of occlusions, where the disappearance of a target cannot be explained but in relationship with the other targets (the event in which the light emitted or reflected by an object is blocked by another object before it reaches the eye or camera where the image is taken, is called occlusion).
On the other hand, principled modeling of the occlusion process is possible when considering the joint configuration of all involved targets, and enables a single tracker in charge of estimating the joint dynamics of the different bodies to interpret images correctly during occlusion. This solution, however, requires a representation size that grows exponentially with the number of bodies, thus leading to an estimation algorithm whose computational complexity grows exponentially as well.
However, the problem of tracking the position and the velocity of a single target is well distinguished from the one of tracking the position of two or more different targets. Although both tasks can be formalized as a joint estimation problem, in the first case physical constraints impose a strong correlation of position and velocity, while in the second case the two components, the locations of the different objects, may depend only weakly from each other, if at all. Their measurements, however, may still be strongly correlated due to occlusions. This is the basic observation that has motivated the invention. In our method we deal with estimates separately, but analyze images jointly.
There are a number of acknowledged approaches described in the literature which address the multitarget/multipart tracking problem.
In particular the article by
Other articles address similar problems, like
The above references disclose implementing principled occlusion reasoning suffering from the problem of dimensionality resulting in heavy computational burden due to exponential complexity increase in the number of targets.
Therefore it is the main object of the present invention to provide a method and apparatus for tracking a number of objects or object parts in image sequences, which offers a considerable reduction in computational complexity.
The subject of this invention is a new method for tracking a number of objects or object parts based on their visual appearance observed in a plurality of successive image frames, which can handle multiple occlusions consistently and at an affordable computational cost.
The present invention offers two main contributions which allow to solve the multitarget tracking problem at an affordable computational cost.
The first contribution provides a novel Bayesian framework tailored to sequential estimation problems of weakly coupled signals, like the ones describing trajectories of different targets to which we will refer from now on. Involved joint distributions are represented by the outer product of single target components, while updates are carried out using joint likelihood model. These updates produce nonseparable distributions which are mapped into single target spaces by a projection that guarantees minimal information loss. The key feature of the resulting model, dubbed Hybrid JointSeparable (HJS), is its economical representation size that scales linearly with the number of targets.
The second contribution is the presentation of an occlusion robust multitarget appearance likelihood and an associated algorithm for an efficient update within the HJS model. The likelihood model is derived according to image formation principles and implements occlusion reasoning at pixel level.
The complexity of the HJS posterior update is quadratic in the number of tracked objects and linear in representation size of single target estimates.
Therefore the present method is unique in the way it handles occlusions in a principled way but maintains an affordable computational cost. The complexity of the method scales quadratically with the number of targets rather than exponentially, as in the known systems.
The method described hereafter addresses both scalability and robustness issues, providing a new solution which is robust to interobject occlusions and remains practical at the same time. The method follows a Bayesian approach to object tracking: it computes, at each time a new image is available, a probability distribution over target configurations for that time. Bayesian tracking then involves two steps:
More specifically, the method in accordance with the present invention computes the posterior distribution value, or probability density, in the Update step, of any hypothetic target configuration associated with a new frame by performing image analysis, in the following way:
With the term "any hypothetic target configuration" it is meant not only the location of a moving subject, but also the posture, the orientation, the leg or arm angles, of any simple or articulated subjects, etc...
The above list of operations has to be repeated for each target configuration on which the value of the probability distribution has to be determined. For example, in a Particle filter implementation of the method, the list of operations has to be executed for each single element of the sample set that has been chosen to represent the distribution (i.e. for each particle). However, the method is by no means limited to being applied to such realization. The present invention is about a new routine to compute the update step of a multibody/multipart Bayes filter, which might be realized on a computer through approximate implementation of it, including, but not limited to, Kalman Filter, Particle Filter, Markov Chain Monte Carlo (MCMC) Filter, PhD Filter, RaoBlackwellised Particle Filter, Grid Filter, Kernel Filter, Belief Propagation, Nonparametric Belief Propagation, PAMPAS, etc.
These and further objects are achieved by means of a method and apparatus for tracking a number of objects or object parts in image sequences, as described in the attached claims, which are considered an integral part of the present description.
The invention will become fully clear from the following detailed description, given by way of a mere exemplifying and non limiting example, to be read with reference to the attached drawing figures, wherein:
The present invention is about a new functional routine which computes the update step of a multibody/multipart Bayes filter (or any approximate implementation of it including, but not limited to, Kalman Filter, Particle Filter, Markov Chain Monte Carlo (MCMC) Filter, PhD Filter, RaoBlackwellised Particle Filter, Grid Filter, Kernel Filter, Belief Propagation, Nonparametric Belief Propagation, PAMPAS, etc.):
In the following each of the above items is specified rigorously, build upon an explicit shape model of the target/s.
Figure 1 shows an example of Bayes filter iteration according to known criteria: the posterior distribution computed for time t1 (Posterior t1) is first projected to time t according to a Prediction model to get a prior distribution for time t (Prior t), and then updated with the likelihood evaluated on the new image, to get a Posterior distribution at time t (Posterior t).
This section describes the type of estimate representation the method calculates.
The configuration of a target within the monitored scene is represented on a computer by an n dimensional vector, x, the target state. A target can be, for example but not in a limiting sense, a single object, or a part of it such as the upper leg of a person, or a joint of a mechanical arm of a robotic platform.
This state can simply be the image coordinates of the centroid of the object, or the spatial position of a person measured on the floor plane, or a highdimensional description of its posture in terms of joint angles, etc.. Let
The method proposed here computes an approximate representation
where distribution
Relation (3) defines an equivalence up to a proportionality factor which is independent of
This section describes the shape rendering function of a target and its representation that is used as input to the method. Each target k has a rendering procedure associated to the numeric description chosen, denoted by g^{k}(x^{k}). This function provides, for a given configuration x^{k} of the target, a depth map of the target as seen from a given camera. Precisely, it computes a realvalued scalar image whose pixels contain the distance of the target surface, or an approximation of it, from the camera optical centre; in pixels where the object is not visible, a reserved value, e.g. ∞, is assigned.
An example of an implementation of this function is given in Fig. 2, showing a functional procedure of an example shape rendering function for a person tracking task. A coarse 3D model is adopted for shape, target state x^{k} is encoded as its 2D position on the floor plane. Given a calibrated model of the camera, a depth map g^{k}(x^{k}) can be computed (i.e. by Computer Graphics techniques, such as raytracing or polygonal mesh rendering). Distance is greencoded, textured regions are at infinity.
This section describes the specific type of visual likelihood function p(zx^{1:K}) on which the method operates. p(zx^{1:K}) is a realvalued function of joint configuration x^{1:K} and image z, and operates as follows. Image z is subdivided into a set of image patches z^{1},···,z^{K}. Each patch z^{k} is composed of image pixels in which g^{k}(x^{k}), the associated shape rendering function instantiated for the camera which captured z and evaluated for the state represented by the kth component of x^{1:K}, is different from ∞ and has smaller value that all the other depth maps g^{m}(x^{m}) with m ≠ k. Patch z^{k}, a shorthand for z^{k}(x^{1:K}), therefore represents the image portion in which target k is expected to be visible under hypothesis x^{1:K}.
An example of image partition is given with reference to Fig. 3 which shows a Twobody image partitioning for an occlusion hypothesis using shape models as defined with reference to Fig. 2.
The likelihood function on which the method operates is given by the following expression
As an example, L^{k}(zw^{k}) could be evaluated by means of Bhattacharryacoefficient based distance between the color histogram of z^{k} and a model histogram of target k.
The method subject of the invention introduces a new functional block that performs the update of approximate distribution defined in Eq. (1) in an efficient way using the likelihood function in Eq. (4).
Manipulations show that computing
Under the specific form of likelihood defined in Equation (4), an approximation to
Equation (6), with Equations (7) and (8), define the mathematical framework of this invention.
An example weight map is shown in Fig. 4, where on the left a highdimensional shape model of a person in a specific pose and a bimodal distribution representing a possible prior estimate of the person's position is shown. The right image shows the corresponding weight map used to update the estimate of a second person hypothesized behind her: the template shape becomes blurred due to estimation uncertainty.
The key claim is that evaluating Eq. (6) can be accomplished in a number of computations that grows quadratically with the number of targets K; evaluating
The gain in efficiency can be understood by rewriting w^{k} (and similarly w^{m\k}) in the following form:
where
This section describes the implementation details of the method in a standalone fashion. The method presents a new way to perform the update step of a Bayes filter for visual multitarget tracking. In general, Bayesian tracking involves recursively computing the posterior distribution p(x_{t}z_{1:t}) of some vectorial description of the configuration x_{t} of a scene at time t in the following way:
where p(z_{t}x_{t}) is a likelihood function for hypothesis x, evaluated on the new image z_{t} and p(x_{t}z_{1:t1}) is the prior distribution usually computed by prediction.
When multiple targets are present, this formula can be applied to compute a posterior distribution
where
Evaluating f in a straightforward way conveys exponential complexity.
The novelty of the method then lies in the efficient implementation of an approximate version of it which is defined in the following way (see also Eq. (6)):
Informally, w^{k},w^{mk} are scalar images computed by the algorithm which attribute to each pixel of z a value in the range [0,1]. This value is interpreted by the method as the importance of that pixel when computing the likelihood score for target k. In other words, it encodes the predicted degree of visibility of k in the considered pixel. Formally, w^{k},w^{mk} are computed for each pixel u as follows:
Here relations
An informal interpretation of the formula presented in Eq. 12, and a description of how to evaluate it, is given in the following.
The equation 12 is composed of several terms, one per target, whose roles are explained next based on Fig. 5, showing a silhouette contour rendered at a hypothetic position
It is computed as the expectation of the foreground term of m under prior
For reference purposes, a possible example of pseudocode of a particle filter implementation of the method is given as follows, applying software criteria known per se.
Algorithm 1 Particle filter implementation of the proposed method 
In this realization, each target is assigned two image buffers,
The method can be used to address any visionbased state estimation problem; among them we can cite: people tracking, vehicle tracking, human pose estimation, tracking of mechanical objects such as a robotic arm, target tracking from a mobile platform such as a robot, etc.
Potential application areas include: Domotics, Ambient Intelligence, Visual Monitoring, Visual Surveillance, Traffic Analysis, Visonbased Control and Automation, HumanComputer Interaction, Sports Event Analysis, Robotics, and others.
A multiperson tracking system based on the present invention can be realized as follows.
The apparatus is composed of a single Personal Computer (PC) with dual Intel XEON 3GHz processor, and 4 nonsynchronized cameras delivering RGB images at 15Hz, connected to the PC via firewire. Image data captured by these cameras is made accessible to software implementation of the method via a software package of a known type (for example the software called "libVHPD1394")
In this realization, the following specifications over the general formulation subject of this invention have been made:
Further implementation details will not be described, as the man skilled in the art is able to carry out the invention starting from the teaching of the above description.
The present invention can be advantageously implemented through a program for computer comprising program coding means for the implementation of one or more steps of the method, when this program is running on a computer. Therefore, it is understood that the scope of protection is extended to such a program for computer and in addition to a computer readable means having a recorded message therein, said computer readable means comprising program coding means for the implementation of one or more steps of the method, when this program is run on a computer.
By means of the present invention, a number of advantages are achieved.
Tracking is based on a sequential Bayesian approach, with the aim of estimating a probability distribution for each target defined over all its possible locations or, more generically, configurations. This has the advantage of providing estimates which can support multiple hypotheses and intrinsic uncertainty, expressed by multiple modes of any shape in the distribution, which is crucial for achieving robust tracking in complex scenes.
When occlusions exist among the targets, such distributions must be computed by considering jointly all targets involved when analyzing the images because of their visual depencencies.
This leads to tracking methods which convey computational complexity that scales exponentially with the number of targets, with obvious implications on realtime applications.
In this invention it is shown that an approximate form of such distributions can be computed much more efficiently, in quadratic complexity, while still applying joint image analysis. The resulting method then handles occlusions robustly, in a principled way, while maintaining an affordable computational cost. Based on the proposed method, a tracking system has been realized that allows realtime 3D tracking of 5 people on complex indoor sequences involving several complete, temporally persistent, occlusions among multiple targets.
Many changes, modifications, variations and other uses and applications of the subject invention will become apparent to those skilled in the art after considering the specification and the accompanying drawings which disclose preferred embodiments thereof. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by this invention.