 Present by 陳群元

 Introduction  Previous work  Predicting motion patterns  Spatio-temporal transition distribution  Discerning pedestrians  Experimental results  conclusion

 Tracking individuals in extremely crowded scenes is a challenging task,  we predict the local spatio-temporal motion patterns that describe the pedestrian movement at each space-time location in the video.  we robustly model the individual’s unique motion and appearance to discern them from surrounding pedestrians.

 Previous work track features and associate similar trajectories to detect individual moving entities within crowded scenes.  We encode many possible motions in the HMM, and derive a full distribution of the motion at each spatio-temporal location in the video.

 Introduction  Previous work  Predicting motion patterns  Spatio-temporal transition distribution  Discerning pedestrians  Experimental results  conclusion

 An example : a 3-state Markov Chain λ o State 1 generates symbol A only, State 2 generates symbol B only, and State 3 generates symbol C only o Given a sequence of observed symbols O={CABBCABC}, the only one corresponding state sequence is {S 3 S 1 S 2 S 2 S 3 S 1 S 2 S 3 }, and the corresponding probability is P(O|λ)=P(q 0 =S 3 ) P(S 1 |S 3 )P(S 2 |S 1 )P(S 2 |S 2 )P(S 3 |S 2 )P(S 1 |S 3 )P(S 2 |S 1 )P(S 3 |S 2 ) =0.1  0.3  0.3  0.7  0.2  0.3  0.3  0.2= s2s2 s3s3 A B C s1s1

 An example : a 3-state discrete HMM λ o Given a sequence of observations O={ABC}, there are 27 possible corresponding state sequences, and therefore the corresponding probability is s2s2 s1s1 s3s3 {A:.3,B:.2,C:.5} {A:.7,B:.1,C:.2}{A:.3,B:.6,C:.1}

 f(Pos) = (f(Pos+1) -f(Pos) + f(Pos) -f(Pos-1))/2 = f(Pos+1)- f(Pos-1)/2;  For each pixel i in cuboid  I is intensity

 the local spatio-temporal motion pattern  represented by a 3D Gaussian of spatio-temporal gradients

 The hidden states of the HMM are represented by a set of motion patterns  The probability of an observed motion pattern given a hidden state s is

 Kullback–Leibler divergence is a non-symmetric measure of the difference between two probability distributions P and Q.

 After training a collection of HMMs on a video of typical crowd motion, we predict the motion pattern at each space- time location that contains the tracked subject.  where S is the set of hidden states, w(s) is defined by

Reference :A Tutorial On Hidden Markov Models andSelected Applications in Speech Recognition.

 a weighted sum of the 3D Gaussian distributions associated with the HMM’s hidden states

 The centroid we are interested in is a multivariate normal density that minimizes the total distortions. Formally, a centroid c is defined as, Reference: On Divergence Based Clustering of Normal Distributions and Its Application to HMM Adaptation

 where and are the mean and covariance of the hidden state s, respectively.

 Introduction  Previous work  Predicting motion patterns  Spatio-temporal transition distribution  Discerning pedestrians  Experimental results  conclusion

 we use the gradient information to estimate the optical flow within each specific sub-volume and track the target in a Bayesian framework.  Bayesian tracking can be formulated as maximizing the posterior distribution of the state x t of the target at time t given available measurements z 1:t = {z i ; i = 1 : : : t} by  z t is the image at time t, p (x t |x t-1 ) is the transition distribution, and p (z t |x t ) is the likelihood.  state vector x t as the width, height, and 2D location of the target within the image.

 we focus on the target’s movement between frames and use a 2nd-degree autoregressive model for the transition distribution of the target’s width and height.  Ideally, the state transition distribution p (x t |x t-1 ) directly reflects the two-dimensional motion of the target between frames t -1 and t.  where is the 2D optical flow vector, and is the covariance matrix.

 Assuming the movement to be small, the image constraint at I(x,y,t) with Taylor series can be developed to get  H.O.T

 The predicted motion pattern is defined by a mean gradient vector and a covariance matrix  The motion information encoded in the spatio-temporal gradients can be expressed in the form of the structure tensor matrix  The optical flow can then be estimated from the structure tensor by solving  where w = [u; v; z] T is the 3D optical flow

 the 2D optical flow is v’ = [u/z; v/z]T

 Introduction  Previous work  Predicting motion patterns  Spatio-temporal transition distribution  Discerning pedestrians  Experimental results  conclusion

 Typical models of the likelihood distribution p (z t |x t )  where is the variance, is a distance measure, and Z is a normalization term.  difference between a region R (defined by state x t ) of the observed image z t and the template.  We assume pedestrians exhibit consistency in their appearance and their motion, and model them in a joint likelihood by  where p A and p M are the appearance and motion likelihoods

 After tracking in frame t, we update each pixel i in the motion template by  where is the motion template at time t,  Is the region of spatio-temporal gradient defined by the tracking result (i.e., the expected value of the posterior)  is the learning rate.

 The error at pixel i and time t becomes  t i and r i are the normalized gradient vectors of the motion template and the tracking result at time t  To reduce the contributions of frequently changing pixels to the computation of the motion likelihood, we weigh each pixel in the likelihood’s distance measure.  where Z is a normalization term such that

 The distance measure of the motion likelihood distribution becomes

 Introduction  Previous work  Predicting motion patterns  Spatio-temporal transition distribution  Discerning pedestrians  Experimental results  conclusion

 The training video for the concourse scene contains 300 frames (about 10 seconds of video),  the video for ticket gate scene contains 350 frames.  We set the cuboid size to 10*10*10 for both scenes.  The learning rate, appearance variance, and motion variance are 0.05.

 Introduction  Previous work  Predicting motion patterns  Spatio-temporal transition distribution  Discerning pedestrians  Experimental results  conclusion

 In this paper, we derived a novel probabilistic method that exploits the inherent spatially and temporally varying structured pattern of a crowd’s motion to track individuals in extremely crowded scenes.

 The end  Thank you