We consider the problem of online 2D multi-object tracking from videos. Given the historical input frames, the goal is to extract a set of 2D object bounding boxes from the current input frame. Each bounding box should have an one-to-one correspondence to an object and thus should not change its identity across different frames.
MOT is a challenging task since one must deal with: (i) unknown number of objects, which requires the tracker to be correctly reinitialized/terminated when the object appears/disappears; (ii) frequent object occlusions, which require the tracker to reason about the depth relationship among objects; (iii) abrupt pose (e.g., rotation, scale, and position), shape, and appearance changes for the same object, or similar properties across different objects, both of which make data association hard; (iv) background noises (e.g., illumination changes and shadows), which can mislead tracking.
To overcome the above issues, one can seek to use expressive features, or improve the robustness of data association. E.g., in the predominant Tracking-by-Detection (TBD) paradigm [Andriluka, Roth, and Schiele2008, Henriques et al.2012, Breitenstein et al.2009, Breitenstein et al.2011], well-performed object detectors are first applied to extract object features (e.g., potential bounding boxes) from each input frame, then appropriate matching algorithms are employed to associate these candidates of different frames, forming object trajectories. To reduce the human effort to manually tune parameters for object detectors or matching algorithms, many machine learning approaches are integrated into the TBD framework and have largely improved the performance [Xiang, Alahi, and Savarese2015, Schulter et al.2017, Sadeghian, Alahi, and Savarese2017, Milan et al.2017]
. However, most of these approaches are based on supervised learning, while manually labeling the video data is very time-consuming. Also, the TBD framework does not consider the feature extraction and data association jointly, i.e., it is not end-to-end, thereby usually leading to suboptimal solutions.
In this paper, we propose a novel framework to achieve both label-free and end-to-end learning for MOT tasks. In summary, we make the following contributions:
We propose a Tracking-by-Animation (TBA) framework, where a differentiable neural model first tracks objects from input frames and then animates these objects into reconstructed frames. Learning is then driven by the reconstruction error through backpropagation.
We propose a Reprioritized Attentive Tracking (RAT) to mitigate overfitting and disrupted tracking, improving the robustness of data association.
We evaluate our model on two synthetic datasets (MNIST-MOT and Sprites-MOT) and one real dataset (DukeMTMC [Ristani et al.2016]), showing its potential.
2 Tracking by Animation
Our TBA framework consists of four components: (i) a feature extractor that extracts input features from each input frame; (ii) a tracker array where each tracker receives input features, updates its state, and emits outputs representing the tracked object; (iii) a renderer (parameterless) that renders tracker outputs into a reconstructed frame; (iv) a loss that uses the reconstruction error to drive the learning of Components (i) and (ii), both label-free and end-to-end.
2.1 Feature Extractor
To reduce the computation complexity when associating trackers to the current observation, we first use a neural network, parameterized by , as a feature extractor to compress the input frame at each timestep :
where is the input frame of height , width , and channel size , and is the extracted input feature of height , width , and channel size , containing much fewer elements than .
2.2 Tracker Array
The tracker array comprises neural trackers indexed by . Let
be the state vector (vectors are assumed to be in row form throughout this paper) of Trackerat time , and be the set of all tracker states. Tracking is performed by iterating over two stages:
- (i) State Update
The trackers first associate input features from to update their states through a neural network parameterized by :
Whilst it is straightforward to set
as a Recurrent Neural Network (RNN)[Rumelhart, Hinton, and Williams1986, Gers, Schmidhuber, and Cummins2000, Cho et al.2014] (with all variables vectorized), we introduce a novel RAT to model in order to increase the robustness of data association, which will be discussed in Sec. 3.
- (ii) Output Generation
Then, each tracker generates its output from via a neural network parameterized by :
where is shared by all trackers and the output is a mid-level representation of objects on 2D image planes, including:
Probability of having captured an object.
One-hot encoding of the image layer possessed by the object. We consider each image comprising object layers and a background layer, where higher layer objects occlude lower layer objects and the background is the 0-th (lowest) layer. E.g., when , denotes the 3-rd layer.
Normalized object pose for calculating the scale and the translation , where are constants.
Binary object shape mask with height , width , and channel size 1.
Object appearance with height , width , and channel size .
In the output layer of , and
are generated by the sigmoid function,is generated by the tanh function, and and Jang, Gu, and Poole2017] to reparameterize both distributions so that backpropagation can still be applied.
The above-defined mid-level representation is not only flexible, but also can be directly used for input frame reconstruction, enforcing the output variables to be disentangled (as would be shown later). Note that through our experiments, we have found that the discreteness of and is also very important for this disentanglement.
To define a training objective with only the tracker outputs but no training labels, we first use a differentiable renderer to convert all tracker outputs into reconstructed frames, and then minimize the reconstruction error through backpropagation. Note that we make the renderer both parameterless and deterministic so that correct tracker outputs can be encouraged to get correct reconstructions, enforcing the feature extractor and tracker array to learn to generate desired outputs. The rendering process contains three stages:
- (i) Spatial Transformation
- (ii) Layer Compositing
Then, we synthesize image layers (), where each layer can contain several objects. The -th layer is composited by:
where is the layer foreground mask, is the layer foreground, and is the element-wise multiplication which broadcasts its operands when they are in different sizes.
- (iii) Frame Compositing
Finally, we iteratively reconstruct the input frame layer-by-layer, i.e., for :
where is initialized as the background (in this paper, we assume the background is either known or easy to extract), and is the final reconstruction. The rendering process is illustrated in Fig. 2, where .
Whilst the layer compositing can be parallelized by matrix operations, it cannot model occlusion since pixel values in overlapped object regions are simply added; conversely, the frame compositing well-models occlusion, but the iteration process cannot be parallelized, consuming more time and memory. Thus, we combine the two to both reduce the computation complexity and maintain the ability of occlusion modeling. As occlusion usually involves only a few objects, we can set the layer number to be small for efficiency, in which case each layer can be shared by several non-occluded objects.
To drive the learning of the feature extractor and tracker array, we define the loss for each timestep:
where, on the RHS, the first term is the reconstruction Mean Squared Error, and the second term, weighted by a constant , is the tightness constraint penalizing large scales to make object bounding boxes more compact. An overview of our TBA framework is shown in Fig. 2.
3 Reprioritized Attentive Tracking
In this section, we focus on designing the tracker state update network defined in (2). Although can be naturally set as a single RNN as mentioned in Sec. 2.2, there can be two issues: (i) overfitting, since there is no mechanism to capture the data regularity that similar patterns are usually shared by different objects; (ii) disrupted tracking, since there is no incentive to drive each tracker to associate its relevant input features. Therefore, we propose the RAT, which tackles Issue (i) by modeling each tracker independently and sharing parameters for different trackers (this also reduces the parameter number and makes learning scalable with the tracker number), and tackles Issue (ii) by utilizing attention to achieve explicit data association (Sec. 3.1). RAT also avoids conflicted tracking by employing memories to allow tracker interaction (Sec. 3.2) and reprioritizing trackers to make data association more robust (Sec. 3.3), and improves efficiency by adapting the computation time w.r.t. the object number (Sec. 3.4).
3.1 Using Attention
To make Tracker explicitly associate its relevant input features from to avoid disrupted tracking, we adopt a content-based addressing. Firstly, the previous tracker state is used to generate key variables and :
is the linear transformation parameterized by, is the addressing key, and is the key strength. Then, is used to match each feature vector of , denoted by where and , to get attention weights:
is the cosine similarity defined as, and is an element of the attention weight , satisfying . Next, a read operation is defined as a weighted combination of all feature vectors of :
where is the read vector, representing the associated input feature for Tracker . Finally, the tracker state is updated with an RNN, taking instead of as its input feature:
While each tracker can now attentively access , it still cannot attentively access if the receptive field of each feature vector is large enough. In this case, it remains hard for the tracker to correctly associate an object from . Therefore, we set the feature extractor as a Fully Convolutional Network (FCN) [Long, Shelhamer, and Darrell2015, Xu et al.2015, Wang et al.2015] purely consisting of convolution layers. By designing the kernel size of each convolution/pooling layer, we can control the receptive field of to be a local region on the image so that the tracker can also attentively access . Moreover, parameter sharing in CNN captures the spatial regularity that similar patterns are shared by objects on different image locations. As local image regions contain little information about the object translation , we add this information by appending the 2D image coordinates as two additional channels to .
3.2 Input as Memory
To allow trackers to interact with each other to avoid conflicted tracking, at each timestep, we take the input feature as an external memory through which trackers can pass messages. Let be the initial memory, we arrange trackers to sequentially read from and write to it, so that records all messages written by the past trackers. In the -th iteration (), Tracker first reads from to update its state by using (10)-(14) (where is replaced by ). Then, an erase vector and a write vector are emitted by:
With the attention weight produced by (12), we then define a write operation, where each feature vector in the memory is modified as:
) is inspired by the Neural Turing Machine[Graves, Wayne, and Danihelka2014, Graves et al.2016]. Since trackers (controllers) interact through the external memory by using interface variables, they do not need to encode messages of other trackers into their working memories (i.e., states), making tracking more efficient.
3.3 Reprioritizing Trackers
Whilst memories are used for tracker interaction, it is hard for high-priority (small ) but low-confidence trackers to associate data correctly. E.g., when the first tracker () is free (), it is very likely for it to associate or, say, ‘steal’ an object from succeeding trackers, since all objects are equally chanced to be associated by a free tracker from the unmodified initial memory .
To avoid this situation, we first update high-confidence trackers so that features corresponding to the tracked objects can be firstly associated and modified. Therefore, we define the priority of Tracker as its previous confidence ranking (in descending order) instead of its index, and then we can update Tracker in the -th iteration to make data association more robust.
3.4 Adapting the Computation Time
Since the object number varies with time and is usually less than the tracker number (assuming is set large enough), iterating over all trackers at every timestep is inefficient. To overcome this, we adapt the idea of Adaptive Computation Time (ACT) [Graves2016] to RAT. At each timestep , we terminate the iteration at Tracker (also disable the write operation) once and , in which case there are unlikely to be more tracked/new objects. While for the remaining trackers, we do no use them to generate outputs. An illustration of the RAT is shown in Fig. 3.
The main purposes of our experiments are: (i) investigating the importance of each component in our model, and (ii) testing whether our model is applicable to real videos. For Purpose (i), we create two synthetic datasets (MNIST-MOT and Sprites-MOT), and consider the following configurations:
TBA with constant computation time, by not using the ACT described in Sec. 3.4.
TBAc without occlusion modeling, by setting the layer number .
TBAc without attention, by reshaping the memory into size , in which case the attention weight degrades to a scalar, i.e., .
TBAc without the tracker reprioritization described in Sec. 3.3.
Our implementation of the ‘Attend, Infer, Repeat’ (AIR) [Eslami et al.2016] for qualitative evaluation, which is a probabilistic generative model that can be used to detect objects from individual images through inference.
Note that it is hard to set a supervised counterpart of our model for online MOT, since calculating the supervised loss with ground truth data itself is an optimization problem which requires to access complete trajectories and thus is usually done offline [Schulter et al.2017]. For Purpose (ii), we evaluate TBA on the challenging DukeMTMC dataset [Ristani et al.2016], and compare it to the state-of-the-art methods.
In this toy task, we aim to test whether our model can robustly handle occlusion and track the pose, shape, and appearance of the object that can appear/disappear from the scene, providing accurate and consistent bounding boxes. Thus, we create a new Sprites-MOT dataset containing 2M frames, where each frame is of size 1281283, consisting of a black background and at most three moving sprites that can occlude each other. Each sprite is randomly scaled from a 21213 image patch with a random shape (circle/triangle/rectangle/diamond) and color (red/green/blue/yellow/magenta/cyan), moves towards a random direction, and appears/disappears only once. To solve this task, for TBA configurations we set the tracker number , layer number , and background .
Training curves are shown in Fig. 5. TBAc-noMem has the highest validation loss, indicating that it cannot well reconstruct the input frames, while other configurations perform similarly and have significantly lower validation losses. However, TBA converges the fastest, which we conjecture benefits from the regularization effect introduced by ACT.
To check the tracking performance, we compare TBA against other configurations on several sampled sequences, as shown in Fig. 4. We can see that TBA consistently performs well on all situations, where in Seq. 1 TBAc perform as well as TBA. However, TBAc-noOcc fails to track objects from occluded patterns (in Frames 4 and 5 of Seq. 2, the red diamond is lost by Tracker 2). We conjecture the reason is that adding occluded pixel values into a single layer can result in high reconstruction errors, and thereby the model just learns to suppress tracker outputs when occlusion occurs. Disrupted tracking frequently occurs on both TBAc-noAtt which does not use attention explicitly (in Sequences 3, trackers frequently change their targets). For TBAc-noMem, all trackers know nothing about each other and compete a same object, resulting in identical tracking with low confidences. For TBAc-noRep, free trackers easily associate the objects tracked by the follow-up trackers. Since AIR does not consider the temporal dependency of sequence data, it fails to track objects across different timesteps.
We further quantitatively evaluate different configurations using the standard CLEAR MOT metrics (Multi-Object Tracking Accuracy (MOTA), Multi-Object Tracking Precision (MOTP), etc.) [Bernardin and Stiefelhagen2008] that count how often the tracker makes incorrect decisions, and the recently proposed ID metrics (Identification F-measure (IDF1), Identification Precision (IDP), and Identification Recall (IDR)) [Ristani et al.2016] that measure how long the tracker correctly tracks targets. Note that we only consider tracker outputs with confidences and convert the corresponding poses into object bounding boxes for evaluation. Table 1 reports the tracking performance. Both TBA and TBAc gain good performance and TBA performs slightly better than TBAc. For TBAc-noOcc, it has a significantly higher FN (False Negative) (472), IDS (209), and Fragmentation (Frag) (202), which is consistent with our conjecture from the qualitative results that using a single layer can sometimes suppress tracker outputs. TBAc-noAtt performs poorly on most of the metrics, especially with a very high ID Switch (IDS) of 9313 potentially caused by disrupted tracking. Note that TBAc-noMem has no valid outputs as all tracker confidences are below 0.5. Without tracker reprioritization, TBAc-noRep is less robust than TBA and TBAc, with a higher False Positive (FP) (623), FN (381), and IDS (446) that we conjecture are mainly caused by conflicted tracking.
To test whether our model can be applied to the real applications involving highly complex and time-varying data patterns, we evaluate the full TBA on the challenging DukeMTMC dataset [Ristani et al.2016]. It consists of 8 videos of resolution 10801920, with each split into 50/10/25 minutes long for training/test(hard)/test(easy). The videos are taken from 8 fixed cameras recording movements of people on various places of Duke university campus at 60fps. For TBA configurations, we set the tracker number , layer number , and use the IMBS algorithm [Bloisi and Iocchi2012] to extract the background . Input frames are down-sampled to 10fps and resized to 108192 to ease processing. Since the hard test set contains very different people statistics from the training set, we only evaluate our model on the easy test set.
Fig. 6 shows sampled results. TBA performs robustly under several situations: (i) frequent object appearing/disappearing; (ii) highly-varying range of object numbers, e.g., a single person (Seq. 4) or ten persons (Frame 1 in Seq. 1); (iii) frequent object occlusions, e.g., when people walk towards each other (Seq. 1); (iv) perspective scale changes, e.g., when people walk close to the camera (Seq. 3); (v) frequent shape/appearance changes; (vi) similar shapes/appearances for different objects (Seq. 6).
Quantitative performances are presented in Table 2. We can see that TBA gains an IDF1 of 80.9%, a MOTA of 76.9%, and a MOTP of 76.2%, being very competitive to the state-of-the-art methods in performance. However, unlike these methods, our model is the first one free of any training labels or extracted features.
|DeepCC [Ristani and Tomasi2018]||89.2||91.7||86.7||87.5||77.1||0.05||1,103||29||37,280||94,399||202||753|
|TAREIDMTMC [Jiang et al.2018]||83.8||87.6||80.4||83.3||75.5||0.06||1,051||17||44,691||131,220||383||2,428|
|MYTRACKER [Yoon, Song, and Jeon2018]||80.3||87.3||74.4||78.3||78.4||0.05||914||72||35,580||193,253||406||1,116|
|MTMC_CDSC [Tesfaye et al.2017]||77.0||87.6||68.6||70.9||75.8||0.05||740||110||38,655||268,398||693||4,717|
|PT_BIPCC [Maksai et al.2017]||71.2||84.8||61.4||59.3||78.7||0.09||666||234||68,634||361,589||290||783|
4.3 Visualizing the RAT
To get more insights into how the model works, we visualize the process of RAT on Sprites (see Fig. 7). At time , Tracker is updated in the -th iteration, using its attention weight to read the memory and then write it as . We can see that the memory content (bright region) related to the associated object is attentively erased (becomes dark) by the write operation, thereby preventing the next tracker from reading it again. Note that at time , Tracker 1 is reprioritized with a priority and thus is updated at the 3-rd iteration, and the memory value has not been modified in the 3-rd iteration by Tracker 1 at which the iteration is terminated (since and ).
5 Related Work
Unsupervised Learning for Visual Data Understanding
There are many works focusing on extracting interpretable representations from visual data using unsupervised learning: some attempt to find disentangled factors ([Kulkarni et al.2015, Chen et al.2016, Rolfe2017] for images and [Ondrúška and Posner2016, Karl et al.2017, Greff, van Steenkiste, and Schmidhuber2017, Denton and others2017, Fraccaro et al.2017] for videos), some aim to extract mid-level semantics ([Le Roux et al.2011, Moreno et al.2016, Huang and Murphy2016] for images and [Jojic and Frey2001, Winn and Blake2005, Wulff and Black2014] for videos), while the remaining seek to discover high-level semantics ([Eslami et al.2016, Yan et al.2016, Rezende et al.2016, Stewart and Ermon2017, Wu, Tenenbaum, and Kohli2017] for images and [Watters et al.2017, Wu et al.2017] for videos). However, none of these works deal with MOT tasks. To the best of our knowledge, the proposed method first achieves unsupervised end-to-end learning of MOT.
Data Association for online MOT
In MOT tasks, data association can be either offline [Zhang, Li, and Nevatia2008, Niebles, Han, and Fei-Fei2010, Kuo, Huang, and Nevatia2010, Berclaz et al.2011, Pirsiavash, Ramanan, and Fowlkes2011, Butt and Collins2013, Milan, Roth, and Schindler2014] or online [Turner, Bottone, and Avasarala2014, Bae and Yoon2014, Wu and Nevatia2007], deterministic [Perera et al.2006, Huang, Wu, and Nevatia2008, Xing, Ai, and Lao2009] or probabilistic [Schulz et al.2003, Blackman2004, Khan, Balch, and Dellaert2005, Vo and Ma2006], greedy [Breitenstein et al.2009, Breitenstein et al.2011, Shu et al.2012] or global [Reilly, Idrees, and Shah2010, Kim et al.2012, Qin and Shelton2012]. Since the proposed RAT deals with online MOT and uses soft attention to greedily associate data based on tracker confidence ranking, it belongs to the probabilistic and greedy online methods. However, unlike these traditional methods, RAT is learnable, i.e., the tracker array can learn to generate matching features, evolve tracker states, and modify input features. Moreover, as RAT is not based on TBD and is end-to-end, the feature extractor can also learn to provide discriminative features to ease data association.
We introduced the TBA framework which achieves unsupervised end-to-end learning of MOT tasks. We also introduced the RAT to improve the robustness of data association. We validated our model on different tasks, showing its potential for real applications such as video surveillance. Our future work is to extend the model to handle videos with dynamic backgrounds.
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Appendix A Supplementary Materials for Experiments
a.1 Implementation Details
There are some common model configurations for all tasks. For the defined in (1defined in (14
), we set it as a Gated Recurrent Unit (GRU)[Cho et al.2014]. For the defined in (3
), we set it as a Fully-Connected network (FC), where the ReLU is chosen as its activation function for each hidden layer. For the loss defined in (9), we set . For the model configurations specified to each task, please see in Table 3.
For MNIST-MOT and Sprites-MOT, we split the data into a proportion of 90/5/5 for training/validation/test; for DukeMTMC, we split the provided training data into a proportion of 95/5 for training/validation. For all tasks, the mini-batch size is set to 64 and early stopping is used to terminate training. To train the model, we minimize the averaged loss on the training set w.r.t. all network parameters using Adam [Kingma and Ba2015] with a learning rate of .
|Size of :||[128, 128, 1]||[128, 128, 3]||[108, 192, 3]|
|Size of :||[8, 8, 50]||[8, 8, 20]||[9, 16, 200]|
|Size of :||[28, 28, 1]||[21, 21, 3]||[9, 23, 3]|
|Size of :||200||80||800|
|Coef. of :||[0, 0]||[0.2, 0.2]||[0.4, 0.4]|
|Layer sizes of (FCN)||[128, 128, 1]||(conv 55)||[128, 128, 3]||(conv 55)||[108, 192, 3]||(conv 55)|
|[64, 64, 32]||(conv 33)||[64, 64, 32]||(conv 33)||[108, 192, 32]||(conv 53)|
|[32, 32, 64]||(conv 11)||[32, 32, 64]||(conv 11)||[36, 64, 128]||(conv 53)|
|[16, 16, 128]||(conv 33)||[16, 16, 128]||(conv 33)||[18, 32, 256]||(conv 31)|
|[8, 8, 256]||(conv 11)||[8, 8, 256]||(conv 11)||[9, 16, 512]||(conv 11)|
|[8, 8, 50]||(out)||[8, 8, 20]||(out)||[9, 16, 200]||(out)|
|Layer sizes of (FC)||200||(fc)||80||(fc)||800||(fc)|
|#Parameters||1.21 M||1.02 M||5.65 M|
As a pilot experiment, we focus on testing whether our model can robustly track the position and appearance of each object that can appear/disappear from the scene. Thus, we create a new MNIST-MOT dataset containing 2M frames, where each frame is of size 1281281, consisting of a black background and at most three moving digits. Each digit is a 2828 image patch randomly drawn from the MNIST dataset [LeCun et al.1998], moves towards a random direction, and appears/disappears only once. When digits overlap, pixel values are added and clamped in . To solve this task, for TBA configurations we set the tracker number , layer number , and background , and fix the scale and shape , thereby only compositing a single layer by adding up all transformed appearances. We also clamp the pixel values of the reconstructed frames in for all configurations.
Training curves are shown in Fig. 8. The TBA, TBAc, and TBAc-noRep have similar validation losses which are slightly better than that of TBAc-noAtt. Similar to the results on Sprites-MOT, TBA converges the fastest, and TBAc-noMem has a significantly higher validation loss as all trackers are likely to focus on a single object, which affects the reconstruction.
Qualitative results are shown in Fig. 9. Similar phenomena are observed as in MNIST-MOT, revealing the importance of the disabled mechanisms. Specifically, as temporal dependency is not considered in AIR, overlapped objects are failed to be disambiguated (Seq. 5).
We further quantitatively evaluate different configurations. Results are reported in Table 4, which are similar to those of the Sprites-MOT.