NATURAL VIDEO SEQUENCE PREDICTION
We propose a deep neural network for the prediction of future frames in natural video sequences. To effectively handle complex evolution of pixels in videos, we propose to decompose the motion and content, two key components generating dynamics in videos. Our model is built upon the Encoder-Decoder Convolutional Neural Network and Convolutional LSTM for pixel-level prediction, which independently capture the spatial layout of an image and the corresponding temporal dynamics. By independently modeling motion and content, predicting the next frame reduces to converting the extracted content features into the next frame content by the identified motion features, which simplifies the task of prediction. Our model is end-to-end trainable over multiple time steps, and naturally learns to decompose motion and content without separate training. We evaluate the proposed network architecture on human activity videos using KTH, Weizmann action, and UCF-101 datasets. We show state-of-the-art performance in comparison to recent approaches. To the best of our knowledge, this is the first end-to-end trainable network architecture with motion and content separation to model the spatio-temporal dynamics for pixel-level future prediction in natural videos.
1 INTRODUCTION
Understanding videos has been one of the most important tasks in the field of computer vision. Compared to still images, the temporal component of videos provides much richer descriptions of the visual world, such as interaction between objects, human activities, and so on. Amongst the various tasks applicable on videos, the task of anticipating the future has recently received increased attention in the research community. Most prior works in this direction focus on predicting high-level semantics in a video such as action (Vondrick et al., 2015; Ryoo, 2011; Lan et al., 2014), event (Yuen and Torralba, 2010; Hoai and Torre, 2013) and motion (Pintea et al., 2014; Walker et al., 2014; Pickup et al., 2014; Walker et al., 2016). Forecasting semantics provides information about what will happen in a video, and is essential to automate decision making. However, the predicted semantics are often specific to a particular task and provide only a partial description of the future. Also, training such models often requires heavily labeled training data which leads to tremendous annotation costs especially with videos.
In this work, we aim to address the problem of prediction of future frames in natural video sequences. Pixel-level predictions provide dense and direct description of the visual world, and existing video recognition models can be adopted on top of the predicted frames to infer various semantics of the future. Spatio-temporal correlations in videos provide a self-supervision for frame prediction, which enables purely unsupervised training of a model by observing raw video frames. Unfortunately, estimating frames is an extremely challenging task; not only because of the inherent uncertainty of the future, but also various factors of variation in videos leading to complicated dynamics in raw pixel values. There have been a number of recent attempts on frame prediction (Srivastava et al., 2015; Mathieu et al., 2015; Oh et al., 2015; Goroshin et al., 2015; Lotter et al., 2015; Ranzato et al., 2014),
∗This work was done while SH and XL were visiting the University of Michigan.
which use a single encoder that needs to reason about all the different variations occurring in videos in order to make predictions of the future, or require extra information like foreground-background segmentation masks and static background (Vondrick et al., 2016).
We propose a Motion-Content Network (MCnet) for robust future frame prediction. Our intuition is to split the inputs for video prediction into two easily identifiable groups, motion and content, and independently capture each information stream with separate encoder pathways. In this architecture, the motion pathway encodes the local dynamics of spatial regions, while the content pathway encodes the spatial layout of the salient parts of an image. The prediction of the future frame is then achieved by transforming the content of the last observed frame given the identified dynamics up to the last observation. Somewhat surprisingly, we show that such a network is end-to-end trainable without individual path way supervision. Specifically, we show that an asymmetric architecture for the two pathways enables such decompositions without explicit supervision. The contributions of this paper are summarized below:
• We propose MCnet for the task of frame prediction, which separates the information streams (motion and content) into different encoder pathways.
• The proposed network is end-to-end trainable and naturally learns to decompose motion and content without separate training, and reduces the task of frame prediction to transforming the last observed frame into the next by the observed motion.
• We evaluate the proposed model on challenging real-world video datasets, and show that it outperforms previous approaches on frame prediction.
The rest of the paper is organized as follows. We briefly review related work in Section 2, and introduce an overview of the proposed algorithm in Section 3. The detailed configuration of the proposed network is described in Section 4. Section 5 describes training and inference procedure. Section 6 illustrates implementation details and experimental results on challenging benchmarks.
2 RELATED WORK
The problem of visual future prediction has received growing interests in the computer vision community. It has led to various tasks depending on the objective of future prediction, such as human activity (Vondrick et al., 2015; Ryoo, 2011; Lan et al., 2014), event (Yuen and Torralba, 2010; Hoai and Torre, 2013) and geometric path (Walker et al., 2014). Although previous work achieved reasonable success in specific tasks, they are often limited to estimating predefined semantics, and require fully-labeled training data. To alleviate this issue, approaches predicting representation of the future beyond semantic labels have been proposed. Walker et al. (2014) proposed a data-driven approach to predict the motion of a moving object, and coarse hallucination of the predicted motion. Vondrick et al. (2015) proposed a deep regression network to predict feature representations of the future frames. These approaches are supervised and provide coarse predictions of how the future will look like. Our work also focuses on unsupervised learning for prediction of the future, but to a more direct visual prediction task: frame prediction.
Compared to predicting semantics, pixel-level prediction has been less investigated due to the difficulties in modeling evolution of raw pixels over time. Fortunately, recent advances in deep learning provide a powerful tool for sequence modeling, and enable the creation of novel architectures for modeling complex sequential data. Ranzato et al. (2014) applied a recurrent neural network developed for language modeling to frame prediction by posing the task as classification of each image region to one of quantized patch dictionaries. Srivastava et al. (2015) applied a sequence-tosequence model to video prediction, and showed that Long Short-Term Memory (LSTM) is able to capture pixel dynamics. Oh et al. (2015) proposed an action-conditional encoder-decoder network to predict future frames in Atari games. In addition to the different choices of architecture, some other works addressed the importance of selecting right objective function: Lotter et al. (2015) used adversarial loss with combined CNN and LSTM architectures, and Mathieu et al. (2015) employed similar adversarial loss with additional regularization using a multi-scale encoder-decoder network. Finn et al. (2016) constructed a network that predicts transformations on the input pixels for next frame prediction. Patraucean et al. (2015) proposed a network that by explicitly predicting optical flow features is able to predict the next frame in a video. Vondrick et al. (2016) proposed a generative adversarial network for video which, by generating a background-foreground mask, is able to generate
realistic-looking video sequences. However, none of the previously mentioned approaches exploit spatial and temporal information separately in an unsupervised fashion. In terms of the way data is observed, the closest work to ours is Xue et al. (2016). The differences are (1) Our model is deterministic and theirs is probabilistic, (2) our motion encoder is based on convolutional LSTM (Shi et al., 2015) which is a more natural module to model long-term dynamics, (3) our content encoder observes a single scale input and theirs observes many scales, and (4) we directly generate image pixels values, which is a more complicated task. We aim to exploit the existing spatio-temporal correlations in videos by decomposing the motion and content in our network architecture.
To the best of our knowledge, the idea of separating motion and content has not been investigated in the task of unsupervised deterministic frame prediction. The proposed architecture shares similarities to the two-stream CNN (Simonyan and Zisserman, 2014), which is designed for action recognition to jointly exploit the information from frames and their temporal dynamics. However, in contrast to their network we aim to learn features for temporal dynamics directly from the raw pixels, and we use the identified features from the motion in combination with spatial features to make pixel-level predictions of the future.
3 ALGORITHM OVERVIEW
In this section, we formally define the task of frame prediction and the role of each component in the proposed architecture. Let xt ∈ Rw×h×c denote the t-th frame in an input video x, where w, h, and c denote width, height, and number of channels, respectively. The objective of frame prediction is to generate the future frame x̂t+1 given the input frames x1:t.
At the t-th time step, our network observes a history of previous consecutive frames up to frame t, and generates the prediction of the next frame x̂t+1 as follows:
• Motion Encoder recurrently takes an image difference input between frame xt and xt−1 starting from t = 2, and produces the hidden representation dt encoding the temporal dynamics of the scene components (Section 4.1). • Content Encoder takes the last observed frame xt as an input, and outputs the hidden
representation st that encodes the spatial layout of the scene (Section 4.2). • Multi-Scale Motion-Content Residual takes the computed features, from both the motion
and content encoders, at every scale right before pooling and computes residuals rt (He et al., 2015) to aid the information loss caused by pooling in the encoding phase (Section 4.3). • Combination Layers and Decoder takes the outputs from both encoder pathways and
residual connections, dt, st, and rt, and combines them to produce a pixel-level prediction of the next frame x̂t+1 (Section 4.4).
The overall architecture of the proposed algorithm is described in Figure 1. The prediction of multiple frames, x̂t+1:t+T , can be achieved by recursively performing the above procedures over T time steps (Section 5). Each component in the proposed architecture is described in the following section.
4 ARCHITECTURE
This section describes the detailed configuration of the proposed architecture, including the two encoder pathways, multi-scale residual connections, combination layers, and decoder.
4.1 MOTION ENCODER
The motion encoder captures the temporal dynamics of the scene’s components by recurrently observing subsequent difference images computed from xt−1 and xt, and outputs motion features by
[dt, ct] = f dyn (xt − xt−1,dt−1, ct−1) , (1)
where xt−xt−1 denotes element-wise subtraction between frames at time t and t−1, dt ∈ Rw ′×h′×c′ is the feature tensor encoding the motion across the observed difference image inputs, and ct ∈ Rw′×h′×c′ is a memory cell that retains information of the dynamics observed through time. f dyn is implemented in a fully-convolutional way to allow our model to identify local dynamics of frames
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rather than complicated global motion. For this, we use an encoder CNN with a Convolutional LSTM (Shi et al., 2015) layer on top.
4.2 CONTENT ENCODER
The content encoder extracts important spatial features from a single frame, such as the spatial layout of the scene and salient objects in a video. Specifically, it takes the last observed frame xt as an input, and produces content features by
st = f cont (xt) , (2)
where st ∈ Rw ′×h′×c′ is the feature encoding the spatial content of the last observed frame, and f cont is implemented by a Convolutional Neural Network (CNN) that specializes on extracting features from single frame.
It is important to note that our model employs an asymmetric architecture for the motion and content encoder. The content encoder takes the last observed frame, which keeps the most critical clue to reconstruct spatial layout of near future, but has no information about dynamics. On the other hand, the motion encoder takes a history of previous image differences, which are less informative about the future spatial layout compared to the last observed frame, yet contain important spatio-temporal variations occurring over time. This asymmetric architecture encourages encoders to exploit each of two pieces of critical information to predict the future content and motion individually, and enables the model to learn motion and content decomposition naturally without any supervision.
4.3 MULTI-SCALE MOTION-CONTENT RESIDUAL
To prevent information loss after the pooling operations in our motion and content encoders, we use residual connections (He et al., 2015). The residual connections in our network communicate motion-content features at every scale into the decoder layers after unpooling operations. The residual feature at layer l is computed by
rlt = f res ([slt,dlt])l , (3)
where rlt is the residual output at layer l, [ slt,d l t ] is the concatenation of the motion and content features along the depth dimension at layer l of their respective encoders, f res (.)l the residual function at layer l implemented as consecutive convolution layers and rectification with a final linear layer.
4.4 COMBINATION LAYERS AND DECODER
The outputs from the two encoder pathways, dt and st, encode a high-level representation of motion and content, respectively. Given these representations, the objective of the decoder is to generate a
pixel-level prediction of the next frame x̂t+1 ∈ Rw×h×c. To this end, it first combines the motion and content back into a unified representation by
ft = g comb ([dt, st]) , (4)
where [dt, st] ∈ Rw ′×h′×2c′ denotes the concatenation of the higher-level motion and content features in the depth dimension, and ft ∈ Rw ′×h′×c′ denotes the combined high-level representation of motion and content. gcomb is implemented by a CNN with bottleneck layers (Hinton and Salakhutdinov, 2006); it first projects both dt and st into a lower-dimensional embedding space, and then puts it back to the original size to construct the combined feature ft. Intuitively, ft can be viewed as the content feature of the next time step, st+1, which is generated by transforming st using the observed dynamics encoded in dt. Then our decoder places ft back into the original pixel space by
x̂t+1 = g dec (ft, rt) , (5)
where rt is a list containing the residual connections from every layer of the motion and content encoders before pooling sent to every layer of the decoder after unpooling. We employ the deconvolution network (Zeiler et al., 2011) for our decoder network gdec, which is composed of multiple successive operations of deconvolution, rectification and unpooling with the addition of the motioncontent residual connections after each unpooling operation. The output layer is passed through a tanh (.) activation function. Unpooling with fixed switches are used to upsample the intermediate activation maps.
5 INFERENCE AND TRAINING
Section 4 describes the procedures for single frame prediction, while this section presents the extension of our algorithm for the prediction of multiple time steps.
5.1 MULTI-STEP PREDICTION
Given an input video, our network observes the first n frames as image difference between frame xt and xt−1, starting from t = 2 up to t = n, to encode initial temporal dynamics through the motion encoder. The last frame xn is given to the content encoder to be transformed into the first prediction x̂t+1 by the identified motion features.
For each time step t ∈ [n+ 1, n+ T ], where T is the desired number of prediction steps, our network takes the difference image between the first prediction x̂t+1 and the previous image xt, and the first prediction x̂t+1 itself to predict the next frame x̂t+2, and so forth.
5.2 TRAINING OBJECTIVE
To train our network, we use an objective function composed of different sub-losses similar to Mathieu et al. (2015). Given the training data D = {x(i)1,...,T }Ni=1, our model is trained to minimize the prediction loss by L = αLimg + βLGAN, (6) where α and β are hyper-parameters that control the effect of each sub-loss during optimization. Limg is the loss in image space from Mathieu et al. (2015) defined by
Limg = Lp (xt+k, x̂t+k) + Lgdl (xt+k, x̂t+k) , (7)
where Lp (y, z) = T∑ k=1 ||y − z||pp, (8)
Lgdl (y, z) = h,w∑ i,j | (|yi,j − yi−1,j | − |zi,j − zi−1,j |) |λ (9)
+ | (|yi,j−1 − yi,j | − |zi,j−1 − zi,j |) |λ . Here, xt+k and x̂t+k are the target and predicted frames, respectively, and p and λ are hyperparameters forLp andLgdl, respectively. Intuitively,Lp guides our network to match the average pixel
values directly, while Lgdl guides our network to match the gradients of such pixel values. Overall, Limg guides our network to learn parameters towards generating the correct average sequence given the input. Training to generate average sequences, however, results in somewhat blurry generations which is the reason we use an additional sub-loss. LGAN is the generator loss in adversarial training to allow our model to predict realistic looking frames and it is defined by
LGAN = − logD ([x1:t, G (x1:t)]) , (10) where x1:t is the concatenation of the input images, xt+1:t+T is the concatenation of the ground-truth future images, G (x1:t) = x̂t+1:t+T is the concatenation of all predicted images along the depth dimension, andD (.) is the discriminator in adversarial training. The discriminative loss in adversarial training is defined by
Ldisc = − logD ([x1:t,xt+1:t+T ])− log (1−D ([x1:t, G (x1:t)])) . (11) LGAN, in addition to Limg, allows our network to not only generate the target sequence, but also simultaneously enforce realism in the images through visual sharpness that fools the human eye. Note that our model uses its predictions as input for the next time-step during the training, which enables the gradients to flow through time and makes the network robust for error propagation during prediction. For more a detailed description about adversarial training, please refer to Appendix D.
6 EXPERIMENTS
In this section, we present experiments using our network for video generation. We first evaluate our network, MCnet, on the KTH (Schuldt et al., 2004) and Weizmann action (Gorelick et al., 2007) datasets, and compare against a baseline convolutional LSTM (ConvLSTM) (Shi et al., 2015). We then proceed to evaluate on the more challenging UCF-101 (Soomro et al., 2012) dataset, in which we compare against the same ConvLSTM baseline and also the current state-of-the-art method by Mathieu et al. (2015). For all our experiments, we use α = 1, λ = 1, and p = 2 in the loss functions.
In addition to the results in this section, we also provide more qualitative comparisons in the supplementary material and in the videos on the project website: https://sites.google. com/a/umich.edu/rubenevillegas/iclr2017.
Architectures. The content encoder of MCnet is built with the same architecture as VGG16 (Simonyan and Zisserman, 2015) up to the third pooling layer. The motion encoder of MCnet is also similar to VGG16 up to the third pooling layer, except that we replace its consecutive 3x3 convolutions with single 5x5, 5x5, and 7x7 convolutions in each layer. The combination layers are composed of 3 consecutive 3x3 convolutions (256, 128, and 256 channels in each layer). The multi-scale residuals are composed of 2 consecutive 3x3 convolutions. The decoder is the mirrored architecture of the content encoder where we perform unpooling followed by deconvolution. For the baseline ConvLSTM, we use the same architecture as the motion encoder, residual connections, and decoder, except we increase the number of channels in the encoder in order to have an overall comparable number of parameters with MCnet.
6.1 KTH AND WEIZMANN ACTION DATASETS
Experimental settings. The KTH human action dataset (Schuldt et al., 2004) contains 6 categories of periodic motions on a simple background: running, jogging, walking, boxing, hand-clapping and hand-waiving. We use person 1-16 for training and 17-25 for testing, and also resize frames to 128x128 pixels. We train our network and baseline by observing 10 frames and predicting 10 frames into the future on the KTH dataset. We set β = 0.02 for training. We also select the walking, running, one-hand waving, and two-hands waving sequences from the Weizmann action dataset (Gorelick et al., 2007) for testing the networks’ generalizability.
For all the experiments, we test the networks on predicting 20 time steps into the future. As for evaluation, we use the same SSIM and PSNR metrics as in Mathieu et al. (2015). The evaluation on KTH was performed on sub-clips within each video in the testset. We sample sub-clips every 3 frames for running and jogging, and sample sub-clips every 20 frames (skipping the frames we have already predicted) for walking, boxing, hand-clapping, and hand-waving. Sub-clips for running, jogging, and walking were manually trimmed to ensure humans are always present in the frames. The evaluation on Weizmann was performed on all sub-clips in the selected sequences.
Results. Figure 2 summarizes the quantitative comparisons among our MCnet, ConvLSTM baseline and their residual variations. In the KTH test set, our network outperforms the ConvLSTM baseline by a small margin. However, when we test the residual versions of MCnet and ConvLSTM on the dataset (Gorelick et al., 2007) with similar motions, we can see that our network can generalize well to the unseen contents by showing clear improvements, especially in long-term prediction. One reason for this result is that the test and training partitions of the KTH dataset have simple and similar image contents so that ConvLSTM can memorize the average background and human appearance to make reasonable predictions. However, when tested on unseen data, ConvLSTM has to internally take care of both scene dynamics and image contents in a mingled representation, which gives it a hard time for generalization. In contrast, the reason our network outperforms the ConvLSTM baseline on unseen data is that our network focuses on identifying general motion features and applying them to a learned content representation.
Figure 3 presents qualitative results of multi-step prediction by our network and ConvLSTM. As expected, prediction results by our full architecture preserves human shapes more accurately than the baseline. It is worth noticing that our network produces very sharp prediction over long-term time steps; it shows that MCnet is able to capture periodic motion cycles, which reduces the uncertainty of future prediction significantly. More qualitative comparisons are shown in the supplementary material and the project website.
6.2 UCF-101 DATASET
Experimental settings. This section presents results on the challenging real-world videos in the UCF-101 (Soomro et al., 2012) dataset. Having collected from YouTube, the dataset contains 101 realistic human actions taken in a wild and exhibits various challenges, such as background clutter, occlusion, and complicated motion. We employed the same network architecture as in the KTH dataset, but resized frames to 240x320 pixels, and trained the network to observe 4 frames and predict a single frame. We set β = 0.001 for training. We also trained our convolutional LSTM baseline in the same way. Following the same protocol as Mathieu et al. (2015) for data pre-processing and
evaluation metrics on full images, all networks were trained on Sports-1M (Karpathy et al., 2014) dataset and tested on UCF-101 unless otherwise stated.1
Results. Figure 4 shows the quantitative comparisons between our network trained for single-stepprediction and Mathieu et al. (2015). We can clearly see the advantage of our network over the baseline. The separation of motion and contents in two encoder pathways allows our network to identify key motion and content features, which are then fed into the decoder to yield predictions of higher quality compared to the baseline.2 In other words, our network only moves what shows motion in the past, and leaves the rest untouched. We also trained a residual version of MCnet on UCF-101, indicated by “MCnet + RES UCF101", to compare how well our model generalizes when trained and tested on the same or different dataset(s). To our surprise, when tested with UCF-101, the MCnet trained on Sports-1M (MCnet + RES) roughly matches the performance of the MCnet trained on UCF-101 (MCnet + RES UCF101), which suggests that our model learns effective representations which can generalize to new datasets. Figure 5 presents qualitative comparisons between frames generated by our network and Mathieu et al. (2015). Since the ConvLSTM and Mathieu et al. (2015) lack explicit motion and content modules, they lose sense of the dynamics in the video and therefore the contents become distorted quickly. More qualitative comparisons are shown in the supplementary material and the project website.
1We use the code and model released by Mathieu et al. (2015) at https://github.com/coupriec/ VideoPredictionICLR2016
2We were not able to get the model fine-tuned on UCF-101 from the authors so it is not included in Figure 4
7 CONCLUSION
We proposed a motion-content network for pixel-level prediction of future frames in natural video sequences. The proposed model employs two separate encoding pathways, and learns to decompose motion and content without explicit constraints or separate training. Experimental results suggest that separate modeling of motion and content improves the quality of the pixel-level future prediction, and our model overall achieves state-of-the-art performance in predicting future frames in challenging real-world video datasets.
8 ACKNOWLEDGEMENTS
This work was supported in part by ONR N00014-13-1-0762, NSF CAREER IIS-1453651, gifts from the Bosch Research and Technology Center, and Sloan Research Fellowship. We also thank NVIDIA for donating K40c and TITAN X GPUs. We thank Ye Liu, Junhyuk Oh, Xinchen Yan, Lajanugen Logeswaran, Yuting Zhang, Sungryull Sohn, Kibok Lee, Rui Zhang, and other collaborators for helpful discussions. R. Villegas was partly supported by the Rackham Merit Fellowship.
9 APPENDIX
A QUALITATIVE AND QUANTITATIVE COMPARISON WITH CONSIDERABLE CAMERA MOTION AND ANALYSIS
In this section, we show frame prediction examples in which considerable camera motion occurs. We analyze the effects of camera motion on our best network and the corresponding baselines. First, we analyze qualitative examples on UCF101 (more complicated camera motion) and then on KTH (zoom-in and zoom-out camera effect).
UCF101 Results. As seen in Figure 9 and Figure 10, our model handles foreground and camera motion for a few steps. We hypothesize that for the first few steps, motion signals from images are clear. However, as images are predicted, motion signals start to deteriorate due to prediction errors. When a considerable amount of camera motion is present in image sequences, the motion signals are very dense. As predictions evolve into the future, our motion encoder has to handle large motion deterioration due to prediction errors, which cause motion signals to get easily confused and lost quickly.
KTH Results. We were unable to find videos with background motion in the KTH dataset, but we found videos where the camera is zooming in or out for the actions of boxing, handclapping, and handwaving. In Figure 11, we display qualitative for such videos. Our model is able to predict the zoom change in the cameras, while continuing the action motion. In comparison to the performance observed in UCF101, the background does not change much. Thus, the motion signals are well localized in the foreground motion (human), and do not get confused with the background and lost as quickly.
B EXTENDED QUANTITATIVE EVALUATION
In this section, we show additional quantitative comparison with a baseline based on copying the last observed frame through time for KTH and UCF101 datasets. Copying the last observed frame through time ensures perfect background prediction in videos where most of the motion comes from foreground (i.e. person performing an action). However, if such foreground composes a small part of the video, it will result in high prediction quality score regardless of the simple copying action.
In Figure 12 below, we can see the quantitative comparison in the datasets. Copying the last observed frame through time does a reasonable job in both datasets, however, the impact is larger in UCF101. Videos in the KTH dataset comprise simple background with minimal camera motion, which allows our network to easily predict both foreground and background motion, resulting in better image quality scores. However, videos in UCF101 contain more complicated and diverse background which in combination with camera motion present a much greater challenge to video prediction networks. From the qualitative results in Section A and Figures 5, 8, 9, and 10, we can see that our network performs better in videos that contain isolated areas of motion compared to videos with dense motion. A simple copy/paste operation of the last observed frame, ensures very high prediction scores in videos where very small motion occur. The considerable score boost by videos with small motion causes the simple copy/paste baseline to outperform MCnet in the overall performance on UCF101.
C UCF101 MOTION DISAMBIGUATION EXPERIMENTS
Due to the observed bias from videos with small motion, we perform experiments by measuring the image quality scores on areas of motion. These experiments are similar to the ones performed in Mathieu et al. (2015). We compute DeepFlow optical flow (Weinzaepfel et al., 2013) between the previous and the current groundtruth image of interest, compute the magnitude, and normalize it to [0, 1]. The computed optical flow magnitude is used to mask the pixels where motion was observed. We set the pixels where the optical flow magnitude is less than 0.2, and leave all other pixels untouched in both the groundtruth and predicted images. Additionally, we separate the test videos by the average `2-norm of time difference between target frames. We separate the test videos into deciles based of the computed average `2-norms, and compute image quality on each decile. Intuitively, the 1st decile contains videos with the least overall of motion (i.e. frames that show the smallest change over time), and the 10th decile contains videos with the most overall motion (i.e. frames that show the largest change over time).
As shown in Figure 13, when we only evaluate on pixels where rough motion is observed, MCnet reflects higher PSNR and SSIM, and clearly outperforms all the baselines in terms of SSIM. The SSIM results show that our network is able to predict a structure (i.e. textures, edges, etc) similar to the grountruth images within the areas of motion. The PSNR results, however, show that our method outperforms the simple copy/paste baseline for the first few steps, but then our method performs slightly worse. The discrepancies observed between PSNR and SSIM scores could be due to the fact that some of the predicted images may not reflect the exact pixel values of the groundtruth regardless of the structures being similar. SSIM scores are known to take into consideration features in the image that go beyond directly matching pixel values, reflecting more accurately how humans perceived image quality.
Figures 15 and 14 show the evaluation by separating the test videos into deciles based on the average `2-norm of time difference between target frames. From this evaluation, it is proven that the copy last frame baseline scores higher in videos where motion is the smallest. The first few deciles (videos with small motion) show that our network is not just copying the last observed frame through time, otherwise it would perform similarly to the copy last frame baseline. The last deciles (videos with large motion) show our network outperforming all the baselines, including the copy last frame baseline, effectively confirming that our network does predict motion similar to the motion observed in the video.
D ADVERSARIAL TRAINING
Mathieu et al. (2015) proposed an adversarial training for frame prediction. Inspired by Goodfellow et al. (2014), they proposed a training procedure that involves a generative model G and a discriminative model D. The two models compete in a two-player minimax game. The discriminator D is optimized to correctly classify its inputs as either coming from the training data (real frame sequence) or from the generator G (synthetic frame sequence). The generator G is optimized to generate frames that fool the discriminator into believing that they come from the training data. At training time, D takes the concatenation of the input frames that go into G and the images produced by G. The adversarial training objective is defined as follows:
min G max D
logD ([x1:t,xt+1:t+T ]) + log (1−D ([x1:t, G (x1:t)])) ,
where [., .] denotes concatenation in the depth dimension, x1:t denotes the input frames toG, xt+1:t+T are the target frames, andG (x1:t) = x̂t+1:t+T are the frames predicted byG. In practice, we split the minimax objective into two separate, but equivalent, objectives: LGAN and Ldisc. During optimization, we minimize the adversarial objective alternating between LGAN and Ldisc. LGAN is defined by
LGAN = − logD ([x1:t, G (x1:t)]) ,
where we optimize the parameters of G to minimize LGAN while the parameters of D stay untouched. As a result, G is optimized to generate images that make D believe that they come from the training data. Thus, the generated images look sharper, and more realistic. Ldisc is defined by
Ldisc = − logD ([x1:t,xt+1:t+T ])− log (1−D ([x1:t, G (x1:t)])) ,
where we optimize the parameters of D to minimize Ldisc, while the parameters of G stay untouched. D tells us whether its input came from the training data or the generator G. Alternating between the two objectives, causes G to generate very realistic images, and D not being able to distinguish between generated frames and frames from the training data.
1) Summary
This paper investigates the usefulness of decoupling appearance and motion information for the problem of future frame prediction in natural videos. The method introduces a novel two-stream encoder-decoder architecture, MCNet, consisting of two separate encoders -- a convnet on single frames and a convnet+LSTM on sequences of temporal differences -- followed by combination layers (stacking + convolutions) and a deconvolutional network decoder leveraging also residual connections from the two encoders. The architecture is trained end-to-end using the objective and adversarial training strategy of Mathieu et al.
2) Contributions
+ The architecture seems novel and is well motivated. It is also somewhat related to the two-stream networks of Simonyan & Zisserman, which are very effective for real-world action recognition.
+ The qualitative results are numerous, insightful, and very convincing (including quantitatively) on KTH & Weizmann, showing the benefits of decoupling content and motion for simple scenes with periodic motions, as well as the need for residual connections.
3) Suggestions for improvement
Static dataset bias:
In response to the pre-review concerns about the observed static nature of the qualitative results, the authors added a simple baseline consisting in copying the pixels of the last observed frame. On the one hand, the updated experiments on KTH confirm the good results of the method in these conditions. On the other hand, the fact that this baseline is better than all other methods (not just the authors's) on UCF101 casts some doubts on whether reporting average statistics on UCF101 is insightful enough. Although the authors provide some qualitative analysis pertaining to the quantity of motion, further quantitative analysis seems necessary to validate the performance of this and other methods on future frame prediction. At least, the results on UCF101 should be disambiguated with respect to the type of scene, for instance by measuring the overall quantity of motion (e.g., l2 norm of time differences) and reporting PSNR and SSIM per quartile / decile. Ideally, other realistic datasets than UCF101 should be considered in complement. For instance, the Hollywood 2 dataset of Marszalek et al would be a good candidate, as it focuses on movies and often contains complex actor, camera, and background motions that would make the "pixel-copying" baseline very poor. Experiments on video datasets beyond actions, like the KITTI tracking benchmark, would also greatly improve the paper.
Additional recognition experiments:
As mentioned in pre-review questions, further UCF-101 experiments on action recognition tasks by fine-tuning would also greatly improve the paper. Classifying videos indeed requires learning both appearance and motion features, and the two-stream encoder + combination layers of the MCNet+Res architecture seem particularly adapted, if they indeed allowed for unsupervised pre-trainining of content and motion representations, as postulated by the authors. These experiments would also contribute to dispelling the aforementioned concerns about the static nature of the learned representations.
4) Conclusion
Overall, this paper proposes an interesting architecture for an important problem, but requires additional experiments to substantiate the claims made by the authors. If the authors make the aforementioned additional experiments and the results are convincing, then this paper would be clearly relevant for ICLR.
5) Post-rebuttal final decision
The authors did a significant amount of additional work, following the suggestions made by the reviewers, and providing additional compelling experimental evidence. This makes this one of the most experimentally thorough ones for this problem. I, therefore, increase my rating, and suggest to accept this paper. Good job!