* They use an implementation of Q-learning (i.e. reinforcement learning) with CNNs to automatically play Atari games.
  * The algorithm receives the raw pixels as its input and has to choose buttons to press as its output. No hand-engineered features are used. So the model "sees" the game and "uses" the controller, just like a human player would.
  * The model achieves good results on various games, beating all previous techniques and sometimes even surpassing human players.

### How
  * Deep Q Learning
    * *This is yet another explanation of deep Q learning, see also [this blog post](http://www.nervanasys.com/demystifying-deep-reinforcement-learning/) for longer explanation.*
    * While playing, sequences of the form (`state1`, `action1`, `reward`, `state2`) are generated.
      * `state1` is the current game state. The agent only sees the pixels of that state. (Example: Screen shows enemy.)
      * `action1` is an action that the agent chooses. (Example: Shoot!)
      * `reward` is the direct reward received for picking `action1` in `state1`. (Example: +1 for a kill.)
      * `state2` is the next game state, after the action was chosen in `state1`. (Example: Screen shows dead enemy.)
    * One can pick actions at random for some time to generate lots of such tuples. That leads to a replay memory.
    * Direct reward
      * After playing randomly for some time, one can train a model to predict the direct reward given a screen (we don't want to use the whole state, just the pixels) and an action, i.e. `Q(screen, action) -> direct reward`.
      * That function would need a forward pass for each possible action that we could take. So for e.g. 8 buttons that would be 8 forward passes. To make things more efficient, we can let the model directly predict the direct reward for each available action, e.g. for 3 buttons `Q(screen) -> (direct reward of action1, direct reward of action2, direct reward of action3)`.
      * We can then sample examples from our replay memory. The input per example is the screen. The output is the reward as a tuple. E.g. if we picked button 1 of 3 in one example and received a reward of +1 then our output/label for that example would be `(1, 0, 0)`.
      * We can then train the model by playing completely randomly for some time, then sample some batches and train using a mean squared error. Then play a bit less randomly, i.e. start to use the action which the network thinks would generate the highest reward. Then train again, and so on.
    * Indirect reward
      * Doing the previous steps, the model will learn to anticipate the *direct* reward correctly. However, we also want it to predict indirect rewards. Otherwise, the model e.g. would never learn to shoot rockets at enemies, because the reward from killing an enemy would come many frames later.
      * To learn the indirect reward, one simply adds the reward value of highest reward action according to `Q(state2)` to the direct reward.
      * I.e. if we have a tuple (`state1`, `action1`, `reward`, `state2`), we would not add (`state1`, `action1`, `reward`) to the replay memory, but instead (`state1`, `action1`, `reward + highestReward(Q(screen2))`). (Where `highestReward()` returns the reward of the action with the highest reward according to Q().)
      * By training to predict `reward + highestReward(Q(screen2))` the network learns to anticipate the direct reward *and* the indirect reward. It takes a leap of faith to accept that this will ever converge to a good solution, but it does.
      * We then add `gamma` to the equation: `reward + gamma*highestReward(Q(screen2))`. `gamma` may be set to 0.9. It is a discount factor that devalues future states, e.g. because the world is not deterministic and therefore we can't exactly predict what's going to happen. Note that Q will automatically learn to stack it, e.g. `state3` will be discounted to `gamma^2` at `state1`.
  * This paper
    * They use the mentioned Deep Q Learning to train their model Q.
    * They use a k-th frame technique, i.e. they let the model decide upon an action at (here) every 4th frame.
    * Q is implemented via a neural net. It receives 84x84x4 grayscale pixels that show the game and projects that onto the rewards of 4 to 18 actions.
    * The input is HxWx4 because they actually feed the last 4 frames into the network, instead of just 1 frame. So the network knows more about what things are moving how.
    * The network architecture is:
      * 84x84x4 (input)
      * 16 convs, 8x8, stride 4, ReLU
      * 32 convs, 4x4, stride 2, ReLU
      * 256 fully connected neurons, ReLU
      * <N_actions> fully connected neurons, linear
    * They use a replay memory of 1 million frames.

### Results
  * They ran experiments on the Atari games Beam Rider, Breakout, Enduro, Pong, Qbert, Seaquest and Space Invaders.
  * Same architecture and hyperparameters for all games.
  * Rewards were based on score changes in the games, i.e. they used +1 (score increases) and -1 (score decreased).
  * Optimizer: RMSProp, Batch Size: 32.
  * Trained for 10 million examples/frames per game.
  * They had no problems with instability and their average Q value per game increased smoothly.
  * Their method beats all other state of the art methods.
  * They managed to beat a human player in games that required not so much "long" term strategies (the less frames the better).
  * Video: starts at 46:05. 
 https://youtu.be/dV80NAlEins?t=46m05s 


![Algorithm](https://raw.githubusercontent.com/aleju/papers/master/neural-nets/images/Playing_Atari_with_Deep_Reinforcement_Learning__algorithm.png?raw=true "Algorithm")

*The original full algorithm, as shown in the paper.*

--------------------

### Rough chapter-wise notes

* (1) Introduction
  * Problems when using neural nets in reinforcement learning (RL):
    * Reward signal is often sparse, noise and delayed.
    * Often assumption that data samples are independent, while they are correlated in RL.
    * Data distribution can change when the algorithm learns new behaviours.
  * They use Q-learning with a CNN and stochastic gradient descent.
  * They use an experience replay mechanism (i.e. memory) from which they can sample previous transitions (for training).
  * They apply their method to Atari 2600 games in the Arcade Learning Environment (ALE).
  * They use only the visible pixels as input to the network, i.e. no manual feature extraction.

* (2) Background
  * blablabla, standard deep q learning explanation

* (3) Related Work
  * TD-Backgammon: "Solved" backgammon. Worked similarly to Q-learning and used a multi-layer perceptron.
  * Attempts to copy TD-Backgammon to other games failed.
  * Research was focused on linear function approximators as there were problems with non-linear ones diverging.
  * Recently again interest in using neural nets for reinforcement learning. Some attempts to fix divergence problems with gradient temporal-difference methods.
  * NFQ is a very similar method (to the one in this paper), but worked on the whole batch instead of minibatches, making it slow. It also first applied dimensionality reduction via autoencoders on the images instead of training on them end-to-end.
  * HyperNEAT was applied to Atari games and evolved a neural net for each game. The networks learned to exploit design flaws.

* (4) Deep Reinforcement Learning
  * They want to connect a reinforcement learning algorithm with a deep neural network, e.g. to get rid of handcrafted features.
  * The network is supposes to run on the raw RGB images.
  * They use experience replay, i.e. store tuples of (pixels, chosen action, received reward) in a memory and use that during training.
  * They use Q-learning.
  * They use an epsilon-greedy policy.
  * Advantages from using experience replay instead of learning "live" during game playing:
    * Experiences can be reused many times (more efficient).
    * Samples are less correlated.
    * Learned parameters from one batch don't determine as much the distributions of the examples in the next batch.
  * They save the last N experiences and sample uniformly from them during training.
  * (4.1) Preprocessing and Model Architecture
    * Raw Atari images are 210x160 pixels with 128 possible colors.
    * They downsample them to 110x84 pixels and then crop the 84x84 playing area out of them.
    * They also convert the images to grayscale.
    * They use the last 4 frames as input and stack them.
    * So their network input has shape 84x84x4.
    * They use one output neuron per possible action. So they can compute the Q-value (expected reward) of each action with one forward pass.
    * Architecture: 84x84x4 (input) => 16 8x8 convs, stride 4, ReLU => 32 4x4 convs stride 2 ReLU => fc 256, ReLU => fc N actions, linear
    * 4 to 18 actions/outputs (depends on the game).
    * Aside from the outputs, the architecture is the same for all games.

* (5) Experiments
  * Games that they played: Beam Rider, Breakout, Enduro, Pong, Qbert, Seaquest, Space Invaders
  * They use the same architecture und hyperparameters for all games.
  * They give a reward of +1 whenever the in-game score increases and -1 whenever it decreases.
  * They use RMSProp.
  * Mini batch size was 32.
  * They train for 10 million frames/examples.
  * They initialize epsilon (in their epsilon greedy strategy) to 1.0 and decrease it linearly to 0.1 at one million frames.
  * They let the agent decide upon an action at every 4th in-game frame (3rd in space invaders).
  * (5.1) Training and stability
    * They plot the average reward und Q-value per N games to evaluate the agent's training progress,
    * The average reward increases in a noisy way.
    * The average Q value increases smoothly.
    * They did not experience any divergence issues during their training.
  * (5.2) Visualizating the Value Function
    * The agent learns to predict the value function accurately, even for rather long sequences (here: ~25 frames).
  * (5.3) Main Evaluation
    * They compare to three other methods that use hand-engineered features and/or use the pixel data combined with significant prior knownledge.
    * They mostly outperform the other methods.
    * They managed to beat a human player in three games. The ones where the human won seemed to require strategies that stretched over longer time frames.

This paper presents a model that can dynamically split computation across coarse, low-capacity sub-networks and fine, high-capacity sub-networks. The coarse model processes the entire input data and is typically shallow while the fine model focuses on a few important regions of the input and is deeper. For images as input, this is a hard attention mechanism that can be trained with stochastic gradient descent and doesn't require a task-specific attention policy trained by reinforcement learning. Key ideas:

- A deep network h can be decomposed into bottom layers f and top layers g such that $h(x) = g(f(x))$. Further, f consists of two alternate sub-networks $f\_c$ and $f\_f$. $f\_c$ is a low-capacity sub-network while $f\_f$ is a high-capacity sub-network.

- g should be able to use representations from $f\_c$ and $f\_f$ dynamically. $f\_c$ processes the entire input while $f\_f$ only a few important regions of the input.

- The coarse model processes the entire input and the norm of the gradient of the entropy with respect to the coarse vector at each spatial region is computed which is a measure of saliency. The use of the entropy gradient as a saliency measure encourages selecting input regions that could affect the uncertainty in the model’s predictions the most.

- The top-k input regions with highest saliency values are processed by the fine model. The refined representation for input to the top layers consists of both coarse and fine vectors. During backpropagation, gradients are computed for the refined model, i.e. propagating gradients at each position into either the coarse or fine features, depending on which was used.

- To make sure $f\_c$ and $f\_f$ representations are interchangeable and input to the top layers has smooth transitions, an additional objective term minimizes the squared distance between coarse and fine representations and this additional term is used only to optimize the coarse layers, not the fine layers.

- Experiments on cluttered MNIST, SVHN and comparison with RAM, DRAW and study with various values of number of patches for fine processing.

## Strengths

- Neat, general way to split computation based on importance of input; a hard-attention mechanism that can be trained with SGD, unlike RAM.

- Entropy gradient as a measure of saliency is an interesting idea, and it doesn't need labels i.e. can be used at test time.

Carlini and Wagner study the effectiveness of adversarial example detectors as defense strategy and show that most of them can by bypassed easily by known attacks. Specifically, they consider a set of adversarial example detection schemes, including neural networks as detectors and statistical tests. After extensive experiments, the authors provide a set of lessons which include:
- Randomization is by far the most effective defense (e.g. dropout).
- Defenses seem to be dataset-specific. There is a discrepancy between defenses working well on MNIST and on CIFAR.
- Detection neural networks can easily be bypassed.
Additionally, they provide a set of recommendations for future work:
- For developing defense mechanism, we always need to consider strong white-box attacks (i.e. attackers that are informed about the defense mechanisms).
- Reporting accuracy only is not meaningful; instead, false positives and negatives should be reported.
- Simple datasets such as MNIST and CIFAR are not enough for evaluation.

Also find this summary at [davidstutz.de](https://davidstutz.de/category/reading/).

This paper introduces a CNN based segmentation of an object that is defined by a user using four extreme points (i.e. bounding box). Interestingly, in a related work, it has been shown that clicking extreme points is about 5 times more efficient than drawing a bounding box in terms of speed. 
https://i.imgur.com/9GJvf17.png

The extreme points have several goals in this work. First, they are used as a bounding box to crop the object of interest. Secondly, they are utilized to create a heatmap with activations in the regions of extreme points. 

The heatmap is created as a 2D Gaussian centered around each of the extreme points. This heatmap is matched to the size of the resized crop (i.e. 512x512) and is concatenated with the original RGB channels of the crop. 

The concatenated input of channel depth=4 is fed to the network which is a ResNet-101 with FC and last two maxpool layers removed. In order to maintain the same receptive field, an astrous convolution is used. Pyramid scene parsing module from PSPNet is used to aggregate global context. The network is trained with a standard cross-entropy loss weighted by a normalization factor (i.e. a frequency of a class in a dataset).

How does it compare to "Efficient Interactive Annotation of Segmentation Datasets with Polygon-RNN++
" paper in terms of accuracy? Specifically, if the polygon is wrong it is easy to correct points on the polygon that are wrong. However, it is unclear how to obtain preferred segmentation when no matter how many (greater than four) extreme points are selected, the object of interest is not segmented properly.

A finding first publicized by Geoff Hinton is the fact that, when you train a simple, lower capacity module on the probability outputs of another model, you can often get a model that has comparable performance, despite that lowered capacity. Another, even more interesting finding is that, if you take a trained model, and train a model with identical structure on its probability outputs, you can often get a model with better performance than the original teacher, with quicker convergence.

This paper addresses, and tries to specifically test, a few theories about why this effect might be observed. One idea is that the "student" model can learn more quickly because getting to see the full probability distribution over a well-trained models outputs gives it a more valuable signal, specifically because the trained model is able to better rank the classes that aren't the true class. For example, if you're training on Imagenet, on an image of a huskies, you're only told "this is a husky (1), and not one of 100 other classes, which are all 0". Whereas a trained model might say "'this is most likely a husky, but the probability of wolf is way higher than that of teapot". This inherently gives you more useful signal to train on, because you’re given a full distribution of classes that an image is most like. This theory goes by the name of the “Dark Knowledge” theory (a truly delightful name), because it pulls all of this knowledge that is hidden in a 0/1 label into the light. An alternative explanation for the strong performance of distillation techniques is that the student model is just benefitting from the implicit importance weighting of having a stronger gradient on examples where the teacher model is more confident. You could think of this as leading the student towards examples that are the most clear or unambiguous examples of a class, rather than more fuzzy and uncertain ones.

Along with a few other tests (which I won’t address here, for sake of time and focus), the authors design a few experiments to test these possible mechanisms of action. The first test involved doing an explicit importance weighting of examples according to how confident the teacher model is, but including no information about the incorrect classes. The second was similar, but instead involved perturbing the probabilities of the classes that weren’t the max probability. In this situation, the student model gets some information in terms of the overall magnitudes of the not-max class, but can’t leverage it as usefully because it’s been randomized.

In both situations, they found that there still was some value - in other words, that the student outperformed the teacher - but it outperformed by less than the case where the teacher could see the full probability distribution. This supports the case that both the inclusion of probabilities for the less probable classes, as well as the “confidence weighting” effect of weighting the student to learn more from examples on which the “teacher” model was more confident.