<Show, Attend and Tell: Neural Image Caption Generation with Visual Attention>

1. **This paper brings about "hard attention". **The authors introduce two attention-based image caption generators under a common framework: 1) a "soft" deterministic attention mechanism trainable by standard back-propagation methods and 2) a "hard" stochastic attention mechanism trainable by maximizing an approximate variational lower bound or equivalently by REINFORCE.

2. **We first introduce their common framework. **The model takes a single raw image and generates a caption y encoded as a sequence of 1-of-K encoded words.

where K is the size of the vocabulary and C is the length of the caption.

3. **We use a CNN to extract a set of feature vectors which we call annotation vectors**.

4. We use LSTM to produce a caption by generating one word at every time step conditioned on a context vector, the previous hidden state and the previously generated words. The structure is like that:

where is a simple affine transformation with parameters that are learned.

The diagram is like that:

5.** In simple terms, the context vector is a dynamic representation of the relevant part of the image input at time t.** We define a mechanism that computes from the **annotation vectors** , I=1,...,L corresponding to the features extracted at different image locations. For each location i, the mechanism generates a positive weight which is the relative importance of it. The is computed by that:

where a is computed by a multilayer perceptron.

6. **Once the weights are computed, the context vector is computed by**

where is a function that returns a single vector given the set of annotation vectors and their corresponding weights.

7. **The initial memory state and hidden state of the LSTM are predicted by an average of the annotation vectors fed through two separate MLPs (init, c and init, h):**

8. **We use a deep output layer to compute the output word probability** given the LSTM state, the context vector and the previous word:

9. **Stochastic "Hard" Attention.**

We define a local variable as where the model decides to focus attention when generating the word. is an indicator one-hot variable representing that i-th location (out of L) is the one used to extract visual features. We can assign a multinoulli distribution parametrized by , and view as a random variable:

We define a new objective function that is a variational lower bound on the marginal log-likelihood of observing the sequence of words given image features

(I think that there is something wrong with the formula... I use the [f(x)g(x)]'=f(x)'g(x) + f(x)g(x)' to conduct it but failed...)

The equation above suggests a Monte Carlo based sampling approximation of the gradient with respect to the model parameters. This can be done by sampling the location from the distribution above, which is:

A **moving average baseline** is used to reduce the variance in the Monte Carlo estimator of the gradient. Upon seeing the mini-batch, the moving average baseline is estimated as an accumulated sum of the previous log likelihoods with exponential decay:

To further reduce the estimator variance, an entropy term on the multinouilli distribution H[s] is added. We set the sampled attention location to its expected value .Both techniques improve the robustness of the stochastic attention learning algorithm.

The final rule is like that:

10. **Deterministic "Soft" Attention**

Learning stochastic attention requires sampling the attention location each time, instead we can take the expectation of the context vector directly,

and formulate a deterministic attention model by computing a soft attention weighted annotation vector:

The whole model is smooth and differentiable under the deterministic attention. So learning end-to-end is trivial by using standard back-propagation.

11. We define the** NWGM**(normalized weighted geometric mean for the softmax word prediction:

12. The model is trained end-to=end by minimizing the following penalized negative log-likelihood:

which encourage , and it is called **DOUBLY STOCHASTIC ATTENTION**.

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