A Deep Decoder Structure Based on WordEmbedding Regression for An Encoder-Decoder Based Model for Image Captioning

06/26/2019 ∙ by Ahmad Asadi, et al. ∙ AUT 1

Generating textual descriptions for images has been an attractive problem for the computer vision and natural language processing researchers in recent years. Dozens of models based on deep learning have been proposed to solve this problem. The existing approaches are based on neural encoder-decoder structures equipped with the attention mechanism. These methods strive to train decoders to minimize the log likelihood of the next word in a sentence given the previous ones, which results in the sparsity of the output space. In this work, we propose a new approach to train decoders to regress the word embedding of the next word with respect to the previous ones instead of minimizing the log likelihood. The proposed method is able to learn and extract long-term information and can generate longer fine-grained captions without introducing any external memory cell. Furthermore, decoders trained by the proposed technique can take the importance of the generated words into consideration while generating captions. In addition, a novel semantic attention mechanism is proposed that guides attention points through the image, taking the meaning of the previously generated word into account. We evaluate the proposed approach with the MS-COCO dataset. The proposed model outperformed the state of the art models especially in generating longer captions. It achieved a CIDEr score equal to 125.0 and a BLEU-4 score equal to 50.5, while the best scores of the state of the art models are 117.1 and 48.0, respectively.

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1 Introduction

Image captioning is the task of generating a textual description for a given image. The generated description is required to be syntactically and grammatically correct and it should suggest a holistic explanation of the image. Image caption generator models can be widely used for providing advanced conceptual search tools on images fang2015captions , assisting blind people wu2017automatic , and human-robot interactions das2017visual .

The problem of generating an appropriate caption for an image is a challenging problem in the computer vision and natural language processing fields. Most methods using deep neural networks for solving this task are based on an encoder-decoder baseline inspired by sequence-to-sequence methods in machine translation

sutskever2014sequence .

The encoder employs convolutional neural networks(CNNs) for understanding the image concept, details, containing objects, and object relations. In fact, the encoder makes a translation from the input space to a latent space, and extracts a feature vector from the given image to be used by a decoder later. The decoder uses recurrent neural networks(RNNs) for generating appropriate sentences in a word by word manner, describing the information and features extracted from the image.

Coping with the long-term dependencies while generating the caption is one of the most important challenges in the simple encoder-decoder based models. These models strive to extract a single feature vector which restricts the encoder to encode all the necessary information in a fixed length vector. In addition, the decoder has to extract all the required information from that single fixed length vector while generating the words of the caption.

Attention mechanism solved the problem of the simple encoder-decoder baseline in the machine translation task bahdanau2014neural . The visual attention mechanism proposed by Mnih et al. mnih2014recurrent applied the idea to the image caption generation task. The keypoint of the attention mechanism solution is to generate more than one feature vector for image representation in the encoding phase and select and use the appropriate feature vectors for word generation in the decoding phase. This mechanism enables models to attend to salient parts of an image while generating its caption.

The visual attention mechanism has recently been widely used in the proposed deep image caption generating models. CNN extracts information from the spatial segments of an image and encodes the extracted information into multiple feature vectors. The employed RNN then uses one of the extracted feature vectors at each step, while creating the words of the caption. It is similar to the process of attending to a specific area of an image while telling a word about that specific part and going to the next area for the next word till the whole caption is generated.

Designing a decoder structure able to generate rich fine-grained sentences plays an important role in improving the performance of image captioning models. Typical encoder-decoder based models for image captioning train the decoder using the log likelihood of each word in the sentence given the previously generated words. Minimizing the log likelihood function requires ”one-hot” vectors for both the decoder output and the desired label at each step. Using the one-hot encoding strongly increases the sparsity of the output space of the model which makes the training phase harder. In this work, instead of using loglikelihood minimization to train the decoder, we utilize word embedding regression which resolves the sparsity of the output space, decreases the number of the decoder’s weights, and allows training deeper structures to extract more complex features and finer-grained captions.

Our work makes the following contributions: 1) We propose a novel method to train decoders to regress the embedding of each word in the sentence instead of predicting the probability distribution of the words at each step. 2) The proposed model resolves the problem of sparsity of the output space and results in faster convergence of the decoder. 3) We introduce a novel attention mechanism that takes the meaning of the word generated in the last step into account. 4) The proposed method outperforms the state-of-the-art on image captioning benchmarks.

The rest of this paper is organized as follows: In section 2 we review related models based on the encoder-decoder framework in the image captioning field. In section 3 the main problems of vanishing gradients and output space sparsity in stacked decoders are presented and a novel technique is introduced to cope with these problems. The proposed model is then evaluated and the evaluation results are reported and discussed in section 4. Finally, the conclusions of the paper are summarized in section 5.

2 Related Work

Recent studies in image caption generation can be categorized into two major categories: the alignment methods and the encoder-decoder based methods. The studies in each category are reviewed in this section.

2.1 Alignment Methods

The general approach of the studies in this group is to make an alignment between images and their corresponding captions. All methods in this category achieve this using the following general pipeline architecture:

  1. A CNN is applied on the input image to extract a good feature vector as the image representation. In fact, the CNN makes a transformation from the input space to a -dimensional latent space.

  2. Simultaneously an RNN is used to extract a good representation of the corresponding caption. The only constraint is that the caption’s extracted representation should be in the same -dimensional space as the image latent space.

  3. In the training phase, the CNN and the RNN are jointly trained in a way that the similarity of the image and its corresponding representation is maximized, while the similarity of the image and other captions’ representations is minimized.

  4. In the testing phase, the input image is fed into the trained CNN to generate an appropriate representation. the generated representation then is used to generate or retrieve the desired caption. Furthermore, if the input is a sentence, the trained RNN is used to generate the appropriate representation and the image with the most similar representation is retrieved as the search result.

Approaches to image-caption alignment differ with respect to the structure of the used CNNs and RNNs. Karpathy et al.karpathy2014deep proposed a deep bidirectional alignment between images and their captions. In this work, the input image is first fragmented to 19 sub-spaces using the R-CNN method girshick2014rich . In the next step, a CNN krizhevsky2012imagenet

is applied to the whole image and the 19 extracted sub-spaces and a 4096-dimensional feature vector is extracted for the 20 images. Meanwhile, a dependency tree of a given caption in the training set is extracted and its relationships are used to identify sentence fragments. A simple linear transformation is applied to each sentence fragment in order to generate a 4096-dimensional meaning vector for all of the extracted sentence fragments. An alignment model is trained in order to maximize the similarity of the related parts of the image and the fragments of the sentence. Figure

1 demonstrates the system architecturekarpathy2014deep .

Figure 1: Structure of alignment method proposed by Karpathy et al.karpathy2014deep

Karpathy et al. also used a bidirectional RNN as a word embedding model and trained it with captions provided in the training set karpathy2015deep .

2.2 Encoder-Decoder Based Methods

The encoder-decoder framework proposed by Cho et al.cho2014learning is one of the most popular models used in machine translation. Methods based on this framework split the translation task into two steps: 1) extract features from the source sentence which is called the encoding phase, 2) generate a new sentence in the destination language given the meaning encoded into the feature vector, which is called the decoding phase.

Since, the encoder-decoder framework yields an end-to-end model to solve sophisticated problems, it is employed as a solution to a wide variety of problems in computer vision and natural language processing fields. Venugopalan et al. venugopalan2014translating first employed the encoder-decoder model in video description generation. Sun et al. sun2018emotional also proposed a model for emotional human-machine conversation generation using the encoder-decoder baseline.

Image captioning task is one of the main application areas of the end-to-end neural encoder-decoder based models. Xu et al. first proposed a model based on this framework in image caption generation, substituting the source sentence in machine translation with the input image xu2015show . Thus, the encoder changed in a way that it generates a feature vector given an image and the remaining parts of the framework remained unchanged.

Encoder-decoder based image captioning baseline model has been employed by many researchers to propose novel image description systems. Wang et al. used a CNN with two novel bidirectional recurrent neural networks to model complicated linguistic patterns using historical and future sentence context

wang2016image . Vinyals et al. used the ”Inception” model proposed by Szegedy et al.szegedy2016rethinking as the encoder and LSTMs as the decoder of its frameworkvinyals2015show . In addition, a linear transformation layer was added at the input of the LSTMs for better training.

More sophisticated models based on the encoder-decoder framework were also proposed. Ding et al. ding2018neural proposed a method based on the encoder-decoder framework, called

Reference based Long Short Term Memory

(R-LSTM), aiming to lead the model to generate a more descriptive sentence for a given image by introducing some reference information. In this work, by introducing reference information, the importance of different words is considered while generating captions.

The first idea of using visual attention was proposed by Mnih et al.mnih2014recurrent to cope with the problem of fixed length context vectors extracted by the encoders. In this work, in order to decrease the overhead of applying convolutional networks at each step, a sequence of feature vectors was extracted from different image regions. At the first step, a convolutional network was applied to all selected image regions and the extracted feature vectors, called annotation vectors, were listed. At each step of generating text, a new feature vector was calculated from the given annotation vectors that helped to predict the next word in the sentence.

Attention mechanism then was exported to machine translation by Bahdanau et al. bahdanau2014neural . Furthermore, Ba et al.ba2014multiple employed the attention mechanism for multiple object recognition. Finally, the well-formed encoder-decoder empowered by attention with two different mechanisms called ”soft attention” and ”hard attention” was proposed by Xu et al.xu2015show .

Advances made through deploying the attention based techniques encouraged researchers to focus on this kind of model. A large number of studies tried to improve the simple attention model in image captioning. Park et al.

park2017attend employed an encoder-decoder method along with a memory slut to generate a personalized caption for an image in social media for specific users. Cornia et al.cornia2017visual

used saliency map estimators to strengthen the attention mechanism. Yang et al.

yang2016stacked used the attention mechanism in visual question answering in order to find the best place to attend in image while generating the answer. In this work, attention layers were structured in a stack. This improved the model’s performance. Furthermore, Donahue et al.donahue2015long proposed an encoder-decoder framework for video description using the attention mechanism.

More complex models were proposed employing the attention mechanism to improve the quality of generated captions. Chen et al. chen2017temporal proposed a novel method to use the attention mechanism and an RNN in order to first observe the input image and create different weights for each word of its caption while training to learn better from the key information of the caption. Chen et al. chen2018show also proposed a technique to focus on training a good attribute-inference model via an RNN and the attention mechanism where the co-occurrence dependencies among attributes are maintained. Chen et al. chen2019image proposed a memory-enhanced captioning model for image captioning to cope with the problem of long-term dependencies. In this work, an external memory is introduced to keep the information of all of the generated words at previous steps. Using the introduced external memory, RNN cells can make better prediction at each step without trying to extract the information from the hidden state.

To the best of our knowledge, decoders in all of the previous studies are used to model the log-likelihood of the next word, given the previously generated words and the image context vector according to expression (1). In this expression is the set of trainable parameters, is the generated word at step and is the image context vector.

(1)

In order to compute the probability distribution of expression (1), the last layer of the decoder should have the same size as that of the vocabulary dictionary.

The expectation of each component of the output vector is extremely small, resulting in extremely small back-propagated gradients of the errors. For the typical image captioning datasets, the expected value of gradient of errors is in the range of . Small gradients of error make learning more difficult and decrease the model’s convergence speed. This is the most important obstacle to make deeper decoders for memorizing long-term dependencies.

Gu et al. proposed a stacked decoder architecture for image caption generation gu2018stack . In this research, the problem of vanishing gradients in training deep stacked decoders is addressed by providing a learning objective function which enforces intermediate supervisions.

In this work, a new method is proposed to cope with the problems of modeling log-likelihood. The proposed method changes the optimization problem used to train the decoder weights in a way that the decoder performs a regression over the word embeddings instead of predicting their probability distributions. In this way, not only the sparsity of the output space is resolved, but also higher gradients are produced at the last layer of the decoder and the gradients are less likely to vanish. In this way, we can use deeper stacked decoders which in general extract more complex features and are able to generate longer qualified captions. In addition, a novel attention mechanism is proposed to take the meaning of the last generated word into account, while creating the visual attention point at each step.

3 Proposed Method

We propose a novel approach to train decoders for image captioning. The main idea is to use a word-embedding vector instead of a one-hot vector representation as the decoder desired output. As a result, the optimization problem changes from predicting the conditional probability distribution of the next word to a word-embedding regression. In addition, a new attention mechanism is proposed to take the last word meaning into consideration.

3.1 Typical Decoder

A typical decoder in image captioning task is an RNN whose parameters are found by solving the optimization problem (2), in which denotes the set of trainable weights, denotes the generated word at time step , denotes the representation vector of the input image, and denotes the number of words in the generated sentence.

(2)

In order to model the probability distribution, a softmax layer is used at the end of the decoder to normalize the output logits according to (

3) in which denotes the th component of the predicted probability distribution, denotes the th element of the output logits and is the output size.

(3)

Typically, the softmax cross entropy loss function

is used in image captioning models. The derivative of the loss function with respect to its weights

can be computed using the chain rule as shown in (

7).

(4)
(5)
(6)
(7)

A typical language model should use about 20,000 words for sentence generation (based on the size of current image captioning benchmarks). This means . Therefore, the average value of is of order

. The small value of the back-propagated error is also multiplied by a small learning rate at each layer and gets even smaller. This means that the backpropagated error makes no significant change in the weights of the first layers of the model.

3.2 Proposed Decoder

Making the decoder deeper results in adding more non-linearity to the model. This is shown to be helpful in vision tasks simonyan2014very . Stacking LSTM layers on top of each other improves the decoder performance if the back-propagated error is large enough to make changes in weight values and train them.

We used the word-embedding vector as the desired output of the decoder at each step instead of the one-hot vector. In order to use the word-embedding vector, the optimization problem (2) should be changed. The problem is changed from predicting the probability distribution of each word to the regression of the word-embedding of the new word. The new optimization problem is formulated as in (8) in which is the embedding vector of the word and is the output logits of the decoder at time step .

(8)

The word embedding model used in this work is the model proposed by Mikolov et al.mikolov2013efficient . The skip-gram model is selected as it can significantly facilitate the training of the decoder. As figure 2 shows, in the skip-gram model, the embedding is trained in a way that the prediction error of the previous and the next word embeddings is minimized. This means that the embedding of a word gives hints to predict the word embedding of the next ones. So, estimating the next word probability distribution will be easier for the decoder.

Figure 2: An illustration of the skip-gram word embedding model proposed in mikolov2013efficient

Using word embedding as the input vector of all LSTM cells results in a notable decrease in the dimensionality of the input space and therefore, decreases the number of parameters (network weights) of the model. Thus, in this case, we can use a stacked multi-RNN cell in the decoder block.

Using a stacked multi-RNN cell structure empowers the model to predict longer captions. In a stack of LSTMs, remembering the far away words will become more convenient because each LSTM cell can be imagined as a cell responsible to remember a far word or context in a sentence.

3.3 Encoder

In this work, the Inception V3 network proposed by Szegedy et al. szegedy2016rethinking is used as the encoder because of its good performance on image classification and feature extraction tasks. The outputs of one of the layers of the Inception V3 which is called transfer learning layer are used as ”annotation vector” and passed to the decoder.

3.4 Proposed Attention Mechanism

We denote the annotation vector set extracted by the encoder (the output of the transfer learning layer of the Inception V3 model) as .

At time step , while generating the th word of the caption, the system computes the used context according to equations (9) to (11). In equation (9), denotes the context vector and the weight of the th annotation vector at step . A SoftMax layer is used to compute each attention coefficient according to equation (10). In this equation, is a measure of how good the annotation vector is for generating a feature vector at step . is computed according to equation (11) in which denotes the hidden state of the decoder at step , is the embedding vector of the word predicted at step , and

is implemented by a multilayer perceptron network with a single hidden layer. The weights of this MLP are trained jointly with the weights of the decoder during the training phase.

(9)
(10)
(11)

The above mentioned attention mechanism is the general structure represented by Xu et al.xu2015show , except that the similarity function takes an extra input parameter to guide the attention considering the previous predicted word.

Function is implemented with an MLP represented with equation (12). In this equation, gives the number of annotation vectors,

denotes the number of MLP’s hidden layer neurons,

and

are bias vectors of the first and second layers of the network, and all

s denote the weight vectors of the network.

(12)

3.5 Regularizations

In order to prevent overfitting on the training dataset, the dropout technique proposed by Srivastava et al.srivastava2014dropout

is employed and a dropout-wrapper is used on all of the units in the proposed network. In addition, The output of the transfer learning layer of the Inception module is pyramided down into a quarter of its original size to decrease the number of required parameters in the attention layer. Figure

3 demonstrates the whole structure of proposed method. In order to make a more readable figure, direct input word vectors at each time step are removed from figure.

Figure 3: The proposed architecture for image caption generation task.

4 Experimental Results

The experiments carried out to evaluate the performance of the proposed method are explained in this section. Section 4.1 takes a brief look at the available dataset and evaluation metrics used to evaluate the model performance. The implementation details are explained in section 4.2. Section 4.3 reports the results obtained from these experiments and presents some discussions regarding the results.

4.1 Dataset and Evaluation Metrics

The MSCOCO dataset intoduced by Lin et al.lin2014microsoft is used to evaluate the model proposed in this work. The training set of MSCOCO consists of more than 82,789 images, each with at most five human generated captions. The validation and testing sets of this dataset have 40,504 and 40,000 images respectively, captioned in the same way.

The proposed method is evaluated based on the popular image captioning evaluation metrics BLEU score papineni2002bleu , CIDEr scorevedantam2015cider , ROUGE_L score lin2004rouge and METEOR score banerjee2005meteor .

4.2 Implementation Details

Each input image is fed to a convolutional network to extract high level features. We used Inception V3 szegedy2016rethinking as the encoder in this work. The output of the Transfer Layer of the Inception model are used as the annotation vectors of the given image. Transfer Layer is the last layer before the fully connected block of the Inception model. The output of this layer shapes a set of feature vectors each associated with a specific spatial segment of the source image. The used Inception model is pretrained on the ImageNet dataset deng2009imagenet . Since the feature maps generated by this model are general enough to extract the necessary information for sentence generation, no additional training is required. Annotation vectors extracted by the Inception

model are sub-sampled one level using an extra max-pooling layer to reduce the weights of the attention layer model. The annotation vectors are fed to the stacked multi-RNN cell through the attention layer in the next step.

The ”Gensim” word2vec model proposed by Mikolov et al.mikolov2013efficient is employed for word embedding in this work. The word-embedding model is trained on all captions of the training set once before all other processes. At each step in caption generation, the embedding vector of the previously generated word is fed to the attention mechanism and the stacked multi-RNN cell as input. Furthermore, all LSTM cells are trained to predict the embedding vector of the next word instead of one-hot vector. This is done using the mean squared error as the cost function and the Adam optimizer. All of our evaluations use a model with 8-layer stacked multi-RNN cells trained with a 1024-dimensional word embedding model and an initial learning rate of 0.001 and a decay rate of 0.1 at each step.

4.3 Results

In this section the experimental methodology and quantitative and qualitative results are described. The effectiveness of the proposed model for image captioning is also validated. In the following experiments, we denote the simple stacked architecture without attention layer with ”stacked”. Also we used ”ATT” for the stacked architecture with a single attention layer only before the first layer. The numbers in parentheses specifies the number of layers in the stacked decoder.

4.3.1 Comparison with the state-of-the-art models

The proposed method is evaluated and compared with other existing models on the MS-COCO dataset. The results are reported in table 1. For the existing methods, both recent best performing methods and the baseline models are chosen and their results are directly taken from the existing literature. The table columns present scores for the metrics BLEU-4 (B4) to BLEU-1 (B1), METEOR (M), CIDEr(C) and ROUGE-L (R).

R M C B4 B3 B2 B1
Vinyals et al. 2017vinyals2017show 53.0 25.4 94.3 30.9 40.7 54.2 71.3
Lu et al. 2017lu2017knowing 55.0 26.4 104.0 33.6 44.4 58.4 74.8
Chen et al. 2017chen2017temporal 54.7 25.9 105.9 32.4 44.1 59.1 75.7
Ren et al. 2017ren2017deep 52.5 25.1 93.7 30.4 40.3 53.9 71.3
Gu et al. 2017gu2017empirical - 25.1 99.1 30.4 40.9 54.6 72.1
Rennie et al. 2017rennie2017self 56.3 27.0 114.7 35.2 - - -
Wang et al. 2017wang2017skeleton 48.9 24.7 96.6 25.9 35.5 48.9 67.3
Gan et al. 2017gan2017semantic 54.3 25.7 100.3 34.1 44.4 57.8 74.1
Liu et al. 2018liu2018show 57.0 27.4 117.1 35.8 48.0 63.1 80.1
Wang et al. 2018wang2018image - 21.1 69.5 23.0 33.3 47.4 65.6
Cornia et al. 2018cornia2018paying 52.1 24.8 89.8 28.4 39.1 53.6 70.8
Ding et al. 2018ding2018neural 55.5 26.1 105.5 34.2 45.8 60.5 76.8
Chen et al. 2018chen2018show 54.9 33.8 104.4 33.8 44.3 57.9 74.3
Gu et al. 2018gu2018stack - 27.4 120.4 36.1 47.9 62.5 78.6
Chen et al. 2019chen2019image 58.7 28.7 125.5 38.4 50.7 65.5 81.9
Stacked + ATT (8) 64.9 34.7 125.0 50.5 57.1 66.4 73.7
Table 1: Performance of the proposed method compared to the state-of-the-art methods on MS-COCO dataset.

According to the table 1, except for BLEU@1 the performance of the method proposed in this paper is better than or roughly equal to those of the reference models. In addition, without introducing any external memory cell, our model is generating relatively better captions than the model proposed by Chen et al. chen2019image . This shows that making deeper stacked decoder structures empowers the RNNs to memorize longer history of the generated words’ information. The proposed method specially outperforms the existing models based on traditional RNNs (without any external memory cell) with respect to BLEU-4 and CIDEr factors measuring the similarity of the same 4-grams and the longest weighted sub-sequence of the same words in suggested and reference captions, respectively. This means our model can generate captions similar to reference human generated image descriptions that are longer than those generated by other methods. It shows that substituting the typical prediction problem at each step to generate caption words with a regression problem resolves the problem of long-term dependencies in the encoder-decoder based models.

4.3.2 On the depth of the decoder

The proposed model is trained with different number of stacked decoder layers. Table 2 reports the number of the trainable parameters of the decoder with respect to the depth of the decoder when using the one-hot vector representation versus when using the word embedding.

Representation Methods Decoder Depth
1 2 5 8 10
Embedding 17.8 M 26.2 M 51.4 M 76.6 M 93.4 M
One-Hot 37.38 M 60.2 M 118.21 M 189.8 M 233.5 M
Table 2: The number of decoder’s trainable parameters with different decoder depths for two representation methods.

In addition, table 3 reports the experimental results of training the stacked decoder structure with different depths. According to this table, we can now stack up to 8 layers of LSTMs on top of each other and use them as the decoder. This means that in our model, the gradients produced at the last layer of the decoder can be back propagated to the first layer without vanishing, while the other models with stacked decoders can have up to only 2 layers gu2018stack donahue2015long .

Model R M C B4 B3 B2 B1
Stacked(5) 62.7 33.1 122.7 46.3 52.7 65.9 71.3
Stacked(8) 64.9 34.7 125.0 50.5 57.1 66.4 73.7
Stacked(10) 60.8 31.3 115.7 40.0 49.8 60.9 70.5
Table 3: Performance of different decoders with different depths compared to each other.

4.3.3 Qualitative Results

Figures 4 and 5 display some samples of correct and incorrect captions generated by the proposed method. In addition, our method outperforms the existing methods using METEOR and ROUGE-L measures. This means better words are used in captions generated by our work. This is the result of training the LSTM cells to predict the embedding of the next words instead of their one-hot vectors. Indeed, it allows the decoder to use alternative synonym words more easily than before at the word generation phase and takes the word co-occurrences and meanings into account while choosing them in the caption generation phase.

(a) LSTM: Giraffe standing in a tree filled area
GT: A giraffe standing next to a forest filled with trees.
A giraffe eating food from the top of the tree.
(b) LSTM: Man on a surf board in the ocean
GT: A‌ man laying on a surfboard in the water.
A man lying on a surfboard in some small waves in water.
(c) LSTM: Plane is on the ground on the tarmac
GT: A modern jet airliner on a snow edged runway.
The airplane is on the ground on the runway.
Figure 4: Samples of correct generated captions
(a) LSTM: Old man in a coat has a giraffis look on his face
GT: A man stands with a frown on his face.
A old man in coat and tie walking down a busy street.
(b) LSTM: Train of an empty intersection with traffic lights
GT: Two traffic lights are posted near the street intersection.
An empty street with some stop lights in a little island.
(c) LSTM: small black and dog with a frisbee by its feet
GT: A small dog standing on a wet ground looking up.
A small black and white dog standing on a sparse grass looking at ahuman.
Figure 5: Samples of incorrect generated captions

5 Conclusions

In this paper, we proposed a novel approach to train the decoders in encoder-decoder structures for the image captioning task. We used a word-embedding vector as the desired output of the decoder instead of a one-hot vector. This enables the stacked decoders to cope with the long-term dependencies and vanishing gradients. In addition, this allows the decoders to be deeper and have more layers stacked on top of each other. Furthermore, we introduced a new attention mechanism to gain better order of attention points during caption generation. We used previously generated words as extra inputs to the attention mechanism and trained a neural network to find the next focus point on the image using the meaning of the previously predicted word and the current state vector of the decoder. This network is trained jointly with the other parts of the decoder during the training phase. In addition, we employed a stacked multi-RNN cell as the decoder and trained it to predict the embedding of the next word instead of its one-hot vector in order to first take word meanings into consideration and second reduce model parameters while generating captions for given images. Results show that the proposed method outperforms the existing models with respect to measures corresponding to length of the caption on the most widely used dataset of image captioning.

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