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Introduction
Neural networks have been successfully used to describe images with text using sequence-to-sequence models BIBREF0. However, the results are simple and dry captions which are one or two phrases long. Humans looking at a painting see more than just objects. Paintings stimulate sentiments, metaphors and stories as well. Therefore, our goal is to have a neural network describe the painting artistically in a style of choice. As a proof of concept, we present a model which generates Shakespearean prose for a given painting as shown in Figure FIGREF1. Accomplishing this task is difficult with traditional sequence to sequence models since there does not exist a large collection of Shakespearean prose which describes paintings: Shakespeare's works describes a single painting shown in Figure FIGREF3. Fortunately we have a dataset of modern English poems which describe images BIBREF1 and line-by-line modern paraphrases of Shakespeare's plays BIBREF2. Our solution is therefore to combine two separately trained models to synthesize Shakespearean prose for a given painting.
Introduction ::: Related work
A general end-to-end approach to sequence learning BIBREF3 makes minimal assumptions on the sequence structure. This model is widely used in tasks such as machine translation, text summarization, conversational modeling, and image captioning. A generative model using a deep recurrent architecture BIBREF0 has also beeen used for generating phrases describing an image. The task of synthesizing multiple lines of poetry for a given image BIBREF1 is accomplished by extracting poetic clues from images. Given the context image, the network associates image attributes with poetic descriptions using a convolutional neural net. The poem is generated using a recurrent neural net which is trained using multi-adversarial training via policy gradient.
Transforming text from modern English to Shakespearean English using text "style transfer" is challenging. An end to end approach using a sequence-to-sequence model over a parallel text corpus BIBREF2 has been proposed based on machine translation. In the absence of a parallel text corpus, generative adversarial networks (GANs) have been used, which simultaneously train two models: a generative model which captures the data distribution, and a discriminative model which evaluates the performance of the generator. Using a target domain language model as a discriminator has also been employed BIBREF4, providing richer and more stable token-level feedback during the learning process. A key challenge in both image and text style transfer is separating content from style BIBREF5, BIBREF6, BIBREF7. Cross-aligned auto-encoder models have focused on style transfer using non-parallel text BIBREF7. Recently, a fine grained model for text style transfer has been proposed BIBREF8 which controls several factors of variation in textual data by using back-translation. This allows control over multiple attributes, such as gender and sentiment, and fine-grained control over the trade-off between content and style.
Methods
We use a total three datasets: two datasets for generating an English poem from an image, and Shakespeare plays and their English translations for text style transfer.
We train a model for generating poems from images based on two datasets BIBREF1. The first dataset consists of image and poem pairs, namely a multi-modal poem dataset (MultiM-Poem), and the second dataset is a large poem corpus, namely a uni-modal poem dataset (UniM-Poem). The image and poem pairs are extended by adding the nearest three neighbor poems from the poem corpus without redundancy, and an extended image and poem pair dataset is constructed and denoted as MultiM-Poem(Ex)BIBREF1.
We use a collection of line-by-line modern paraphrases for 16 of Shakespeare’s plays BIBREF2, for training a style transfer network from English poems to Shakespearean prose. We use 18,395 sentences from the training data split. We keep 1,218 sentences in the validation data set and 1,462 sentences in our test set.
Methods ::: Image To Poem Actor-Critic Model
For generating a poem from images we use an existing actor-critic architecture BIBREF1. This involves 3 parallel CNNs: an object CNN, sentiment CNN, and scene CNN, for feature extraction. These features are combined with a skip-thought model which provides poetic clues, which are then fed into a sequence-to-sequence model trained by policy gradient with 2 discriminator networks for rewards. This as a whole forms a pipeline that takes in an image and outputs a poem as shown on the top left of Figure FIGREF4. A CNN-RNN generative model acts as an agent. The parameters of this agent define a policy whose execution determines which word is selected as an action. When the agent selects all words in a poem, it receives a reward. Two discriminative networks, shown on the top right of Figure FIGREF4, are defined to serve as rewards concerning whether the generated poem properly describes the input image and whether the generated poem is poetic. The goal of the poem generation model is to generate a sequence of words as a poem for an image to maximize the expected return.
Methods ::: Shakespearizing Poetic Captions
For Shakespearizing modern English texts, we experimented with various types of sequence to sequence models. Since the size of the parallel translation data available is small, we leverage a dictionary providing a mapping between Shakespearean words and modern English words to retrofit pre-trained word embeddings. Incorporating this extra information improves the translation task. The large number of shared word types between the source and target sentences indicates that sharing the representation between them is beneficial.
Methods ::: Shakespearizing Poetic Captions ::: Seq2Seq with Attention
We use a sequence-to-sequence model which consists of a single layer unidrectional LSTM encoder and a single layer LSTM decoder and pre-trained retrofitted word embeddings shared between source and target sentences. We experimented with two different types of attention: global attention BIBREF9, in which the model makes use of the output from the encoder and decoder for the current time step only, and Bahdanau attention BIBREF10, where computing attention requires the output of the decoder from the prior time step. We found that global attention performs better in practice for our task of text style transfer.
Methods ::: Shakespearizing Poetic Captions ::: Seq2Seq with a Pointer Network
Since a pair of corresponding Shakespeare and modern English sentences have significant vocabulary overlap we extend the sequence-to-sequence model mentioned above using pointer networks BIBREF11 that provide location based attention and have been used to enable copying of tokens directly from the input. Moreover, there are lot of proper nouns and rare words which might not be predicted by a vanilla sequence to sequence model.
Methods ::: Shakespearizing Poetic Captions ::: Prediction
For both seq2seq models, we use the attention matrices returned at each decoder time step during inference, to compute the next word in the translated sequence if the decoder output at the current time step is the UNK token. We replace the UNKs in the target output with the highest aligned, maximum attention, source word. The seq2seq model with global attention gives the best results with an average target BLEU score of 29.65 on the style transfer dataset, compared with an average target BLEU score of 26.97 using the seq2seq model with pointer networks.
Results
We perform a qualitative analysis of the Shakespearean prose generated for the input paintings. We conducted a survey, in which we presented famous paintings including those shown in Figures FIGREF1 and FIGREF10 and the corresponding Shakespearean prose generated by the model, and asked 32 students to rate them on the basis of content, creativity and similarity to Shakespearean style on a Likert scale of 1-5. Figure FIGREF12 shows the result of our human evaluation.
The average content score across the paintings is 3.7 which demonstrates that the prose generated is relevant to the painting. The average creativity score is 3.9 which demonstrates that the model captures more than basic objects in the painting successfully using poetic clues in the scene. The average style score is 3.9 which demonstrates that the prose generated is perceived to be in the style of Shakespeare.
We also perform a quantitative analysis of style transfer by generating BLEU scores for the model output using the style transfer dataset. The variation of the BLEU scores with the source sentence lengths is shown in Figure FIGREF11. As expected, the BLEU scores decrease with increase in source sentence lengths.
Results ::: Implementation
All models were trained on Google Colab with a single GPU using Python 3.6 and Tensorflow 2.0. The number of hidden units for the encoder and decoder is 1,576 and 256 for seq2seq with global attention and seq2seq with pointer networks respectively. Adam optimizer was used with the default learning rate of 0.001. The model was trained for 25 epochs. We use pre-trained retrofitted word embeddings of dimension 192.
Results ::: Limitations
Since we do not have an end-to-end dataset, the generated English poem may not work well with Shakespeare style transfer as shown in Figure FIGREF12 for "Starry Night" with a low average content score. This happens when the style transfer dataset does not have similar words in the training set of sentences. A solution would be to expand the style transfer dataset, for a better representation of the poem data.
Results ::: Conclusions and Future Work
In conclusion, combining two pipelines with an intermediate representation works well in practice. We observe that a CNN-RNN based image-to-poem net combined with a seq2seq model with parallel text corpus for text style transfer synthesizes Shakespeare-style prose for a given painting. For the seq2seq model used, we observe that it performs better in practice using global attention as compared with local attention. We make our models and code publicly available BIBREF12. In future work we would like to experiment with GANs in the absence of non-parallel datasets, so that we can use varied styles for text style transfer. We would also like to experiment with cross aligned auto-encoders, which form a latent content representation, to efficiently separate style and content.
Please answer the question based on the text provided above. What limitations do the authors demnostrate of their model?

Answer: Since we do not have an end-to-end dataset, the generated English poem may not work well with Shakespeare style transfer we do not have an end-to-end dataset, the generated English poem may not work well with Shakespeare style transfer as shown in Figure FIGREF12 for "Starry Night" with a low average content score