Towards Fully Automated Manga Translation

12/28/2020 ∙ by Ryota Hinami, et al. ∙ 60

We tackle the problem of machine translation of manga, Japanese comics. Manga translation involves two important problems in machine translation: context-aware and multimodal translation. Since text and images are mixed up in an unstructured fashion in Manga, obtaining context from the image is essential for manga translation. However, it is still an open problem how to extract context from image and integrate into MT models. In addition, corpus and benchmarks to train and evaluate such model is currently unavailable. In this paper, we make the following four contributions that establishes the foundation of manga translation research. First, we propose multimodal context-aware translation framework. We are the first to incorporate context information obtained from manga image. It enables us to translate texts in speech bubbles that cannot be translated without using context information (e.g., texts in other speech bubbles, gender of speakers, etc.). Second, for training the model, we propose the approach to automatic corpus construction from pairs of original manga and their translations, by which large parallel corpus can be constructed without any manual labeling. Third, we created a new benchmark to evaluate manga translation. Finally, on top of our proposed methods, we devised a first comprehensive system for fully automated manga translation.

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Introduction

Comics are popular all over the world. There are many different forms of comics around the world, such as manga in Japan, webtoon in Korea, and manhua in China, all of which have their own unique characteristics. However, due to the high cost of translation, most comics have not been translated and are only available in their domestic markets. What if all comics could be immediately translated into any language? Such a panacea for readers could be made possible by machine translation (MT) technology. Recent advances in neural machine translation (NMT) 

Cho et al. (2014); Sutskever, Vinyals, and Le (2014); Bahdanau, Cho, and Bengio (2015); Wu et al. (2016); Vaswani et al. (2017) have increased the number of applications of MT in a variety of fields. However, there are no successful examples of MT for comics.

Figure 1: Given a manga page, our system automatically translates the texts on the page into English and replaces the original texts with the translated ones. ©Mitsuki Kuchitaka.

What makes the translation of comics difficult? In comics, an utterance by a character is often divided up into multiple bubbles. For example, in the manga page shown on the left side of Fig. 1, the female’s utterance is divided into bubbles #1 to #3 and the male’s into #4 to #5. Since both the subject “I” and the verb “know” are omitted in the bubble #3, it is essential to exploit the context from the previous or next bubbles. What makes the problem even more difficult is that the bubbles are not simply aligned from right to left, left to right, or top to bottom. While the bubble #4 is spatially closed to #1, the two utterances are not continuous. Thus, it is necessary to parse the structure of the manga to recognize the texts in the correct order. In addition, the visual semantic information must be properly captured to resolve the ambiguities. For example, some Japanese word can be translated into both “he”, “him”, “she”, or “her” so it is crucial to capture the gender of the characters.

These problems are related to context and multimodality; considering context is essential in comics translation and we need to understand an image to capture the context. Context-aware Jean et al. (2017); Tiedemann and Scherrer (2017); Wang et al. (2017) and multimodal translation Specia et al. (2016); Elliott et al. (2017); Barrault et al. (2018) are both hot topics in NMT but researched independently. Both are important in various kinds of application, such as movie subtitles or face-to-face conversations. However, there have not been any study on how to exploit context with multimodal information. In addition, there are no public corpus and benchmarks for training and evaluating models, which prevents us from starting the research of multimodal context-aware translation.

Contributions

This paper addresses the problem of translating manga, meeting the grand challenge of fully automated manga translation. We make the following four contributions that establish a foundation for research on manga translation.

Multimodal context-aware translation. Our primary contribution is a context-aware manga translation framework. This is the first approach that incorporates context information obtained from an image into manga translation. We demonstrated it significantly improves the performance of manga translation and enables us to translate texts that are hard to be translated without using context information, such as the example presented above with Fig. 1.

Automatic parallel corpus construction.

Large in-domain corpora are essential to training accurate NMT models. Therefore, we propose a method to automatically build a manga parallel corpus. Since, in manga, the text and drawings are mixed up in an unstructured manner, we integrate various computer vision techniques to extract parallel sentences from images. A parallel corpus containing four million sentence pairs with context information is constructed automatically without any manual annotation.

Manga translation dataset. We created a multilingual manga dataset, which is the first benchmark of manga translation. Five categories of Japanese manga were collected and translated. This dataset is publicly available.

Fully automatic manga translation system. On the basis of the proposed methods, we built the first comprehensive system that translates manga fully automatically from image to image. We achieved this capability by integrating text recognition, machine translation, and image processing into a unified system.

Related Work

Context-aware machine translation

Despite the recent rapid progress in NMT Cho et al. (2014); Sutskever, Vinyals, and Le (2014); Bahdanau, Cho, and Bengio (2015); Wu et al. (2016); Vaswani et al. (2017), most models are not designed to capture extra-sentential context. The sentence-level NMT models suffer from errors due to linguistic phenomena such as referential expressions (e.g., outputting ‘him’ when correct output is ‘her’) or omitted words in the source text Voita, Sennrich, and Titov (2019b). There have been interests in modeling extra-sentential context in NMT to cope with these problems. The previously proposed methods aimed at context-aware NMT can be categorized into two types: (1) extending translation units from a single sentence to multiple sentences  Tiedemann and Scherrer (2017); Bawden et al. (2018); Scherrer, Tiedemann, and Loáiciga (2019); and (2) adding modules that capture context information to NMT models Jean et al. (2017); Wang et al. (2017); Tu et al. (2018); Werlen et al. (2018); Voita et al. (2018); Maruf and Haffari (2018); Zhang et al. (2018); Maruf, Martins, and Haffari (2019); Xiong et al. (2019); Voita, Sennrich, and Titov (2019a, b).

While the various methods have been evaluated on different language pairs and domains, we mainly focused on Japanese-to-English translation in manga domains. Our scene-based translation is deeply related to 2+2 translation Tiedemann and Scherrer (2017), which incorporates the preceding sentence by prepending it to be the current one. While it captures the context in the previous sentence, our scene-based model considers all the sentences in a single scene.

Multimodal machine translation

The manga translation task is also related to multimodal machine translation (MMT). The goal of the MMT is to train a visually grounded MT model by using sentences and images Harnad (1990); Glenberg and Robertson (2000). More recently, the NMT paradigm has made it possible to handle discrete symbols (e.g., text) and continuous signals (e.g., images) in a single framework Specia et al. (2016); Elliott et al. (2017); Barrault et al. (2018).

The manga translation can be considered as a new challenge in the MMT field for several reasons. First, the conventional MMT assumes a single image and its description as inputs Elliott et al. (2016). However, manga consists of multiple images with context, and the texts are drawn in the images. Second, the commonly used pre-trained image encoders Russakovsky et al. (2015) cannot be used to encode manga images as they are all trained on natural images. Third, no parallel corpus is available in the manga domain. We tackled these problems by developing a novel framework to extract visual/textual information from manga images and an automatic corpus construction method.

Figure 2: Proposed manga translation framework. N’ represents the translation of a source sentence N. ©Mitsuki Kuchitaka

Context-Aware Manga Translation

Now let us introduce our approach to manga translation that incorporates the multimodal context. In this section, we will focus on the translation of texts with the help of image information, assuming that the text has already been recognized in an input image. Specifically, suppose we are given a manga page image and unordered texts on the page. The texts are denoted as , where . We are also given a bounding box for each text: . Our goal is to translate each text into another language .

The most challenging problem here is that we cannot translate each independently of each other. As discussed in the introduction, incorporating texts in other speech bubbles is indispensable to translate each . In addition, visual semantic information such as the gender of the character sometimes helps translation. We first introduce our approach to extracting such context from an image and then describe our translation model using those contexts.

Extraction of Multimodal Context

We extract three types of context, i.e., scene, reading order, and visual information, which are all useful information for multimodal context-aware translation. The left side of Fig. 2 illustrates the three procedures explained below 1)–3).

1) Grouping texts into scenes:

A single manga page includes multiple frames, each of which represents a single scene. In the translation of the story, the texts included in the same scene are usually more useful for translation than the texts in a different scene. Therefore, we group texts into scenes to determine the ones useful as contexts. First, we detect frames in a manga page using an object detector by regarding each frame as an object in the manner of Ogawa et al. (2018). In particular, we trained the Faster R-CNN detector Ren et al. (2015) with the Manga109 dataset Matsui et al. (2017). Given a manga page, the detector outputs a set of scenes . Each scene is represented as a bounding box of a frame: . For each text , we find the scene that the text belongs to. Such a scene is defined as one that maximally overlaps the bounding box of the text. This is determined by an assignment function , where , where computes the intersection over the union for two boxes.

2) Ordering texts:

Next, we estimate the reading order of the texts. More formally, we sort the unordered set

to make an ordered set as shown in the left side of Fig. 2. Since, in manga, a single sentence is usually divided up into multiple text regions, it is quite important to ensure the text order is correct. Manga is read on a frame-by-frame basis. Therefore, the reading order of the texts is determined from the order of 1) the frames and 2) the texts in each frame. We estimate the order of the frames from the general structure of manga: each page consists of one or more rows, each consisting of one or more columns, recursively repeating. Each page is read sequentially from the top row, and each row is read from the right column. On the basis of this knowledge, we estimate the reading order by recursively splitting mange page vertically and horizontally. Afterward, the reading order of the texts in each frame is determined by the distance from the upper right point of each frame. Even though this approach does not use any supervised information, it accurately estimates the reading order of the frames. Some examples are shown in Fig. D We confirmed that it could identify the correct reading order of 91.9% of the 258 pages we tested (we evaluate with PubManga dataset introduced in the experiments section). The remaining 8.2% were irregular cases (e.g., diagonally separated, multiple frames overlapping, etc.).

  
Figure 3: Results of our text and frame order estimation. The bounding boxes of text and frame are shown in the green and red rectangles, respectively. The estimated orders of text and frame are depicted at the upper left corner of bounding boxes. ©Mitsuki Kuchitaka

3) Extracting visual semantic information:

Finally, we extract visual semantic information, such as the objects appearing in the scene. To exploit the visual semantic information in each scene, we predict semantic tags for each scene by using the illustration2vec model Saito and Matsui (2015). Given a target scene , the illustration2vec module describes the scene by predicting semantic tags: . In the illustration2vec model, contains 512 pre-defined semantic tags: . Several tags can be predicted from a single scene. Although we tried integrating a deep image encoder as is done in many multimodal tasks Zhou et al. (2018); Fukui et al. (2016); Vinyals et al. (2015), it did not improve performance on our tasks.

We should emphasize that this framework is not limited to manga. It can be extended to any kind of media having multimodal context, including movies and animations, by properly defining the scene. For example, it can be easily applied to movie subtitle translation by extracting contexts in three steps: 1) segmenting videos into scenes, 2) ordering texts by time, and 3) extracting semantic tags by video classification.

Context-aware Translation Model

To incorporate the extracted multimodal context into MT, we take a simple yet effective concatenation approach Tiedemann and Scherrer (2017); Junczys-Dowmunt (2019): concatenate multiple continuous texts and translate them with a sentence-level NMT model all at once. Note that any NMT architecture can be incorporated with this approach. In this study, we chose the Transformer (big) model and set its default parameters in accordance with Vaswani et al. (2017). The right side of Fig. 2 illustrates the three models explained below.

Model1: 2+2 translation.

The simplest method utilizes the previous text as context. To train and test the model, we prepend the previous text in the source and target languages Tiedemann and Scherrer (2017). That is, to translate into , two texts and are fed into the translation model, which outputs and . The boundary of the two texts is marked with a special token <SEP>.

Model2: Scene-based translation.

Considering only the previous text as the context is not always sufficient. We may want to consider two or more previous texts or even the subsequent texts in the same scene. To enable this, we generalize the 2+2 translation by concatenating all the texts in each frame and translating them all at once. This procedure is illustrated as follows. Suppose we would like to translate to . Unlike Model1 that makes use of only, we feed into the model.

Model3: Scene-based translation with visual feature

To incorporate the visual information into Model2, we prepend the predicted tags to the sequence of the input texts. Each tag is represented as a special token, such as <1GIRL> or <1BOY>. Note that this does not lead to any changes in the model itself. By adding the tags as input, we let the model consider the visual information when needed. This means that, to translate into , we additionally input .

Parallel Corpus Construction

We propose the approach to automatic corpus construction for training our translation model. Given a pair of manga books as input: a Japanese manga and its English-translation, our goal is to extract parallel texts with context information that can be used to train the proposed model. This is a challenging problem because manga is regarded as a sequence of images without any text data. Since texts are scattered all over the image and are written in various styles, it is difficult to accurately extract texts and group them into sentences. In addition, even when sentences are correctly extracted from manga images, it is difficult to find the correct correspondence between sentences in different languages. The differences in text direction from one language to another (e.g., vertical in Japanese and horizontal in English) makes this problem harder. We solve this problem by using computer vision techniques by fully utilizing the structural features of manga images, such as the pixel-level locations of the speech bubbles.

Terms and available labeled data

First though, let us define the terms associated with manga text; Fig. 4 illustrates speech bubbles, text regions, and text lines. One speech bubble contains one or more text regions (i.e., paragraph), each comprising one or more text lines. We assume that only the annotation of speech bubbles is available for training models; annotations of text lines and text regions are unavailable. In addition, segmentation masks of speech bubbles and any data in the target language are also unavailable. This is a natural assumption because current public datasets only have speech bubble-level bounding box annotations of the Japanese version for manga Matsui et al. (2017) and those of English version for American-style comics Iyyer et al. (2017); Guérin et al. (2013). This limitation on labeled data is one of the challenges of parallel text extraction from comics. Note that our approach does not depend on specific languages. We also applied it to Chinese as a target language in addition to English, which is demonstrated later in Fig. 9.

Figure 4: Term definition for manga text. ©Mitsuki Kuchitaka
Figure 5: Proposed framework of parallel corpus construction.

Training of Detectors

We train two object detectors: speech bubble and text line detectors, which is the basic building block of our corpus construction pipeline. We use Faster R-CNN model with ResNet101 backbone He et al. (2016) for both object detectors. The object detectors are trained with the annotation of bounding boxes. While the speech bubble detector could be trained with public datasets (e.g., Manga 109), the annotations of the text lines were not available. Therefore, we devised a way to generate annotations of text lines from the speech bubble-level annotation in a weakly supervised manner. Fig. 6 illustrates the process of generating annotations. Suppose we have images with annotations of the speech bubbles’ bounding boxes and texts. In this paper, we use the annotations of Manga109 dataset Ogawa et al. (2018); Aizawa et al. (2020). We detect the text line with a rules-based approach (whose algorithm is described in supplementary material); then we recognize characters using our text line recognition module. If the recognized text perfectly matches the ground truth, we consider the text lines to be correct and use them as the annotation of text lines. Other text regions that were not recognized are filled in with white (e.g., inside the dotted rectangle in Fig. 6). The object detector is trained with the generated images and bounding box annotations. Although the rules-based approach sometimes misses complicated patterns such as bubbles containing multiple text regions, the object detector can detect them by capturing the intrinsic properties of text lines.

Figure 6: Generation of textline annotations.©Yasuyuki Ohno

Extraction of Parallel Text Regions

Fig. 5 (a)–(g) illustrates the proposed pipeline for extracting parallel text regions.

(a) Pairing pages. Let us define an input Japanese manga as a set of images (pages), denoted as . Similarly, let us define the English manga as a set of pages: . Note that typically , because pages such as the front cover, table of contents, and illustration can be optionally included or removed during the production of the translation. Owing to such inconsistencies, we must find page-wise correspondences first as shown in Fig. 5

(a). We find the correspondences by global descriptor-based image retrieval combined with spatial verification 

Radenović et al. (2018). For each Japanese page , we first retrieve the English image with the highest similarity to from , where the similarity of two pages is computed as the

distance of global features extracted by the deep image retrieval (DIR) model 

Gordo et al. (2016). We then apply spatial verification Philbin et al. (2007) to reject false matching pairs. The homography matrix between two pages is estimated by RANSAC Fischler and Bolles (1981) with AKAZE descriptors Alcantarilla, Nuevo, and Bartoli (2013). If the number of inliers in RANSAC is more than 50, we decide that and are corresponding.

(b) Detection of text boxes. After the page-aligning step, we obtain a set of corresponding pairs of English and Japanese pages. Hereafter, we discuss how to extract a parallel corpus from a single pair, and . First, the bounding boxes of the speech bubbles are obtained by applying the speech bubble detector to .

(c) Pixel-level estimation of speech bubbles. We estimate the precise pixel-level mask for each bubble from the bounding box. We employ edge detection with canny detector Canny (1986) to detect the contour of speech bubbles. For each bounding box of a speech bubble, we select the connected component of non-edge pixels that shares the largest area with the bounding box, which is the blank area inside the speech bubble. In this way, we precisely estimate the masks of the speech bubbles without having to worry about how to train a semantic segmentation model that cannot be trained with the currently available dataset.

(d) Splitting connected speech bubbles. As illustrated in Fig. 4, sometimes a speech bubble includes multiple text regions (i.e., paragraphs). We split up such speech bubbles in order to identify the text regions by clustering the text lines. The text lines obtained by the object detector are then grouped into paragraphs by clustering the vertical coordinates at the top of text lines with MeanShift Comaniciu and Meer (2002). Finally, masks are split so that all text regions are perfectly separated, and the length of the boundary (i.e., splitting length) is minimized.

(e) Alignment between languages. We then estimate the masks of text regions for by aligning and . Since the scales and margins are often different between and , is transformed so that the two images overlap exactly. We update by applying a perspective transformation: , where indicates the transformation computed in the previous page pairing step. The resulting page has a better pixel-level alignment so that text regions in can be easily localized from the text regions in . Such a correspondence is made possible by the distinctive nature of manga: the translated text is located in the same bubble. Note that we do not use any learning-based models for in steps 1)–5), so our method can be used for any target language even if a dataset for learning detectors is unavailable.

(f) Text recognition. Given the segmentation masks of the text regions, we recognize the characters for each image pair and . Since we found that existing OCR systems perform very poorly on manga text due to the variety of fonts and styles that are unique to manga text, we developed an OCR module optimized for manga. We developed our own text rendering engine that generates text images optimized for manga. Five millions of text images are generated with the engine, by which we train the OCR module based on the model of Baek et al. Baek et al. (2019). Technical details of this component are described in the supplementary material.

(g) Context extraction. We extract the context information (i.e., the reading order and scene labels of each text) from in the manner described in the previous section.

Experiments

Dataset

Although there are no manga/comics datasets comprising of multiple languages, we created two new manga datasets, i.e., OpenMantra and PubManga, one to evaluate the MT, the other to evaluate the constructed corpus.

OpenMantra:

While we need a ground-truth dataset to evaluate the NMT models, no parallel corpus in the manga domain is available. Thus, we started by building OpenMantra, an evaluation dataset for manga translation. We selected five Japanese manga series across different genres, including fantasy, romance, battle, mystery, and slice of life. In total, the dataset consists of 1593 sentences, 848 frames, and 214 pages. After that, we asked professional translators to translate the whole series into English and Chinese. This dataset is publicly available for research purposes.111https://github.com/mantra-inc/open-mantra-dataset

PubManga:

OpenMantra is not appropriate for evaluating the constructed corpus because translated versions are created by ourselves. Thus, we selected nine Japanese manga series across different categories, each having 18–40 pages (258 pages in total), and created another dataset of published translations (PubManga). This dataset includes annotations of 1) bounding boxes of the text and frame, 2) texts (character sequence) in both Japanese and English, and 3) the reading order of the frames and texts. The annotations and full list of manga titles are available upon request.

System Training corpus Translation unit Human BLEU
Without context
  Google Translate222We ran the test on Apr. 17, 2020. N/A sentence - 8.72
  Sentence-NMT (OS18) OpenSubtitles2018 sentence 2.11 9.34
  Sentence-NMT (Manga) Manga Corpus sentence 2.76 14.11
With context
  2 + 2  Tiedemann and Scherrer (2017) Manga Corpus 2 sentences 2.85 12.73
  Scene-NMT Manga Corpus frame 2.98* 12.65
  Scene-NMT w/ visual Manga Corpus frame 2.91* 12.22
Table 1: System description and translation performances on the OpenMantra Ja–En dataset. * indicates the result is significantly better than Sentence-NMT (Manga) at .

Evaluation of Machine Translations

To confirm the effectiveness of our models and Manga corpus, we ran translation experiments on the OpenMantra dataset.

Training corpus:

To train the NMT model for manga, we collected training data by the proposed corpus construction approach. We prepared 842,097 pairs of manga pages that were published in both Japanese and English. Note that all the pages are in digital format without textual information. 3,979,205 pairs of Japanese–English sentences were obtained automatically. We randomly excluded 2,000 pairs for validation purposes.

In addition, we used OpenSubtitles2018 (OS18) Lison, Tiedemann, and Kouylekov (2018), a large-scale parallel corpus to train a baseline model. Most of the data in OS18 are conversational sentences extracted from movie and TV subtitles, so they are relatively similar to the text in manga. We excluded 3K sentences for the validation and 5K for the test and used the remaining 2M sentences for training.

Methods:

Table 1 shows the six systems used in our evaluation. Google Translate333https://translate.google.com/ is an NMT system used in several domains, but the sizes and domains of its training corpus have not been disclosed. We chose the Sentence-NMT (OS18) as another baseline. The model is trained with the OS18 corpus; therefore, there are no manga domain texts included in its training data. The Sentence-NMT (Manga) was trained on our automatically constructed Manga corpus described in the previous section. Sentence-NMT (OS18) and Sentence-NMT (Manga) use the same sentence-level NMT model.

While the first three systems are sentence-level NMTs, the fourth to sixth ones are proposed context-aware NMT models. We set 2 + 2 Tiedemann and Scherrer (2017) (Model1) as the baseline and compared their performance with those of our Scene-NMT models with and without visual features (Model3 & Model2, respectively).

Evaluation procedure:

Manga translation differs from plain text translation because the content of the images influences the “feeling” of the text. To examine how readers actually feel when reading a translated page, we conducted a manual evaluation of translated pages instead of plain texts. We recruited En–Ja bilingual manga readers. They were given a Japanese page and translated English ones, and they were asked to evaluate the quality of the translation of each English page. Following the procedure in the Workshop on Asian Translation Nakazawa et al. (2018), we asked five participants to score the texts from 1 (worst; less than 20% of the important information is correctly translated) to 5 (best; all important information is correctly translated). All the methods explained above other than Google Translate were compared. The order of presenting the methods was randomized. In total, we collected 5 participants 5 methods 214 pages samples. See the supplemental material for the details of the evaluation system. We also conducted an automatic evaluation using the BLEU Papineni et al. (2002).

Results:

Table 1 shows the results of the manual and automatic evaluation. The huge improvement of the Sentence-NMT (Manga) over Google Translate and Sentence-NMT (OS18) indicates the effectiveness of our strategy of Manga corpus construction.

A pair-wise bootstrap resampling test Koehn (2004) on the results of the human evaluation shows that the Scene-NMT outperformed the Sentence-NMT (Manga). On the other hand, there is no statistically significant difference between 2 + 2 and Sentence-NMT (Manga). These results suggest that not only the contextual information but also the appropriate way to group them is essential for accurate translation.

Figure 7: Outputs of the sentence-based (center) and frame-based (right) models. The values after H and B are respectively the human evaluation and BLEU scores for each page. ©Mitsuki Kuchitaka

In contrast to the results of the human evaluation, the BLEU scores of the context-aware models (fourth to sixth lines in Table 1) are worse than that of Sentence-NMT (Manga). These results suggest that the BLEU is not suitable for evaluating manga translations. Fig. 7 shows an example where the Scene-NMT outperformed Sentence-NMT (Manga) in the manual evaluation but had lower BLEU scores. Here, we can see that only the Scene-NMT has swapped the order of the texts. This flexibility naturally resolves the differences in word order between Japanese and English. However, it results in a worse BLEU score since the references usually maintain the original order of the texts.

Figure 8: Translation results with and without visual features. ©Miki Ueda, ©Satoshi Arai.
Figure 9: Results of fully automatic manga translation from Japanese to English and Chinese. ©Masami Taira, ©Syuji Takeya

Although there is no statistically significant difference between Scene-NMT and Scene-NMT w/ visual, Fig. 8 shows some promising results; pronouns (“you” and “her”) that cannot be estimated from textual information are correctly translated by using visual information. These examples indicate that we need to combine textual and visual information to appropriately translate the content of manga. However, we found that a large portion of the errors of Scene-NMT w/ visual are caused by the incorrect visual features. To fully understand the impact of the visual feature (i.e., semantic tags) on translation, we conducted an analysis in Fig. 10: (i) and (ii) in the figure show the outputs of the Scene-NMT and Scene-NMT w/ visual, respectively. The pronoun errors in (ii) are caused by the incorrect visual feature “Multiple Girls” extracted from the original image. When we overwrote the character face with a male face, Scene-NMT w/ visual output the correct pronouns, as shown in (iii). This result proved that Scene-NMT w/ visual model consider visual information to determine translation results, and it would be improved if we devise a way to extract visual features more accurately. Designing such a good recognition model for manga images remains as future work.

Figure 10: Translations output by the sentence-based model without (i) and with visual information (ii). By overwriting the character face in the input image (a) with a male face (b), the pronouns in the translation results (iii) are also changed. ©Nako Nameko

Evaluation of Corpus Construction

To evaluate the performance of corpus construction, we compared the following four approaches: 1) Box: Bounding boxes by the speech bubble detector are used as text regions instead of segmentation masks. This is the baseline of a simple combination of speech bubble detection and OCR. 2) Box-parallel: Bounding box of speech bubbles are detected in both Japanese and English images by applying detector to both images. For each detected Japanese box, the English box that overlaps it most is selected as the corresponding box. 3) Mask w/o split: Segmentation masks of speech bubbles are estimated, but the process of splitting the masks is not done. 4) Mask w/ split (the full proposed method): Segmentation masks of speech bubbles are estimated, and connected bubbles are split. This fully utilizes the structural feature of the manga images. In 1) and 2) the regions of bounding boxes are regarded as the mask of text regions.

The corpus construction performances were evaluated on the PubManga dataset; the results are listed in Tab. 2. For a % and % match, the text pair with a normalized edit distance Karatzas et al. (2013)

between the ground truth and extracted texts of more than 0.9 and 0.7 were considered true positives, respectively; this allowed for some OCR mistakes because the accuracy of the OCR module is not the main focus of this experiment. This result shows that our approach that uses mask estimation is significantly better than the two approaches that use only bounding-box regions. Mask splitting also significantly improved both precision and recall. The bounding box-based approaches fail to identify the regions of English text, especially when the shapes of the text regions are different from those of the Japanese text; this problem is caused by the difference in text direction. These results indicate that parallel corpus extraction from manga cannot be done with the simple combination of OCR and object detection; exploiting structural information manga is effective. Note that we use the same OCR and detection modules in these experiments. The details of the evaluations are provided in the supplementary material.

% match % match
Method Recall Prec. Recall Prec.
Box 0.267 0.365 0.434 0.594
Box-parallel 0.246 0.614 0.289 0.722
Mask w/o split 0.381 0.522 0.480 0.657
Mask w/ split 0.584 0.653 0.688 0.769
Table 2: Corpus construction performance on the PubManga.

Fully Automated Manga Translation System

We launch a fully automated manga translation system on top of the proposed model trained with the constructed corpus. Given a Japanese manga page, the system automatically recognizes texts, translates them into target language, and replaces the original texts with the corresponding translated texts. It performs the following steps.

1) Text detection and recognition: Given a Japanese input page, the system recognizes texts in the same way as in the corpus construction. This step predicts masks of the text regions and Japanese texts with their contexts.

2) Translation: Japanese texts are translated into the target languages by using the trained NMT model. Since our approach to translation and corpus construction does not depend on a specific language, we can translate the Japanese text into any target language if unlabeled manga book pairs for constructing corpus are available.

3) Cleaning:

The original Japanese texts are removed from the translation. We employ an image inpainting model for this; the regions of text lines are replaced by the inpainting model, by which texts are removed clearly even when they are on image texture or drawing. We used edge-connect 

Nazeri et al. (2019), because its edge-first approach is very good at complementing defects of drawings.

4) Lettering: Finally, the translated texts are rendered with optimized font size and location on the cleaned image. The location is one that maximizes the font size under the condition that all texts are inside the text region.

Examples. Fig. 9 shows the translations produced by our system. It demonstrates that our system can automatically translate Japanese manga into English and Chinese.

Conclusion & Future Work

We established a foundation for the research into manga translation by 1) proposing multimodal context-aware translation method and 2) automatic parallel corpus construction, 3) building benchmarks, and 4) developing a fully automated translation system. Future work will look into 1) an image encoding method that can extract continuous visual information that helps translation, 2) an extension of scene-based NMT to capture longer contexts in other scenes and pages, and 3) a framework to train the image recognition models and the NMT model jointly for more accurate end-to-end performance.

Acknowledgement

This work was partially supported by IPA Mitou Advanced Project, FoundX Founders Program, and the UTokyo IPC 1st Round Program.

The authors would like to appreciate Ito Kira, Mitsuki Kuchitaka, and Nako Nameko for providing their manga for research use, and Morisawa Inc. for providing their font data. We also thank Naoki Yoshinaga and his research group for the fruitful discussions before the submission. Finally, we thank the anonymous reviewers for their careful reading of our paper and insightful comments.

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Dataset details

Table A describes the details of datasets used in our experiments. Manga corpus is used to train our machine translation models. OpenMantra is used to evaluate machine translation. PubManga is used to evaluate corpus extraction and text ordering. Manga109 is used to train and evaluate object detectors.

Rule-based Text Line Detection

Here, we introduce the rule-based approach to detect text lines without learning. The benefit of this approach is that it does not require any labeled data. We use this method in two parts: 1) detecting text lines of target languages (e.g., English and Chinese) in the corpus construction process and 2) generating training data for the Japanese text line detector. The steps of rule-based text line detection are visualized in Fig. A (a) and (b) for vertical texts, and in (c) and (d) for horizontal texts. Given a text region, we first apply an edge detector in order to obtain connected components that are candidates for characters (or parts of characters). For each pixel line (a column for vertical texts, or a row in horizontal texts) inside the text region, we check whether connected components are included (Figs. A (a) or (c)). Activated consecutive columns/rows are flagged as text line candidates, which are visualized as red or orange lines (Figs. A (b) and (d)). Candidates whose widths are less than half of the max widths of other candidates are removed, which removes ruby for Japanese text.

Character Recognition for Manga

Let us explain the details of text recognition of each text region, which is described as the step 6) of Fig. 5. We found that state-of-the-art OCR systems, such as the google cloud vision API, perform very poorly on manga text due to the variety of fonts and styles that are unique to manga text. Therefore, we developed a text recognition module optimized for manga. The characters in each text region are recognized in two steps: 1) the text lines are detected; 2) characters on each text line are recognized. Text lines for the Japanese image can be detected by a text line detector. For the image of the target language, we apply a rule-based text line detection explained in the above section; since the text regions are separated into paragraphs by a mask splitting process, we can accurately detect text lines even with a simple rule-based approach. Detected text lines are then fed into recognition models. For the recognition of each text line, we use the models trained on the data generated by our developed manga text rendering engine described below.

Figure A: Rules-based text-line detection.
vocab. augmentation score
random corpus color ruby bg Acc. NED
Tesseract footnote:tesseract n/a n/a 1.5 0.53
google cloud vision footnote:gcv n/a n/a 21.5 0.35
Ours w/o augmentation 33.6 0.30
38.4 0.28
39.3 0.28
Ours w/ augmentation 44.1 0.26
56.1 0.22
61.7 0.19
Table B: Text recognition performance on the Manga109.
Figure B: Procedures of rendering the training data.

Synthetic data generation

Noting the success of the synthetic dataset for scene text recognition Jaderberg et al. (2014, 2016), we decided to generate the training images synthetically. We developed our own text rendering engine to generate text images optimized for manga. Fig. B shows the rendering process and examples of synthetic text lines. The steps below correspond to each process in Fig. B (a)–(g).

  1. Text sampling. The character sequence to be rendered is generated in two ways: sampling from the manga text corpus or randomly generating characters. By using a corpus built from manga, the model can implicitly learn the language model. However, this procedure cannot handle certain irregular patterns. Therefore, we decided to combine these two approaches, i.e., choosing 90% from the corpus and 10% at random. The text length is randomly chosen from 2 to 10.

  2. Font rendering. A font is randomly selected from 586 and 1156 fonts for Japanese and English, respectively. The font size and weight are also varied randomly.

  3. Ruby. Ruby, i.e., phonetic characters placed above Chinese characters, is added to 50% of the data for the Japanese text.

  4. Coloring. Foreground and background are filled with random colors.

  5. Background image composition. Since the texts in manga are sometimes overlaid on the images, the images from the Manga109 dataset are used as the background images for 20% of the data.

  6. Noise. JPEG noise is added to the images.

  7. Distortion. An affine transformation is used to distort the image.

We generate five million of cropped line images with the processes above, which are used to train the model described below. As shown below, each of the above processes helps to improve text recognition accuracy.

Model

We follow the text recognition models introduced by Baek et al. Baek et al. (2019). The images of the text line are resized to and fed into the model. The vertical text lines are rotated 90 degrees before resizing. We tried the various combinations of modules described in Baek et al. (2019)

and found that the combination of spatial transformer network 

Jaderberg et al. (2015), ResNet backbone He et al. (2016), Bi-LSTM Cheng et al. (2017); Shi et al. (2016); Shi, Bai, and Yao (2017), and attention-based sequence prediction Cheng et al. (2017) performed the best.

Evaluation

We evaluated the text recognition module with the annotated text in the Manga109 dataset. Given each cropped speech bubble in the dataset, we recognized the characters in the bubble using our text recognition module. The metrics used here were the accuracy and normalized edit distance (NED) Karatzas et al. (2013). The accuracy was computed as the number of correctly recognized texts (perfect match) divided by the total number of text regions (=12,542 regions). Since there is no previous research on manga text recognition, we compared with two existing OCR systems as baselines: Tesseract OCR,444https://github.com/tesseract-ocr/tesseract

, one of the most popular open-source OCR engines, and

Google cloud vision API,555https://cloud.google.com/vision/, a pre-trained OCR API provided by Google.

text frame
Method input image size backbone
SSD-fork Ogawa et al. (2018) 300 VGG n/a 84.1 n/a 96.9
Faster R-CNN 500 ResNet-101 65.0 92.5 91.6 97.5
800 ResNet-101 69.3 94.4 92.5 97.6
1170 ResNet-101 71.2 94.9 92.5 97.7
1170 ResNet-50 70.9 94.8 90.7 97.5
1170 ResNet-101-FPN 70.3 94.4 92.5 97.7
1170 ResNeXt-101 70.4 94.5 92.9 98.5
RetinaNet 1170 ResNet-101 70.6 95.4 89.8 98.3
Table C: Text and frame detection performance on the Manga109 dataset
Figure C: GUI of the user study. ©Mitsuki Kuchitaka
        
(a)
(b)
(c)
(d)
Figure D: Results of our text and frame order estimation. The bounding boxes of text and frame are shown in green and red rectangle, respectively. Estimated order of text and frame are depicted at the upper left corner of bounding boxes. ©Ito Kira ©Mitsuki Kuchitaka ©Nako Nameko

Table B shows the text recognition performance. Our method achieves much better accuracy than the existing systems. This is because 1) we train the model using images obtained by our manga text rendering engine, and 2) the text line detection model optimized for manga can properly discriminate the main characters and ruby characters. We also performed an ablation study on our data augmentation process. It demonstrated that every component significantly improved accuracy. In particular, adding ruby characters and the background image improved accuracy by 12.0 and 5.6 points, respectively. Since ruby and the background image tended to be mistakenly recognized as part of a character, our model learns to ignore them by adding them to the training data.

Evaluation of Object Detection

We here evaluate the performance of object detection. We use a Faster R-CNN object detector to detect speech bubbles and frames in the manga image. In accordance with Ogawa et al. Ogawa et al. (2018)

, we use Manga109 annotation to train and test our method, where a speech bubble containing multiple text lines is considered as a single text area. We use the average precision (AP) as an evaluation metric of this task.

 Lin et al. (2014), average AP for IoU from 0.5 to 0.95 with a step size of 0.05, and with a threshold of are computed.

Table C shows the performance of object detection. We compare our method with the one proposed by Ogawa et al. Ogawa et al. (2018) because it is the state-of-the-art method of text detection in Manga images. Our system achieves significant improvements over the state-of-the-art method Ogawa et al. (2018); for text and for frame. Although Ogawa et al. Ogawa et al. (2018) reported that the performance of Faster R-CNN is much poorer than that of their SSD-based model, this is because they trained the Faster R-CNN as a multiclass object detector. They mentioned that it is usually difficult to train a multiclass detection model on comic images in the same way as a generic detection because some objects are overlapping significantly. Instead, we trained a Faster R-CNN with a single class. The table also shows several important tips for object detection in manga. For example, using a larger input size is effective for text classes, while it is not effective for frame classes because frames tend to be larger. Therefore, in practice, the computational time can be reduced by using a small-sized input for the frame class. In addition, several architectures that have had success in object detection tasks (RetinaNet and ResNeXt/FPN backbone Lin et al. (2017b); Xie et al. (2017); Lin et al. (2017a)) does not improve the accuracy of this task.

Hyperparameters of the NMT module

We implement a Transformer (big) model with the fairseq Ott et al. (2019) toolkit and set its default parameters in accordance with Vaswani et al. (2017). The model is trained using an Adam Kingma and Ba (2015)

optimizer. The hyperparameters of the model and optimizer are detailed in Tab. 

D.

# Layers of encoder 6
# Layers of decoder 6
# Dimensions of encoder embeddings 1024
# Dimensions of decoder embeddings 1024
# Dimensions of FFN encoder embeddings 4096
# Dimensions of FFN decoder embeddings 4096
# Encoder attention heads 16
# Decoder attention heads 16
of Adam 0.9
of Adam 0.98
Learning rate 0.001
Learning rate for warm-up 1e-07
Warm-up steps 4000

Dropout probability

0.3
Table D: Hyperparameters for training the NMT model.

GUI of User Study

The evaluation system for the user study was developed as a web application. The whole GUI is visualized in Fig. C. A Japanese page and its English translated page are shown to a participant. He/she selects the score for each sentence in the check box. Unlike the usual plain text translation, this study directly compares the translated pages.

More Examples of Text Ordering

Examples of our text and frame order estimation are shown in Fig. D. Fig. D (a)–(c) are successful cases; our approach can correctly estimate the order of texts and frames in a manga image with complex structure. Fig. D (d) shows a failure case; the system cannot handle some irregular cases such as the frames diagonally separated. Such irregular cases should be detected and processed separately, which has remained as future work.

Text Cleaning Examples

Fig. E shows the examples of text cleaning. Our inpainting-based method removes Japanese texts even if texts are on textures, although the complemented texture is a little different from original one.

Original Cleaned           Original Cleaned
Original Cleaned           Original Cleaned
Figure E: Examples of text cleaning. ©Mitsuki Kuchitaka ©Masako Yoshi ©Masaki Kato ©Miki Ueda

More End-to-End Translation Examples

Fig. F and G shows more results of our fully automatic manga translation system. The left images show the input pages, while the center and right figures show the translated results to English and Chinese.

(a) Original (Japanese)
(b) Translated (English)
(c) Translated (Chinese)
(d) Original (Japanese)
(e) Translated (English)
(f) Translated (Chinese)
Figure F: Examples of our translation. ©Syuji Takeya ©Hidehisa Masaki
(a) Original (Japanese)
(b) Translated (English)
(c) Translated (Chinese)
(d) Original (Japanese)
(e) Translated (English)
(f) Translated (Chinese)
Figure G: Examples of our translation. ©Masami Taira, ©Naoya Matsumori