Enhancing performance of mathematical expression detection in scientific document images

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY BUI HAI PHONG ENHANCING PERFORMANCE OF MATHEMATICAL EXPRESSION DETECTION IN SCIENTIFIC DOCUMENT IMAGES DOCTORAL DISSERTATION IN COMPUTER SCIENCE Hanoi−2021 MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY BUI HAI PHONG ENHANCING PERFORMANCE OF MATHEMATICAL EXPRESSION DETECTION IN SCIENTIFIC DOCUMENT IMAGES Major: Computer Science Code: 9480101 DOCTORAL DISSERTATION IN COMPUTER SCIENCE SUPERVISORS: 1.Assoc Prof Hoang Manh Thang 2.Assoc Prof Le Thi Lan Hanoi−2021 DECLARATION OF AUTHORSHIP I, Bui Hai Phong, declare that the thesis titled "Enhancing performance of mathematical expression detection in scientific document images" has been entirely composed by myself I assure some points as follows: This work was done wholly or mainly while in candidature for a Ph.D research degree at Hanoi University of Science and Technology The work has not be submitted for any other degree or qualifications at Hanoi University of Science and Technology or any other institutions Appropriate acknowledge has been given within this thesis where reference has been made to the published work of others The thesis submitted is my own, except where work in the collaboration has been included The collaborative contributions have been clearly indicated Hanoi, November, 2021 PhD Student SUPERVISORS 1.Assoc Prof Hoang Manh Thang 2.Assoc Prof Le Thi Lan i ACKNOWLEDGEMENT I decided to pursue a PhD in Computer Science at MICA International Research Institute, Hanoi University of Science and Technology (HUST) in 2017 It has been one of the best decisions I could have made HUST is a really special place where I have accumulated immense knowledge I would like to thank Executive Board and all members of MICA Research Institute, HUST for the kind support in the PhD course I wish to express my deepest gratitude to my supervisors Assoc.Prof Hoang Manh Thang and Assoc.Prof Le Thi Lan for their continuous instruction, advice and support in the PhD course The thesis cannot be fulfilled without the specific direction of my supervisors I wish to thank all members of Computer Vision Department, MICA Research Institute, HUST for the frequent support in the PhD course I wish to thank Executive Board and all members of School of Graduate Education; School of School of Electronics and Telecommunications and School of Information and Communication Technology, HUST for the specific comments and suggestion for the thesis I wish to thank all members of Faculty of Information Technology, Hanoi Architectural University for the support in the professional work in the completion of the PhD I wish to thank Professor Akiko Aizawa and members of Aizawa Laboratory, National Institute of Informatics, Tokyo, Japan where I have obtained many scientific experiences during the internship of the PhD I wish to thank anonymous reviewers for valuable comments during the completion of the PhD I gratefully acknowledge the funding from SAHEP HUST project number T2020SAHEP-008 and Domestic Master/ PhD Scholarship Programme of Vingroup Innovation Foundation 2019-2021 I wish to express my sincere gratitude to my family and friends for the continuous support and encouragement in the completion of the PhD Hanoi, 2021 Ph.D Student ii ABSTRACT Mathematical expressions (MEs) play an important role in scientific documents and a huge number of scientific documents have been produced over years Therefore, the demand of document digitization for researching and studying purposes has continuously increased Detection and recognition of MEs in documents are considered as essential steps for document digitization The detection of expressions aims to locate the position of expressions within documents Meanwhile, the recognition of MEs aims at converting expressions from image format to string In the documents, mathematical expressions are classified in two categories: isolated (displayed) and inline (embedded) expressions An isolated expression displays in a separate line, an inline expression is mixed with other components (texts) Mathematical expressions may consist of mathematical operators (e.g +, -, ì, ữ), functions (log, sin, cos) and variables (i, j, r) Large expressions may consist of multiple text lines Meanwhile, small expressions may consist of one character The accuracy of the detection of isolated expressions has been gradually improved However, the detection of inline expressions is considered as a challenging task In practice, the detection and recognition of MEs in document images are closely related The accuracy of the detection allows to obtain accuracy of the recognition In contrast, the incorrect detection may cause errors in the recognition of MEs This thesis presents three main contributions in the detection and recognition of MEs in scientific document images: (1) First, a hybrid method of two stages has been proposed for the effective detection of MEs At first stage, the layout analysis of entire document images is introduced to improve the accuracy of text line and word segmentation At second stage, both isolated and inline MEs in document images are detected Both hand-crafted and deep learning features are extensively investigated and combined to improve the detection accuracy In the handcrafted feature extraction approach, the Fast Fourier Transform (FFT) is applied for text line images for the detection of isolated MEs The Gaussian parameters of projection profile are applied as the feature extraction for the detection of inline MEs After the feature extraction, various machine learning classifiers have been fine tuned for the detection In the deep learning approach, the CNNs (Alexnet and ResNet) have been optimized for the detection of MEs The fusion of handcrafted and deep learning features based on the prediction scores has been applied The merit of the method is that it can operate directly on the ME images without the employment of character recognition (2) Second, an end-to-end framework for mathematical expression detection in sciiii entific document images is proposed without using any Optical Character Recognition (OCR) or Document Analysis techniques as in conventional methods The distance transform is firstly applied for input document images in order to take advantages of the distinguished features of spatial layout of MEs Then, the transformed images are fed into the Faster Region with Convolutional Neural Network (Faster R-CNN) that has been optimized to improve the accuracy of the detection Specifically, the optimization and generation strategies of anchor boxes of the Region Proposal Network have been proposed to improve the accuracy of expression detection of various sizes The proposed methods for the detection of MEs have been tested on two public datasets (Marmot and GTDB) The obtained accuracies of isolated and inline expressions in the Marmot dataset are 92.09% and 85.90% while those in the GTDB dataset are 91.04% and 85.15%, respectively The performance comparison with conventional methods shows the effectiveness of the proposed method (3) Finally, the detection and recognition of MEs have been integrated in a system The MEs in document images have been detected and recognized The recognition results are represented in Latex The application aims to support end users to use the detection and recognition of MEs in document images conveniently Hanoi, 2021 Ph.D Student iv CONTENTS DECLARATION OF AUTHORSHIP i ACKNOWLEDGEMENT ii ABSTRACT iii CONTENTS viii ABBREVIATIONS viii LIST OF TABLES LIST OF FIGURES xi xviii INTRODUCTION 0.1 Motivation 0.2 Scope of the thesis 0.3 Objectives of the thesis 0.4 Introduction of the ME detection and recognition 0.4.1 Introduction of MEs 0.4.2 Introduction of ME detection 0.4.3 Introduction of ME recognition 0.5 Contributions of this thesis 0.6 Structure of this thesis CHAPTER LITERATURE REVIEW 10 1.1 Document analysis 11 1.2 ME detection methods in document images 13 1.2.1 Rule-based detection 1.2.2 Handcrafted feature extraction methods for the ME detection 13 14 1.2.3 Deep neural network for ME detection 15 1.2.3.1 Deep neural networks 15 1.2.3.2 Deep neural network models for ME detection 21 1.3 ME recognition 22 1.3.1 Traditional approaches for ME recognition 22 1.3.2 Neural network approaches for ME recognition 25 1.4 Datasets and evaluation metrics 27 1.4.1 Datasets 27 1.4.2 Evaluation metrics 30 v 1.5 Existing systems for ME recognition 33 1.6 Summary of the chapter 35 CHAPTER THE DETECTION OF MEs USING THE LATE FUSION OF HANDCRAFTED AND DEEP LEARNING FEATURES 37 2.1 Overview of the proposed method 37 2.2 Page segmentation 38 2.3 Handcrafted feature extraction for ME detection 2.3.1 Handcrafted feature extraction for isolated ME detection 41 43 2.3.2 Handcrafted feature extraction for inline ME detection 47 2.4 Deep learning method for ME detection 53 2.5 Late fusion of handcrafted and deep learning features for ME detection 55 2.6 Post-processing for ME detection 59 2.7 Experimental results 61 2.7.1 Performance evaluation of the detection of MEs using different machine learning algorithms 61 2.7.2 Performance evaluation of the detection of MEs using the fusion of handcrafted and deep learning features with different operations 63 2.7.3 Performance evaluation of the detection of isolated and inline MEs on different public datasets 63 2.7.4 Evaluation of the impact of image resolution on the ME detection 66 2.7.5 Evaluation of the impact of the post-processing 68 2.7.6 Visualization of extracted features of images using the handcrafted and deep learning feature approaches 68 2.7.7 Error analysis and discussion 70 2.7.8 Measurement of execution time 74 2.8 Summary of the chapter 76 CHAPTER THE DETECTION OF MEs USING THE COMBINATION OF THE DISTANCE TRANSFORM AND FASTER R-CNN 78 3.1 Overview of the proposed method for ME detection using the DT and the Faster R-CNN 78 3.2 The detection of MEs using the DT and the Faster R-CNN 3.2.1 Distance transform of document image 79 79 3.2.2 ME detection using a Faster R-CNN 82 3.2.2.1 Region proposal network 83 3.2.2.2 Fully connected detection network 86 vi 3.2.2.3 Loss function of the training Faster R-CNN 87 3.3 Experimental results 89 3.3.1 Loss function of the training process of Faster R-CNN 89 3.3.2 Evaluation of the impact of the DT and anchor box generation to the performance of the ME detection 91 3.3.3 Comparison of Faster R-CNN models in ME detection 94 3.3.4 Comparison of the proposed and state-of-the-art methods used in ME detection 95 3.3.5 Performance comparison of the proposed method on cross datasets 97 3.3.6 Illustration of feature extraction of the Resnet-50 98 3.3.7 Error analysis and discussion 102 3.3.8 Measurement of execution time 103 3.4 Summary of the chapter 104 CHAPTER THE DETECTION AND RECOGNITION OF MEs IN DOCUMENT IMAGES 105 4.1 Overview of the proposed system for the detection and recognition of MEs 105 4.2 ME recognition using the WAP network 4.2.1 Watcher module of the WAP network 106 107 4.2.2 Parser module of the WAP network 108 4.2.3 Training the WAP network 112 4.3 Experimental results 4.3.1 Performance evaluation of the detection and recognition of MEs 112 114 4.3.2 Error analysis and discussion 119 4.3.3 Measurement of execution time 119 4.4 Summary of the chapter 120 CONCLUSIONS 121 PUBLICATIONS 123 Bibliography 125 vii ABBREVIATIONS No Abbreviation Meaning CNN Convolutional Neural Network DT Distance Transform ExpRate Expression Error Rate FFT Fast Fourier Transform Faster R-CNN Faster Regions Convolutional Neural Network GRU Gated Recurrent Unit HOG Histogram of Oriented Gradients HPP Horizontal Projection Profile IoU Intersection over Union 10 kNN k-Nearest Neighbour 11 LSTM Long-Short Term Memory 12 Mask R-CNN Mask Region with Convolutional Neural Network 13 ME Mathematical Expression 14 OCR Optical Charater Recognition 15 ResNet Residual Neural Network 16 RF Random Forest 17 RNN Recurrent Neural Network 18 ROIs Region of Interests 19 RPN Region Proposal Network 20 SSD Single Shot Detector 21 SVM Support Vector Machine 22 t-SNE t- Distributed Stochastic Neighbor Embedding 23 VPP Vertical Projection Profile 24 WAP Watcher Attend Parser Neural Network 25 WER Word Error Rate 26 YOLO You Only Look One viii 108, 11 GB GPU The average execution time of the ME recognition is 1.2 second per page One page consists of average 23 expressions The ME recognition using the WAP network obtains higher accuracy, however, the method performs slower than Tesseract and Infty Reader Tesseract and Infty Reader applied traditional image processing and handcrafted features for the recognition Therefore, the performance of Tesseract and Infty Reader is better than that of the WAP network 4.4 Summary of the chapter This chapter has presented a system for detection and recognition of expressions in document images The detection of expressions is performed by the DT and the Faster R-CNN The recognition expression is performed by using the WAP network The overall performance of the detection and recognition of MEs was evaluated on the Marmot datasets The accuracies of the recognition for isolated and detected MEs are 51.77% and 45.50%, respectively Comparing with traditional ME recognition systems, the proposed system obtained better performance The results have shown the promising application of the system The main results in this chapter have been published in the publication [C4] 120 CONCLUSION AND FUTURE WORKS Conclusion The detection and recognition of MEs in scientific document images have provided attractive and challenging tasks for researchers Moreover, results of the detection and recognition of MEs allow to develop many real applications for users such as: document digitization, mathematical information retrieval, text to speech This thesis has presented the contributions in the field of ME detection and recognition in scientific document images Concretely, three main approaches have been proposed in the thesis to improve the accuracy of the ME detection and recognition in scientific document images: (1) The fusion of handcrafted and deep learning features has been proposed for the detection of MEs In particular, the accuracy of the detection of inline MEs has been significantly improved compared with traditional handcrafted feature extraction methods The performance of overall system is evaluated on two public datasets those are the Marmot and GTDB The generic performance metrics based on IoU are applied to evaluate the system clearly The performance evaluation has shown that the proposed fusion strategy is an efficient way to improve the accuracy of the detection of MEs in scientific document images without using the OCR techniques (2) To further improve the detection accuracy of MEs, the end-to-end framework for ME detection has been proposed In the framework, the DT and Faster R-CNN are proposed to detect MEs in document images The distance transform with various distance metrics including Euclidean, City Block and Chessboard is applied for document images in order to take advantages of the mathematical expression layouts Moreover, the optimization and generation strategies of anchor boxes of RPN of Faster R-CNN are proposed to improve the accuracy of the detection The proposed system has been tested on two public datasets those are Marmot and GTDB Comparing with conventional methods, the uses of the DT with Euclidean metric and Faster R-CNN have shown higher accuracy of the detection with the reduction of human-resources Obtained results have shown that the DT significantly enhances the discriminative features of MEs and the optimisation of anchor boxes plays an important role to improve the accuracy of ME detection in document images (3) Finally, the detection and recognition of MEs have been integrated in an applicable system The MEs in document images have been detected and recognized simultaneously In the system, MEs have been detected using the DT and Faster R-CNN Then, the advanced network that is WAP has been applied to recognize detected MEs 121 The recognition results are represented in Latex that is a popular format for scientific documents The application supports end users to use the detection and recognition of MEs conveniently Compared with conventional methods for ME recognition, the WAP network has recognized MEs more accurately Moreover, the network has shown the efficient performance in the structure analysis of complex layout MEs Future works The thesis has gained competitive results in the detection and recognition of MEs in scientific document images However, there are rooms for improvement In the future, the following directions can be extended in order to improve the accuracy of the expression detection and recognition: (1) The performance of the ME detection can be improved by the combination of different deep neural networks Actually, the strategies in the detection of tiny symbols can be apply to further improve the detection accuracy of inline MEs Moreover, the contextual information of MEs can be investigated to improve the accuracy of the detection (2) The ME detection and recognition can be further improved to obtain the applicable and robust system Particularly, the ME recognition can be investigated A large number of mathematical notations can be inserted to train the recognition models Moreover, recent advances in the encoder-decoder network architecture can be applied to improve the recognition accuracy The improvement of attention mechanisms can be applied to improve the accuracy of the ME recognition (3) The document images considered in the thesis are assumed to be non-skew Therefore, the deskew algorithms will be investigated for the proposed approaches in the thesis Deskew algorithms are considered as preprocessing steps for document images Then, the detection and recognition approaches can be applied for the document images 122 PUBLICATIONS Conferences [C1] Bui Hai Phong, Thang Manh Hoang, Thi-Lan Le (2017), A new method for displayed mathematical expression detection based on FFT and SVM, 4th NAFOSTED Conference on Information and Computer Science (NICS), IEEE, Hanoi, Vietnam, ISBN: 978-1-5386-3210-9, DOI: 10.1109/NAFOSTED.2017.8108044, pp.90-96, 2017 [C2] Bui Hai Phong, Thang Manh Hoang and Thi-Lan Le (2019), Mathematical variable detection based on CNN and SVM, 2nd International Conference on Multimedia Analysis and Pattern Recognition (MAPR), ISBN: 978-1-7281-1829-1, DOI: 10.1109/MAPR.2019.8743543, pp 1-5, 2019 [C3] Bui Hai Phong, Thang Manh Hoang and Thi-Lan Le (2019), A unified system for for mathematical expression detection in scientific document images , KoreaVietnam International Joint Workshop on Communications and Information Sciences (KICS), ISBN: 978-89-950043-7-1[93560], Hanoi, Viet Nam, pp.14-16, 2019 [C4] Bui Hai Phong, Luong Tan Dat, Nguyen Thi Yen, Thang Manh Hoang and Thi-Lan Le (2020), A deep learning based system for mathematical expression detection and recognition in scientific document images, The 12th IEEE International Conference on Knowledge and Systems Engineering (KSE), ISBN:978-1-7281-3003-3, pp.85-90, 2020, DOI:10.1109/KSE.2019.8919461 Journals [J1] Bui Hai Phong, Thang Manh Hoang and Thi-Lan Le (2020), A hybrid method for mathematical expression detection in scientific document images, IEEE Access, vol 8, pp.83663 - 83684, 2020, ISSN: 2169-3536 (Print) 2169-3536 (Online), DOI: 10.1109/ACCESS.2020.2992067 (ISI, Q1, IF=4.098) [J2 ] Bui Hai Phong, Thang Manh Hoang and Thi-Lan Le (2021), Mathematical variable detection in scientific document images, International Journal of Computational Vision and Robotics, Vol 11, No 1, pp.66-89, 2021, ISSN online: 1752-914X, ISSN print: 1752-9131, DOI:10.1504/IJCVR.2021.111876 (SCOPUS) [J3] Bui Hai Phong, Thang Manh Hoang and Thi-Lan Le (2021), An end-to-end framework for the detection of mathematical expressions in scientific document images, Expert Systems, Online ISSN:1468-0394, DOI: 10.1111/exsy.12800 (ISI, Q2, IF=2.587) Related publication of the thesis 123 [C5] Bui Hai Phong, Thang Manh Hoang, Thi-Lan Le and Akkiko Aizawa (2019), Mathematical variable detection in PDF scientific documents, 11th Asian conference on intelligent information and database systems (ACIIDS), Springer, Cham, Indonesia, DOI: https://doi.org/10.1007/978-3-030-14802-7-60, ISBN: 978-3-030-14802-7, 2019 124 Bibliography [1] K.Iwatsuki and A.Aizawa (2017) Detecting in-line mathematical expressions in scientific documents ACM Symposium on Document Engineering, pp 141–144 doi:10.1145/3103010.3121041 [2] Mahdavi M et al (2019) Icdar 2019 crohme + tfd: competition on recognition of handwritten mathematical expressions and typeset formula detection International Conference on Document Analysis and Recognition doi:10.1109/icdar 2019.00247 [3] Muno F.A (2015) Mathematical expression 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IEEE Transactions on Multimedia, 17.11, pp 1875–1886 [121] Zeiler M.D (2012) Adadelta: https://arxiv.org/abs/1212.5701 134 An adaptive learning rate method ...MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY BUI HAI PHONG ENHANCING PERFORMANCE OF MATHEMATICAL EXPRESSION DETECTION IN SCIENTIFIC DOCUMENT IMAGES Major:... outstanding performance in the recognition and detection of mathematical expressions tasks [63, 64] The work in [63] takes the advantages of CNNs in the detection of isolated and inline expressions in. .. detection and recognition of expressions in documents The detection of expressions aims to locate the expressions in document images In scientific documents, MEs are classified in two categories, i.e