{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,5,20]],"date-time":"2026-05-20T02:47:20Z","timestamp":1779245240152,"version":"3.51.4"},"reference-count":39,"publisher":"MDPI AG","issue":"2","license":[{"start":{"date-parts":[[2020,1,9]],"date-time":"2020-01-09T00:00:00Z","timestamp":1578528000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100010418","name":"Institute for Information and Communications Technology Promotion","doi-asserted-by":"publisher","award":["IITP-2019-2015-0-00448"],"award-info":[{"award-number":["IITP-2019-2015-0-00448"]}],"id":[{"id":"10.13039\/501100010418","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001321","name":"National Research Foundation","doi-asserted-by":"publisher","award":["NRF-2017R1A2B4012096"],"award-info":[{"award-number":["NRF-2017R1A2B4012096"]}],"id":[{"id":"10.13039\/501100001321","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001321","name":"National Research Foundation","doi-asserted-by":"publisher","award":["NRF-2018R1D1A3B07044041"],"award-info":[{"award-number":["NRF-2018R1D1A3B07044041"]}],"id":[{"id":"10.13039\/501100001321","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Trajectory-based writing system refers to writing a linguistic character or word in free space by moving a finger, marker, or handheld device. It is widely applicable where traditional pen-up and pen-down writing systems are troublesome. Due to the simple writing style, it has a great advantage over the gesture-based system. However, it is a challenging task because of the non-uniform characters and different writing styles. In this research, we developed an air-writing recognition system using three-dimensional (3D) trajectories collected by a depth camera that tracks the fingertip. For better feature selection, the nearest neighbor and root point translation was used to normalize the trajectory. We employed the long short-term memory (LSTM) and a convolutional neural network (CNN) as a recognizer. The model was tested and verified by the self-collected dataset. To evaluate the robustness of our model, we also employed the 6D motion gesture (6DMG) alphanumeric character dataset and achieved 99.32% accuracy which is the highest to date. Hence, it verifies that the proposed model is invariant for digits and characters. Moreover, we publish a dataset containing 21,000 digits; which solves the lack of dataset in the current research.<\/jats:p>","DOI":"10.3390\/s20020376","type":"journal-article","created":{"date-parts":[[2020,1,10]],"date-time":"2020-01-10T04:06:51Z","timestamp":1578629211000},"page":"376","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":79,"title":["Trajectory-Based Air-Writing Recognition Using Deep Neural Network and Depth Sensor"],"prefix":"10.3390","volume":"20","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-8413-5428","authenticated-orcid":false,"given":"Md. Shahinur","family":"Alam","sequence":"first","affiliation":[{"name":"Department of Computer and Communication Engineering, Chungbuk National University, Cheongju, Chungbuk 28644, Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0334-504X","authenticated-orcid":false,"given":"Ki-Chul","family":"Kwon","sequence":"additional","affiliation":[{"name":"Department of Computer and Communication Engineering, Chungbuk National University, Cheongju, Chungbuk 28644, Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Md. Ashraful","family":"Alam","sequence":"additional","affiliation":[{"name":"Department of Computer Science and Engineering, BRAC University, Dhaka 1212, Bangladesh"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Mohammed Y.","family":"Abbass","sequence":"additional","affiliation":[{"name":"Department of Computer and Communication Engineering, Chungbuk National University, Cheongju, Chungbuk 28644, Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7001-1667","authenticated-orcid":false,"given":"Shariar Md","family":"Imtiaz","sequence":"additional","affiliation":[{"name":"Department of Computer and Communication Engineering, Chungbuk National University, Cheongju, Chungbuk 28644, Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Nam","family":"Kim","sequence":"additional","affiliation":[{"name":"Department of Computer and Communication Engineering, Chungbuk National University, Cheongju, Chungbuk 28644, Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2020,1,9]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"24","DOI":"10.1109\/MCG.2002.1046626","article-title":"Motion tracking: No silver bullet, but a respectable arsenal","volume":"22","author":"Welch","year":"2002","journal-title":"IEEE Comput. Graph. Appl."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"355","DOI":"10.1016\/j.ergon.2017.02.004","article-title":"Gesture recognition for human-robot collaboration: A review","volume":"68","author":"Liu","year":"2018","journal-title":"Int. J. Ind. Ergon."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"50","DOI":"10.1016\/j.patcog.2016.03.026","article-title":"Towards hand gesture based writing support system for blinds","volume":"57","author":"Modanwal","year":"2016","journal-title":"Pattern Recognit."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"871","DOI":"10.1109\/TSMCB.2012.2217324","article-title":"Most Probable Longest Common Subsequence for Recognition of Gesture Character Input","volume":"43","author":"Frolova","year":"2013","journal-title":"IEEE Trans. Cybern."},{"key":"ref_5","doi-asserted-by":"crossref","unstructured":"De, O., Deb, P., Mukherjee, S., Nandy, S., Chakraborty, T., and Saha, S. (2016, January 13\u201315). Computer vision based framework for digit recognition by hand gesture analysis. Proceedings of the 2016 IEEE 7th Annual Information Technology, Electronics and Mobile Communication Conference, Vancouver, BC, Canada.","DOI":"10.1109\/IEMCON.2016.7746361"},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"2094","DOI":"10.1109\/TCYB.2015.2464195","article-title":"Low-Complexity Hand Gesture Recognition System for Continuous Streams of Digits and Letters","volume":"46","author":"Poularakis","year":"2016","journal-title":"IEEE Trans. Cybern."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"413","DOI":"10.12733\/jics20105578","article-title":"Online Kinect Handwritten Digit Recognition Based on Dynamic Time Warping and Support Vector Machine","volume":"12","author":"Qu","year":"2015","journal-title":"J. Inf. Comput. Sci."},{"key":"ref_8","doi-asserted-by":"crossref","unstructured":"Mohammadi, S., and Maleki, R. (2019). Air-writing recognition system for Persian numbers with a novel classifier. arXiv.","DOI":"10.1007\/s00371-019-01717-3"},{"key":"ref_9","unstructured":"Tian, J., Qu, C., Xu, W., and Wang, S. (2013, January 24\u201327). KinWrite: Handwriting-Based Authentication Using Kinect. Proceedings of the 20th Annual Network and Distributed System Security Symposium, San Diego, CA, USA."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"85","DOI":"10.1109\/MMUL.2013.50","article-title":"A new writing experience: Finger writing in the air using a kinect sensor","volume":"20","author":"Zhang","year":"2013","journal-title":"IEEE Multimed."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"1293","DOI":"10.1109\/JSEN.2016.2643165","article-title":"Study of Text Segmentation and Recognition Using Leap Motion Sensor","volume":"17","author":"Kumar","year":"2017","journal-title":"IEEE Sens. J."},{"key":"ref_12","doi-asserted-by":"crossref","unstructured":"Zhou, Y., Dai, Z., and Jing, L. (2016, January 8\u201310). A controlled experiment between two methods on ten-digits air writing. Proceedings of the 2016 16th IEEE International Conference on Computer and Information Technology, Nadi, Fiji.","DOI":"10.1109\/CIT.2016.112"},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"403","DOI":"10.1109\/THMS.2015.2492598","article-title":"Air-Writing Recognition\u2014Part I: Modeling and Recognition of Characters, Words, and Connecting Motions","volume":"46","author":"Chen","year":"2016","journal-title":"IEEE Trans. Hum.-Mach. Syst."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"2998","DOI":"10.1109\/TIE.2011.2167895","article-title":"An accelerometer-based digital pen with a trajectory recognition algorithm for handwritten digit and gesture recognition","volume":"59","author":"Wang","year":"2012","journal-title":"IEEE Trans. Ind. Electron."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"1737","DOI":"10.1587\/transfun.E101.A.1737","article-title":"Air-writing recognition based on fusion network for learning spatial and temporal features","volume":"E101A","author":"Yana","year":"2018","journal-title":"IEICE Trans. Fundam. Electron. Commun. Comput. Sci."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"473","DOI":"10.1109\/TMC.2018.2831709","article-title":"Writing in the Air with WiFi Signals for Virtual Reality Devices","volume":"18","author":"Fu","year":"2019","journal-title":"IEEE Trans. Mob. Comput."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"436","DOI":"10.1109\/THMS.2015.2492599","article-title":"Air-Writing Recognition\u2014Part II: Detection and Recognition of Writing Activity in Continuous Stream of Motion Data","volume":"46","author":"Chen","year":"2016","journal-title":"IEEE Trans. Human-Machine Syst."},{"key":"ref_18","doi-asserted-by":"crossref","unstructured":"Tordesillas, J., Lopez, B.T., and How, J.P. (2019, January 3\u20138). FaSTraP: Fast and Safe Trajectory Planner for Flights in Unknown Environments. Proceedings of the International Conference on Intelligent Robots and Systems, Macau, China.","DOI":"10.1109\/IROS40897.2019.8968021"},{"key":"ref_19","doi-asserted-by":"crossref","unstructured":"Wobbrock, J.O., Wilson, A.D., and Li, Y. (2007, January 7\u201310). Gestures without libraries, toolkits or training: A $1 recognizer for user interface prototypes. Proceedings of the 20th Annual ACM Symposium on User Interface Software and Technology\u2014UIST \u201907, Newport, RI, USA.","DOI":"10.1145\/1294211.1294238"},{"key":"ref_20","doi-asserted-by":"crossref","unstructured":"Luo, Y., Tsang, C.C., Zhang, G., Dong, Z., Shi, G., Kwok, S.Y., Li, W.J., Leong, P.H.W., and Wong, M.Y. (2006, January 18\u201321). An Attitude Compensation Technique for a MEMS Motion Sensor Based Digital Writing Instrument. Proceedings of the 2006 1st IEEE International Conference on Nano\/Micro Engineered and Molecular Systems, Zhuhai, China.","DOI":"10.1109\/NEMS.2006.334563"},{"key":"ref_21","doi-asserted-by":"crossref","unstructured":"Nguyen, H., and Bartha, M.C. (2012, January 22\u201326). Shape writing on tablets: Better performance or better experience?. Proceedings of the Human Factors and Ergonomics Society, Boston, MA, USA.","DOI":"10.1037\/e572172013-330"},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"191","DOI":"10.1007\/s00779-013-0637-3","article-title":"Airwriting: A wearable handwriting recognition system","volume":"18","author":"Amma","year":"2014","journal-title":"Pers. Ubiquitous Comput."},{"key":"ref_23","doi-asserted-by":"crossref","unstructured":"Kratz, S., and Rohs, M. (2011, January 13\u201316). Protractor3D. Proceedings of the 15th International Conference on Intelligent User Interfaces\u2014IUI \u201911, Palo Alto, CA, USA.","DOI":"10.1145\/1943403.1943468"},{"key":"ref_24","doi-asserted-by":"crossref","unstructured":"Guerra-Casanova, J., Avila, C.S., Bailador, G., and de-Santos-Sierra, A. (2011, January 18\u201321). Time series distances measures to analyze in-air signatures to authenticate users on mobile phones. Proceedings of the 2011 Carnahan Conference on Security Technology, Barcelona, Spain.","DOI":"10.1109\/CCST.2011.6095931"},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"3508","DOI":"10.1109\/TIE.2009.2038339","article-title":"An Inertial-Measurement-Unit-Based Pen With a Trajectory Reconstruction Algorithm and Its Applications","volume":"57","author":"Wang","year":"2010","journal-title":"IEEE Trans. Ind. Electron."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"154","DOI":"10.1109\/JSEN.2014.2339843","article-title":"An inertial pen with dynamic time warping recognizer for handwriting and gesture recognition","volume":"15","author":"Hsu","year":"2015","journal-title":"IEEE Sens. J."},{"key":"ref_27","unstructured":"Cho, S.J., Oh, J.K., Bang, W.C., Chang, W., Choi, E., Jing, Y., Cho, J., and Kim, D.Y. (2004, January 26\u201329). Magic Wand: A Hand-Drawn Gesture Input Device in 3-D Space with Inertial Sensors. Proceedings of the Ninth International Workshop on Frontiers in Handwriting Recognition, Tokyo, Japan."},{"key":"ref_28","doi-asserted-by":"crossref","unstructured":"Chiu, L.-W., Hsieh, J.-W., Lai, C.-R., Chiang, H.-F., Cheng, S.-C., and Fan, K.-C. (2018, January 24\u201326). Smart Multimedia. Proceedings of the Person Authentication by Air-Writing Using 3D Sensor and Time Order Stroke Context, Toulon, France.","DOI":"10.1007\/978-3-030-04375-9_22"},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"8855","DOI":"10.1109\/JSEN.2019.2922395","article-title":"Character Recognition in Air-Writing Based on Network of Radars For Human-Machine Interface","volume":"19","author":"Arsalan","year":"2019","journal-title":"IEEE Sens. J."},{"key":"ref_30","first-page":"4693","article-title":"Deep Belief Networks for Recognizing Handwriting Captured by Leap Motion Controller","volume":"8","author":"Setiawan","year":"2018","journal-title":"Int. J. Electr. Comput. Eng."},{"key":"ref_31","doi-asserted-by":"crossref","unstructured":"Castellucci, S.J., and MacKenzie, I.S. (2008, January 5\u201310). Graffiti vs. unistrokes. Proceedings of the Twenty-Sixth Annual CHI Conference on Human Factors in Computing Systems\u2014CHI \u201908, Florence, Italy.","DOI":"10.1145\/1357054.1357106"},{"key":"ref_32","doi-asserted-by":"crossref","unstructured":"Xu, S., and Xue, Y. (2017, January 9\u201315). A Long Term Memory Recognition Framework on Multi-Complexity Motion Gestures. Proceedings of the 2017 14th IAPR International Conference on Document Analysis and Recognition (ICDAR), Kyoto, Japan.","DOI":"10.1109\/ICDAR.2017.41"},{"key":"ref_33","doi-asserted-by":"crossref","unstructured":"Chen, M., Alregib, G., and Juang, B.H. (2012, January 22\u201324). 6DMG: A new 6D motion gesture database. Proceedings of the 3rd Multimedia Systems Conference, Chapel Hill, NC, USA.","DOI":"10.1145\/2155555.2155569"},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"1735","DOI":"10.1162\/neco.1997.9.8.1735","article-title":"Long Short-Term Memory","volume":"9","author":"Hochreiter","year":"1997","journal-title":"Neural Comput."},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"343","DOI":"10.1007\/BF01386329","article-title":"The norm of the Schur product operation","volume":"4","author":"Davis","year":"1962","journal-title":"Numer. Math."},{"key":"ref_36","first-page":"1929","article-title":"Dropout: A Simple Way to Prevent Neural Networks from Overfitting","volume":"15","author":"Srivastava","year":"2014","journal-title":"J. Mach. Learn. Res."},{"key":"ref_37","unstructured":"Kingma, D.P., and Ba, J. (2015, January 7\u20139). Adam: A Method for Stochastic Optimization. Proceedings of the 3rd International Conference for Learning Representations, San Diego, CA, USA."},{"key":"ref_38","doi-asserted-by":"crossref","first-page":"561","DOI":"10.1109\/TMM.2012.2237024","article-title":"Feature processing and modeling for 6D motion gesture recognition","volume":"15","author":"Chen","year":"2013","journal-title":"IEEE Trans. Multimed."},{"key":"ref_39","first-page":"142","article-title":"Fusion networks for air-writing recognition","volume":"10705 LNCS","author":"Yana","year":"2018","journal-title":"Lecture Notes in Computer Science (Including Subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics)"}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/20\/2\/376\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,13]],"date-time":"2025-10-13T13:42:34Z","timestamp":1760362954000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/20\/2\/376"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2020,1,9]]},"references-count":39,"journal-issue":{"issue":"2","published-online":{"date-parts":[[2020,1]]}},"alternative-id":["s20020376"],"URL":"https:\/\/doi.org\/10.3390\/s20020376","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2020,1,9]]}}}