Identifying 3D Prints Using Slicing Parameters by Mustafa Doğa Doğan B.S. in Electrical and Electronics Engineering, Boğaziçi University (2018), Submitted to the Department of Electrical Engineering and Computer Science in partial fulfillment of the requirements for the degree of Master of Science in Computer Science at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY May 2020 © Massachusetts Institute of Technology 2020. All rights reserved. Author . Department of Electrical Engineering and Computer Science May 11, 2020 Certified by . Stefanie Mueller Assistant Professor of Electrical Engineering and Computer Science Thesis Supervisor Accepted by . Leslie A. Kolodziejski Professor of Electrical Engineering and Computer Science Chair, Department Committee on Graduate Students 1 Identifying 3D Prints Using Slicing Parameters by Mustafa Doğa Doğan submitted to the Department of Electrical Engineering and Computer Science on May 11, 2020, in partial fulfillment of the requirements for the degree of Master of Science in Computer Science Abstract In this thesis, we propose a method for identifying 3D prints using slicing parameters. Called “G- ID,” the method utilizes the subtle patterns left by the 3D printing process to identify objects. The key idea is to mark different instances of a 3D model by varying slicing parameters that do not change the model geometry but can be detected as machine-readable differences in the print. As a result, G-ID does not add anything to the object but exploits the unobtrusive textures resulting as a byproduct of slicing, an essential step of the 3D printing pipeline. We introduce the G-ID slicing & labeling user interface that varies the settings for each instance, and the G-ID mobile application, which uses image processing techniques to retrieve the param- eters and their associated labels from a photo of the 3D printed object. We also evaluate our method’s accuracy under different lighting conditions, with objects printed using different fila- ments and printers, and with pictures taken from various positions and angles. Thesis Supervisor: Stefanie Mueller Title: Assistant Professor of Electrical Engineering and Computer Science 2 3 4 Acknowledgments First, I would like to thank my research advisor, Professor Stefanie Mueller, who first gave me a chance to visit her group in CSAIL back in 2017. From being a visiting student to starting my PhD journey, I have always felt privileged to be able to learn so many things from her, be it about the field of digital fabrication or soft skills for a researcher’s life. I would not be able to make it through without the support of my lab mates, as well as my dearest friends at MIT and in the Greater Boston Area, including Andrea, Dishita, Jack, Jon, Jules, Miranda, Ola, Ryan, Safa, Soya, Ticha, Ulya, and Ximo. I am also extremely grateful for my encouraging friends from my hometown, Ecem, Ege, Elif, and Utku, who have always made themselves available to me through phone calls. A big thank you goes to all my collaborators on this project — my UROP Andrew Churchill who did excellent work in Android development, Kenny Friedman who was always there to answer my questions about the intricacies of JavaScript, and Prof. Sriram Subramanian whose insight really helped us polish our CHI 2020 publication. I also sincerely thank Alexandre Kaspar, Liane Makatura, and Danielle Pace from CSAIL for their technical feedback. Finally, I would like to express my heartfelt gratitude to my parents, Neşe and Rifat, and my sister, Bengü, who have always been supportive of my academic endeavors throughout the years. Publications This Master’s thesis led to the following publication: Mustafa Doga Dogan, Faraz Faruqi, Andrew Day Churchill, Kenneth Friedman, Leon Cheng, Sriram Subramanian, Stefanie Mueller. G-ID: Identifying 3D Prints Using Slicing Param- eters. In Proceedings of the ACM Conference on Human Factors in Computing Systems (CHI) 2020 [9]. This work was supported in part by NSF Award IIS-1716413. 5 6 Table of Contents I. INTRODUCTION ........................................................................................................................... 10 II. BACKGROUND: TAGGING 3D OBJECTS .................................................................................. 12 2.1 PURPOSE OF THE TAG (DATA STORAGE VS. IDENTIFICATION) ............................................... 12 2.2 MODEL GEOMETRY (COLOR, SURFACE & INTERNAL GEOMETRY) .......................................... 13 2.3 FABRICATION PROCESS (FILAMENT, SLICING, & PRINTING) ................................................... 14 2.4 DETECTION MECHANISM (CAMERA/SCANNER, MICROPHONE) ................................................ 15 III. G-ID: LABELING AND IDENTIFYING OBJECTS BY THEIR SLICING PARAMETERS ....... 16 3.1 MAIN BENEFITS OF USING DIFFERENT SLICING PARAMETERS ................................................ 17 3.2 G-ID WORKFLOW FOR AN IDENTIFICATION APPLICATION ...................................................... 17 3.3 LABELING INTERFACE (SLICER) ............................................................................................... 17 3.4 IDENTIFICATION INTERFACE (MOBILE APP + OBJECT ALIGNMENT) ...................................... 19 3.5 SURFACE & INTERIOR: DETECTING INFILL USING A LIGHT SOURCE ....................................... 19 IV. SLICING PARAMETERS USED FOR LABELING .................................................................. 20 4.1 SURFACE PARAMETERS ........................................................................................................... 21 Bottom Surface: Resolution & Angle ..................................................................................... 21 Intermediate Layers: Resolution & Angle ............................................................................... 21 4.2 INFILL PARAMETERS ................................................................................................................ 22 Infill: Resolution, Angle, & Pattern ....................................................................................... 22 4.3 SELECTING PARAMETERS THAT MINIMIZE PRINT TIME & MATERIAL ..................................... 23 V. DETECTING AND IDENTIFYING SLICING PARAMETERS .................................................... 23 5.1 ALIGNING THE OBJECT’S BASE IN HANDHELD CAMERA IMAGES ............................................. 23 Images Used for Alignment ................................................................................................... 24 Removing Overhangs ........................................................................................................... 24 Reducing Perspective from the Image (Undistorting) ............................................................... 25 5.2 DETECTING BOTTOM LINE ANGLE AND WIDTH ...................................................................... 25 5.3 ERROR CHECKING .................................................................................................................... 26 5.4 DETECTING INFILL ANGLE, WIDTH, AND PATTERN ................................................................. 26 VI. SPACING OF SLICING PARAMETERS .................................................................................. 27 6.1 SELECTING 3D MODELS TO EVALUATE PARAMETER SPACINGS .............................................. 28 6.2 DETERMINING THE RANGE FOR EACH SLICER SETTING .......................................................... 29 6.3 SLICING WITH DIFFERENT SPACINGS AND CAPTURING PHOTOS .............................................. 29 6.4 RESULTS OF THE EXPERIMENTS ............................................................................................... 30 Bottom Line Angle & Width ................................................................................................. 30 Infill Angle & Line Distance (Width) .................................................................................... 31 Infill Pattern ....................................................................................................................... 31 7 6.5 TOTAL NUMBER OF INSTANCES POSSIBLE ............................................................................... 31 6.6 CROSS-VALIDATION ................................................................................................................. 32 VII. SYSTEM IMPLEMENTATION .................................................................................................. 32 VIII. EVALUATION ............................................................................................................................ 33 8.1 DIFFERENT MATERIALS, LIGHTING CONDITIONS, THICKNESSES ............................................. 33 8.2 DIFFERENT 3D PRINTERS ........................................................................................................ 34 8.3 CAMERA DISTANCE AND ANGLE .............................................................................................. 35 IX. APPLICATION SCENARIOS ........................................................................................................ 36 X. DISCUSSION AND LIMITATIONS ............................................................................................... 38 10.1 OTHER SLICING PARAMETERS ................................................................................................. 38 10.2 ROTATIONAL SYMMETRY OF OUTLINES ................................................................................... 38 10.3 NON-FLAT SIDE SURFACES