Modeling by Example Thomas Funkhouser, 1 Michael Kazhdan, 1 Philip Shilane, 1 Patrick Min, 2 William Kiefer, 1 Ayellet Tal, 3 Szymon Rusinkiewicz, 1 and David Dobkin 1 1 Princeton University 2 Utrecht University 3 Technion - Israel Institute of Technology

Abstract In this paper, we investigate a data-driven synthesis approach to constructing 3D geometric surface models. We provide methods with which a user can search a large database of 3D meshes to find parts of interest, cut the desired parts out of the meshes with intel- ligent scissoring, and composite them together in different ways to form new objects. The main benefit of this approach is that it is both easy to learn and able to produce highly detailed geometric models – the conceptual design for new models comes from the user, while the geometric details come from examples in the database. The focus of the paper is on the main research issues motivated by the proposed approach: (1) interactive segmentation of 3D surfaces, (2) shape-based search to find 3D models with parts matching a query, and (3) composition of parts to form new models. We provide new research contributions on all three topics and incorporate them into a prototype modeling system. Experience with our prototype sys- tem indicates that it allows untrained users to create interesting and detailed 3D models. Keywords: databases of geometric models, 3D shape matching, interactive modeling tools 1 Introduction One of the most significant obstacles in computer graphics is pro- viding easy-to-use tools for creating detailed 3D models. Most commercial modeling systems are difficult to learn, and thus their use has been limited to a small set of trained experts. Conversely, 3D sketching programs are good for novices, but practical for cre- ating only simple shapes. Our goal is to provide a tool with which almost anybody can create detailed geometric models quickly and easily. In this paper, we investigate “modeling by example,” a data- driven approach to constructing new 3D models by assembling parts from previously existing ones. We have built an interac- tive tool that allows a user to find and extract parts from a large database of 3D models and composite them together to create new 3D models. This approach is useful for creating objects with inter- changeable parts, which includes most man-made objects (vehi- cles, machines, furniture, etc.) and several types of natural objects (faces, fictional animals). Our current implementation employs a database of more than 10,000 models, including multiple examples of almost every type of household object. The main motivation for this approach is that it allows untrained users to create detailed geometric models quickly. Unlike previous interactive modeling systems, our users must only search, select, and combine existing parts from examples in the database – i.e.,

Figure 1: Modeling by example: geometric parts extracted from a database of 3D models can be used to create new objects. The large brown chair was built from the circled parts of the others.

they rarely have to create new geometry from scratch. As a result, the user interface can be simpler and accessible to a wider range of people. For example, when making the rocking chair shown in Figure 1, the user started with a simple chair (top-left), and then simply replaced parts. The commands were very simple, but the result has all the geometric details created by the expert modelers who populated the database. This approach provides a new way to make 3D models for students, designers of virtual worlds, and participants in on-line 3D games. In the following sections, we address the main research issues in building such a system: segmenting 3D surfaces into parts, search- ing a database of 3D models for parts, and compositing parts from different models. Specifically, we make the following research contributions: (1) an intelligent scissors algorithm for cutting 3D meshes, (2) a part-in-whole shape matching algorithm, (3) a method for aligning 3D surfaces optimally, and (4) a prototype system for data-driven synthesis of 3D models. Experience with our prototype system indicates that it is both easy to learn and useful for creating interesting 3D models.

2 Related Work

This paper builds upon related work in several sub-fields of com- puter graphics, geometric modeling, and computer vision. Geometric modeling: Our system is a 3D modeling tool. How- ever, its purpose is quite different than most previous modeling sys- tems (e.g., [Wavefront 2003]). It is intended for rapidly combining

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