Copyright © 2005 by the Association for Computing Machinery, Inc. Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, to republish, to post on servers, or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from Permissions Dept, ACM Inc., fax +1 (212) 869-0481 or e-mail [email protected]. © 2005 ACM 1-59593-203-6/05/0005 $5.00 Sketch-based Modeling with Few StrokesJoseph Jacob Cherlin†Faramarz Samavati†Mario Costa Sousa†Joaquim A. Jorge‡†University of Calgary‡Technical University of Lisbon{cherlin,samavati,mario}@cpsc.ucalgary.ca [email protected] present a novel sketch-based system for the interactive model-ing of a variety of free-form 3D objects using just a few strokes.Our technique is inspired by the traditional illustration strategy fordepicting 3D forms where the basic geometric forms of the sub-jects are identified, sketched and progressively refined using fewkey strokes. We introduce two parametric surfaces, rotational andcross sectional blending, that are inspired by this illustration tech-nique. We also describe orthogonal deformation and cross sectionaloversketching as editing tools to complement our modeling tech-niques. Examples with models ranging from cartoon style to botan-ical illustration demonstrate the capabilities of our system.CR Categories: I.3.5 Computer Graphics Geometric algorithms,languages, and systems I.3.6 Computer Graphics Interaction tech-niquesKeywords: sketch-based interfaces/modeling, free-form surfaces1 IntroductionIn traditional illustration, the depiction of 3D forms is usuallyachieved by a series of drawing steps using few strokes. The artistinitially draws the outline of the subject to depict its basic massesand boundaries. This initial outline is known as constructive curvesand usually results in very simple geometric forms. Outline detailsand internal lines are then progressively added to suggest featuressuch as curvatures, wrinkles, slopes, folds, etc [Dease et al. 1999;Goldstein 1999; Guptill 1977]. Three examples of methods typi-cally used to achieve such drawing progression are shown in Fig-ure 1:The spiral method, where shape depiction is achieved by the useof quickly formed spiral strokes connecting the constructive curves,creating a visual “blend” of the overall volume between the con-structive curves. Spiral strokes are helpful when irregular roundedforms are involved, such as fruits, vegetables, animals, and people.The scribble method, involves the use of continuous stroke(s)placed between constructive curves. The scribbled strokes are typ-ically used to depict specific folds, bumps, etc., across the subject.In Figure 1, a single scribbled stroke defines the fold pattern at theboundary end of the skirt.The bending (or distortion) method, adds unique variations to theinitial sketch of the subject. These variations aid on depicting theoverall shape of subjects which naturally present a large variety ofSpiral methodScribble methodBending (or distortion) methodFigure 1: Traditional hand-drawn techniques for progressive shapedepiction [Dease et al. 1999; Goldstein 1999; Guptill 1977]. Skirtdrawing, Copyright 1998-2004 Rio Aucena. Used with permission.twists, turns, and growth patterns, such as botanical and anatomicalparts.In this paper we present a sketch-based modeling system inspiredby the three traditional illustration methods above. We have de-veloped new algorithms to facilitate the rapid modeling of a widevariety of free-form 3D objects, constructed and edited from just afew freely sketched 2D line segments, without imposing any con-straints regarding the order in which lines are entered as well astheir spatial relationship.Our system takes as input strokes sketched by the user using amouse or tablet. Each stroke is then captured and properly filtered(Section 3) to allow efficient and robust modeling of 3D objects,represented as parametric surfaces, in two phases, creation and edit-ing. The techniques we developed, at each phase, were inspiredby the traditional shape sketching methods of spiral, scribble andbending (Figure 1). In the creation phase (Section 4), surfaces canbe generated using techniques for two types of parametric surfaces,rotational blending surface (Section 4.1) − approximating the spi-ral method −, and cross sectional blending surface (Section 4.2)− approximating the scribble method. In the editing phase (Sec-tion 5), subtle or drastic variations to the surfaces can be added byusing a single deformation stroke (Section 5.1)− approximating thebending method. Surfaces can also be modified by oversketchingcross-sections of the model (Section 5.2) − also related to the scrib-ble method.The rest of this paper is organized as follows: related research isreviewed in Section 2. Details of our approach are provided in Sec-tions 3, 4 and 5. Results are discussed in Section 6, and conclusionspresented in Section 7. An appendix is also provided with mathe-matical details of the techniques presented in Sections 4 and 5.1372 Related WorkSketch-based systems are a relatively new area in modeling, in par-ticular for 3D content creation of free-form objects in design, pro-duction and science. The main goal of sketch-based systems is to al-low the creation of 3D models by using strokes extracted from userinput and/or existing drawing scans. Refer to [Naya et al. 2002] fora complete classification of sketch-based systems. Our system fitsin the sketch-based category of gestural modeling where hand ges-tures are used as commands for generating and editing 3D shapesfrom 2D line segments.SKETCH [Zeleznik et al. 1996] combines mouse gestures and sim-ple geometric recognition to create and modify 3D models. To thisend, SKETCH uses a gesture grammar to create simple extrusion-like primitives in orthogonal view. It is also possible to specify CSGoperations and defining quasi- free-form shapes such as ducts in alimited manner.Quick-Sketch [Eggli et al. 1997] is based on parametric surfaces.The system creates extrusion primitives from sketched curves,which are segmented into line and circle primitives