Skippy: Single View 3D Curve Interactive Modeling

1 Skippy: Single view 3D Curve Interactive ModelingVOJT CH KRS,Purdue UniversityERSIN YUMER,Adobe ResearchNATHAN CARR,Adobe ResearchBEDRICH BENES,Purdue UniversityRADOM R M CH,Adobe ResearchFig. 1. The user draws a 2D stroke in front of the 3D model (left). The 2D stroke is converted into a 3D Curve (middle). A complete 3D model from mutiplecurves is generated within a few seconds (right).We introduce Skippy, a novel algorithm for 3D Interactive Curve modelingfrom a Single view . While positing curves in space can be a tedious task, ourrapid sketching algorithm allows users to draw curves in and around existinggeometry in a controllable manner. The key insight behind our system is toautomatically infer the 3D Curve coordinates by enumerating a large set ofpotential Curve trajectories. More specifically, we partition 2D strokes intocontinuous segments that land bothonandoffthe geometry, duplicatingsegments that could be placed in front or behind, to form a directed use distance fields to estimate 3D coordinates for our Curve segments andsolve for an optimally smooth path that follows the curvature of the scenegeometry while avoiding intersections.

2 Using our Curve design frameworkwe present a collection of novel editing operations allowing artists to rapidlyexplore and refine the combinatorial space of solutions. Furthermore, weinclude the quick placement of transient geometry to aid in guiding the3D Curve . Finally we demonstrate our Interactive design Curve system on avariety of applications including geometric Modeling , and camera motionpath Concepts: Computing methodologies Parametric Curve andsurface models; Human-centered computing Graphical user in-terfaces;This work has been sponsored by Adobe Research and by the National Science Foun-dation grant #1606396 Haptic-Based Learning Experiences as Cognitive Mediators forConceptual Understanding and Representational Competence in Engineering EducationAuthor s addresses: Vojt ch Krs and Bedrich Benes, Computer Graphics Technology,401 N Grant St, West Lafayette, IN 47907.

3 Ersin Yumer, Nathan Carr and Radom r M ch,Adobe Research, 345 Park Ave, San Jose, CA to make digital or hard copies of all or part of this work for personal orclassroom use is granted without fee provided that copies are not made or distributedfor profit or commercial advantage and that copies bear this notice and the full citationon the first page. Copyrights for components of this work owned by others than ACMmust be honored. Abstracting with credit is permitted. To copy otherwise, or republish,to post on servers or to redistribute to lists, requires prior specific permission and/or afee. Request permissions from 2017 ACM. 0730-0301/2017/7-ART128 $ : Key Words and Phrases: Single view , 3D Curve , Geometric Mod-elingACM Reference format:Vojt ch Krs, Ersin Yumer, Nathan Carr, Bedrich Benes, and Radom r M Skippy: Single view 3D Curve Interactive Trans.

4 , 4, Article 128 (July 2017), 12 : INTRODUCTIONC omputer graphics has achieved impressive results in areas suchas rendering and computer animation. However, 3D Modeling stillposes many challenges; one of them is expressing user intent bysimple means. User interaction, which is at the heart of Modeling ,is where most of the related complexity still exists. This is mainlydue to the fact that most input and display devices currently in useare 2D, which is not intuitive to human interaction with the world,where we operate and think in 3D. To overcome the loss of depth,the user is usually forced to change the viewpoint, rotate the object,or use multiple viewports at once [Bae et ]; which can leadto a loss of particularly difficult problem is drawing of 3D curves . Thesespace curvesare important to a variety of tasks such as planningof trajectories of dynamic objects, for example, particle systems orvirtual cameras, design of curved surface patches, such as NURBS,or swept surfaces, or generalized cylinders.

5 The main problem ofdrawing 3D curves in 2D is that there is an infinite number ofpossible configurations of the Curve in the missing dimension, and,as explored by Schmidt et al. [2009a], even expert users have troublewith drawing 3d objects and curves . The foreshortening causedby perspective projection is especially difficult to get right andthe resulting objects hardly match user s intent. While the use ofshadows as visual depth cues has been shown to improve spatialACM Transactions on Graphics, Vol. 36, No. 4, Article 128. Publication date: July :2 V. Krs et. [Cohen et ]; specifying the shape of curveswhen drawing in 3D poses an additional set of work has addressed this issue by using constraints and ad-ditional sources of information to infer the desired Curve s of the common problems not addressed by the previous workis drawing behind occluding surfaces or drawing curves with highcurvature and torsion, such as spirals.

6 One of the underlying mech-anisms that makes this such a hard task is that our prior knowledgeof the object s shape alters our perception [Taylor and Mitchell1997]. As noted by Matthews and Adams [2008], people seem todraw what they know rather than what they see .One way to make drawing in 3D easier is to impose assumptionsabout the underlying form, such as regularity, symmetry, planarity,and orthogonality [Bae et ; Kara and Shimada 2007; Schmidtet ; Xu et ]. While these approaches work well inpractice they are often restricted to a certain class of shapes, whichmay limit artistic about the 3D representation of the sketched curveusing other 3D objects in the scene is a more practical and lessdisturbing approach from the user s point of view since it does notrequire any change of viewport at the time of drawing.

7 De Paoli andSingh [2015] used this insight for Modeling shapes around alreadyexisting 3D geometry. Their approach is limited to shorter and fullyvisible or partially visible symmetrical Curve segments, which applywell to local shape presentSkippy, a new method for sketching 3D curves froma Single viewpoint using both existing or transient 3D geometry forshape inference. Our method enables the user to draw arbitrarilylong, smooth curves that are placed at various depths between ob-jects in the scene and that can also be partially occluded at authoringtime (Figure 1). After being drawn the 3D Curve canskipbetween dif-ferent depths around the objects by clicking on the part of the curvethat is visible or obstructed by an object. The user can also clickon any surface and add transient guiding objects. Such temporarygeometry is then also used to guide drawing of the Curve withouthaving to change the viewpoint.

8 Furthermore, our system allowsa quick intuitive way for users to guide and control theskippingbehavior, allowing curves to be easily woven in a complex mannerthrough the negative space surrounding any geometry. We do notplace underlying assumptions about these curves ( , that theyform surfaces, or represent regular geometric forms), and as suchour curves can be used for a variety of applications from motionpath planning, surface decoration, and even shape design. Finallywe demonstrate a number of intuitive overdrawing mechanismsthat enable iterative refinement of Curve solutions interactively. Ourmain contributions are as follows: an intuitive approach for drawing 3d curves with varyingdepth using only 2D input by leveraging existing 3D shapesas guides, an efficient graph data structure that stores valid variationsof the 3D Curve for the input 2D stroke and enables real-time interaction with the Curve , and a set of novel editing operations specifically forskipediting,re-drawing, and via anchoring through template 3D RELATED WORKP ositioning 3D curves using a 2D interface poses numerous chal-lenges, a number of which have been tackled in the sketch basedmodeling literature.

9 We refer the reader to [Olsen et ] and arecent survey [Kazmi et ] for a more complete overview ofthis domain. To highlight the most relevant work in this space, wedivide related work into a set of broad categories; each relying ondifferent sets of underlying assumptions which ease the 3D curvedrawing Curve early work of Cohen et al. [1999]presented a Single view interface for designing 3D space curves . Thenovel idea of this work was to rely on shadows as additional depthcues. This conceptual idea is orthogonal to our approach and weleverage a form of shadowing ( , real-time screen space ambientocclusion) in our interface to improve spatial understanding. Thework of Cohen et al. [1999] did not directly address the tediousnature of specifying Curve shape during the drawing way to make drawing in 3D easier is to choose an angle thatminimizes the foreshortening.

10 A popular approach, investigated fordrawing 3D curves , is to rely on epipolar geometry. This allowssketching from a second viewpoint to find a unique solution to thecurve s shape [Bae et ], or choosing two view orthogonaldirections [Karpenko et ]. Such approaches either require asymmetric structure to be drawn, or consistent change of viewpoint;limiting artistic important aspect that is often considered during object sketch-ing are occlusions. Cordier and Seo [2007] construct self-occludingobjects from free-form sketches using constrained , McCrae and Singh [2008; 2009] , use clothoid splines,to infer 3D road networks from sketches with self-crossings. Mc-Cann and Pollard [2009] order interactively 2D objects with localoverlaps, which Igarashi and Mitani [2010] extended to 3D, and Lay-erPaint [Fu et ] allows users to paint on occluded surfacesusing a multi-layer approach that helps with 3D Modeling is leveragingexisting geometry and environment.