Active Strokes: Coherent Line Stylization for Animated 3D Models
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The document discusses various techniques for non-photorealistic rendering (NPR) of 3D shapes, highlighting approaches like stylized line-based rendering and image space buffers. It emphasizes motion coherence, parameterization, and optimization for real-time rendering while addressing the limitations of connectivity and computational expense. Additionally, it covers vectorization, tracking features, and advancements in animated line representation, focusing on achieving temporal continuity and coherent stylization.
![• Image space active contours [Kass et al. 1988]
Shape
Topology
Density
• Brush paths
Coherent parameterization
Shape abstraction
• Interactive frame rates
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Active Strokes: Coherent Line Stylization for Animated 3D Models
- 1. Pierre Bénard1,2 Jingwan Lu3 Forrester Cole4 Adam Finkelstein3 Joëlle Thollot1,2 1Grenoble University, LJK 2INRIA 3Princeton University 4MIT CSAIL
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- 4. • Image space buffers • Object space line processing extraction Comprehensible rendering of 3-D shapes, Real-Time Nonphotorealistic Rendering, Saito and Takahashi, SIGGRAPH 1990 Markosian et al., SIGGRAPH 1997 4
- 5. Simple and fast Natural coherence and LoD Restricted stylization effects Implicit Brushes for stylized line-based rendering, Vergne et al., CGF 2011 5
- 6. Wide range of styles Computationally expensive Complex LoD No natural coherence A Procedural Approach to Style for NPR Line Drawing from 3D models, Grabli et al., TOG 2010 6
- 7. Line texture l 0 Brush path 7
- 8. Fixed Line View-Dependent Lines Creases Silhouettes Ridges and valleys Suggestive contours Apparent ridges Parameterization = arc-length Parameterization = ? 8
- 9. • Flatness Remain linear in image space • Motion coherence Evolve according to the motion of the object • Temporal continuity Adapt to the topological events 9
- 10. Optimization between 2D and 3D Multiple brush strokes per line Dependent on the input connectivity Coherent Stylized Silhouettes, Kalnins et al., SIGGRAPH 2003 10
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- 12. Kalnins et al., Coherent Stylized Silhouettes, SIGGRAPH 2003 12
- 13. 2D Infinite zoom: Self-Similar Line Artmap (SLAM) Dependent on the input connectivity One brush stroke per line Self-Similar Texture for Coherent Line Stylization, Bénard et al. NPAR 2010 13
- 14. Spatio-Temporal Analysis for Ease propagation Parameterizing Animated Lines, of parameterization Buchholz et al., NPAR 2011 with CSS Input connectivity Optimization over the Snaxels on a Plane, entire animation Kevin Karsch and John C. Hart, Offline computation NPAR 2011 14
- 15. • Image space active contours [Kass et al. 1988] Shape Topology Density • Brush paths Coherent parameterization Shape abstraction • Interactive frame rates 15
- 16. 3D scene Feature samples Image space lines extraction Line drawing Snakes Brush Paths Tracking Geometry Advection Relaxation Parameterization Vectorization Stylization Coverage Connectivity 16
- 17. • Feature samples extracted in image space 2D position Local tangent 2D velocity 17
- 18. 3D scene Feature samples Image space lines extraction Line drawing Snakes Brush Paths Tracking Geometry Advection Relaxation Parameterization Vectorization Stylization Coverage Connectivity 18
- 19. Vectorization Tracking Coherence continuous evolution across frames Accuracy faithfully representation of shape Coverage level of detail Simplicity simple topology Length stylization freedom 19
- 20. • Color regions tracking in videos Off-line computation User corrections SnakeToonz, Agrawala NPAR 2002 Keyframe-Based Tracking for Rotoscoping and Animation, Agrawala et al. SIGGRAPH 2004 20
- 21. • Advection + Relaxation Frame f Frame f+1 21
- 22. • Reprojection similar to Bousseau et al. 2007 and Lu et al. 2010 Image f f+1 22
- 23. • Minimize the energy: Internal External – Continuity Attraction by the – Smoothness features samples • Semi-explicit Euler scheme 23
- 24. • Grow / shrink re-sampling (similar to Delingette et al. 2000) • Contour shrinking mass-spring forces between the vertices almost constant length if no tangential external force is applied 24
- 25. • Advection + Relaxation 25
- 26. • Local vectorization operators • Applied sequentially in a greedy fashion 26
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- 29. 3D scene Feature samples Image space lines extraction Line drawing Snakes Brush Paths Tracking Geometry Advection Relaxation Parameterization Vectorization Stylization Coverage Connectivity 29
- 30. • Linear image space parameterization: with = slope = phase = arc-length • Evolve according to the motion and topology of the snakes parameterization at each vertex 30
- 31. • Propagation at each vertex Parameterization stored between two frames 31
- 32. • Propagation at each vertex • Linearization (in the least-square sense) parameterization arc-length 32
- 33. • Propagation at each vertex • Vectorization events Linearization Split Extend propagated directly Trim Merge: mechanism to avoid parameterization discontinuities 33
- 34. • Only if the slope and phase match • Leveling mechanism pushes the 2 parameterizations to their mid-value: with parameterization arc-length 34
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- 44. Input samples Active strokes Robustness toward noise / outliers Temporal smoothing 44
- 45. • New propagation primitive for image space lines fully automatic running at interactive frame rates • Temporally coherent basis for complex effects Arc-length variations (texture, offsets, tapering) Shape abstraction 45
- 46. Thank you for your attention 46