active strokes: coherent line stylization for animated 3d models
DESCRIPTION
These slides presents a method for creating coherently animated line drawings that include strong abstraction and stylization effects. These effects are achieved with active strokes: 2D contours that approximate and track the lines of an animated 3D scene. Active strokes perform two functions: they connect and smooth unorganized line samples, and they carry coherent parameterization to support stylized rendering. Line samples are approximated and tracked using active contours ("snakes") that automatically update their arrangment and topology to match the animation. Parameterization is maintained by brush paths that follow the snakes but are independent, permitting substantial shape abstraction without compromising fidelity in tracking. This approach renders complex models in a wide range of styles at interactive rates, making it suitable for applications like games and interactive illustrations.TRANSCRIPT
ACTIVE STROKES:COHERENT LINE STYLIZATION FOR ANIMATED 3D MODELS
Pierre Bénard1,2 Jingwan Lu3 Forrester Cole4 Adam Finkelstein3 Joëlle Thollot1,2
1Grenoble University, LJK 2INRIA 3Princeton University 4MIT CSAIL
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Line Stylization
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Line Stylization in CG
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Line Stylization in CG• Image space buffers
processing
Comprehensible rendering of 3-D shapes,Saito and Takahashi, SIGGRAPH 1990
• Object space line extraction
Real-Time Nonphotorealistic Rendering,Markosian et al., SIGGRAPH 1997
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Line Stylization: Image Space
Implicit Brushes for stylized line-based rendering, Vergne et al., CGF 2011
Simple and fastNatural coherence and LoD
Restricted stylization effects
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Line Stylization: Object SpaceWide 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
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Brush Path Parameterization
0
l
Brush path
Line texture
Feature lines
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Creases Ridges and valleys
Silhouettes Suggestive contours Apparent ridges
Fixed Line View-Dependent Lines
Parameterization = arc-length Parameterization = ?
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Coherent Parameterization Goals
• Flatness Remain linear in image space
• Motion coherence Evolve according to the motion of the object
• Temporal continuity Adapt to the topological events
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Previous Work
Coherent Stylized Silhouettes, Kalnins et al., SIGGRAPH 2003
Optimization between 2D and 3D
Multiple brush strokes per line
Dependent on the input connectivity
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Previous Work
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Previous Work
Kalnins et al., Coherent Stylized Silhouettes, SIGGRAPH 2003
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Previous Work
Self-Similar Texture for Coherent Line Stylization, Bénard et al. NPAR 2010
2D Infinite zoom:Self-Similar Line Artmap (SLAM)
Dependent on the input connectivity
One brush stroke per line
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Previous WorkEase propagation
of parameterizationwith CSS
Input connectivity
Snaxels on a Plane,Kevin Karsch and John C. Hart,
NPAR 2011
Spatio-Temporal Analysis for Parameterizing Animated Lines,
Buchholz et al., NPAR 2011
Optimization over the entire animation
Offline computation
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Our Approach: Active Strokes• Image space active contours [Kass et al. 1988]
Shape Topology Density
• Brush paths Coherent parameterization Shape abstraction
• Interactive frame rates
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SnakesTracking
Pipeline
Advection
3D scene
Image space lines extraction
Feature samples
Relaxation
Vectorization
Coverage Connectivity
Line drawing
Brush PathsGeometry
Parameterization
Stylization
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Input• Feature samples extracted in image space
2D position Local tangent 2D velocity
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SnakesTracking
Pipeline
Advection
3D scene
Image space lines extraction
Feature samples
Relaxation
Vectorization
Coverage Connectivity
Line drawing
Brush PathsGeometry
Parameterization
Stylization
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Snakes: Properties
Coherence continuous evolution across frames
Accuracy faithfully representation of shape
Coverage level of detail Simplicity simple topology Length stylization freedom
Trac
king
Vect
oriz
ation
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Active Contours in NPR• Color regions tracking
in videos Off-line computation User corrections
SnakeToonz, AgrawalaNPAR 2002
Keyframe-Based Tracking for Rotoscoping and Animation,
Agrawala et al. SIGGRAPH 2004
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Snakes: Tracking• Advection + Relaxation
Frame f Frame f+1
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Snakes: Advection• Reprojection
similar to Bousseau et al. 2007 and Lu et al. 2010
Image fImage f+1
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Snakes: Relaxation [Kass et al. 1988]
• Minimize the energy:
• Semi-explicit Euler scheme
ExternalAttraction by the features samples
Internal– Continuity– Smoothness
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Snakes: Relaxation• 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
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Snakes: Tracking• Advection + Relaxation
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Snakes: Cleaning• Local vectorization operators
• Applied sequentially in a greedy fashion
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Snakes: Tracking + Cleaning
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Snakes: Level of Detail
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SnakesTracking
Pipeline
Advection
Feature samples
Relaxation
Vectorization
Coverage Connectivity
Brush PathsGeometry
Parameterization
Stylization
3D scene
Image space lines extraction
Line drawing
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Brush Paths: Parameterization• Linear image space parameterization:
with = slope= phase= arc-length
• Evolve according to the motion and topology of the snakes parameterization at each vertex
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Brush Paths: Parameterization• Propagation at each vertex
Parameterization stored between two frames
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Brush Paths: Parameterization• Propagation at each vertex• Linearization (in the least-square sense)
para
met
eriza
tion
arc-length
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Brush Paths: Parameterization• Propagation at each vertex• Vectorization events
Split Extend Trim Merge: mechanism to avoid parameterization
discontinuities
propagated directly
• Linearization
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Brush Paths: Merging
• Only if the slope and phase match• Leveling mechanism
pushes the 2 parameterizations to their mid-value:
with
para
met
eriza
tion
arc-length
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Stylization: SLAM texture
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Stylization: SLAM offsets
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Stylization: Shape abstraction• Segments fitting
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Stylization: Shape abstraction• Arcs fitting
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Stylization: Shape abstraction
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Stylization: Implicit Brushes
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Stylization: Steerable filters
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Stylization: Deformable models
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Stylization: Deformable models
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Limitations & Future Work
Robustness toward noise / outliers Temporal smoothing
Input samples Active strokes
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Conclusions• 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
QUESTIONS?Thank you for your attention
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