Interactive control of deformable-object animations through control metaphor pattern adherence

1. Interactive control of deformable-object animations through controlmetaphor pattern adherence Shane Transue and Min-Hyung Choi Department of Computer Science and Engineering, University of Colorado Denver, Denver, USA

2. Introduction • Motivation • Related Work • Method Overview • Deformation Control Metaphors • Results • Discussion and Evaluation

3. Motivation • Deformation Control within a Physical Simulation • Goal: Editable deformation behavior that appears physically plausible • Interactive editing of motion in animations using control metaphors • Artistic control resides within a high control domain (general motion) • Artistic process is generally iterative (small modifications to existing motion) • Reinforces iterative process • For existing animations: Make fine adjustments to the existing behavior • Deformation Control Domains: • State-driven objectives • Motion-oriented control

4. Motivation • Goal State Methods • Define an exact deformation state at times tiand interpolate between them: • Force-based Methods • Define the exact application of external forces at time ti to provide a natural modifications to the original behavior: • Key: Tradeoff between goal state accuracy and artistically derived force-based techniques [Hildebrandt et al., 2012]

5. Motivation • State-based Keyframe Optimization (Goal Oriented) + Can be used to define an exact deformation state + Optimization to reach intended deformation - Deriving static deformation states is challenging - Can lead to unrealistic behavior • Force-based Deformation (Motion Oriented) + Naturally provides physically plausible behavior + Higher level motions can be described - Less obvert control over resulting motion - Difficult to define force direction and magnitude

6. Interactive Deformation Editing (video)

7. Related Work • Rest-shape Adaptations • Deformable Objects Alive! [Coros et al., 2012] • Internal elastic potential • Goal-driven motion derivation • Deformations personify resulting behaviors • Harmonic Coordinates [Joshi et al., 2007] • Cage-based deformation control • Limits artistically controlled degrees of freedom • Intended for deformable character animation Cage-based Deformation (Harmonics) Internal Elastic Potential (Alive!)

8. Related Work • State-driven and Dynamic Keyframes • Space-time Optimization [Hildebrandt et al., 2012] • Optimization of keyframes, velocity, forces • Target states obtained through keyframe interpolation • Assumes animation driving keyframes are provided • Difficulties with collision (non-continuous) • Interactive Editing of Deformable Simulations [Barbič et al., 2012] • Interactively set goal states (such as nodal positions) • Oscillations drive intended deformations • Gradually achieves intended goal state

9. Method Overview • Control Metaphor Pattern Adherence: • Intent: Generate or modify an existing animation by editing deformation behaviors through a set of intuitive control metaphors that impose predefined behaviors on an object • Control Metaphors: • Define a set of high-level controls that define deformation behaviors over time • Intuitive motions that easily convey meaning (bending, twisting, etc). • Configurable to derive different behaviors within an animation • Provides artistic freedom • Compound Deformations • Localized Deformations Ex: Bend Control Metaphor applied to a cylindrical model

10. Method Overview • Animation Framework • Interactive multi-view animation studio • Generate Animations from deformable models • Tetrahedral-based Volumetric Models • Shell-based Surface Models • Supports multiple control metaphors per object • Intuitive control metaphor customization • Animation previews • Generates next n % m simulation frames to display future motions • Existing animation is not modified • Real-time Simulation recording • Timeline-based recording easily allows for animation editing • Manual timeline progress (for accurate deformation analysis) Animation Preview for 3 future states of the simulated cloth (with bend metaphor)

11. Method Overview • Control Metaphor-based Animation Process Overview: • Generate or obtain original animation • Select and apply various control metaphors For each applied control metaphor • Define deformation position and orientation • Edit force curve to define deformation duration and magnitude • Generate animation preview to view resulting deformations • Record modified simulation • Iterate (until desired behavior is achieved)

12. Deformation Control Metaphors • Abstract Control Metaphor: CM = (R, O, V, F) • Control Regions (R) • Defines the region of influence of the applied control • Force Orientations (O) • For each control region, an associated force orientation is defined • Visual Representation (V) • Intuitive widgets that can be interactively configured for each CM • Force Curve (F) • Defines both the duration and magnitude of the applied forces

13. Deformation Control Metaphors • Control Regions (R) • Regions of a deformable body influenced by a control metaphor • Customizable through the use of the visual representation • Region separation • Spherical volume, cylindrical radius, etc. • Click-and-drag operations modify influence regions Bend control metaphor Deformation Regions: 3 Spherical components. Influenced nodes are highlighted in red, green, and blue.

14. Deformation Control Metaphors • Force Orientations (O) • Define in force diagrams for each control metaphor • Each unique set of force orientations maps to a high-level deformation • Stretch separates nodes on opposing sides of a separation axis • Twist rotates nodes in opposing directions • Bend creases an object using a pivot point • Bend Deformation Force Diagram: • 3 control regions • 3 force orientations • Left, right sets, Pivot set • Opposing forces derive benddeformation at the pivot Configurable Bend-control Force Diagram

15. Deformation Control Metaphors • Visual Representation (V) • Primitives compose a basic widget tool • Each component region can be adjusted • Customization tied to metaphor definition • Intuitive Widget Interaction • Click-and-drag modifies widget state • Highlight indicates component within thewidget that will be modified • Localization using Node selection • Point-and-click Node selection • Multiple metaphors / different regions Interactive Bend Control Widget – Applied to Bunny Ear Highlighted node represents localized deformation position

16. Deformation Control Metaphors • Force Curve (F) • Defines both force duration and magnitude • Editable Bezier curve with control points • Snap to frame (provides per-frame resolution of force magnitude) • Provides intuitive ease-in/out for smooth deformations • Animation Time (simulation time-step) [vs] Force Magnitude

17. Interactive Animation Studio Interactive Animation Editing Studio: Animation simulation, timeline, and control metaphor toolbar (right-hand side)

18. Interactive Animation Studio (video)

19. Results • Localized Deformations + Targeted regions of an object can be easily deformed + Regarded as a challenge by current methods – partially addressed - Large force magnitudes may collapse effected region - Challenge in selecting appropriate node (none may be suitable) • Compound Deformations + Application of multiple control metaphors + Natural blending between applied deformation behaviors + Introduces a larger set of complex deformations - Multiple metaphors can cancel out imposed behavior (forces) - Imposed trajectory may be modified - May impose unintended torque

20. Results • Localized Deformation Demonstration • Bend and Stretch Control Metaphors • Applied to specific regions within the underlying geometry (node selection) • Control metaphor bounding regions defined resulting deformation Bend and Stretch control metaphors applied to a cloth and cylinder models respectively. Resulting deformations blend naturally with the surrounding geometry.

21. Results • Compound Deformation Demonstration • Multiple twist control metaphors applied to the bunny model • Results in a naturally blended compound deformation Control Metaphor Configuration: Neck rotation and ear twist Resulting Animation: Head is rotated as the right ear is twisted

22. Results • Compound Deformation (Dragon Model) • Intended Result: Wing flap, Head turn • Obtained using 1-twist and 2-bend metaphors • Resulting Deformation Behavior: Wing Bend Metaphor Configuration Dragon Model Animation (Frames [0 – 480]) Compound Deformation Result

23. Evaluation and Discussion • Inverse Dynamics + High level of control of keyframe deformation states + Effective for goal-oriented motion - Derivation of deformation states that lead to physically plausible behavior can be difficult (cloth, complex models, etc) - Unrealistic behaviors may occur during the interpolation process • Cage-based Control + Assists in the process of defining complex static deformation states + Utility in combination with dynamic keyframe interpolation methods - Does not inherently define motions between states • Rest-shape Adaptations (Internal Elastic Potential) + Technique inherently addresses unintended torque / modified trajectory - Internal elasticity methods result in very specific motion behavior (self-automated objects) - Example-based techniques also rely on realistic deformation keyframes

24. Evaluation and Discussion • Challenges in External Force Application • Introduction of unintended torque • Unbalanced application of forces • Misaligned Control Metaphors • Do not provide the requirements to impose the correct behavior • External force magnitude/duration • How much force is required to impose a deformation? • Mitigated through force curves and simulation previews • Unintended modification of global trajectory • Some controls may intend to modify the trajectory (poke, push)

25. Evaluation and Discussion • Metaphor Alignment • Applied control metaphor can be misaligned with the underlying geometry • Metaphor position limited to node selection • May generate unintended torque • Intended deformation may not be obtained • Can (incorrectly) modify objects global trajectory (push or pull the object) • Additional merit: Provided set of control metaphors can be applied in unintended ways (one tool can be used in multiple ways) Misaligned Control Metaphor: This will not impose the intended bend deformation

26. Evaluation and Discussion • Force Curves (magnitude/duration) • Intuitive for general motion modification • Difficult to impose exact deformation behaviors • High-level deformation behaviors can be interpreted in several ways • Given a desired deformation - Difficulties in forming a force curve: • Difficult to determine force magnitude required for desired behavior • Force duration required for intended behavior • Desired deformation depends on several factors: • Material Properties • Control Metaphor Configuration • Geometric composition of the deformable object • Collision Events

27. Conclusion • Interactive editing of physically plausible deformations • Introduced an intuitive method for editing deformations within existing animations using controlled deformation metaphors • High-level artistically controlled deformation • Bending, twisting, stretching, compression, etc. • Duration and deformation intensity provided by force curves • Animation previews illustrate imposed deformations • Compound deformations obtained through multiple metaphors • Naturally blended deformation behaviors • Real-time interactive recording and animation editing • Interactive control metaphor configuration • Iterative artistic process • Animation editing and simulation recording