Interactive control of deformable-object animations through control metaphor pattern adherence Shane Transue and Min-Hyung Choi Department of Computer Science and Engineering, University of Colorado Denver, Denver, USA Introduction Motivation Related Work Method Overview Deformation Control Metaphors Results Discussion and Evaluation 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 Motivation Goal State Methods Define an exact deformation state at times t i and interpolate between them: Force-based Methods Define the exact application of external forces at time t i 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] 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 Interactive Deformation Editing (video) 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!) 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 [Barbi et al., 2012] Interactively set goal states (such as nodal positions) Oscillations drive intended deformations Gradually achieves intended goal state 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 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) Method Overview Control Metaphor-based Animation Process Overview: 1. Generate or obtain original animation 2. 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 3. Generate animation preview to view resulting deformations 4. Record modified simulation 5. Iterate (until desired behavior is achieved) 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 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. 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 bend deformation at the pivot Configurable Bend-control Force Diagram 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 the widget 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 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 Interactive Animation Studio Interactive Animation Editing Studio: Animation simulation, timeline, and control metaphor toolbar (right-hand side) Interactive Animation Studio (video) 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 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. 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 Results Compound Deformation (Dragon Model) Intended Result: Wing flap, Head turn Obtained using 1-twist and 2-bend metaphors Resulting Deformation Behavior: Dragon Model Animation (Frames [0 480]) Compound Deformation Result Wing Bend Metaphor Configuration 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 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) 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 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 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