Overview
This lecture introduces a method for dynamic, implicit muscle modeling for character skinning, enabling real-time, anatomically plausible deformations with efficient simulation and collision handling.
Background & Motivation
- Character animation realism benefits from simulating underlying anatomical structure (bones, muscles, soft tissues).
- Existing skinning methods: geometric (fast, simple, less realistic), data-driven (tedious, limited control), and physics-based (realistic, slow).
- Implicit Skinning uses scalar fields to simulate elastic skin and contact in real-time.
Proposed Approach
- New muscle primitives are defined as implicit surfaces (scalar fields) attached to skeletal bones.
- Muscle shapes are based on swept profiles along a central polyline, with minimal intuitive parameters.
- Muscle deformation (contraction, bulging, activation) preserves nearly constant volume.
- Position Based Dynamics (PBD) drives the muscle central curve, adding dynamic effects (e.g. jiggling) and handling collisions.
- Muscle-muscle, muscle-bone, and muscle-skin collisions are efficiently resolved via field-based constraints.
Muscle Model Details
- Each muscle primitive has two endpoints attached to bones, user-defined geometry, and rest/activated shapes.
- The muscle profile is controlled by longitudinal and radial functions, and adjustable eccentricity.
- Volume is globally preserved by updating width based on muscle length changes.
- Non-fusiform muscles (e.g. pectorals) are modeled by combining multiple primitives.
Dynamics & Collision Handling
- Internal muscle polyline points act as particles animated via PBD, constrained by elasticity and collision rules.
- Mass and stiffness control muscle tone and inertial response; higher stiffness yields tenser muscles.
- Field-based constraints keep muscles above bones, prevent inter-muscle penetration, and keep muscles inside skin.
- Global damping and friction model viscoelasticity and energy loss on collision.
Integration with Implicit Skinning
- Muscle fields are compacted and blended with bone fields using smooth operators to avoid gradient discontinuities.
- At each animation frame, muscle endpoints and parameters are updated, PBD is solved, and the mesh deforms accordingly.
Results & Performance
- Muscles simulate plausible dynamics (e.g. bulging, jiggling) with <3% volume variation during animation.
- The method maintains interactive rates (under 1 sec/frame for complex scenes), with adjustable particle count for performance.
- User parameters (shape, activation, stiffness) are intuitive; parameter tuning can be keyframed or interactive.
- Collision constraints double PBD solve time but are critical for realistic deformations.
Limitations & Future Work
- Main computation bottleneck is scalar field evaluation for mesh vertices.
- Current model approximates complex muscle shapes and is limited for non-fusiform muscles.
- Muscle dynamics rely on time step and stiffness tuning; some jiggling effects are challenging to achieve.
- Future directions include GPU optimization and extending the approach to other tissues like fat or cartilage.
Key Terms & Definitions
- Implicit Skinning — A method using 3D scalar fields to define and deform a character’s skin, handling elasticity and contact.
- Muscle Primitive — An implicit surface representing a muscle, defined by a swept profile along a polyline.
- Position Based Dynamics (PBD) — A physics simulation method that enforces constraints (e.g. elasticity, collisions) on particles’ positions.
- HRBF (Hermite Radial Basis Function) — A mathematical function used to define smooth scalar fields for modeling surfaces.
- Volume Preservation — Maintaining a constant muscle volume during shape changes (activation/stretching).
Action Items / Next Steps
- Practice adjusting muscle parameters (shape, eccentricity, stiffness) interactively in a modeling tool.
- Review the Implicit Skinning pipeline and how muscle primitives integrate into it.
- Explore further readings on Position Based Dynamics and implicit modeling for animation.