Unit
- Hair structure: A single hair is a flexible, thin cylinder with a complex internal fiber structure. In simulation, hair is often modeled as a chain of linked segments or as a continuous curve with defined mass, stiffness, and damping properties.
- Physical properties: Key parameters include density, bending stiffness (Young's modulus), and torsional stiffness. Real hair has a density near 1.1 g/cm3 and bending stiffness that varies with hydration and damage.
- Fur versus hair: Fur typically has shorter, thinner, and more numerous fibers with varying densities across the body. Simulation of fur often uses simplified dynamics compared to long hair because individual strands are less visible.
- Forces on hair: Gravity, inertial forces from body movement, air resistance, friction between strands, and contact forces with surfaces all affect hair motion. The relative importance of each force changes with hair length and style.
- Time scale: Hair dynamics occur over milliseconds to seconds. Stable simulation requires time steps on the order of 1/240 second or smaller, especially for fast-moving characters.
- Numerical integration: Common integration schemes include Euler (simple but unstable), Verlet (energy-conserving), and implicit methods (stable for stiff systems). Implicit integration is preferred for production hair due to stability with large time steps.
- Collision basics: Hair must collide with the body, cloth, and other hair. Efficient collision detection uses spatial hashing or bounding volume hierarchies to find contacts between hundreds of thousands of strands.
- Rendering link: The output of simulation is a set of positions and orientations for each strand. These are passed to a renderer that shades each fiber using specialized hair BSDF models like the Marschner model.
- Guide hair interpolation: To avoid simulating every strand, production systems simulate a sparse set of guide hairs and then interpolate the motion to thousands or millions of render hairs. Interpolation uses skinning weights or radial basis functions to propagate position and orientation from guides to final strands.
- Damping and plasticity: Hair exhibits internal damping that dissipates energy over time, preventing perpetual oscillation. Plastic deformation models permanent bending from styling or collisions. Both are implemented as additional forces or constraints in the simulation to achieve realistic rest shapes.
- Wind and external forces: Environmental forces like wind are applied as directional velocity fields or noise-driven forces across strands. The magnitude and frequency of wind gusts can be artistically controlled to create natural-looking motion without requiring full fluid simulation.
- Performance optimization: Simulating hundreds of thousands of strands is computationally expensive. Techniques include level-of-detail where distant hair uses fewer segments, parallel computation on GPUs, and adaptive time stepping that reduces computation when motion is slow.
Design Principle
Principle: Physical Plausibility vs. Artistic Control
Definition: Hair simulation exists on a spectrum from physically accurate (using real-world parameters) to fully art-directed (using exaggerated forces or constraints). The right balance depends on the style of the project: realistic VFX requires accurate physics, while stylized animation often benefits from artistic override.
Application: For a realistic character in a live-action film, use measured hair stiffness and mass values, and simulate with implicit integration to maintain stability. For a cartoon character with bouncy hair, reduce damping and increase spring forces to create exaggerated motion that reads clearly on screen.
Quick Revision
- Hair is modeled as flexible chains or curves with defined physical properties.
- Key forces include gravity, inertia, air resistance, friction, and contact.
- Stable simulation requires small time steps and appropriate integration methods.
- Collision detection is essential and uses spatial data structures.
- Balance physical accuracy with artistic control based on project style.