175 lines
7.7 KiB
Rust
175 lines
7.7 KiB
Rust
use crate::math::Real;
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/// Parameters for a time-step of the physics engine.
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#[derive(Copy, Clone, Debug)]
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#[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
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pub struct IntegrationParameters {
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/// The timestep length (default: `1.0 / 60.0`)
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pub dt: Real,
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/// Minimum timestep size when using CCD with multiple substeps (default `1.0 / 60.0 / 100.0`)
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///
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/// When CCD with multiple substeps is enabled, the timestep is subdivided
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/// into smaller pieces. This timestep subdivision won't generate timestep
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/// lengths smaller than `min_ccd_dt`.
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///
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/// Setting this to a large value will reduce the opportunity to performing
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/// CCD substepping, resulting in potentially more time dropped by the
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/// motion-clamping mechanism. Setting this to an very small value may lead
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/// to numerical instabilities.
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pub min_ccd_dt: Real,
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/// 0-1: multiplier for how much of the constraint violation (e.g. contact penetration)
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/// will be compensated for during the velocity solve.
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/// (default `0.8`).
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pub erp: Real,
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/// 0-1: the damping ratio used by the springs for Baumgarte constraints stabilization.
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/// Lower values make the constraints more compliant (more "springy", allowing more visible penetrations
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/// before stabilization).
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/// (default `0.25`).
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pub damping_ratio: Real,
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/// 0-1: multiplier for how much of the joint violation
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/// will be compensated for during the velocity solve.
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/// (default `1.0`).
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pub joint_erp: Real,
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/// The fraction of critical damping applied to the joint for constraints regularization.
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/// (default `0.25`).
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pub joint_damping_ratio: Real,
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/// Amount of penetration the engine wont attempt to correct (default: `0.001m`).
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pub allowed_linear_error: Real,
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/// Maximum amount of penetration the solver will attempt to resolve in one timestep.
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pub max_penetration_correction: Real,
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/// The maximal distance separating two objects that will generate predictive contacts (default: `0.002`).
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pub prediction_distance: Real,
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/// Maximum number of iterations performed to solve non-penetration and joint constraints (default: `4`).
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pub max_velocity_iterations: usize,
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/// Maximum number of iterations performed to solve friction constraints (default: `8`).
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pub max_velocity_friction_iterations: usize,
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/// Maximum number of iterations performed to remove the energy introduced by penetration corrections (default: `1`).
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pub max_stabilization_iterations: usize,
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/// If `false`, friction and non-penetration constraints will be solved in the same loop. Otherwise,
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/// non-penetration constraints are solved first, and friction constraints are solved after (default: `true`).
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pub interleave_restitution_and_friction_resolution: bool,
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/// Minimum number of dynamic bodies in each active island (default: `128`).
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pub min_island_size: usize,
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/// Maximum number of substeps performed by the solver (default: `1`).
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pub max_ccd_substeps: usize,
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}
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impl IntegrationParameters {
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/// The inverse of the time-stepping length, i.e. the steps per seconds (Hz).
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///
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/// This is zero if `self.dt` is zero.
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#[inline(always)]
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pub fn inv_dt(&self) -> Real {
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if self.dt == 0.0 {
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0.0
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} else {
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1.0 / self.dt
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}
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}
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/// Sets the time-stepping length.
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#[inline]
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#[deprecated = "You can just set the `IntegrationParams::dt` value directly"]
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pub fn set_dt(&mut self, dt: Real) {
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assert!(dt >= 0.0, "The time-stepping length cannot be negative.");
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self.dt = dt;
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}
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/// Sets the inverse time-stepping length (i.e. the frequency).
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///
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/// This automatically recompute `self.dt`.
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#[inline]
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pub fn set_inv_dt(&mut self, inv_dt: Real) {
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if inv_dt == 0.0 {
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self.dt = 0.0
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} else {
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self.dt = 1.0 / inv_dt
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}
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}
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/// The ERP coefficient, multiplied by the inverse timestep length.
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pub fn erp_inv_dt(&self) -> Real {
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self.erp * self.inv_dt()
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}
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/// The joint ERP coefficient, multiplied by the inverse timestep length.
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pub fn joint_erp_inv_dt(&self) -> Real {
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self.joint_erp * self.inv_dt()
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}
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/// The CFM factor to be used in the constraints resolution.
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pub fn cfm_factor(&self) -> Real {
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// Compute CFM assuming a critically damped spring multiplied by the damping ratio.
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let inv_erp_minus_one = 1.0 / self.erp - 1.0;
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// let stiffness = 4.0 * damping_ratio * damping_ratio * projected_mass
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// / (dt * dt * inv_erp_minus_one * inv_erp_minus_one);
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// let damping = 4.0 * damping_ratio * damping_ratio * projected_mass
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// / (dt * inv_erp_minus_one);
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// let cfm = 1.0 / (dt * dt * stiffness + dt * damping);
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// NOTE: This simplies to cfm = cfm_coefff / projected_mass:
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let cfm_coeff = inv_erp_minus_one * inv_erp_minus_one
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/ ((1.0 + inv_erp_minus_one) * 4.0 * self.damping_ratio * self.damping_ratio);
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// Furthermore, we use this coefficient inside of the impulse resolution.
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// Surprisingly, several simplifications happen there.
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// Let `m` the projected mass of the constraint.
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// Let `m’` the projected mass that includes CFM: `m’ = 1 / (1 / m + cfm_coeff / m) = m / (1 + cfm_coeff)`
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// We have:
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// new_impulse = old_impulse - m’ (delta_vel - cfm * old_impulse)
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// = old_impulse - m / (1 + cfm_coeff) * (delta_vel - cfm_coeff / m * old_impulse)
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// = old_impulse * (1 - cfm_coeff / (1 + cfm_coeff)) - m / (1 + cfm_coeff) * delta_vel
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// = old_impulse / (1 + cfm_coeff) - m * delta_vel / (1 + cfm_coeff)
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// = 1 / (1 + cfm_coeff) * (old_impulse - m * delta_vel)
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// So, setting cfm_factor = 1 / (1 + cfm_coeff).
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// We obtain:
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// new_impulse = cfm_factor * (old_impulse - m * delta_vel)
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//
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// The value returned by this function is this cfm_factor that can be used directly
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// in the constraints solver.
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1.0 / (1.0 + cfm_coeff)
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}
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/// The CFM (constranits force mixing) coefficient applied to all joints for constraints regularization
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pub fn joint_cfm_coeff(&self) -> Real {
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// Compute CFM assuming a critically damped spring multiplied by the damping ratio.
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let inv_erp_minus_one = 1.0 / self.joint_erp - 1.0;
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inv_erp_minus_one * inv_erp_minus_one
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/ ((1.0 + inv_erp_minus_one)
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* 4.0
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* self.joint_damping_ratio
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* self.joint_damping_ratio)
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}
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}
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impl Default for IntegrationParameters {
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fn default() -> Self {
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Self {
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dt: 1.0 / 60.0,
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min_ccd_dt: 1.0 / 60.0 / 100.0,
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erp: 0.8,
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damping_ratio: 0.25,
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joint_erp: 1.0,
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joint_damping_ratio: 1.0,
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allowed_linear_error: 0.001, // 0.005
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max_penetration_correction: Real::MAX,
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prediction_distance: 0.002,
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max_velocity_iterations: 4,
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max_velocity_friction_iterations: 8,
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max_stabilization_iterations: 1,
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interleave_restitution_and_friction_resolution: true, // Enabling this makes a big difference for 2D stability.
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// TODO: what is the optimal value for min_island_size?
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// It should not be too big so that we don't end up with
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// huge islands that don't fit in cache.
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// However we don't want it to be too small and end up with
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// tons of islands, reducing SIMD parallelism opportunities.
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min_island_size: 128,
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max_ccd_substeps: 1,
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}
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}
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}
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