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