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feat: apply ground friction on predictive velocity correction too #7

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feat: apply ground friction on predictive velocity correction too #7

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30 changes: 15 additions & 15 deletions crates-ptx/Cargo.lock
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43 changes: 24 additions & 19 deletions src_kernels/cuda/grid_update.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -108,11 +108,12 @@ fn update_single_cell(
}
}
BoundaryHandling::Friction | BoundaryHandling::FrictionZUp => {
if let Some((mut normal, dist)) =
if let Some((mut normal, mut dist)) =
Unit::try_new_and_get(cell.projection_scaled_dir, 1.0e-5)
{
if is_inside {
normal = -normal;
dist = 0.0;
}

#[cfg(feature = "dim2")]
Expand All @@ -125,25 +126,29 @@ fn update_single_cell(

if apply_friction {
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This could be a better name

Suggested change
if apply_friction {
if apply_friction_and_cancel_penetration {

let normal_vel = cell_velocity.dot(&normal);
let dist_with_margin = (dist - cell_width * 0.5).max(0.0);

if normal_vel < 0.0 {
let dist_with_margin = dist - cell_width;
if is_inside || dist_with_margin <= 0.0 {
let tangent_vel =
cell_velocity - normal_vel * normal.into_inner();
let tangent_vel_norm = tangent_vel.norm();

cell_velocity = tangent_vel;

if tangent_vel_norm > 1.0e-10 {
let friction = collider.friction;
cell_velocity = tangent_vel / tangent_vel_norm
* (tangent_vel_norm + normal_vel * friction).max(0.0);
}
} else if -normal_vel * dt > dist_with_margin {
cell_velocity -=
(dist_with_margin / dt + normal_vel) * normal.into_inner();
}
if -normal_vel * dt > dist_with_margin {
// NOTE: if we enter this code, normal_vel is negative.
let tangent_vel = cell_velocity - normal_vel * normal.into_inner();
let tangent_vel_norm = tangent_vel.norm();

let new_normal_vel = normal_vel.max(-dist_with_margin / dt);
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Even if we zero the normal velocity out completely, particles can still approach the boundary as their non-zero-kernel space goes further. We may want to extend the margin s.t. we cover the whole range of an approaching particle.

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This amounts to changing the line above to let dist_with_margin = (dist - cell_width * 1.5).max(0.0);.

This also restores the previous behavior in the fluid example btw.


// NOTE: removed_normal_vel is always positive.
let removed_normal_vel = new_normal_vel - normal_vel;

let new_tangent_vel = if tangent_vel_norm > 1.0e-6 {
tangent_vel / tangent_vel_norm
* (tangent_vel_norm
- removed_normal_vel * collider.friction)
.max(0.0)
} else {
Vector::zeros()
};

cell_velocity =
new_normal_vel * normal.into_inner() + new_tangent_vel;
}
}
}
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