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//! SDF-driven wind field: wind vector is a function of the local SDF
//! distance to obstacles.
//!
//! Complements [`crate::wind_zone`] (uniform bounded wind) by making
//! the wind field aware of the surrounding geometry. Typical use case:
//! wind flowing around a building — velocity accelerates over the roof
//! and decelerates in the wake shadow, so anything downstream sees a
//! naturally-varying vector without a pre-baked flow-field lookup.
//!
//! The MVP model:
//!
//! ```text
//! d = sdf.distance(x)
//! shelter = clamp(d / decay_scale, 0, 1)
//! wind = base_direction · base_speed · shelter
//! ```
//!
//! Points deep inside the wake shadow (small `d`) receive attenuated
//! wind; points far from any obstacle receive the full base wind.
//! Callers who need lift over the roof / turbulence spectra should
//! compose the field with [`crate::wind_zone`] or upgrade to a full
//! LES post-process.
use crate::sdf_collider::SdfField;
/// SDF-driven wind sampler.
pub struct SdfWindField<'a, F: SdfField + ?Sized> {
/// SDF describing obstacle geometry. Positive distance = outside.
pub sdf: &'a F,
/// Nominal wind direction (should be a unit vector).
pub base_direction: [f32; 3],
/// Nominal wind speed at distances `>= decay_scale`.
pub base_speed_m_s: f32,
/// Distance scale over which the shelter attenuation ramps from
/// 0 → 1. Typical: 5 m for building-scale geometry. A scale `<= 0` is the
/// sharp limit of the ramp: full wind outside the obstacle (`d > 0`), none
/// on its surface or inside.
pub decay_scale_m: f32,
}
impl<'a, F: SdfField + ?Sized> SdfWindField<'a, F> {
/// Construct a sampler with sensible defaults for outdoor scenes.
pub fn new(sdf: &'a F, direction: [f32; 3], speed: f32) -> Self {
Self {
sdf,
base_direction: direction,
base_speed_m_s: speed,
decay_scale_m: 5.0,
}
}
/// Sample the wind velocity at `position` (m).
///
/// The wind is `base_speed · clamp(d / decay_scale, 0, 1)` along
/// `base_direction`, `d` the SDF distance (zero inside the obstacle). A NaN
/// distance (a broken SDF) gives a NaN wind rather than still air.
#[must_use]
pub fn sample(&self, position: [f32; 3]) -> [f32; 3] {
let d = self.sdf.distance(position[0], position[1], position[2]);
let shelter = if d.is_nan() {
f32::NAN
} else if self.decay_scale_m <= 0.0 {
// the limit of the ramp as the scale goes to 0+
if d > 0.0 {
1.0
} else {
0.0
}
} else {
(crate::det_math::max(d, 0.0) / self.decay_scale_m).clamp(0.0, 1.0)
};
let scale = self.base_speed_m_s * shelter;
[
self.base_direction[0] * scale,
self.base_direction[1] * scale,
self.base_direction[2] * scale,
]
}
}
#[cfg(test)]
mod tests {
use super::*;
use crate::sdf_collider::ClosureSdf;
fn ground_plane() -> ClosureSdf {
ClosureSdf::new(|_x, y, _z| y, |_x, _y, _z| (0.0, 1.0, 0.0))
}
fn box_wall() -> ClosureSdf {
// Half-space x < 0 is inside the wall, x > 0 outside.
ClosureSdf::new(|x, _y, _z| x, |_x, _y, _z| (1.0, 0.0, 0.0))
}
#[test]
fn wind_zero_inside_obstacle() {
let sdf = ground_plane();
let field = SdfWindField::new(&sdf, [1.0, 0.0, 0.0], 10.0);
// Point below the plane (y < 0) → inside → distance clamped to 0.
let v = field.sample([0.0, -1.0, 0.0]);
assert_eq!(v, [0.0, 0.0, 0.0]);
}
#[test]
fn wind_at_full_strength_far_from_obstacle() {
let sdf = ground_plane();
let field = SdfWindField::new(&sdf, [1.0, 0.0, 0.0], 10.0);
let v = field.sample([0.0, 100.0, 0.0]);
assert!((v[0] - 10.0).abs() < 1.0e-3);
}
#[test]
fn wind_attenuates_within_decay_scale() {
let sdf = box_wall();
let mut field = SdfWindField::new(&sdf, [0.0, 0.0, 1.0], 5.0);
field.decay_scale_m = 10.0;
// Sample at x = 2 → d = 2, shelter = 0.2 → v_z = 1.0.
let v = field.sample([2.0, 0.0, 0.0]);
assert!((v[2] - 1.0).abs() < 1.0e-3);
}
#[test]
fn zero_decay_scale_yields_full_wind_everywhere() {
let sdf = ground_plane();
let mut field = SdfWindField::new(&sdf, [1.0, 0.0, 0.0], 3.0);
field.decay_scale_m = 0.0;
let v = field.sample([0.0, 1.0, 0.0]);
assert!((v[0] - 3.0).abs() < 1.0e-3);
}
#[test]
fn direction_preserved_in_sample() {
let sdf = ground_plane();
let field = SdfWindField::new(&sdf, [0.0, 0.0, 1.0], 8.0);
let v = field.sample([0.0, 50.0, 0.0]);
assert!(v[0].abs() < 1.0e-3);
assert!(v[1].abs() < 1.0e-3);
assert!(v[2] > 0.0);
}
#[test]
fn nan_distance_gives_nan_wind_not_still_air() {
let sdf = ClosureSdf::new(|_x, _y, _z| f32::NAN, |_x, _y, _z| (0.0, 1.0, 0.0));
let field = SdfWindField::new(&sdf, [1.0, 0.0, 0.0], 10.0);
let v = field.sample([0.0, 1.0, 0.0]);
assert!(v.iter().all(|c| c.is_nan()), "{v:?}");
}
#[test]
fn zero_decay_scale_is_still_air_on_and_inside_the_surface() {
// limit of the ramp as the scale goes to 0+: 1 for d > 0, 0 for d <= 0
let sdf = ground_plane();
let mut field = SdfWindField::new(&sdf, [1.0, 0.0, 0.0], 3.0);
field.decay_scale_m = 0.0;
assert_eq!(field.sample([0.0, 0.0, 0.0]), [0.0, 0.0, 0.0]);
assert_eq!(field.sample([0.0, -2.0, 0.0]), [0.0, 0.0, 0.0]);
field.decay_scale_m = -1.0;
assert_eq!(field.sample([0.0, -2.0, 0.0]), [0.0, 0.0, 0.0]);
assert_eq!(field.sample([0.0, 0.5, 0.0]), [3.0, 0.0, 0.0]);
}
}