Cleanup flocking example to use composition

crates/processing_ffi/src/lib.rs

@@ -3694,14 +3694,6 @@ pub extern "C" fn processing_particles_kernel_impulse() -> u64 {
         .unwrap_or(0)
 }
 
-#[unsafe(no_mangle)]
-pub extern "C" fn processing_particles_kernel_flock() -> u64 {
-    error::clear_error();
-    error::check(particles_kernel_flock)
-        .map(|e| e.to_bits())
-        .unwrap_or(0)
-}
-
 #[unsafe(no_mangle)]
 pub extern "C" fn processing_particles_kernel_orient() -> u64 {
     error::clear_error();

crates/processing_pyo3/examples/flocking_duck.py

@@ -1,4 +1,4 @@
-# Flocking inside a duck: the GPU boids from flocking_gpu.py, seeded from
+# Flocking inside a duck: GPU boids seeded from
 # the vertices of the Duck glTF mesh. Each boid remembers its spawn vertex
 # in a `home` attribute; a homing force that is negligible near home but
 # grows quadratically with distance lets the boids swirl and flock locally

crates/processing_pyo3/examples/flocking_gpu.py

@@ -1,26 +1,23 @@
-# GPU flocking: the boids from flocking.py, moved entirely onto the GPU.
-# Positions and velocities live in particle attribute buffers, two compute
-# kernels update them each frame, and the flock renders instanced — nothing
-# is ever read back to the CPU. Brute-force O(N²) neighbor search is trivial
-# for a GPU at this scale; a spatial hash grid is the next step past ~100k.
+# GPU flocking: the boids from flocking.py, built from generic particle ops
+# over each boid's neighbors. Nothing is read back to the CPU.
 from mewnala import *
 from math import cos, sin
 from random import uniform
 
-BOID_COUNT = 10000
+BOID_COUNT = 100000
 BOUND = 30.0  # half-extent of the wrapping box
 NEIGHBOR_DIST = 5.0
 SEPARATION_DIST = 2.5
 MAX_SPEED = 10.0  # units per second
 MAX_FORCE = 6.0  # units per second²
 DT = 1.0 / 60.0
 
-# Pass 1: every boid reads the whole flock's state and writes only its
-# steering force. Splitting the read from the write mirrors the CPU
-# example's two loops — no boid sees a half-updated neighbor.
-
-# Pass 2: integrate the steering force, wrap at the box edges, and point
-# each instanced boid along its velocity via the rotation quaternion.
+# Separation steers away from the summed offsets, so its speed is negative.
+RULES = [
+    ("separation", -MAX_SPEED, 1.5, "close"),
+    ("alignment", MAX_SPEED, 1.0, "near"),
+    ("cohesion", MAX_SPEED, 1.0, "near"),
+]
 
 p = None
 boid = None
@@ -60,15 +57,7 @@ def setup():
 
     directional_light((0.95, 0.9, 0.85), 800.0)
 
-    p = create_particles(
-        BOID_COUNT,
-        attributes=[
-            Attribute.position(),
-            Attribute.rotation(),
-            Attribute.color(),
-            Attribute.velocity(),
-        ],
-    )
+    p = create_particles(BOID_COUNT)
 
     positions = []
     velocities = []
@@ -98,6 +87,38 @@ def setup():
     )
 
 
+# Reynolds steering. `mask` zeroes it for boids with no neighbors in range,
+# which would otherwise brake.
+def steer(desired, speed, mask):
+    p.apply(MAP, desired, op=NORMALIZE, length=speed)
+    p.apply(COMBINE, desired, "velocity", op=SUB)
+    p.apply(MAP, desired, op=LIMIT, max_length=MAX_FORCE * DT)
+    p.apply(COMBINE, desired, mask, op=MUL)
+
+
+def flock():
+    p.apply(FIND_NEIGHBORS, grid=grid, radius=NEIGHBOR_DIST)
+    # what each rule steers toward, summed over the neighbors
+    p.apply(NEIGHBOR, "position", out="separation", op=SUM,
+            relative=True, radius=SEPARATION_DIST, falloff=INVERSE_SQUARE)
+    p.apply(NEIGHBOR, "velocity", out="alignment", op=SUM)
+    p.apply(NEIGHBOR, "position", out="cohesion", op=SUM, relative=True)
+    # which boids have any neighbors for each rule
+    p.apply(NEIGHBOR, out="close", op=COUNT, radius=SEPARATION_DIST)
+    p.apply(NEIGHBOR, out="near", op=COUNT)
+    p.apply(MAP, "close", op=GREATER, threshold=0)
+    p.apply(MAP, "near", op=GREATER, threshold=0)
+
+    for i, (rule, speed, weight, mask) in enumerate(RULES):
+        steer(rule, speed, mask)
+        if i == 0:
+            p.apply(MAP, rule, out="force", op=AFFINE, scale=weight)
+        else:
+            p.apply(COMBINE, "force", rule, op="add", b_scale=weight)
+    p.apply(COMBINE, "velocity", "force", op="add")
+    p.apply(MAP, "velocity", op=LIMIT, min_length=MAX_SPEED * 0.25, max_length=MAX_SPEED)
+
+
 def draw():
     global title_last_time, title_last_frame
 
@@ -114,20 +135,11 @@ def draw():
     camera_look_at(0.0, 0.0, 0.0)
     background(10, 12, 18)
 
+    flock()
+
     material(mat)
     particles(p, boid)
 
-    p.flock(
-        grid,
-        sep_distance=SEPARATION_DIST,
-        neighbor_distance=NEIGHBOR_DIST,
-        weight_separation=1.5,
-        weight_alignment=1.0,
-        weight_cohesion=1.0,
-        max_speed=MAX_SPEED,
-        max_force=MAX_FORCE * DT,
-        min_speed=MAX_SPEED * 0.25,
-    )
     p.apply(INTEGRATE, dt=DT)
     p.apply(BOUNDS_BOX, aabb_min=[-BOUND] * 3, aabb_max=[BOUND] * 3, mode=2)
     p.apply(ORIENT, forward=[0.0, 0.0, 1.0], up=[0.0, 1.0, 0.0])

crates/processing_pyo3/src/particles.rs

@@ -150,18 +150,6 @@ impl Drop for Grid {
     }
 }
 
-static FLOCK_COMPUTE: std::sync::Mutex<Option<Entity>> = std::sync::Mutex::new(None);
-
-fn flock_compute() -> PyResult<Entity> {
-    let mut guard = FLOCK_COMPUTE.lock().unwrap();
-    if let Some(e) = *guard {
-        return Ok(e);
-    }
-    let e = particles_kernel_flock().map_err(|e| PyRuntimeError::new_err(format!("{e}")))?;
-    *guard = Some(e);
-    Ok(e)
-}
-
 static PHYSICS_COMPUTES: std::sync::Mutex<Option<HashMap<String, Entity>>> =
     std::sync::Mutex::new(None);
 
@@ -1113,24 +1101,6 @@ impl Particles {
         Ok(Grid { entity })
     }
 
-    #[pyo3(signature = (grid, **kwargs))]
-    pub fn flock(&self, grid: &Grid, kwargs: Option<&Bound<'_, PyDict>>) -> PyResult<()> {
-        let flock = flock_compute()?;
-        if let Some(kwargs) = kwargs {
-            crate::compute::set_compute_kwargs(flock, kwargs)?;
-        }
-        let cell = grid.cell_size()?;
-        let neighbor_distance = kw_f32(kwargs, "neighbor_distance", cell)?.min(cell);
-        compute_set(
-            flock,
-            "neighbor_distance",
-            shader_value::ShaderValue::Float(neighbor_distance),
-        )
-        .map_err(|e| PyRuntimeError::new_err(format!("{e}")))?;
-        particles_flock(self.entity, flock, grid.entity)
-            .map_err(|e| PyRuntimeError::new_err(format!("{e}")))
-    }
-
     #[staticmethod]
     pub fn orient() -> PyResult<Compute> {
         let entity =

crates/processing_render/src/lib.rs

@@ -41,11 +41,11 @@ pub use particles::{
     particles_apply, particles_attribute_add, particles_attributes, particles_buffer,
     particles_capacity, particles_connectivity_indirect, particles_create,
     particles_create_from_geometry, particles_destroy, particles_emit, particles_emit_gpu,
-    particles_ensure_attribute, particles_flock, particles_gather, particles_kernel_age,
+    particles_ensure_attribute, particles_gather, particles_kernel_age,
     particles_kernel_attr_combine, particles_kernel_attr_linear, particles_kernel_attr_lookup1d,
     particles_kernel_attr_lookup2d, particles_kernel_attr_mix, particles_kernel_attract,
     particles_kernel_bounds_box, particles_kernel_bounds_geometry, particles_kernel_bounds_sphere,
-    particles_kernel_drag, particles_kernel_field, particles_kernel_flock, particles_kernel_force,
+    particles_kernel_drag, particles_kernel_field, particles_kernel_force,
     particles_kernel_impulse, particles_kernel_integrate, particles_kernel_noise,
     particles_kernel_orient, particles_kernel_transform, particles_kernel_vortex,
     particles_reset_indices, particles_scatter_create, particles_scatter_volume_create,

crates/processing_render/src/particles/emit.rs

@@ -143,34 +143,6 @@ pub fn particles_emit(
     })
 }
 
-pub fn particles_flock(
-    particles_entity: Entity,
-    flock_entity: Entity,
-    grid: Entity,
-) -> error::Result<()> {
-    let position = app_mut(|app| {
-        let world = app.world();
-        let field = world
-            .get::<Particles>(particles_entity)
-            .ok_or(error::ProcessingError::ParticlesNotFound)?;
-        for (&attr_entity, &buf_entity) in &field.buffers {
-            let attr = world
-                .get::<geometry::Attribute>(attr_entity)
-                .ok_or(error::ProcessingError::InvalidEntity)?;
-            if attr.name == "position" {
-                return Ok(buf_entity);
-            }
-        }
-        Err(error::ProcessingError::InvalidArgument(
-            "particles_flock requires a `position` attribute".to_string(),
-        ))
-    })?;
-
-    grid_build(grid, position)?;
-    compute_set(flock_entity, "grid", ShaderValue::Grid(grid))?;
-    particles_apply(particles_entity, flock_entity)
-}
-
 static NEIGHBOR_COMPUTE: Mutex<Option<Entity>> = Mutex::new(None);
 
 fn neighbor_compute() -> error::Result<Entity> {

crates/processing_render/src/particles/kernels/mod.rs

@@ -42,7 +42,6 @@ impl Plugin for ParticlesKernelsPlugin {
         embedded_asset!(app, "bounds_sphere.wgsl");
         embedded_asset!(app, "bounds_box.wgsl");
         embedded_asset!(app, "impulse.wgsl");
-        embedded_asset!(app, "flock.wgsl");
         embedded_asset!(app, "orient.wgsl");
         embedded_asset!(app, "field.wgsl");
         embedded_asset!(app, "attr_linear.wgsl");
@@ -255,21 +254,6 @@ pub fn particles_kernel_impulse() -> error::Result<Entity> {
     Ok(entity)
 }
 
-pub fn particles_kernel_flock() -> error::Result<Entity> {
-    let shader = shader_load("embedded://processing_render/particles/kernels/flock.wgsl")?;
-    let entity = compute_create(shader)?;
-    set_requires(entity, &["position", "velocity"])?;
-    compute_set(entity, "sep_distance", ShaderValue::Float(1.2))?;
-    compute_set(entity, "neighbor_distance", ShaderValue::Float(2.5))?;
-    compute_set(entity, "weight_separation", ShaderValue::Float(1.5))?;
-    compute_set(entity, "weight_alignment", ShaderValue::Float(1.0))?;
-    compute_set(entity, "weight_cohesion", ShaderValue::Float(1.0))?;
-    compute_set(entity, "max_speed", ShaderValue::Float(0.1))?;
-    compute_set(entity, "max_force", ShaderValue::Float(0.003))?;
-    compute_set(entity, "min_speed", ShaderValue::Float(0.02))?;
-    Ok(entity)
-}
-
 pub fn particles_kernel_orient() -> error::Result<Entity> {
     let shader = shader_load("embedded://processing_render/particles/kernels/orient.wgsl")?;
     let entity = compute_create(shader)?;