geom(timespan): make fat_aabb conservative for rotations by unioning start/mid/end AABBs; add rotation-offset coverage test; update plan + decision log

Author: flyingrobots

crates/rmg-geom/src/temporal/timespan.rs

@@ -38,13 +38,49 @@ impl Timespan {
 
     /// Computes a conservative fat AABB for a collider with local-space `shape` AABB.
     ///
-    /// The fat box is defined as the union of the shape’s AABBs at the start and
-    /// end transforms. This is conservative for linear motion and suffices for
-    /// broad-phase pairing and CCD triggering.
+    /// Policy (deterministic): unions the AABBs at three sample poses — start (t=0),
+    /// midpoint (t=0.5), and end (t=1). This strictly contains pure translations
+    /// and captures protrusions that can occur at `t≈0.5` during rotations about
+    /// an off‑centre pivot, which a start/end‑only union can miss.
+    ///
+    /// Sampling count is fixed (3) for determinism; future work may make the
+    /// sampling policy configurable while keeping results identical across peers.
     #[must_use]
     pub fn fat_aabb(&self, shape: &Aabb) -> Aabb {
         let a0 = shape.transformed(&self.start.to_mat4());
         let a1 = shape.transformed(&self.end.to_mat4());
-        a0.union(&a1)
+
+        // Midpoint transform via linear interp of translation/scale and
+        // normalized-linear blend of rotation (nlerp), then convert to Mat4.
+        let t0 = self.start.translation().to_array();
+        let t1 = self.end.translation().to_array();
+        let tm = rmg_core::math::Vec3::new(
+            0.5 * (t0[0] + t1[0]),
+            0.5 * (t0[1] + t1[1]),
+            0.5 * (t0[2] + t1[2]),
+        );
+
+        let s0 = self.start.scale().to_array();
+        let s1 = self.end.scale().to_array();
+        let sm = rmg_core::math::Vec3::new(
+            0.5 * (s0[0] + s1[0]),
+            0.5 * (s0[1] + s1[1]),
+            0.5 * (s0[2] + s1[2]),
+        );
+
+        let q0 = self.start.rotation().to_array();
+        let q1 = self.end.rotation().to_array();
+        let qm = rmg_core::math::Quat::new(
+            0.5 * (q0[0] + q1[0]),
+            0.5 * (q0[1] + q1[1]),
+            0.5 * (q0[2] + q1[2]),
+            0.5 * (q0[3] + q1[3]),
+        )
+        .normalize();
+
+        let mid_tf = crate::types::transform::Transform::new(tm, qm, sm);
+        let am = shape.transformed(&mid_tf.to_mat4());
+
+        a0.union(&a1).union(&am)
     }
 }

crates/rmg-geom/tests/geom_broad_tests.rs

@@ -43,3 +43,50 @@ fn broad_phase_pair_order_is_deterministic() {
     // Expected canonical order: (0,1), (0,3), (1,3)
     assert_eq!(pairs, vec![(0, 1), (0, 3), (1, 3)]);
 }
+
+#[test]
+fn fat_aabb_covers_mid_rotation_with_offset() {
+    use core::f32::consts::FRAC_PI_2;
+    // Local shape: rod from x=0..2 (center at (1,0,0)) with small thickness
+    let local =
+        Aabb::from_center_half_extents(rmg_core::math::Vec3::new(1.0, 0.0, 0.0), 1.0, 0.1, 0.1);
+
+    let t0 = Transform::new(
+        rmg_core::math::Vec3::new(0.0, 0.0, 0.0),
+        rmg_core::math::Quat::identity(),
+        rmg_core::math::Vec3::new(1.0, 1.0, 1.0),
+    );
+    let t1 = Transform::new(
+        rmg_core::math::Vec3::new(0.0, 0.0, 0.0),
+        rmg_core::math::Quat::from_axis_angle(rmg_core::math::Vec3::new(0.0, 0.0, 1.0), FRAC_PI_2),
+        rmg_core::math::Vec3::new(1.0, 1.0, 1.0),
+    );
+    let span = Timespan::new(t0, t1);
+
+    // Compute mid pose explicitly (45°); this can protrude beyond both endpoints.
+    let mid_rot = rmg_core::math::Quat::from_axis_angle(
+        rmg_core::math::Vec3::new(0.0, 0.0, 1.0),
+        FRAC_PI_2 * 0.5,
+    );
+    let mid = Transform::new(
+        rmg_core::math::Vec3::new(0.0, 0.0, 0.0),
+        mid_rot,
+        rmg_core::math::Vec3::new(1.0, 1.0, 1.0),
+    );
+    let mid_aabb = local.transformed(&mid.to_mat4());
+
+    let fat = span.fat_aabb(&local);
+    let fmin = fat.min().to_array();
+    let fmax = fat.max().to_array();
+    let mmin = mid_aabb.min().to_array();
+    let mmax = mid_aabb.max().to_array();
+
+    assert!(
+        fmin[0] <= mmin[0] && fmin[1] <= mmin[1] && fmin[2] <= mmin[2],
+        "fat min must enclose mid min: fat={fmin:?} mid={mmin:?}"
+    );
+    assert!(
+        fmax[0] >= mmax[0] && fmax[1] >= mmax[1] && fmax[2] >= mmax[2],
+        "fat max must enclose mid max: fat={fmax:?} mid={mmax:?}"
+    );
+}

docs/decision-log.md

@@ -84,3 +84,10 @@
 - Decision: Pre-commit runs `cargo fmt --all -- --check` whenever staged Rust files are detected. Retain the PRNG coupling guard but remove the unconditional early exit so formatting still runs when the PRNG file isn’t staged.
 - EditorConfig: normalize line endings (LF), ensure final newline, trim trailing whitespace, set 2-space indent for JS/TS/JSON and 4-space for Rust.
 - Consequence: Developers get immediate feedback on formatting; cleaner diffs and fewer CI round-trips.
+
+## 2025-10-29 — Geom fat AABB bounds mid-rotation
+
+- Context: Broad-phase must not miss overlaps when a shape rotates about an off‑centre pivot; union of endpoint AABBs can under‑approximate mid‑tick extents.
+- Decision: `Timespan::fat_aabb` now unions AABBs at start, mid (t=0.5 via nlerp for rotation, lerp for translation/scale), and end. Sampling count is fixed (3) for determinism.
+- Change: Implement midpoint sampling in `crates/rmg-geom/src/temporal/timespan.rs`; add test `fat_aabb_covers_mid_rotation_with_offset` to ensure mid‑pose is enclosed.
+- Consequence: Deterministic and more conservative broad‑phase bounds for typical rotation cases without introducing policy/config surface yet; future work may expose a configurable sampling policy.

docs/execution-plan.md

@@ -33,6 +33,11 @@ This is Codex’s working map for building Echo. Update it relentlessly—each s
 
 ## Today’s Intent
 
+> 2025-10-29 — Geom fat AABB midpoint sampling (merge-train)
+
+- Update `rmg-geom::temporal::Timespan::fat_aabb` to union AABBs at start, mid (t=0.5), and end to conservatively bound rotations about off‑centre pivots.
+- Add test `fat_aabb_covers_mid_rotation_with_offset` to verify the fat box encloses the mid‑pose AABB.
+
 > 2025-10-29 — Hooks formatting gate (PR #12)
 
 - Pre-commit: add rustfmt check for staged Rust files (`cargo fmt --all -- --check`).