arcattackandClaude Fable 5
d4e139a0d9
User-reported recurrence of "legs stutter and lose sync". Live [sync] showed
advSum/legSum drifting 5->92 during walk-up churn, then locking. Reproduced
headlessly (BT_FORCE_OSC throttle feathering + BT_FORCE_TURN + the new
per-frame BT_SYNC_LOG probe) and root-caused in two parts:
- SEED: the bring-up turn-in-place trigger arms only the LEG channel, so the
walk re-entry lands one frame apart -> a persistent clip phase offset.
- AMPLIFIER: with offset clips the two end-of-clip callbacks fire on different
frames; a demand change landing between them picks DIFFERENT next states
(one channel winds down, the other keeps walking) -> opposite-phase churn
(observed bs=6 ls=7 -- body on walk-R, leg on walk-L).
Under v4 (display+travel = BODY) the out-of-phase LEG pose showed through on
every frame the body didn't write joints (Standing/wind-down) = the visible
stutter/pop.
DISASM GROUND TRUTH (neither Advance fn has a static decomp caller; found by
byte-scanning the CODE section for e8 calls):
- master perf (0x4a9b5c gap) -> AdvanceLegAnimation @0x4aa399 (air @0x4aa388)
-> -dist/dt into localVelocity (+0x1cc): the LEG drives LOCAL travel and
(writing last) the displayed pose;
- IntegrateMotion (0x4ab1c8, body advance @0x4ab312, caller FUN_004ab430 =
the projected-origin updater) -> -dist/dt into projectedVelocity (+0x2a0):
the BODY is the dead-reckoning PROJECTOR -- locally invisible.
v4's reading of FUN_004ab430 as the travel source was wrong.
FIX: advance body FIRST (projection; its writes get overwritten), leg LAST
(displayed pose); travel = legAdv under the two-channel split. Display ==
travel through the LEG by construction -- body-channel drift can no longer
become visible, structurally (not by input-symmetry luck).
Verified: trn+feathering repro clean; straight drive locked (adv==proj);
solo goto=enemy kill chain intact; MP 2-node drive-to-range cross-pod kill
intact (241 hits, DESTROYED); no crashes. KB corrected (locomotion.md roles
+ P3_LOCOMOTION.md v5 entry). New gated instruments: BT_SYNC_LOG,
BT_FORCE_OSC.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
98 lines
7.2 KiB
Markdown
98 lines
7.2 KiB
Markdown
---
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id: locomotion
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title: "Locomotion — gait, the ground model, collision, shadow"
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status: established
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source_sections: "PROGRESS_LOG.md §7, §10 (gait/ground/collision/shadow); docs/P3_LOCOMOTION.md"
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related_topics: [asset-formats, decomp-reference, reconstruction-gotchas, combat-damage, rendering]
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key_terms: [RootTranslation, ground-model, SequenceController, MechControlsMapper, SLD, joint]
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open_questions:
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- "Authentic per-mech turn-rate constant (currently bring-up constant)"
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- "Body-callback gimp handlers (states 16-19); airborne callbacks"
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---
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# Locomotion
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Walking, the gait controller, the ground/collision model, and the shadow. All DEFAULT-ON now
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(`BT_GAIT_CUTOVER`, `BT_GAIT_SM`, `BT_COLLISION`, `BT_REAL_CONTROLS` — set `=0` to fall back). Full
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detail: `docs/P3_LOCOMOTION.md` + `docs/PROGRESS_LOG.md §7, §10`.
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## Walk speed is animation-driven (not physics)
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Each `.ANI` has a `[RootTranslation]` where **`.z` = forward root SPEED** (world units/s). The
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engine `AnimationInstance::Animate(dt, move_joints)` integrates `movement += Δt·rootTrans.z` and
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moves the body that far → feet plant by construction (no stride constant; k=1). Idle anims have
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z=0. The `.MOD` physics governed thrust VEHICLES, NOT walk gait. [T1]
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## The two-channel gait (real controls)
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The BT `SequenceController` (`seqctl.cpp`, from `SelectSequence@004277a8`+`Advance@0042790c`) drives
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locomotion. Under `BT_REAL_CONTROLS` two channels run — **roles corrected 2026-07-09 (task #49) from
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raw disasm** (both Advance fns had NO static decomp caller; found by byte-scanning for `e8` calls):
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- **LEG channel = the LOCAL sim.** The master perf (the `0x4a9b5c` gap region) calls
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`AdvanceLegAnimation` @0x4aa399 (airborne @0x4aa388) and stores `-dist/dt` into **localVelocity
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(+0x1cc)** — the leg drives the local mech's TRAVEL and, advancing LAST (mj=1), the DISPLAYED pose.
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It reads the LIVE `speedDemand` and owns trn/turn-in-place + the early-stand wind-down guard. [T1]
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- **BODY channel = the replication PROJECTOR.** `IntegrateMotion` (0x4ab1c8; body advance @0x4ab312,
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from `FUN_004ab430`, the projected-origin/dead-reckon updater) stores `-dist/dt` into
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**projectedVelocity (+0x2a0)** — the dead-reckoning feed for update records. Its phase drift is
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locally INVISIBLE in the binary. (The recon does not yet wire this store — follow-up.) [T1]
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**v5 (current): display == travel BY CONSTRUCTION through the LEG** (body advances first, leg last).
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History: v3 (travel=leg, display=body) foot-slipped — two channels, different consumers; v4 unified
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BOTH on the body (mis-reading 0x4ab430 as the travel source) — feet planted, but whenever the two
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state machines phase-split, the out-of-phase LEG pose showed through on frames the body didn't write
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(Standing/wind-down) = the RECURRING "legs stutter / lose sync" report. The split mechanism [T2,
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reproduced via `BT_FORCE_OSC` + `BT_FORCE_TURN` + `BT_SYNC_LOG`]: (a) SEED — the bring-up trn trigger
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arms only the leg (authentic trn dispatcher still unlocated), so the walk re-entry lands 1 frame
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apart; (b) AMPLIFIER — with offset clips the end-of-clip callbacks fire on different frames, so a
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demand change landing between them picks DIFFERENT next states (one winds down, one keeps walking)
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→ opposite-phase churn. Under v5 both effects still exist but only skew the projection channel. [T2]
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The gait STATE MACHINE (`AdvanceBodyAnimation`, mech2.cpp) slews `bodyCycleSpeed` toward the demand
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through walk/run caps (`LoadLocomotionClips`, mech3.cpp:326 — measures `walkStride`/`standSpeed` via
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`legAnimation.SelectSequence`). Transition callbacks (`Mech::BodyClipFinished @0x4a6d8c`,
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`LegClipFinished @0x4a6928` — PE-parsed jump tables) alternate walk 6↔7 / run 12↔13 + walk→run. [T2]
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⚠ **Aliasing bug class (namedClip):** `namedClip[]`@0x5e0 == `animationClips[]`@0x5cc+0x14 (ONE array);
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`gimpBaseClip`@0x64c == `animationClips[0x20]`; `bodyAnimationState`@0x728 == `bodyStateAlarm.level`.
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Declared-separate members don't see each other's writes — alias them.
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## The AUTHENTIC 1995 ground model (default-on, `BT_GROUND_REAL`)
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Decoded by the 10-agent `ground-model-decode` workflow (binary `FUN_004a9b5c` = the master perf).
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**NO GRAVITY anywhere** — accelerations zeroed every frame. Vertical position = an ABSOLUTE per-frame
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ground SNAP:
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1. ctor lifts `collisionTemplate->minY` by 5% of volume X-width (the probe height + wall-vs-floor separation).
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2. per frame `MoveCollisionVolume()` re-finds the containing node in the zone's **BoundingBoxTree**
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(BOXTREE.cpp — NOT a heightfield).
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3. probe `q = origin + (0, lift, 0)` → `FindBoundingBoxUnder(q, &h)` (h = distance down to the highest
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solid top; −1 = miss).
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4. snap gate `h > 1e-4`: `origin.y -= (h − lift)` ⇒ y = surfaceY exactly. **MISS ⇒ NOTHING** (y holds,
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no runaway). Up-slope bounded by the lift window (implicit step allowance); walk-offs drop instantly.
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5. `GetCurrentCollisions` → `ProcessCollisionList` → `Mech::ProcessCollision` (@0x4abb40, vtbl+0x3c):
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BoxedSolid resolver → StaticBounce → owner classification. Crushable icon (flags&0x8000) = damage
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0.00123f sentinel = move stands; damage>0 = **FULL FRAME REJECTION** (walls block, never slide);
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impactVel²>40 = crash-anim (SetLegAnimation(0x20)). [T2]
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KEY: `FindSmallestNodeContainingColumn`/`FindBoundingBoxUnder` (`FUN_0040e36c`/`0040e5f0`, BOXTREE.CPP)
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are ALIVE in the 2007 engine — the "heightfield" labels were wrong (caused two reverted attempts). A
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master mech needs a **CollisionAssistant** (GetCurrentCollisions iterates it unchecked). BT solids
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ship ZERO tiles (h never negative). [T2]
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## Knockdown / crash + the desync/stutter fixes
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Wall impacts iv²>40 (~6.3 u/s) bind the `bmp` stagger clip. Two documented bug fixes: (a) the
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knockdown must stagger BOTH channels (`SetBodyAnimation(0x20)` + `SetLegAnimation(0x20)`) or display
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+ travel split permanently (foot-slip); (b) a contact-hysteresis gate (0.4 s `gBlockCooldown`)
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stops the knockdown re-firing on sustained/glancing contact. `localVelocity` is zeroed while crashed
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(spec-faithful) so a knocked mech can't keep advancing. [T2]
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## Controls (`BT_REAL_CONTROLS`, default-on)
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`MechControlsMapper` (mechmppr.cpp @004afbe0; btl4mppr.cpp mappers) interprets input → `speedDemand`
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/ `turnDemand`. ⚠ **WndProc NEVER receives WM_KEYUP** (the engine's per-frame reader `GetMessage`s
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them out) → poll `GetAsyncKeyState` per frame, gated on foreground. Virtual controls: W/S sweep a
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persistent throttle LEVER with a zero detent; A/D a momentary auto-centering stick. [T2]
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## Shadow (default-on, `BT_SHADOW_TILT`)
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BT 4.10's shadow is the flat `*_tshd.bgf` proxy drawn as blend-pass geometry, posed by `jointshadow`
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(terrain tilt) + `jointtshadow` (torso twist). Terrain-tilt samples the collision surface gradient;
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a depth-bias (`D3DRS_DEPTHBIAS`, decal-style) stops it z-fighting/vanishing on inclines. Polar maps
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have STUB `*_tshd` (shadow authored-off on snow). Also the visual-ground conform (`btvisgnd.cpp`,
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render-only lift to the visible terrain triangles — presentation only, never touches localOrigin). [T2]
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## Key Relationships
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- Detail: `docs/P3_LOCOMOTION.md`. Uses: [[asset-formats]] (SKL/ANI), [[decomp-reference]] (offsets).
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- Feeds: [[combat-damage]] (collision→damage), [[rendering]] (shadow/visual-conform).
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