arcattackandClaude Fable 5
b6fe686b22
User report: the ground shadow painted OVER the mech's feet. Root cause: the
flat quad + big depth-bias (-0.004 ~= 2-4 world units) workaround for slope
burial -- a bias big enough to beat the TERRAIN also beat the FEET in the
depth test, and the shadow drew in the late alpha-blend pass (after the mech),
so z-arbitration was the only layering control.
Fix, four co-dependent parts (user-verified live: feet on top, opaque,
survives inclines/declines):
- TILT (mech.cpp UpdateShadowJoint): apply the SKL's own contract for
jointshadow ("apply terrain angle to pitch and roll") -- quad up aligned to
the surface normal via an orthonormal basis fed to the ENGINE's own
LinearMatrix->EulerAngles conversion. Hand-derived Euler signs are exactly
how the prior tilt attempts "dug into the hillside"; the engine conversion
is convention-proof. ~35 deg cliff-guard cap.
- SAMPLER (mech4.cpp): surface gradient from the collision probes PLUS
BTVisualGroundLift per probe -- the quad hugs the VISIBLE terrain, whose
lift varies across a slope; world->local rotation by the TRUE yaw from
localToWorld, not the drift-prone gDriveHeading mirror (the task-#48 bug
class). BT_SHADOW_LOG traces the normals.
- DRAW ORDER (new PASS_SHADOW, l4d3d.h/L4VIDRND/L4VIDEO): shadows draw
between the STATIC terrain and the DYNAMIC opaque bodies -- the classic
decal order. The mech, drawn after, z-passes over the shadow: the bias
can never paint the shadow over the feet, structurally.
- BIAS pairing (L4D3D.cpp): tilt on (default) -> decal epsilon -0.0008;
BT_SHADOW_TILT=0 flat fallback -> the old -0.004. BT_SHADOW_BIAS overrides.
KB (locomotion.md): the four-part arrangement recorded as co-dependent, with
the gait-desync-symptom diagnostic hint retained.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
115 lines
8.6 KiB
Markdown
115 lines
8.6 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 `*_tshd.bgf` proxy, posed by `jointshadow` (terrain tilt — the SKL's own
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contract: "apply terrain angle to pitch and roll") + `jointtshadow` (torso twist). The working 2026-07-09
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arrangement (user-verified live: feet layer over it, opaque, survives inclines/declines) has FOUR
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co-dependent parts — change one, re-check the others:
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1. **TILT** (mech.cpp `UpdateShadowJoint`): quad up aligned to the surface normal via an orthonormal
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basis fed to the ENGINE's own LinearMatrix→EulerAngles conversion — hand-derived Euler signs are
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exactly how two prior tilt attempts "dug into the hillside"/vanished. ~35° cliff-guard cap.
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2. **SAMPLER** (mech4.cpp): gradient from the collision probes PLUS `BTVisualGroundLift` per probe
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(the quad hugs the VISIBLE terrain, whose lift varies across a slope); world→local by the TRUE yaw
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from `localToWorld` (NOT the `gDriveHeading` scalar mirror — the task-#48 drift bug class).
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`BT_SHADOW_LOG` traces the normals.
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3. **DRAW ORDER** (`PASS_SHADOW`, l4d3d.h/L4VIDRND/L4VIDEO): shadows draw between the STATIC terrain
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and the DYNAMIC opaque bodies (classic decal order), so the mech z-passes over the shadow — the
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bias can never paint the shadow OVER the feet, structurally.
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4. **BIAS pairing** (L4D3D.cpp): tilt on (default) → decal epsilon −0.0008; `BT_SHADOW_TILT=0`
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(flat A/B fallback) → the old −0.004 it needs to survive slope burial. `BT_SHADOW_BIAS` overrides.
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Polar maps have STUB `*_tshd` (shadow authored-off on snow). The visual-ground conform (`btvisgnd.cpp`)
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stays render-only (never touches localOrigin). [T2]
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⚠ **"Shadow vanishes on inclines" can also be a GAIT-DESYNC symptom.** The first 2026-07-09 recurrence
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CO-RESOLVED with the gait v5 fix (no shadow code changed): a popping root pose (out-of-phase leg
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channel showing through) can push the quad past the bias margin. Check `[sync]`/`BT_SYNC_LOG` for
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channel drift before touching the shadow itself. [T2 observation; mechanism T4]
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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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