acdream/docs/research/2026-07-30-landing-bounce-family.md

# The landing bounce family — retail bounce vs ground vs slide (investigation, report-only)

**Date:** 2026-07-30 · **Status:** IMPLEMENTED (same day — see §Implementation)
**Symptoms (user, live gate):** (1) downhill jumps glide instead of bouncing;
(2) flat-ground jumps at speed/height don't bounce; (3) uphill jumps get stuck
in weird animations, flapping and gliding. Speed (#266) and roof slide
(#265's freeze) are fixed and unaffected.

## The retail mechanism (decomp, read end-to-end this session)

Three functions compose the whole behavior:

### 1. The floor-touch dual record (plane handler, 0x0050d100-0x0050d30c)

Touching a floor plane records **two independent facts**:

```c
if (step_down || !(state & CONTACT) || is_valid_walkable(plane))
    set_contact_plane(plane)                 // grounding fact
if (!(state & CONTACT) && !step_down) {
    set_collision_normal(plane.N);           // collision fact
    collided_with_environment = 1;
}
```

A landing (not already in contact, not a step-down probe) is BOTH a contact
AND an environment collision carrying the floor normal. Ordinary walking
(already in contact / step-down glue probes) records only the contact —
that is why walking never bounces.
**acdream's transition already ports this faithfully**
(`TransitionTypes.cs:3410-3415` — `!oi.Contact && !sp.StepDown` →
`SetCollisionNormal` + `CollidedWithEnvironment = true`).

### 2. SetPositionInternal (0x00515330, read fully — VELOCITY-SIGN-FREE)

```
contact     = collision_info.contact_plane_valid          (no velocity test)
on_walkable = contact && contact_plane.N.z >= floor_z     (set_on_walkable → HitGround/LeaveGround)
handle_all_collisions(collision_info, prevContact, prevOnWalkable)   ← velocity UNMODIFIED
```

There is **no `Velocity.Z <= 0` landing gate and no velocity zeroing**
anywhere in retail's commit. Contact is a per-frame fact from the
transition's contact plane; the bounce is the velocity reflect; they are
independent and coexist — you can be "landed" this frame AND carry
reflected +Z that lifts you off next frame. That IS the bounce chain.
(Our Core `PhysicsObjUpdate.CommitSetPositionTransition` is already a
faithful port of this function — used by teleport/remote placement, NOT by
the local player's per-tick path.)

### 3. handle_all_collisions (0x00514780) + elasticity

For fsf≤1, should-reflect (NOT(was-walkable AND still-walkable) or the
garbled state-flag override), valid collision normal, and `v·n < 0`:

```
v += -(v·n) · (elasticity + 1) · n        // pc:282712
```

`DEFAULT_ELASTICITY = 0.05` (byte constant @0x007c6a7c; ctor writes at
0x005124d3/0x0051d537; `set_elasticity` clamps to [0, 0.1]). So every
landing reverses 5% of the impact's normal component and keeps the full
tangential component:

- **Flat ground at speed/height:** v=(6,0,−7) → v'=(6,0,+0.35) — forward
  carry plus a visible pop at speed. Symptom (2).
- **Downhill:** reflect is off the SLOPE normal — each contact pops the body
  off-slope while tangential speed persists → contact/airborne chain =
  the characteristic downhill bounce. Symptom (1).
- **Uphill:** the reflect kills the into-slope component at impact, contact
  stands, HitGround fires once, land animation plays. Symptom (3)'s clean
  retail counterpart.

## Why acdream glides/flaps instead (the adaptation stack)

`PlayerMovementController.cs:2032-2079` (the per-tick commit) replaces
retail's SetPositionInternal with a hand-rolled block:

1. **AD-25 landing gate:** `if (resolveResult.IsOnGround && Velocity.Z <= 0)`
   — needed because *our resolver reports IsOnGround even during an UPWARD
   jump (it always step-downs)*. Retail has no such gate: an ascending mover
   simply finds no contact plane (it moves away from it; the touch test
   fails), so contact clears naturally.
2. **The bounce killer:** `if (Velocity.Z < 0) Velocity.Z = 0` on landing,
   whose comment says its purpose plainly: *"makes handle_all_collisions'
   landing reflect a no-op — dot(v,n)=0."* This retired the old
   "micro-bounce death spiral" — but that spiral was caused by our OWN
   gate (reflected +Z defeating the `Velocity.Z<=0` landing test), not by
   the reflect being wrong. The workaround deleted retail's legitimate
   bounce.
3. With the reflect suppressed, the new #265 residual-velocity fix correctly
   preserves landing momentum — which now SLIDES via calc_friction instead
   of bouncing. Hence "I glide but that's incorrect."
4. **Uphill flap:** during the up-leg our resolver glues to the slope
   (IsOnGround true) while the gate refuses to ground (v.z > 0) →
   Contact/OnWalkable and HitGround/LeaveGround edges cycle against the
   animation state machine → "weird animations, flapping and gliding."

## Hypotheses (ranked)

1. **H1 (root, high confidence — every link read this session):** the
   AD-25 landing gate + Velocity.Z hand-zero must be REPLACED by retail's
   SetPositionInternal semantics, which requires first fixing the underlying
   resolver divergence: **the transition must not produce a contact plane
   for a mover ascending away from the ground** (retail's step-down/touch
   conditions do this naturally; ours "always step-downs"). With that fixed,
   route the per-tick commit through the already-ported
   `CommitSetPositionTransition` and delete the hand-rolled block — reflect,
   contact, HitGround/LeaveGround, and land animation then compose exactly
   as retail.
   - Falsify by: cdb trace on retail (bp SetPositionInternal +
     handle_all_collisions, dump v before/after while jumping downhill) —
     expect unmodified impact v entering, 5% normal reversal exiting.
2. **H2 (contributing detail):** the garbled `state & <mush>` override in
   handle_all_collisions' gate (our port maps it to Sledding) and the
   `0x20000` Inelastic mapping need byte-decode confirmation before the
   rework — a wrong flag here changes when reflects fire while grounded.
3. **H3 (animation-side residual):** if flap persists after H1, the
   MotionInterp land/fall transition (LandJump vs falling-hold) has its own
   gate to audit — deferred until H1 is in.

## What we've ruled out

- The transition's landing dual-record being missing — ours is faithful
  (TransitionTypes.cs:3410).
- HandleAllCollisions' reflect math/elasticity — ported correctly
  (PhysicsObjUpdate.cs:198, elasticity 0.05 default present).
- The #265 residual-velocity fix being wrong — it exposed the missing
  bounce; it didn't cause it.

## Recommended next step

Approve H1 for implementation: (a) byte-decode the two garbled flags (H2)
first; (b) find + port retail's exact ascent/step-down gating in the
transition (the one remaining unread mechanism); (c) cut the per-tick commit
over to `CommitSetPositionTransition`; (d) re-run the roof/downhill/flat/
uphill matrix live. Optional pre-implementation confirmation: the H1 cdb
trace against live retail.

## What this is NOT

Not a missing-elasticity port and not a missing collision-record — both
exist and are faithful; the bounce is suppressed by our own landing-commit
adaptation (AD-25 family), whose reason-for-being is the resolver's
ascent-glue divergence.


## Implementation (2026-07-30, user-approved)

All three retail mechanisms are now live; the AD-25 adaptation stack is
deleted:

1. **check_contact seeding** (`PhysicsEngine.ResolveWithTransition`): a body
   in transient CONTACT seeds the transition's contact state ONLY while
   `v · contactPlane.N <= ε` (0.0002 = PhysicsGlobals.EPSILON, retail
   0x0050f5b0); a failing body seeds the last-known plane alone (retail
   get_object_info's init_last_known_contact_plane branch). The plane
   requirement is strict — Contact-without-plane is unrepresentable in
   retail. Body-less callers keep the legacy isOnGround seed (test rigs).
2. **SetPositionInternal-shaped commit** (`PlayerMovementController`): the
   `Velocity.Z <= 0` landing gate and the landing `Velocity.Z = 0` hand-zero
   are DELETED. Contact commits purely from `resolveResult.InContact` /
   `OnWalkable`, HitGround fires on the airborne→walkable edge, and
   `HandleAllCollisions` runs with the UNMODIFIED impact velocity — the 5%
   elasticity reflect is live. The whole commit is gated on
   `resolveResult.Ok && candidateMoved` (retail runs SetPositionInternal
   only when the transition succeeded AND the candidate moved — pc:283657;
   AD-41's row updated accordingly). Zero-move frames leave contact state
   untouched (this is what keeps a standing body stable: a zero-move
   resolve cannot re-derive a plane because no sweep runs).
3. **Byte decodes** (this doc's H2): the handle_all_collisions gate override
   is `state & 0x800000` = Sledding; the zero branch is `state & 0x20000` =
   Inelastic; the reflect fires strictly on `dot < 0` (`test ah, 5; jp`).
   Our port had all three correct already — no change.

Settle behavior: a real landing (|v| ≥ 0.25 m/s) bounces at 5% and the hop
chain decays geometrically; sub-0.25 m/s impacts are consumed by retail's
unconditional small-velocity zero (PhysicsBody.UpdatePhysicsInternal), so a
standing body never micro-bounces. calc_acceleration turns gravity off for
Contact+OnWalkable bodies, which is what makes rest bit-stable.

Test re-baselines (each documented in place): the landing-survival pin now
measures decay after the hop chain settles; `LiveCompare_Tick0/376` pin the
new IsOnGround=false on their zero-move ticks (the captured `true` was the
retired seed echo — tick 376's captured body even carries an 11.8 m/s
grounded velocity from the deleted get_state_velocity-overwrite era);
`RemoteDeOverlapMechanismTests.GroundedBody` now carries the plane a real
grounded body always has (the big-creature 1.80 m expectation was calibrated
against the unrepresentable flags-without-plane fixture; production settles
at 1.58 m, unchanged before/after). New pins:
`LandingBounceSeedingTests` (ascent no-seed, rest keeps-contact, strict
plane, slope 5% reversal + tangential preservation, Sledding override).

Verification: complete Release suite 10,031 passed / 5 skips / 0 failures.
Live gate (downhill bounce chain, flat-ground pop, uphill clean landing,
roof slide intact, walking intact) pends the user's next session.