acdream/docs/research/2026-07-05-ccylsphere-collision-family-pseudocode.md

# CCylSphere collision family — retail pseudocode (port prep)

**Date:** 2026-07-05 · **Trigger:** the Holtburg town-network portal platform
(stab `0xC0A9B465`, Setup `0x020019E3`, CylSphere r=2.597 m h=0.256 m) blocks
the player with an endless rim slide instead of the retail step-up-onto-top.
Surfaced the moment #149 (`4cf6eeb`) started registering BSP-less stab
CylSpheres — the collision SHAPE is right; the RESPONSE family was never
ported. Feeds #137 (dungeon door feet flow through the same dispatcher).

**Sources:** named-retail pseudo-C (addresses below) = ground truth;
`references/ACE/Source/ACE.Server/Physics/CylSphere.cs` = cross-reference
(settles BN x87 garbles; one ACE bug found, noted in §8).

## Retail function inventory

| Function | Address | pseudo-C line |
|---|---|---|
| `CCylSphere::intersects_sphere(CTransition*)` — dispatcher | `0x0053b440` | :324558 |
| `CCylSphere::intersects_sphere(Position*, float scale, CTransition*)` — wrapper | `0x0053b8f0` | :324744 |
| `CCylSphere::collides_with_sphere` | `0x0053a880` | :323943 |
| `CCylSphere::normal_of_collision` | `0x0053ab50` | :324102 |
| `CCylSphere::collide_with_point` | `0x0053acb0` | :324173 |
| `CCylSphere::slide_sphere` | `0x0053b2a0` | :324502 |
| `CCylSphere::step_sphere_up` | `0x0053b310` | :324516 |
| `CCylSphere::land_on_cylinder` | `0x0053b3d0` | :324542 |
| `CCylSphere::step_sphere_down` | `0x0053a9b0` | :324032 |
| `COLLISIONINFO::set_contact_plane(plane, is_water)` | `0x00509d80` | :271925 |

## 1. Wrapper (0x0053b8f0) — globalize the cylinder

```
intersects_sphere(cyl, Position* objPos, float scale, CTransition* t):
    SPHEREPATH::cache_localspace_sphere(&t->sphere_path, objPos, 1f)
    world_cyl = { low_pt: objPos.localtoglobal(cyl.low_pt * scale),
                  radius: cyl.radius * scale,
                  height: cyl.height * scale }
    return world_cyl.intersects_sphere(t)      // axis stays world-Z
```

**acdream mapping:** `ShadowEntry` already stores the globalized base point
(`Position` = entity pos + rotated scaled local offset, registration sites in
`GameWindow.cs`) and pre-scaled Radius/CylHeight — the wrapper's work is done
at registration. `cache_localspace_sphere` matters only for
`localspace_pos` (used by step_sphere_up's normal rotation, §6).

## 2. collides_with_sphere (0x0053a880) — pure overlap test

```
collides_with_sphere(cyl, CSphere* sphere, Vector3* disp, float radsum):
    // disp = sphere.center − cyl.low_pt (caller computes)
    if (disp.x² + disp.y² <= radsum²)                       // XY overlap
        halfH = cyl.height * 0.5
        if (|halfH − disp.z| <= sphere.radius − F_EPSILON + halfH)  // Z band
            return 1
    return 0
```

`radsum` at every call site = `cyl.radius − F_EPSILON + sphere.radius`
(ε shaved ONCE, in the dispatcher preamble). The ε is what makes "resting
exactly on the top" a non-overlap, so landings settle instead of re-colliding.

## 3. Dispatcher (0x0053b440)

```
intersects_sphere(cyl, CTransition* t):        // cyl in world frame
    sp = t.sphere_path;  oi = t.object_info
    s0 = sp.global_sphere[0];  disp0 = s0.center − low_pt
    if sp.num_sphere > 1: s1 = sp.global_sphere[1]; disp1 = s1.center − low_pt
    radsum = cyl.radius − F_EPSILON + s0.radius

    // ── branch 1: placement / ethereal — detection only ──
    if (sp.insert_type == PLACEMENT_INSERT || sp.obstruction_ethereal):
        if collides(s0, disp0) → COLLIDED
        if num_sphere>1 && collides(s1, disp1) → COLLIDED
        return OK

    // ── branch 2: step-down probe — land on the top ──
    if (sp.step_down): return step_sphere_down(t, s0, disp0, radsum)

    // ── branch 3: walkable probe — cylinder occupancy blocks ──
    if (sp.check_walkable):
        if collides(s0, disp0) → COLLIDED
        if num_sphere>1 && collides(s1, disp1) → COLLIDED
        return OK

    // ── branch 4: normal sweep (collide flag clear) ──
    if (!sp.collide):
        if (oi.state & (CONTACT|ON_WALKABLE)):              // grounded
            if collides(s0, disp0) → step_sphere_up(t, s0, disp0, radsum)
            if num_sphere>1 && collides(s1, disp1)
                → slide_sphere(t, s1, disp1, radsum, sphereNum=1)   // §8: retail passes disp1
        elif (oi.state & PATH_CLIPPED):
            if collides(s0, disp0) → collide_with_point(t, s0, disp0, radsum, 0)
        else:                                                // airborne
            if collides(s0, disp0) → land_on_cylinder(t, s0, disp0, radsum)
            if num_sphere>1 && collides(s1, disp1)
                → collide_with_point(t, s1, disp1, radsum, 1)
        return OK

    // ── branch 5: collide-flag re-test — exact-TOI cap landing ──
    if collides(s0,disp0) || (num_sphere>1 && collides(s1,disp1)):
        movement = sp.global_curr_center[0] − s0.center − block_offset(cur→check)
        if |movement.z| < F_EPSILON → COLLIDED
        timecheck = (height + s0.radius − disp0.z) / movement.z
        offset = movement * timecheck
        if radsum² < |xy(offset + disp0)|² → OK             // rewound off the cap
        t2 = (1 − timecheck) * sp.walk_interp
        if t2 >= sp.walk_interp || t2 < −0.1 → COLLIDED
        pt = s0.center + offset;  pt.z −= s0.radius
        ci.set_contact_plane(Plane(n=(0,0,1), d=−pt.z), is_water=1)   // literal 1, §7
        ci.contact_plane_cell_id = sp.check_pos.objcell_id
        sp.walk_interp = t2
        sp.add_offset_to_check_pos(offset)
        return ADJUSTED
    return OK
```

State bits (verified against our `ObjectInfoState`): CONTACT=0x1,
ON_WALKABLE=0x2, PATH_CLIPPED=0x8, PERFECT_CLIP=0x40.

## 4. step_sphere_down (0x0053a9b0) — land on the top during a step-down probe

```
step_sphere_down(t, s0, disp0, radsum):
    if !collides(s0,disp0) && !(num_sphere>1 && collides(s1,disp1)) → OK
    stepScale = sp.step_down_amt * sp.walk_interp
    if |stepScale| < F_EPSILON → COLLIDED
    deltaz = height + s0.radius − disp0.z          // lift so bottom rests on top
    interp = (1 − deltaz / stepScale) * sp.walk_interp    // divisor = stepScale (BN garbled; ACE)
    if interp >= sp.walk_interp || interp < −0.1 → COLLIDED
    contactPt = (s0.center.x, s0.center.y, s0.center.z + deltaz − s0.radius)
    ci.set_contact_plane(Plane(n=(0,0,1), d=−contactPt.z), is_water=1)   // §7
    ci.contact_plane_cell_id = sp.check_pos.objcell_id
    sp.walk_interp = interp
    sp.add_offset_to_check_pos((0,0,deltaz))
    return ADJUSTED
```

This is THE missing piece that made step-up-onto-a-wide-cylinder impossible:
`CTransition::step_up`'s internal step-down probe needs branch 2 to produce a
walkable contact plane ON the cylinder top.

## 5. normal_of_collision (0x0053ab50)

```
normal_of_collision(cyl, sp, sphere, dispCheck, radsum, sphereNum, out n) → bool definite:
    dispCurr = sp.global_curr_center[sphereNum] − low_pt
    if (radsum² < dispCurr.x² + dispCurr.y²):               // curr was XY-OUTSIDE → side hit
        n = (dispCurr.x, dispCurr.y, 0)                     // radial, horizontal
        // definite unless the contact could actually be a diagonal cap hit:
        zBandOverlapAtCurr = |halfH − dispCurr.z| <= sphere.radius − F_EPSILON + halfH
        noZMovement        = |dispCurr.z − dispCheck.z| <= F_EPSILON
        return zBandOverlapAtCurr || noZMovement
    // curr was XY-INSIDE the footprint → cap hit
    n = (0, 0, (dispCheck.z − dispCurr.z <= 0) ? +1 : −1)   // descending → top (+1)
    return true
```

Cap polarity settled by ACE + geometry (BN's x87 branch rendering is
untrustworthy here — [[feedback_bn_decomp_field_names]] class 2).

## 6. step_sphere_up (0x0053b310) / land_on_cylinder (0x0053b3d0) / slide_sphere (0x0053b2a0)

```
step_sphere_up(t, s0, disp0, radsum):
    if (oi.step_up_height < s0.radius + height − disp0.z)   // too tall
        → slide_sphere(t, s0, disp0, radsum, 0)
    definite = normal_of_collision(..., 0, out n)
    if normalize_check_small(n) → COLLIDED
    nWorld = localspace_pos.localtoglobalvec(n)             // rotate by the OBJECT's frame
    if CTransition::step_up(t, nWorld) → OK
    else → sp.step_up_slide(t)

land_on_cylinder(t, s0, disp0, radsum):                     // airborne foot hit
    normal_of_collision(..., 0, out n)
    if normalize_check_small(n) → COLLIDED
    sp.set_collide(n)                                        // backup + Collide flag
    sp.walkable_allowance = LANDING_Z (0.0871557)
    return ADJUSTED

slide_sphere(t, sphere, disp, radsum, sphereNum):
    normal_of_collision(..., sphereNum, out n)
    if normalize_check_small(n) → COLLIDED
    return CSphere::slide_sphere(sphere, sp, ci, n, sp.global_curr_center[sphereNum])
```

The airborne landing closes through the retry loop: land_on_cylinder
(ADJUSTED, sets `sp.collide`) → next attempt → branch 5 exact-TOI rests the
sphere on the top + CP → next attempt → ε-shaved overlap now misses → OK →
TransitionalInsert Phase 3 `sp.Collide` placement re-test validates on the
CP → landing completes.

## 7. collide_with_point (0x0053acb0) — PathClipped / head-sphere hits

Port per ACE `CylSphere.CollideWithPoint` verbatim (self-contained TOI math):
non-PerfectClip movers → `set_collision_normal` + COLLIDED. PerfectClip →
exact time-of-impact reposition (`add_offset_to_check_pos`) + ADJUSTED, with
the not-definite branch deriving cap-vs-side from the movement.

## 8. Divergences + settled ambiguities (register-relevant)

1. **`is_water=1` on cylinder-top contact planes is RETAIL** (literal 1 at
   0x0053aae2 and the branch-5 site; `set_contact_plane` 0x00509d80 stores
   arg3 → `contact_plane_is_water`). Port verbatim; do not "fix".
2. **ACE bug (do NOT copy):** ACE's grounded head-sphere leg passes the FOOT
   disp to `SlideSphere`; retail 0x0053b843 passes the HEAD disp (`x_2`).
   Retail wins. (Class: [[feedback_bn_decomp_field_names]] #3 — ACE decode
   wrong in a branch ACE rarely exercises.)
3. **Block offset in branch 5:** retail subtracts the cur→check landblock
   offset; acdream's physics frame is continuous world-space → offset = 0.
   Standing frame adaptation (same as SlideSphere's gDelta note).
4. **Ethereal targets:** branch 1 returns COLLIDED on overlap even for
   ethereal; passability comes from the caller's Layer-2 override
   (pc:276961-276989, non-static + !step_down → forced OK) plus the #150
   step-down skip. The previous port consumed ObstructionEthereal with an
   early OK before any test — response-equivalent for non-static targets,
   but branch 1 is the faithful shape and also gives placement inserts the
   retail blocked-by-cylinder semantics. Ported faithfully now.
5. **`normalize_check_small`** = normalize; returns true (fail) when |v| < ε
   before normalizing — maps to `LengthSquared() < EpsilonSq` guard.
6. **step_sphere_up normal rotation:** retail rotates the collision normal by
   the target OBJECT's frame (`localspace_pos` = the object's Position cached
   by the wrapper) before `CTransition::step_up`. For yaw-only AC objects
   this only affects yawed radial normals; ported faithfully via
   `Vector3.Transform(n, obj.Rotation)`.

## 9. acdream port surface

`Transition.CylinderCollision` (TransitionTypes.cs) becomes the branch-4/5
dispatcher body; new private siblings `CylCollidesWithSphere`,
`CylNormalOfCollision`, `CylStepSphereUp`, `CylStepSphereDown`,
`CylSlideSphere`, `CylLandOnCylinder`, `CylCollideWithPoint`. Callers
unchanged (`FindObjCollisionsInCell` Cylinder branch; the BspOnlyDispatch
gate and the #150 ethereal step-down skip sit ABOVE this dispatch and are
unaffected). `DoStepUp` (= CTransition::step_up, A6.P6) and
`SpherePath.StepUpSlide` are reused as-is.