# obstruction_ethereal — set / clear / consume contract

**Research date:** 2026-06-24  
**Oracle:** `docs/research/named-retail/acclient_2013_pseudo_c.txt`  
**Phase:** Task 3 of the collision-inclusion verbatim-retail port

---

## 1. Gate 1 — `CPhysicsObj::FindObjCollisions` (pc:276782 / 0x0050f050)

```
// pc:276782
if (state & 4) AND (state & 0x10):   // ETHEREAL_PS | IGNORE_COLLISIONS_PS
    return 1   // OK_TS — instant-skip, no further work
// else fall through: ETHEREAL-alone goes into the block below

// pc:276802
int32_t var_c;
if ((state_2 & 4) != 0                              // target ETHEREAL
    || (ebx->object_info.ethereal != 0              // mover is ethereal
        && (state_2 & 1) == 0)):                    // AND target not REPORTS_COLLISIONS
    var_c = 1
    if (sphere_path.step_down == 0):
        goto label_50f0c9   // set obstruction_ethereal = 1 and continue
else:
    var_c = 0
// label_50f0c9:
ebx->sphere_path.obstruction_ethereal = var_c  // pc:276806
// ... continue with shape dispatch (FindObjCollisions body: BSP / Sphere / Cyl) ...

// At the END of the FindObjCollisions body (after all shape tests):
// pc:276989 / 0x0050f31e:
ebx->sphere_path.obstruction_ethereal = 0   // clear after each object's test
```

**Translation (C#):**
- `ShouldSkip` returns `true` ONLY when `(targetState & 0x4) != 0 && (targetState & 0x10) != 0`.
- ETHEREAL-alone falls through. Before the shape dispatch (BSP/Sphere/Cyl call), set
  `sp.ObstructionEthereal = true` when `(targetState & 0x4) != 0`.
- After the shape dispatch, retail clears the flag (`= 0`). In acdream we clear it at the
  END of the per-target loop iteration (mirrors the per-object clear at pc:276989).

**Note on `step_down` gate:** retail's `var_c` assignment also checks `step_down == 0`
before jumping to `label_50f0c9`. When `step_down != 0`, var_c stays 1 but hits the
`else var_c = 0` branch at pc:276804 (which sets var_c = 0). Reading the full
branching tree: the ONLY path that sets `obstruction_ethereal = 1` is when
`(state_2 & 4) != 0` (target ETHEREAL) AND `step_down == 0`. For acdream's
transitional player insert, `step_down` is false at the outer loop entry, so the flag
fires when target is ETHEREAL. We match retail exactly by gating on target ETHEREAL
(the `(obj.State & 0x4) != 0` check in the loop).

---

## 2. D5 clear — `CEnvCell::find_env_collisions` (pc:309580 / 0x0052c144)

```
// 0x0052c144:
arg2->sphere_path.obstruction_ethereal = 0;
// ... then: BSP dispatch for the ENV cell walls (not objects) ...
```

**Translation (C#):** At the top of `FindEnvCollisions`, clear `sp.ObstructionEthereal = false`
before any BSP dispatch. The ENV path clears it because ENV walls are always solid — the
flag only applies to object (CPhysicsObj) tests. This prevents a stale flag from a
prior object loop from weakening ENV wall tests.

---

## 3. Consume site 1 — `CSphere::intersects_sphere` (pc:321692 / 0x00537ae4)

```
// pc:321692:
if (obstruction_ethereal != 0 || insert_type == PLACEMENT_INSERT):
    // sphere_intersects_solid test — allows passage if sphere is NOT inside solid
    if (collides_with_sphere(center, pos, radius_sum)):
        return 2   // COLLIDED — solid containment  
    // else: sphere is NOT inside solid, passage allowed — no push-back
else if (step_down == 0):
    // normal walkable / check_walkable / slide path (the BLOCKING path)
    ...
```

**What this means:** when `obstruction_ethereal` (target is passable-ethereal) or
`PLACEMENT_INSERT`, the test only fails if the sphere FULLY OVERLAPS the solid region
of the sphere target. In practice for a Sphere-type entity with no BSP this means
you can walk through it as long as your sphere center isn't inside its solid — which
is almost never true during normal movement. The blocker path (slide/push-back) is
bypassed entirely.

**acdream scope note:** `SphereCollision` in `TransitionTypes.cs` does NOT currently
implement this gate; it always does the slide path. That is a separate task (Task 5 or
equivalent). For Task 3 scope: BSP objects are the priority for opened doors; the
Sphere path is a separate port.

---

## 4. Consume site 2 — `BSPTREE::find_collisions` (pc:323742 / 0x0053a496)

```
// pc:323742:
if (insert_type == PLACEMENT_INSERT || obstruction_ethereal != 0):
    // sphere_intersects_solid with bldg_check center_solid flag
    ebp_4 = 1
    if (bldg_check != 0):
        ebp_4 = (hits_interior_cell == 0)  // center_solid weakens inside buildings
    if (root_node->sphere_intersects_solid(localspace_sphere, ebp_4)):
        return 2   // COLLIDED
    if (num_sphere > 1):
        if (root_node->sphere_intersects_solid(localspace_sphere[1], ebp_4)):
            return 2
else:
    // check_walkable / step_down / normal blocking path
    ...
```

**This is the BSPQuery Path 1 at `BSPQuery.cs:1717`.** The existing code:
```csharp
if (path.InsertType == InsertType.Placement || obj.Ethereal)
```
`obj.Ethereal` is `ObjectInfo.Ethereal` (the MOVER's ethereal flag) which is NEVER
set — it's always false. The correct translation of `obstruction_ethereal` is
`path.ObstructionEthereal` (on `SpherePath`).

**Fix:** change to:
```csharp
if (path.InsertType == InsertType.Placement || path.ObstructionEthereal)
```

This is the ONLY consume site for Task 3 scope (BSP objects). For the BSP path:
when the target is ETHEREAL, `path.ObstructionEthereal` is set before calling
`BSPQuery.FindCollisions`, so Path 1 fires instead of the normal blocking path.
Path 1 uses `sphere_intersects_solid` which only returns COLLIDED if the player
is inside a BSP solid leaf — during forward movement toward a door (which has no
solid wall when open), the BSP finds no solid intersection, returns OK, player passes
through.

---

## 5. Consume site 3 — `CCylSphere::intersects_sphere` (pc:324573 / 0x0053b4a0)

```
// pc:324573:
if (insert_type == PLACEMENT_INSERT || obstruction_ethereal != 0):
    // collides_with_sphere test (3D overlap only)
    if (CCylSphere::collides_with_sphere(this, global_sphere, ...)):
        return  // early-out, no Collided return — void return here
    // else: no collision, return (void — passable)
else:
    // step_down / check_walkable / normal blocking path
    ...
```

**acdream implementation (Task 3 follow-up, 2026-06-24):** `CylinderCollision` in
`TransitionTypes.cs` now implements this gate as `if (sp.ObstructionEthereal) return OK`
at the top of the method — mirroring exactly how `SphereCollision` (consume site 1) is
handled. The retail function is `void __thiscall`, so all returns in the ethereal branch
are void (= OK). An ethereal Cylinder (e.g. an NPC ghost) is now fully passable.

---

## 3b. Consume site 1 — `CSphere::intersects_sphere` (pc:321692) — full contract

The full decomp at pc:321692 / 0x537ae4 confirms:

```
// pc:321692:
if (obstruction_ethereal != 0 || insert_type == PLACEMENT_INSERT):
    // distSq check — if sphere NOT overlapping: return (void = passable)
    if (distSq < radiusSumSq):   // overlapping:
        // calls collides_with_sphere on sphere[1] if num_sphere > 1
        // returns void — no COLLIDED
    return  // void = passable in all cases
else:
    // step_down / check_walkable / slide blocking paths
```

**Key:** the `CSphere::intersects_sphere` function is `void __thiscall` — there is NO
`return 2` (COLLIDED) from the ethereal branch. The inner `collides_with_sphere` call
(for the penetrating case) also produces no blocking result (it may contribute to a
side-channel contact; it does NOT stop the player). The player is passable in ALL
sub-cases of the ethereal branch.

**acdream implementation (Task 3 follow-up, 2026-06-24):** `SphereCollision` implements
`if (sp.ObstructionEthereal) return OK` at the top. The inner overlap/slide path is
bypassed — matches the void-return semantics of the retail ethereal branch.

---

## Summary: set / clear / consume flow (complete after Task 3 follow-up)

```
FindObjCollisionsLoop():
    for each obj in cell.shadow_entries:
        // Gate 1: instant-skip needs BOTH bits
        if obj.State has ETHEREAL AND IGNORE_COLLISIONS:
            continue  // pass through, no obstruction_ethereal

        // Set flag for ETHEREAL-alone
        sp.ObstructionEthereal = (obj.State & 0x4) != 0

        // Shape dispatch — ALL three shapes now consume the flag:
        //   BSP:      BSPQuery.FindCollisions Path 1 → sphere_intersects_solid (passable)
        //   Sphere:   SphereCollision → if (ObstructionEthereal) return OK
        //   Cylinder: CylinderCollision → if (ObstructionEthereal) return OK

        // Clear after per-object test (retail pc:276989)
        sp.ObstructionEthereal = false

FindEnvCollisions():
    sp.ObstructionEthereal = false  // D5 clear — ENV walls are always solid
    // BSP dispatch against cell walls ...
```

**Contracts (complete):**
- ETHEREAL-alone BSP target (e.g. open door): `ShouldSkip` = false → flag set → BSP
  Path 1 fires → `sphere_intersects_solid` → player walks through open door. Passable.
- ETHEREAL-alone Sphere target (e.g. ghost NPC with sphere shape): flag set →
  `SphereCollision` returns OK immediately. Passable.
- ETHEREAL-alone Cylinder target (e.g. ghost NPC with cyl shape): flag set →
  `CylinderCollision` returns OK immediately. Passable.
- Non-ethereal wall/object: flag = false → normal blocking paths. Blocked.
- ENV cell walls: flag cleared before BSP dispatch → ENV BSP always blocking. Blocked.