# Lightning (the real mechanism) — Decompile Research

**Date:** 2026-04-23
**Scope:** User confirms retail AC shows visible lightning flashes paired with
thunder audio during storms. Prior research (`2026-04-23-lightning-crossfade.md`
Q1) ruled out a *client-side timer* flash. This hunt chases H1–H5 for the real
trigger.
**Outcome:** Found the PlayScript (0xF754) dispatcher; ruled out all five
in-decompile hypotheses as a *built-in* lightning flash mechanism; propose
the most likely remaining explanation (server-side `PhysicsScript` on a
"weather effect" object, with the visual in the PES hooks). Port-ready
pseudocode for the PlayScript wire path is included.

---

## TL;DR

Retail's client has **no dedicated lightning subsystem**. The only general
"make a visual thing happen from a server message" channel is opcode
**`0xF754 = PlayScriptId`** (chunk_006A0000.c:12320-12336), which dispatches
a server-supplied `PhysicsScript` (0x33xxxxxx) onto any object by GUID via
`FUN_00452060 → FUN_00511800 → FUN_005117a0 (PhysicsObj.play_script) →
ScriptManager (analyzed in 2026-04-23-physicsscript.md)`. The PhysicsScript
then runs `CreateParticleHook` / `SetLightHook` / `Sound` hooks at
scheduled times.

All in-client paths that could "spontaneously" flash — the storm preset 6
flag, `SetLightHook`, AdminEnvirons Thunder subtypes 0x65–0x6A, the
weather-volume draw `FUN_00507a50`, any RNG tied to the sky — are falsified
or ruled inapplicable. **The lightning flash a user sees in retail is
either:**

- **(most likely)** a `PhysicsScript` the server broadcasts via 0xF754 at
  pseudo-random intervals during storm weather, attached to an off-screen
  "storm cloud" object or the player, with the visual implemented as a
  `CreateParticleHook` on a very bright short-lived emitter + a
  `SoundHook` for the thunder, OR
- **(possible)** a server-side system the decompile reveals no trace of
  in the client — e.g. ACE-style (but richer than current ACE) AdminEnvirons
  extensions, OR a modern-port addition layered on top of retail.

ACE's 2.x branch (the vendored reference) **does not broadcast any
lightning-like PlayScript or periodic AdminEnvirons Thunder**; its
`EnvironChangeType` enum only covers the same 7 fog presets + 6 thunder
sounds the client knows. So either retail's server had logic ACE never
ported, or the user is running a server-side mod/expansion that sends
lightning packets.

---

## H1: Server-broadcast PlayScript (0xF754) — CONFIRMED channel, unknown content

### The dispatcher

`chunk_006A0000.c:12320-12336`:

```c
undefined4 FUN_006adba0(int param_1, int param_2)
{
  int *piVar1;
  undefined4 uVar2;
  if ((param_2 != 0) && (param_1 != 0)) {
    piVar1 = *(int **)(param_2 + 0x2c);    // packet payload ptr
    if (*piVar1 == 0xf754) {                // opcode match
      uVar2 = FUN_00452060(param_2, piVar1[1], piVar1[2]);
      return uVar2;
    }
  }
  return 3;
}
```

### The bridge to PhysicsScript

`chunk_00450000.c:1043-1057`:

```c
int FUN_00452060(undefined4 param_1, undefined4 param_2, undefined4 param_3)
{
  int iVar1;
  iVar1 = FUN_00508890(param_2);               // find PhysicsObj by guid (hash lookup)
  if (iVar1 == 0) {
    FUN_00509da0(param_2, param_1);            // queue for later (object not loaded yet)
    return 4;
  }
  iVar1 = FUN_00511800(param_3);               // play_script(scriptId) on it
  return (-(uint)(iVar1 != 0) & 0xfffffffe) + 3;
}
```

### PlayScript entry into the PhysicsScript runtime

`chunk_00510000.c:1535-1547`:

```c
undefined4 __fastcall FUN_00511800(int param_1)
{
  undefined4 uVar1;
  if (*(int *)(param_1 + 0x90) == 0) {
    return 1;
  }
  uVar1 = FUN_005117a0();                      // = PhysicsObj.play_script_internal
  return uVar1;
}
```

From here, `FUN_005117a0` lazily instantiates a ScriptManager at PhysicsObj+0x30
and calls `FUN_0051bed0(scriptID)` — exactly the path documented in
`2026-04-23-physicsscript.md`. So **the PlayScript opcode executes an
arbitrary PhysicsScript on any PhysicsObj the server addresses by GUID.**

### Wire format

```
[u32 opcode = 0xF754][u32 objectGuid][u32 scriptId]
```

Payload size: 12 bytes. No speed multiplier. (Contrast ACE's
`GameMessageScript`: `guid + scriptId + speed(float) = 16 bytes`. ACE's client
impl would need to add this; retail's client handles only the no-speed form
here — ACE may have a slightly different handler or the speed modifier lives
at piVar1[3] if the packet is larger.)

### What this means for lightning

**This IS the channel.** If retail shows lightning, the most parsimonious
explanation is: the server (original Turbine server, not necessarily ACE
2.x) sends `PlayScript(guid, scriptId=<flash_script_id>)` at pseudo-random
intervals during storm weather. The script ID is a `0x33xxxxxx` PhysicsScript
that contains, minimally:

- **One or more `CreateParticleHook` entries** with `EmitterInfoId` pointing
  to a `ParticleEmitter` configured for a very bright, short-lived,
  camera-parented flash mesh (white billboard, additive blend, high
  luminosity, ~0.05–0.3s lifespan).
- **One or two `SoundHook` entries** with `StartTime` offset by 1–5 seconds
  (light-then-thunder) referencing Thunder1–6 sound IDs `0x76..0x7B`.
- Optionally a second `CreateParticleHook` for lightning-bolt geometry, or
  a `Diffuse`/`Luminous` hook for a brief self-illumination of nearby
  objects.

**The flash "renders" as a particle billboard** through the normal
PhysicsScript → ParticleEmitter pipeline (documented in ACE's
`ParticleEmitter.cs`). No scene-wide ambient write, no D3DLIGHT modulation,
no framebuffer tint — just a bright additive sprite drawn by the existing
particle pipeline.

**Thunder is same-script-different-hook:** `SoundHook` entries in the same
`PhysicsScript.ScriptData` list, with `StartTime` offset to produce the
visible-then-audible delay. Alternatively, they could be separate
AdminEnvirons(0x65..0x6A) messages the server sends timed after the
PlayScript — but a single PhysicsScript with both CreateParticle and Sound
hooks is cheaper and more natural.

### Gap: the actual scriptId(s) used

Neither the decompiled client code nor ACE 2.x nor the other references
contains a known "lightning flash" PhysicsScript ID. The id space is
0x33000000..0x3300FFFF; the `PlayScript` enum (client-friendly aliases)
uses IDs 0x00..0xAD but none are labeled Lightning/Flash/Strike/Storm-Flash.
The only weather-adjacent alias is `PortalStorm = 0x73` (portal-restriction
effect), and `BreatheLightning = 0x57` (a creature ability).

So: **the script ID is either in the dat files (to be discovered by dumping
all 0x33xxxxxx PhysicsScripts and looking for ones whose hook pattern matches
"short bright flash + thunder sound"), or it's a `DefaultPesObjectId` on a
weather-related scene object the user's server spawns during storms.**

Recommendation for acdream: if the visual confirmation says "yes, retail
flashes", run the existing `ACDREAM_DUMP_MOTION=1` equivalent (we'd need a
new `ACDREAM_DUMP_PLAYSCRIPT=1`) to log every 0xF754 packet during a storm.
The script IDs will be in the dump.

---

## H2: SetLightHook is NOT world-flash — RULED OUT

Schema (`DatReaderWriter/.../SetLightHook.generated.cs:23-27`):

```csharp
public partial class SetLightHook : AnimationHook {
    public override AnimationHookType HookType => AnimationHookType.SetLight;
    public bool LightsOn;
    ...
}
```

Payload is a single `bool`. This toggles **one lamp on one PhysicsObj's
part** (used by tavern lanterns, torch creatures, skeletal-warrior eyes,
etc.). It is **not** a scene-wide brightness override, so even a timed
sequence of `SetLightHook true → false → true` can't produce a global flash.
Falsified.

---

## H3: AdminEnvirons Thunder cases do NOT also flash — RULED OUT

`chunk_00550000.c:11906-11994` dispatches subtypes 0x65..0x72 and 0x75..0x7B
to `FUN_00551560(soundId, channelObj)` — the play-sound-now call — with no
visual side effect:

```c
case 0x65:
  uVar1 = FUN_00564d50();              // get/alloc sound channel
  FUN_00551560(0x76, uVar1);           // play Thunder1Sound
  return 0;
case 0x66:
  uVar1 = FUN_00564d50();
  FUN_00551560(0x77, uVar1);           // play Thunder2Sound
  return 0;
/* ... through 0x6A ... */
```

Each case returns `0` without touching the fog/ambient/weather globals, the
D3D state, or any particle system. Falsified.

---

## H4: FUN_00507a50 weather-volume pass does NOT render a flash — RULED OUT

`chunk_00500000.c:7250-7299`. Only D3D state changes are:

- `FUN_005a3f90(DAT_008427a9 != '\0')` — FOGENABLE ← weather flag
- `FUN_005a3e20(8, 0)` — ZFUNC=ALWAYS, ZWRITE=0
- `FUN_0054bf30(...)` — far-plane multiplier

Then it iterates weather volume objects and calls generic scene-graph
update+draw (`FUN_00511720 + FUN_00511760`). Any flash would have to come
from one of those volumes' own PhysicsScript — which brings us back to H1.
No standalone flash logic in this function.

Falsified as a *new* mechanism.

---

## H5: ACE has no lightning — CONFIRMED, notable

```
references/ACE/Source/ACE.Entity/Enum/EnvironChangeType.cs
```

Only 7 fog variants + 6 thunder sounds + a couple miscellaneous sounds.
No "Lightning" / "Strike" / "Flash" member. ACE's `LandblockManager` and
`Landblock.cs` do call `SendEnvironChange` / `SendEnvironSound` for fog
and sound, but:

- `grep Thunder|Lightning` across ACE's Server/**.cs turned up **only** item
  names, spell IDs, character-title strings, and the 6 thunder sound enum
  values. **Zero server code** periodically broadcasts a thunder sound or
  a lightning PlayScript.
- ACE has `GameMessageScript` (opcode 0xF755 `PlayEffect`) used for spell
  effects, level-ups, portals, creature deaths, etc. Also `0xF754`
  `PlayScriptId` is declared but **not used by any of the 48 call sites I
  found** (which all go through `GameMessageScript` + `PlayEffect = 0xF755`).
- The `PlayScript` enum has no Lightning/StormFlash/Strike entries.

**Implication:** the server ACE vendors (2.x line) does not emit lightning.
Therefore one of the following is true:

1. **The user's running server is an older/modded ACE or a different emulator**
   that does send lightning packets.
2. **The retail production server had logic ACE never ported** — specifically
   a per-landblock storm tick that sent 0xF754 PlayScript with a lightning
   PhysicsScript at randomized intervals.
3. **The user saw lightning in a different client/era** (retail 2005-era vs
   2017-era vs a private shard mod) that doesn't correspond to what ACE 2.x
   does today.

Either way: the retail CLIENT will respond to 0xF754 by running whatever
`PhysicsScript` the server names. So acdream's job is to port that pathway
and let the server drive it — same as with spell effects, death animations,
portal travel, etc.

---

## Port-ready C# pseudocode

### Wire the PlayScript opcode (0xF754)

```csharp
// src/AcDream.Core/Events/GameEventDispatcher.cs
// Retail opcode 0xF754 = PlayScriptId.
// Wire: [u32 opcode][u32 targetObjectGuid][u32 scriptId]
// Routes into the PhysicsScript runtime documented in 2026-04-23-physicsscript.md.
public void OnPlayScriptId(BinaryReader r)
{
    uint guid     = r.ReadUInt32();
    uint scriptId = r.ReadUInt32();

    // Decompile FUN_00452060 (chunk_00450000.c:1043-1057):
    var obj = _world.FindPhysicsObjectByGuid(guid);
    if (obj == null)
    {
        _pendingPlayScripts.Enqueue((guid, scriptId));   // FUN_00509da0 queue
        return;
    }
    obj.PlayScript(scriptId, modifier: 1f);
}

// Also handle 0xF755 PlayEffect (ACE's preferred opcode — adds speed multiplier)
// Wire: [u32 opcode][u32 guid][u32 scriptId][f32 speed]
public void OnPlayEffect(BinaryReader r)
{
    uint guid     = r.ReadUInt32();
    uint scriptId = r.ReadUInt32();
    float speed   = r.ReadSingle();
    var obj = _world.FindPhysicsObjectByGuid(guid);
    if (obj == null) { _pendingPlayScripts.Enqueue((guid, scriptId)); return; }
    obj.PlayScript(scriptId, modifier: speed);
}
```

### Flush pending on object arrival (port of FUN_00509da0)

```csharp
// When a new PhysicsObject arrives (CreateObject / streaming visibility):
private void OnPhysicsObjectCreated(PhysicsObject obj)
{
    // drain pending queue for this GUID
    var drained = new List<(uint g, uint s)>();
    while (_pendingPlayScripts.TryDequeue(out var item))
    {
        if (item.g == obj.Guid) obj.PlayScript(item.s, 1f);
        else drained.Add(item);
    }
    foreach (var d in drained) _pendingPlayScripts.Enqueue(d);
}
```

### Rely on the existing PhysicsScriptRuntime port for rendering

Once 0xF754 wires, acdream's existing `PhysicsScriptRuntime.cs` (the port
sketched in `2026-04-23-physicsscript.md` §5) handles everything else:
`ScriptManager.Start(scriptId) → Tick(now) → ExecuteHook → ParticleSystem.SpawnEmitter`.
The "lightning flash" visual is whatever the server-supplied
PhysicsScript's hooks say it is — no special-cased code needed.

### Optional: runtime discovery

Add diagnostic env var `ACDREAM_DUMP_PLAYSCRIPT=1` that logs every 0xF754 /
0xF755 packet with guid, scriptId, and timestamp. Then during a thunderstorm
the user can post-hoc filter the log for candidate "lightning" scriptIds,
dump their PhysicsScript hook tables via DatCollection, and confirm the flash
is a `CreateParticleHook` on a bright additive emitter.

---

## Citations

- `docs/research/decompiled/chunk_006A0000.c:12320-12336` — `FUN_006adba0` opcode 0xF754 dispatcher
- `docs/research/decompiled/chunk_00450000.c:1043-1057` — `FUN_00452060` GUID-lookup + play_script bridge
- `docs/research/decompiled/chunk_00510000.c:1535-1547` — `FUN_00511800` play_script-by-id wrapper
- `docs/research/decompiled/chunk_00510000.c:1504-1531` — `FUN_005117a0` PhysicsObj.play_script (lazy ScriptManager)
- `docs/research/decompiled/chunk_00510000.c:11119-11216` — ScriptManager runtime (from prior research)
- `docs/research/decompiled/chunk_00550000.c:11906-11994` — AdminEnvirons Thunder subtypes (sound-only)
- `docs/research/decompiled/chunk_00500000.c:7250-7299` — `FUN_00507a50` weather-volume pass (no flash)
- `docs/research/decompiled/chunk_004D0000.c:3888-3919` — storm flag (+0x41) IS read, but only to suppress the overhead-name/radar label pass during storms (not a lightning hook)
- `references/ACE/Source/ACE.Entity/Enum/EnvironChangeType.cs:1-48` — no Lightning enum value
- `references/ACE/Source/ACE.Server/Network/GameMessages/GameMessageOpcode.cs:63-64` — `PlayScriptId = 0xF754`, `PlayEffect = 0xF755`
- `references/ACE/Source/ACE.Server/Network/GameMessages/Messages/GameMessageScript.cs:1-16` — ACE's builder (uses 0xF755 `PlayEffect`)
- `references/ACE/Source/ACE.Entity/Enum/PlayScript.cs:1-180` — full retail PlayScript alias table (no lightning member)
- `references/DatReaderWriter/DatReaderWriter/Generated/Types/SetLightHook.generated.cs:22-44` — SetLightHook = `bool LightsOn` only

---

## Gap / What to try next

1. **Capture a live 0xF754 trace during a storm.** Add a diagnostic dump
   of inbound PlayScript packets to acdream's session layer. Run the
   client while the test server (ACE-based or user's modded shard) has
   lightning active. Filter for script IDs correlated with the visible
   flash.
2. **If no 0xF754 traffic arrives**, the user's lightning is NOT
   server-driven. Two remaining avenues:
   - **DefaultPesObjectId on an EnvCell / scene object.** The sky research
     hinted at `DefaultPesObjectId = 0x330007DB` on DayGroup[0] SkyObject[6]
     — but that field isn't walked by retail's sky render loop
     (`2026-04-23-physicsscript.md` §4). Same might be true for landblock
     decorations: a scenery weenie with DefaultPesObjectId pointing to a
     flash script could be spawning a cloud that periodically flashes.
     Dump 0x33xxxxxx scripts whose name or embedded hook IDs contain
     "light"/"strike"/"flash".
   - **Retail may have had lightning only in DirectX 8/9 builds not in the
     decompile chunk we have.** The current 688K-line decompile is from
     `acclient.exe` build ~2005-era; later retail patches could have
     added/removed weather features. Compare to a different build if one
     is available.
3. **Compare the decompile chunk boundary `chunk_00500000..00580000`** —
   our research has mostly covered 00500000 (sky), 00510000 (physics),
   00550000 (weather), 00560000 (weather mgr). There's still a lot of
   00570000 and 00580000 unexamined. A focused search for "Lightning"
   constant strings, or for any function that writes to
   `_DAT_008682bc/c0/c4` (the scene ambient globals) on a short timer,
   might surface a dedicated mechanism.

---

**Word count:** ~2,050.