# CoopAllTheThings

Steam **Remote Play Together (RPT)** for any XInput game — without breaking DRM,
achievements, or playtime.

Existing "donor game" tools (e.g. RemotePlayWhatever) copy a target game's files
into a donor game's folder and rename the executable so Steam streams the target
under the donor's appid. That breaks DRM-protected games, breaks achievements,
and credits playtime to the donor.

CoopAllTheThings takes a different approach: the **real game runs normally under
its own appid** (so DRM, achievements, and playtime all work), while a lightweight
**mirror app runs under the donor appid**. The mirror presents a borderless
window that is a live copy of the game's video + audio, and forwards the guests'
input back into the real game. Steam's RPT captures the mirror window — so any
XInput game becomes Remote-Play-Together-able.

The end-to-end path is working: launched under a donor appid, the host streams a
live video + audio mirror of a separately-running game over Remote Play Together
and forwards guest controllers back into it.

## Architecture

| Concern | Mechanism | Component |
| --- | --- | --- |
| Receive guest input | XInput (RPT delivers guest pads to the focused window); optional, opt-in Steam Input when built with the Steamworks SDK | `coop_host.exe` |
| Forward input to game | DLL injection + XInput hook (SafetyHook) — game sees *only* our pad | `coop_hook.dll` |
| Forward mouse + keyboard | Opt-in MKB subsystem: host streams its window's clicks/keys, the hook posts the matching window messages and synthesizes `GetAsyncKeyState`/`GetKeyboardState`/`GetCursorPos` for polling games | `coop_hook.dll` + `coop_host.exe` |
| Keep game running unfocused | Hook spoofs focus so the game polls while the host holds OS focus | `coop_hook.dll` |
| Mirror video (default) | Windows Graphics Capture of the game window, letterboxed into the host window | `coop_host.exe` |
| Mirror video (hooked) | Injected Present / OpenGL hook copies the backbuffer into a shared keyed-mutex texture the host samples (lower latency, no capture border) | `coop_hook.dll` + `coop_host.exe` |
| Mirror audio | Injected render-hook copies each of the game's WASAPI render streams into its own shared ring and silences the game locally (no echo); the host mixes the streams (soft-clipped); WASAPI process loopback is the automatic fallback | `coop_hook.dll` + `coop_host.exe` |
| Host ↔ hook IPC | Named shared memory (seqlock for input, status back-channel, video/audio/log shares) | `common/` |

The hooked video path has two producers: **Direct3D (DXGI)** hooks
`IDXGISwapChain::Present` / `Present1` and copies the backbuffer — directly for
D3D10/11 games (the backbuffer is an `ID3D11Texture2D`), and via a **D3D11On12
bridge** for D3D12 games (wrap the `ID3D12Resource` backbuffer, `CopyResource` into
the shared texture); **OpenGL** hooks `SwapBuffers` / `wglSwapBuffers` and reads the
backbuffer with `glReadPixels` (for games that never touch DXGI, e.g. Phantom
Brave). The host samples the copy as plain UNORM (`srgb_to_unorm`) so
`*_SRGB`-backbuffer games mirror at correct brightness. **WGC remains the default**
and covers anything the hooked path doesn't (Vulkan, D3D9 — see Roadmap).

## Limitations

- **Anti-cheat:** the input path injects `coop_hook.dll` into the target game.
  Games protected by kernel-level anti-cheat (Easy Anti-Cheat, BattlEye,
  Vanguard, etc.) will detect the injected module and may **kick the player or
  issue a ban**. Such games are explicitly **out of scope and unsupported** — do
  not use CoopAllTheThings with them. The tool targets single-player and
  co-op/local-multiplayer titles without active anti-cheat.
- **XInput only:** the game must read controllers via XInput (the common case).
  DirectInput-only / RawInput-only games are not handled.
- **32-bit games supported via a helper:** the host is x64, but the build also
  produces an x86 hook DLL (`coop_hook_x86.dll`) and a 32-bit injector helper
  (`coop_inject_x86.exe`). When the target is a 32-bit (WOW64) process the host
  detects it (`IsWow64Process2`) and shells out to the helper to load the x86 DLL
  (a 64-bit process can't cleanly inject a 32-bit one). The shared-memory IPC is
  fixed-width / bitness-stable, so the x64 host and x86 hook interoperate.
- **Local audio echo on the fallback path:** when the render-hook is active it
  silences the game's local playback while mirroring it, so there is no echo. If
  the hook can't attach or the game uses an unhooked render path, the host falls
  back to process-loopback capture, which does *not* mute the game — so the local
  machine hears the audio twice (guests hear it once). The Audio panel shows which
  path is active.
- **Debug-oriented UI:** the ImGui overlay is laid out for diagnosing the
  pipeline, not for end use. F1 hides it entirely so the window is a clean mirror
  for RPT.

## Roadmap

### Planned (next up)

Nothing queued — the previous backlog (bin restructure, terminated/hung detection,
re-attach, window-based target picker, overlay auto-layout, Audio "live" column,
moving the synthetic-input toggle, mouse + keyboard forwarding, rumble forwarding,
per-backend input debug view, cursor release, capture metrics + latency, DX12 hooked
capture, multi-stream audio + per-stream formats) is all shipped. See Future work
for what's left.

### Future work

- **Vulkan video hook.** Vulkan games present via `vkQueuePresentKHR`; hooking
  them needs a Vulkan layer / device-dispatch hook plus a `vkCmdCopyImage` to a
  readable image. Use WGC in the meantime.
- **D3D9 hooked path.** Covered by WGC today; a dedicated `IDirect3DDevice9::Present`
  hook would be the lower-latency upgrade.
- **Mouse + keyboard forwarding for Raw Input / DirectInput games.** The MKB
  subsystem forwards via window messages (`PostMessage`) plus synthesized
  `GetAsyncKeyState` / `GetKeyboardState` / `GetCursorPos`, which covers message-loop
  and polling games. Games that read keyboard/mouse via **Raw Input** (`WM_INPUT` /
  `GetRawInputData`, e.g. Trails through Daybreak) or **DirectInput**
  (`IDirectInputDevice8::GetDeviceState/GetDeviceData`) don't see it. Add hooks for
  those paths to synthesize the forwarded input there too.

## Building

Requirements: Windows 10/11, Visual Studio 2022 (MSVC + C++ workload), CMake ≥ 3.21.

```sh
git clone --recurse-submodules <repo-url>
# or, if already cloned:
git submodule update --init --recursive

cmake -S . -B build -G "Visual Studio 17 2022" -A x64
cmake --build build --config Debug
# output: bin/Debug/coop_host.exe (+ coop_hook.dll, test exes)
```

The x64 build also drives a nested Win32 sub-build (CMake `ExternalProject`,
configured into `build/x86/`) that produces `coop_hook_x86.dll` and
`coop_inject_x86.exe` for 32-bit games, staged next to the x64 binaries. Disable
it with `-DCOOP_BUILD_X86_HELPER=OFF` if you don't need 32-bit support.

Third-party dependencies (Dear ImGui, SafetyHook) are git submodules under
`third_party/`. No vcpkg / package manager is used.

**Steam Input is optional.** It's enabled automatically when the Steamworks SDK is
vendored at `third_party/steamworks_sdk/` (extract the `steamworks_sdk_*.zip`
there). The SDK isn't redistributable, so it's gitignored and never committed; if
it's absent the host builds XInput-only (no other features depend on it). When
present, the build links `steam_api64.lib`, stages `steam_api64.dll` and the
action manifest next to the host, and also builds `coop_steam_input_probe`.

Steam Input is **off by default and XInput is the primary path**: merely
initializing Steam Input activates Steam's in-process XInput interception, which
hides controllers from XInput unless they're bound to our action set for the
running appid. Enable it (Controllers panel → **Use Steam Input**) only once a
controller is bound to Steam Input for the donor appid.

### clangd / IDE setup

The Visual Studio CMake generator does **not** emit `compile_commands.json`, so
clangd has no include paths and reports false errors. Run
[`gen-compile-commands.bat`](gen-compile-commands.bat) once (and after adding
sources or include dirs); it configures a parallel Ninja build in `build-clangd/`
that produces the database, which [`.clangd`](.clangd) points clangd at. clangd's
clang-cl driver resolves the MSVC / Windows SDK system includes on its own.

## Tests

```sh
ctest --test-dir build -C Debug --output-on-failure
```

- **`hook_selftest`** — in-process check of the IPC + XInput hook core (no game,
  no controller needed).
- **`audio_ring_test`** — unit test of the shared audio ring (lock-free SPSC
  push/pop, wrap-around, format handshake, overrun/drop). No device needed.
- **`audio_mix_test`** — unit test of the multi-stream mixer math (decode / sum /
  soft-clip / encode for float32 + int16). No device needed.
- **`audio_hook_test`** — in-process self-test of the WASAPI render-hook: installs
  the hooks, renders a tone through WASAPI in the same process, and asserts the
  COM vtables were discovered, the frames reached the ring (non-silent), the
  primary stream was silenced, and the render stream was counted. Skips cleanly if
  the machine has no audio endpoint.
- **`srgb_format_test`** — unit test of the `srgb_to_unorm` mapping the hooked
  video path uses so `*_SRGB`-backbuffer games aren't darkened. No device.
- **`opengl_hook_test`** — in-process self-test of the OpenGL capture path:
  installs the swap hooks, drives a real OpenGL context (clears the backbuffer to a
  known color, calls `SwapBuffers`), and asserts the detour fired, the frame was
  `glReadPixels`'d into the shared texture, and a second device reads the exact
  pixels back by name. Skips cleanly without an OpenGL / D3D11 device.
- **`dx12_present_hook_test`** — in-process self-test of the Present hook's **D3D12**
  path: drives a real D3D12 swapchain through the (shared) `IDXGISwapChain::Present`
  vtable and asserts the D3D11On12 bridge wraps the `ID3D12Resource` backbuffer and
  copies it into the shared texture, then reads the exact rendered color back by
  name. Skips cleanly without a D3D12 device.
- **`present_hook_test`** — in-process self-test of the Present-hook video path:
  installs the hook, drives a real D3D11 swapchain in the same process (clears the
  backbuffer to a known color and calls `Present`), and asserts the detour fired,
  the backbuffer reached the shared keyed-mutex texture, and a second device can
  open it by name and read the exact pixels back. Skips cleanly if the machine has
  no D3D11 device.
- **`audio_loopback_test`** — spawns `coop_tone.exe` (a standalone WASAPI
  sine-wave source under [`tools/audio_tone`](tools/audio_tone)) and verifies the
  shipping process-loopback capture (the fallback path) receives its audio by
  PID. Skips cleanly if the machine has no audio endpoint.

### Debugging the hooks against a real game

[`tools/audio_probe`](tools/audio_probe) (`coop_audio_probe.exe <pid> [seconds]`)
brings up the audio render-hook without Steam / RPT / the host UI: it creates the
IPC block + audio ring the hook expects, injects `coop_hook.dll` into the target
game, then drains the ring and prints per-stream format, captured-frame counts,
peak amplitude (proves the audio is real, not silence), and overruns. It enables
the hook's file trace (`%TEMP%\coop_hook.log`) for the run.

[`tools/input_probe`](tools/input_probe)
(`coop_input_probe.exe <pid> [seconds] [disable_mask]`) does the same for input: it
injects, reports one connected pad, and toggles a button each second so the game's
input layer sees a real state change. `disable_mask` (hex bits `0x1`=input
`0x2`=focus `0x4`=audio `0x8`=video) skips installing a subsystem, so you can
**bisect which injected subsystem affects a game** — this is how the 32-bit
Present-hook crash was isolated.

Both auto-detect a 32-bit (WOW64) target and inject via `coop_inject_x86.exe` +
`coop_hook_x86.dll`, exactly like the host. The probes build into
`bin/<config>/tools/` (the deployable `bin/<config>/` root holds only shipping
artifacts; tests build into `bin/<config>/tests/`) and resolve `coop_hook.dll` from
the root one level up, so run them from there. **Kill the game between runs** — the
loaded DLL locks `coop_hook.dll` against the next rebuild.

## Running the tool (manual, end-to-end)

This needs Steam, a donor game that supports Remote Play Together, and a second
person/account to receive the stream.

1. **Launch the host under a donor appid.** Find the donor's appid (the number in
   its store URL); the donor only needs RPT support and is never actually played:

   ```text
   "C:\Program Files (x86)\Steam\steam.exe" -applaunch <donorAppId> "D:\dev\CoopAllTheThings\bin\Debug\coop_host.exe"
   ```

   The borderless window appears and Steam marks the donor "running". If the donor
   ignores the trailing path, set the host as the donor's **Launch Options**
   (`"D:\...\coop_host.exe" %command%`) or use a launcher like RemotePlayDetached.

2. **Start the real game** windowed or borderless (not exclusive fullscreen — see
   Lessons learned). In the host's **Injection** panel, filter for the game's
   `.exe`, select it, and click **Inject & Connect**. Watch **Hook status** for
   **Attached**, a non-zero **XInput polled: N/s**, and **Focus spoof: active**.

3. **Mirror video:** in the **Video mirror** panel, tick **Mirror game window** —
   the host window now shows a live, letterboxed copy of the game. **Source**
   picks how the frames are grabbed: **WGC** (default, Windows Graphics Capture —
   works for any window) or **Hooked (Present)** (the injected hook's shared
   texture — lower latency and no capture border, for DXGI / D3D11 and OpenGL
   games; selecting it installs the video subsystem in the game).

4. **Mirror audio:** in the **Audio mirror** panel, tick **Mirror game audio**.
   With the hook injected, **Source** shows **Hooked (no echo)** and the game's
   local playback goes silent while guests still hear it. If it shows **Loopback
   (echo)** the hook's render path wasn't caught and you'll hear the game twice
   locally (guests still hear it once). The **Render streams** table shows how
   many WASAPI streams the game emits.

5. **Start Remote Play Together** from Steam and invite a guest. Verify the guest
   sees the mirrored video, hears the audio, and that their controller drives the
   real game.

Useful checks while developing without RPT: tick **Forward synthetic test input**
in the Injection panel to make the game move on its own (proving forwarding is the
source), and click away from the game to confirm focus spoofing keeps it running.

> Injection access error → run the host as administrator. A 32-bit (WOW64) target
> is injected automatically via `coop_inject_x86.exe` + `coop_hook_x86.dll`; if
> those aren't next to the host, rebuild (the x86 sub-build stages them there).

## Lessons learned

Non-obvious things that cost time and constrain the design:

- **RPT only streams the *focused* window.** The game can't hold focus itself, so
  the hook spoofs it (`GetForegroundWindow` / `GetActiveWindow` / `GetFocus` +
  swallowing deactivation messages) to keep the game polling and rendering while
  the host owns real OS focus.
- **Run target games windowed or borderless, never exclusive fullscreen** —
  exclusive fullscreen minimizes on focus loss (defeating the spoof) and can't be
  window-captured. While unfocused the game gets no OS keyboard/mouse, only the
  forwarded pad.
- **WGC captures occluded windows but not minimized ones.**
- **Process-loopback capture doesn't mute the source.** Capturing a process's
  render doesn't stop it reaching the speakers, so the no-echo path instead injects
  a WASAPI render-hook that copies each buffer then releases it with
  `AUDCLNT_BUFFERFLAGS_SILENT`; loopback stays as the (echoing) fallback.
- **`ActivateAudioInterfaceAsync` needs an *agile* completion handler.** If the
  handler doesn't answer `QueryInterface` for `IAgileObject`, the call is rejected
  **synchronously** with `E_ILLEGAL_METHOD_CALL` (`0x8000000E`) — regardless of
  apartment, device path, or activation params. (WRL/wil samples make the handler
  agile for you.) Process loopback also needs the Win10 20H1 headers
  (`NTDDI_VERSION ≥ 0x0A00000B`).
- **COM methods have no exports, so hooks walk vtables by frozen-ABI index — count
  exactly.** All instances of a coclass share one vtable, so hooking one object's
  slot catches every instance; but `IAudioClient::GetService` is **14**, not 13
  (`SetEventHandle` sits at 13 between `Reset` and `GetService`). Count every
  inherited `IUnknown`/base method when adding a hook.
- **D3D12 capture copies the *rotating* back buffer, not `GetBuffer(0)`.** D3D11
  flip-model keeps `GetBuffer(0)` pointing at the live back buffer, but D3D12 rotates
  buffers explicitly — the game renders into the buffer at
  `IDXGISwapChain3::GetCurrentBackBufferIndex()`, which advances each `Present`.
  Grabbing buffer 0 copies a stale buffer on N-1 of every N frames, so the mirror
  silently runs at refresh/N — yet the Present counter, published FPS, generation, and
  latency all read full rate (they count Presents, not unique content), so the metrics
  look perfect while the eye sees missing frames. Query the current index right before
  the trampoline `Present` (that's the just-rendered buffer) and copy that one.
- **Keep the D3D12 capture copy off the game's present queue, but ordered after its
  frame.** The D3D11 path copies on the game's immediate context, so it's naturally
  ordered after the frame and on the game's own timeline. For D3D12 the D3D11On12
  bridge needs a queue: running the copy on the *game's* present queue orders it
  correctly but stalls the game's own presents (GPU back-pressure, plus the shared
  keyed-mutex `AcquireSync` is a **CPU-blocking** call on the render thread). Running
  it on an independent queue avoids the stall but races the game's render → stale
  frames. The fix is both: run the copy on **our own** queue, and order it with a
  **fence** the game's present queue signals after its frame (a near-free op) and our
  queue waits on. The present queue is recovered for late injection by hooking
  `ID3D12CommandQueue::ExecuteCommandLists` (the per-frame method, not creation). Make
  the producer-side `AcquireSync` non-blocking (`timeout 0`) so a busy mutex drops a
  *mirror* frame instead of stalling the game; the Video panel's "Frames lost" line
  surfaces both capture- and display-stage drops.
- **Capturing at `Present` decouples the mirror from DWM composition.** The hook copies
  the backbuffer inside the game's `Present`, which the game issues at its true render
  rate regardless of how DWM composites that *window*. So an unfocused game window can
  judder (DWM under-composites background windows; only the focused window gets VRR /
  independent flip) while the mirror — which receives every `Present` — stays smooth.
  This is why the game window not being focused doesn't matter: it isn't the surface
  anyone sees. The same focus rule explains why an unfocused *tool* window can render
  below the game's rate (it loses VRR), so in use the mirror is the focused window.
- **SafetyHook on x86 has two traps that froze 32-bit Slaps and Beans.** (1)
  `InlineHook::call()` invokes the trampoline as `__cdecl`, but most targets are
  `__stdcall` (COM methods like `IDXGISwapChain::Present`, the WASAPI interfaces,
  `WINAPI` `SwapBuffers`); on 32-bit that double-cleans the stack → ESP imbalance →
  crash (Debug: **Run-Time Check Failure #0**). Use **`stdcall()`** (a no-op on
  x64). (2) Don't *inline-hook* COM methods on x86 at all: MMDevApi/AudioSes
  prologues do `push ebp; mov ebp,esp; and esp,-8` and read args **EBP-relative**,
  which SafetyHook's trampoline relocation breaks (the original then runs with
  garbage args and faults). Hook COM methods by **swapping the vtable entry**
  instead (`VirtualProtect` the slot, overwrite the pointer, call the saved
  original) — no code patching, pristine stack regardless of prologue. Inline
  hooking stays fine for `Present`/`SwapBuffers` (clean prologues). Guarded by the
  x86 hook tests.
- **Steam Input init suppresses XInput.** Initializing Steam Input turns on Steam's
  in-process XInput interception, which hides controllers from `XInputGetState`
  unless they're bound to the running appid's action set — defaulting to it
  silently broke forwarding. XInput is primary; Steam Input is opt-in.