# Windows native client — bootstrap handoff

> **Status:** SHIPPED — `clients/windows` (binary `punktfunk-client`), WinUI 3 via `windows-reactor`;
> commits `0a3b92d..0cc36fa`. Build + clippy + fmt green on `x86_64-pc-windows-msvc` and
> `aarch64-pc-windows-msvc` (ARM64 cross-compiled off the one x64 runner; signed MSIX for both arches
> via `windows-msix.yml`). This doc is trimmed to design rationale, the HDR reference, hard-won
> gotchas, and open items. The shipped source under `clients/windows/src/` is the truth.

The native Windows punktfunk/1 client connects to a host (`serve` / `punktfunk1-host`), decodes HEVC,
presents it low-latency on a `SwapChainPanel`, plays Opus audio, and captures local
mouse/keyboard/gamepad to send back — the Windows analogue of the GTK4 Linux client
(`clients/linux`), which was the architectural template. The locked decisions below are the durable
"why"; the HDR section is the evergreen present reference.

## Locked decisions (the "why")

- **Pure Rust.** `windows-rs` + **Windows App SDK "Reactor"** (WinUI 3 from Rust, merged windows-rs
  PR #4479). No C++/C#. Reactor + `SwapChainPanel` was the only novel/uncertain piece and was
  de-risked first; everything else is a known-good port of the Linux client.
  - Reactor is viable: windows-rs [PR #4499](https://github.com/microsoft/windows-rs/pull/4499)
    (merged 2026-06-01) added a `SwapChainPanel` widget to `windows-reactor` with `set_swap_chain`
    over `CreateSwapChainForComposition`, so a DXGI presenter *can* be hosted. (An earlier read that
    Reactor had no swapchain hatch was wrong/stale.) The UI is a declarative React-like tree
    (`App::new().render(app)`, `use_state`/`use_resource`/`use_effect` hooks); the video page is
    `swap_chain_panel().on_ready(|p| p.set_swap_chain(&sc))` driven by `on_rendering`.
- **Links `punktfunk-core` directly** (Cargo path dep, `features = ["quic"]`) — **no C ABI**, exactly
  like the GTK client, *unlike* the Apple path. `NativeClient` is already `Sync` (mutexed plane
  receivers), so it drops into a UI app cleanly. The C ABI (`punktfunk_connect` + `next_au`/
  `next_audio`/`next_rumble`/`next_hidout`/`send_input`/`send_rich_input`) is the *Apple* path; the
  native Rust clients call `crates/punktfunk-core/src/client.rs` (`NativeClient`) methods directly.
- **Video widget = WinUI 3 `SwapChainPanel`** (built-in), fed a D3D11 swapchain via
  `ISwapChainPanelNative::SetSwapChain`. `present.rs` owns the D3D11 composition swapchain (WARP
  fallback, runtime shaders, Contain-fit) — the same renderer, bound to the panel instead of an HWND.
- **Decode = FFmpeg-next + D3D11VA** (HEVC; **Main10** for 10-bit/HDR — see below).
- **Audio playback = WASAPI render** + Opus decode (`opus` crate, vendors libopus via cmake).
- **Input capture→send**: the client captures LOCAL input and sends it. Mouse (abs + relative) +
  keyboard via the **inverse VK table** (Windows VK is the native source, so simpler than Linux);
  gamepad via **SDL3** (already a workspace dep, cross-platform) → `NativeClient::send_input`/
  `send_rich_input`. (`SendInput`/`ViGEm` are HOST-side injection — not used by the client.)
  - **Stream input is Win32 low-level hooks**, not XAML: reactor exposes only keyboard *accelerators*
    + pointer *button-state* (no raw key-down/up, no pointer position, no wheel), insufficient for a
    game stream. `input.rs` installs `WH_KEYBOARD_LL`/`WH_MOUSE_LL` on the stream page (uninstalled
    on exit), maps the pointer through the window client rect, sends native VK + abs mouse + wheel,
    with a Ctrl+Alt+Shift+Q capture toggle. (A future alternative: generate
    `Microsoft.UI.Xaml.UIElement` bindings from the staged winmd and subscribe to `KeyDown`/
    `PointerMoved` — scoped to the panel.)
- **Discovery = `mdns-sd`** (cross-platform, browses `_punktfunk._udp`).
- **Trust = shared client identity + SPAKE2 PIN pairing + TOFU** (`trust.rs`; same identity
  files/logic as the other native clients).

## 10-bit + HDR (the present reference)

The host negotiates and emits **HEVC Main10 + BT.2020 PQ HDR10** when the captured desktop is HDR
(and 10-bit SDR Main10 when negotiated). The Windows client mirrors the Apple present:

- **Advertise caps** in the `Hello`: `video_caps = VIDEO_CAP_10BIT | VIDEO_CAP_HDR`
  (`crates/punktfunk-core/src/quic.rs`). The host enables 10-bit only if the client advertised it.
- **Detect HDR in-band** from the HEVC VUI (transfer characteristics = SMPTE ST 2084 / PQ), exactly
  like the Apple client's `VideoDecoder.isHDRFormat` (`clients/apple/Sources/PunktfunkKit/`). This
  handles a mid-session HDR toggle without renegotiation. `Welcome.bit_depth` (8/10) is also
  available.
- **Decode** Main10 → **P010** (10-bit) via D3D11VA.
- **Present HDR**: swapchain in `DXGI_FORMAT_R10G10B10A2_UNORM` (or `R16G16B16A16_FLOAT`),
  `IDXGISwapChain3::SetColorSpace1(DXGI_COLOR_SPACE_RGB_FULL_G2084_NONE_P2020)` + `SetHDRMetaData`
  for HDR10; the host's stream is BT.2020 PQ, so present PQ. For SDR, the existing
  `DXGI_FORMAT_B8G8R8A8_UNORM` + BT.709 path. (The host-side HDR conversion math is in
  `crates/punktfunk-host/src/capture/dxgi.rs` `HDR_PS`/`HdrConverter` if you need the inverse.)

## Build gotchas (hard-won)

- **`CARGO_HOME` must be an ASCII path** (`C:\Users\Public\.cargo`). SDL3's `build-from-source` PCH
  embeds the registry source path; the `ü` in the dev box's username makes MSVC fail (`MSB8084` /
  `C4828`). Build under an ASCII path generally (the same `ü` triggers `LNK1201` PDB-write failures
  under `~/Developer`).
- **`CMAKE_POLICY_VERSION_MINIMUM=3.5`** in the build env (CMake 4 rejects libopus's old minimum).
- **Toolchain:** `winget install NASM.NASM Kitware.CMake LLVM.LLVM` (NASM for aws-lc-rs on the quic
  path; libclang/LLVM for ffmpeg-sys).
- **windows-reactor is unpublished** (`version 0.0.0`) and fast-moving — depend on it as a **git dep
  pinned to a commit** (`b4129fcc`), and pin the `windows` crate to the **same commit** so the
  `IDXGISwapChain1` you pass to `set_swap_chain` satisfies reactor's `windows_core::Interface`. Its
  `build.rs` downloads the Windows App SDK NuGets (Foundation/Interactive/Runtime), stages the
  bootstrap DLL + `resources.pri` next to the exe, and **`.unwrap()`s `CARGO_WORKSPACE_DIR`** — set
  it in the build env (`CARGO_WORKSPACE_DIR=C:\Users\Public\punktfunk`). It writes `/temp` + `/winmd`
  to the workspace root (gitignored). The App SDK runtime must be installed to *run*.
- **Windows clippy is stricter** than Linux CI and `cfg(windows)` code is excluded from Linux CI →
  run `cargo clippy -p punktfunk-client-windows -- -D warnings` ON the Windows box before committing.

## Open items

- **On-glass validation on the RTX box** of **D3D11VA hardware decode** + **10-bit/HDR present** +
  the **WinUI GUI** — the dev VM is headless / SSH Session 0 / WARP, so the WinUI window can't show
  there and there is no hardware decode. Validate over RDP or on the RTX box against a live HDR host.
- **RAWINPUT relative-mouse pointer-lock** for the stream view.
- **Per-host speed-test widget** in the UI.

## Key references

- **Full Windows plan + SudoVDA/host details:** `design/windows-host.md`.
- **Template ported from:** `clients/linux/src/*`.
- **Apple HDR present** (the pattern mirrored): `clients/apple/Sources/PunktfunkKit/{VideoDecoder,
  MetalVideoPresenter,Stage2Pipeline}.swift` — in-band PQ detection, P010 decode, EDR present.
- **Core client API:** `crates/punktfunk-core/src/client.rs` (`NativeClient`).
- **Protocol:** `crates/punktfunk-core/src/quic.rs` (`Hello.video_caps`, `Welcome.bit_depth`,
  `VIDEO_CAP_10BIT`/`VIDEO_CAP_HDR`).
- **Host HDR conversion (inverse math):** `crates/punktfunk-host/src/capture/dxgi.rs` (`HDR_PS`,
  `HdrConverter`) + `crates/punktfunk-host/src/encode/nvenc.rs` (BT.2020/PQ VUI).
