A 4-agent read of the host crate: a Windows host is an "add a backend" job, not a parallel port — ~95% reuse (core/protocol/FEC/crypto/C-ABI, QUIC, GameStream, mgmt, m3/pipeline are all platform-agnostic and already cfg-isolated). New cfg(windows) backends behind the existing traits: DXGI Desktop Duplication (capture), Media Foundation / NVENC-SDK (encode), SendInput + ViGEm (input), WASAPI loopback + virtual mic (audio). The blocker is the virtual-display feature — no user-mode Windows API; it needs a signed kernel-mode IDD driver (XL). docs/windows-host.md records the per-subsystem effort + a phased plan (Phase 0 = a "basic Windows host" capturing an existing monitor, smallest surface). Deferred: large and unbuildable on the Linux dev box, per the request to only take it on if manageable. roadmap.md marks #1/#2/#4 done, #3 packaged, and adds #7 Windows. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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# Windows as a host — feasibility & scoping
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**Status: scoped, deferred.** A Windows host is architecturally an *"add a backend"* job, not a
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parallel port — but it is a **large** implementation effort across five GPU/driver subsystems, and
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the project's headline feature (a per-client *virtual* output at the client's exact mode) has **no
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user-mode Windows API**: it needs a signed kernel-mode Indirect Display Driver (IDD). This doc
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records what it takes so the work can be picked up deliberately later.
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(Grounded in a 4-agent read of the host crate, 2026-06-10.)
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## What's already done for us
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punktfunk is cleanly layered. **~95% of the codebase is platform-agnostic and reuses verbatim:**
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| Reusable as-is | Why |
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|---|---|
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| `punktfunk-core` (protocol, FEC, crypto, session, transport, **C ABI**) | Zero platform deps — no `cfg(linux)` anywhere; the C ABI is already cross-platform |
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| QUIC control plane (`quic.rs`, pairing, mode negotiation) | quinn + tokio are portable |
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| GameStream P1.1 (mDNS, serverinfo, pairing, RTSP, ENet) — *except* `stream.rs`/`audio.rs` | pure wire logic |
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| Management REST API (`mgmt.rs`) + OpenAPI | axum/tokio, portable |
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| Pipeline + `m3.rs` orchestration | trait-generic — calls `capturer.next_frame()`, `encoder.submit/poll()`; **needs zero changes** |
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| The **trait boundaries** themselves: `Capturer`, `Encoder`, `VirtualDisplay`, `InputInjector`, `AudioCapturer`, `VirtualMic` | platform-neutral signatures; Linux deps are already isolated under `[target.'cfg(target_os="linux")'.dependencies]` |
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So a Windows host is **new `#[cfg(target_os = "windows")]` backend modules behind the existing
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traits** — the per-frame path, protocol, and control plane don't move. No architectural refactor is
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required; the boundaries are already in the right places.
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## What a Windows host needs (new code)
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Each row is a Linux backend that needs a Windows sibling. Effort is the *implementation* effort;
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all reuse the existing trait.
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| Subsystem | Linux today | Windows equivalent | Effort | Notes |
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|---|---|---|---|---|
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| **Capture** | xdg ScreenCast portal → PipeWire (dmabuf) | **DXGI Desktop Duplication** (or Windows.Graphics.Capture) → D3D11 texture | M | DXGI gives a GPU `B8G8R8A8` texture directly |
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| **Virtual display** | KWin/Mutter/Sway/gamescope protocols | **Indirect Display Driver (IDD)** — kernel UMDF mini-driver | **XL** | ⚠️ **the blocker**: no user-mode API; C++ driver + **code signing** (test-sign or WHQL). Fallback: capture an existing monitor (loses the native-resolution feature) or a borderless window |
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| **Encode** | `ffmpeg-next` NVENC, CUDA hwframes | Media Foundation H.264/HEVC/AV1, **or** NVENC SDK direct with a D3D11 device context (`AVD3D11VADeviceContext`) | M–L | `encode.rs` AU/codec logic + NVENC option strings are portable; only the hwdevice + frame-pool glue swaps |
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| **Zero-copy bridge** | dmabuf → EGL/Vulkan → CUDA | D3D11 texture → NVENC (shared texture / `cudaImportExternalMemory` + D3D12 fence) | M | **optional** — a portable CPU-copy path already exists, so v1 can skip this |
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| **Input (ptr/kbd)** | libei (RemoteDesktop portal) / wlr protocols | **SendInput** (`keybd_event`/`mouse_event`) | S | the VK→evdev table just becomes VK→`VIRTUAL_KEY` (already Win32-native) |
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| **Input (gamepads)** | uinput X-Box-360 pad + FF rumble | **ViGEm** (Virtual Gamepad Emulation) + HID reports | M | rumble back-channel maps to ViGEm notifications |
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| **Audio capture** | PipeWire sink-monitor | **WASAPI loopback** (`IAudioCaptureClient`) | S–M | also produces interleaved f32 — same `AudioCapturer` contract |
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| **Virtual mic** | PipeWire `Audio/Source` | virtual audio device (VB-Cable-style WDM driver) or WASAPI render-to-fake-device | M | needs a driver or a bundled 3rd-party cable |
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| **`sendmmsg` batching** | `gamestream/stream.rs` | already has a `cfg(not(linux))` per-packet fallback | — | nothing to do |
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**Rough total: ~2,000–4,000 LOC of new Rust** (+ a C++ IDD driver if the virtual-display feature is
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kept), spread over capture/encode/vdisplay/input/audio. Every reader rated the overall effort
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**large**; the input+audio layer alone is *medium*.
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## Recommended phasing (when picked up)
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1. **Phase 0 — "basic Windows host" (no virtual display).** Capture an *existing* monitor (DXGI
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Desktop Duplication) → Media Foundation/NVENC encode → SendInput + WASAPI loopback. This proves
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the whole stack on Windows with the smallest surface, reusing all of core/QUIC/GameStream/mgmt.
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It loses the per-client native-resolution output but is a working Windows host quickly.
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2. **Phase 1 — input + audio parity.** ViGEm gamepads + rumble; WASAPI virtual mic; D3D11→NVENC
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zero-copy.
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3. **Phase 2 — the virtual display (IDD).** The XL piece: a signed Indirect Display Driver that
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surfaces a client-sized monitor, captured via DXGI. This restores punktfunk's differentiator on
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Windows. Gated on solving driver signing/distribution.
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## Why it's deferred (not started now)
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- It's **large**, and the virtual-display blocker (IDD) is a kernel driver + signing problem
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outside Rust — not "somewhat manageable" as a side effort.
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- None of it is **buildable or testable on the Linux dev box** — it would be unvalidated code.
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The architecture is ready whenever the work is scheduled; this doc + the clean trait boundaries are
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the down payment. Start at **Phase 0** for the fastest path to a working Windows host.
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