feat(windows-client): D3D11VA zero-copy hw decode + HDR10 present + GUI polish

The client was pure software HEVC decode + CPU swscale->RGBA + a full-frame
dynamic-texture upload every frame -- the reason performance was poor on a GPU
box (the GPU sat idle while the CPU churned). This adds a hardware path, HDR,
and a GUI pass.

Performance -- D3D11VA zero-copy:
- gpu.rs (new): one D3D11 device (hardware + VIDEO_SUPPORT, WARP fallback,
  multithread-protected) shared by decoder and presenter via a Send/Sync
  OnceLock. Sharing is mandatory -- a decoded texture is only bindable on the
  device that created it. windows-rs COM interfaces are !Send/!Sync, so the
  unsafe impl is sound only under the multithread protection + disjoint
  decode(video ctx)/present(immediate ctx) split.
- video.rs: D3d11vaDecoder (raw FFI mirroring the Linux VAAPI module). The
  COM-typed AVD3D11VA{Device,Frames}Context are declared here (stable FFmpeg
  ABI) to avoid ffmpeg-sys binding the d3d11 headers; get_format builds a frames
  ctx with BindFlags=SHADER_RESOURCE so the NV12/P010 array slices are
  sampleable. av_frame_clone guard keeps each surface out of the reuse pool
  until the presenter drops it. Software decode stays as the fallback
  (DecoderPref Auto/Hardware/Software; auto falls back on init/decode error).
- present.rs: shared device; per-plane SRVs over the array slice
  (NV12->R8/R8G8, P010->R16/R16G16) + three pixel shaders (RGBA passthrough,
  NV12/BT.709, P010/BT.2020-PQ). present() now takes the frame by value so the
  GPU surface survives re-presents.

HDR:
- Detected in-band (transfer == SMPTE2084), same signal as the other clients.
  Swapchain flips to R10G10B10A2 + ST.2084 + HDR10 metadata. New Settings toggle
  gates advertising VIDEO_CAP_10BIT|HDR; host still gates 10-bit behind its own
  PUNKTFUNK_10BIT + actual-HDR-content checks.

GUI (windows-reactor):
- Host cards with accent-monogram avatars + colored status pills, InfoBar for
  errors/pairing hints, ToggleSwitch settings (+ HDR, decoder, bitrate), button
  icons, a richer connecting screen, and a stream HUD with GPU/CPU-decode + HDR
  status chips.

Not yet on-glass validated: the Linux dev box can't compile the cfg(windows)
code (ffmpeg/windows crates unfetched; WARP has no hw decode) -- only
cargo fmt checks it here. API shapes verified against the windows-rs/reactor
source and the YUV->RGB coefficients checked by hand, but D3D11VA + shaders +
the GUI need a real build (Windows CI / build VM) and on-glass test on the RTX
box. The host-side HDR encode path is unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -123,11 +123,21 @@ Low-latency desktop/game streaming stack, Linux-first, with a shared Rust protoc
    framework backed by WinUI; PR #4499 added the `SwapChainPanel` widget + `set_swap_chain`). The
    video is a **`SwapChainPanel`** bound to a **D3D11 composition swapchain** (WARP fallback for
    the GPU-less dev box; runtime-compiled fullscreen-triangle shaders, Contain-fit letterbox),
-   driven by reactor's per-frame `on_rendering`. **FFmpeg software HEVC decode** (D3D11VA hw decode
+   driven by reactor's per-frame `on_rendering`. **FFmpeg HEVC decode with a D3D11VA
-   is the follow-up), **WASAPI** render + mic capture, **SDL3** gamepads (rumble/lightbar/DualSense),
+   zero-copy hardware path** (`gpu.rs` shares one D3D11 device — hardware+`VIDEO_SUPPORT`, WARP
-   `mdns-sd` discovery, and the full trust surface — all **in-app**: host list (live mDNS + saved +
+   fallback, multithread-protected — between the decoder and presenter; the decoder outputs
-   manual), settings (resolution/refresh/mic), SPAKE2 PIN pairing screen, TOFU, pinned-fp-mismatch
+   NV12/P010 `ID3D11Texture2D` array slices with `BIND_SHADER_RESOURCE` and the presenter samples
-   re-pair. **Stream input** is Win32 low-level hooks (`WH_KEYBOARD_LL`/`WH_MOUSE_LL`) — reactor
+   them via per-plane SRVs + YUV→RGB shaders — NV12/BT.709, P010/BT.2020-PQ; **software CPU decode
+   stays as the robust fallback**, auto-selected with a `DecoderPref` override). **HDR10**: the
+   client advertises 10-bit/HDR (Settings toggle), detects PQ in-band (`transfer == SMPTE2084`),
+   and flips the swapchain to `R10G10B10A2` + ST.2084 with HDR10 metadata. **WASAPI** render + mic
+   capture, **SDL3** gamepads (rumble/lightbar/DualSense), `mdns-sd` discovery, and the full trust
+   surface — all **in-app**: a polished WinUI shell (host cards w/ monogram + status pills,
+   `InfoBar` errors/hints, `ToggleSwitch` settings, status-chip stream HUD showing GPU/CPU decode +
+   HDR), host list (live mDNS + saved + manual), settings (resolution/refresh/decoder/bitrate/HDR/
+   mic), SPAKE2 PIN pairing screen, TOFU, pinned-fp-mismatch re-pair. **(D3D11VA + HDR present + the
+   GUI polish are written against the windows-rs/reactor APIs but not yet on-glass validated — the
+   dev VM is headless/WARP; needs the RTX box.)** **Stream input** is Win32 low-level hooks (`WH_KEYBOARD_LL`/`WH_MOUSE_LL`) — reactor
    exposes no raw key/pointer events; native Windows VK + absolute mouse (client-rect Contain-fit) +
    wheel, Ctrl+Alt+Shift+Q capture toggle. `--headless`/`--discover` keep CLI paths. Builds + clippy
    + fmt green on `x86_64-pc-windows-msvc` (on the dev VM). **windows-reactor is unpublished** (git
@@ -135,9 +145,9 @@ Low-latency desktop/game streaming stack, Linux-first, with a shared Rust protoc
    with `set_swap_chain`); its `build.rs` downloads the Win App SDK NuGets + needs `CARGO_WORKSPACE_DIR`
    set (in the VM build env; `/temp`+`/winmd` gitignored). Gotcha: `CARGO_HOME` must be an ASCII path
    — the `ü` in the dev box's username breaks SDL3's MSVC precompiled-header build. Next: **on-glass
-   validation** (the dev VM is headless/Session-0 → the WinUI window needs a display: RDP or the RTX
+   validation** of the D3D11VA decode + HDR present + GUI on the RTX box (the dev VM is
-   box), D3D11VA hw decode + 10-bit/HDR present, RAWINPUT relative-mouse pointer-lock, and a per-host
+   headless/Session-0/WARP → the WinUI window + hardware decode need a real display+GPU: RDP or the
-   speed test in the UI.
+   RTX box), then RAWINPUT relative-mouse pointer-lock and a per-host speed test in the UI.
 2. **Sub-frame pipelining**: overlap encode and transmit within a frame. Requires a direct
    NVENC SDK wrapper (libavcodec only emits whole AUs) — the next big latency lever (~2–4 ms
    at high res).

clients/windows/src/app.rs

@@ -16,7 +16,7 @@ use crate::gamepad::GamepadService;
 use crate::present::Presenter;
 use crate::session::{self, SessionEvent, SessionParams, Stats};
 use crate::trust::{self, KnownHost, KnownHosts, Settings};
-use crate::video::DecodedFrame;
+use crate::video::{DecodedFrame, DecoderPref};
 use punktfunk_core::client::NativeClient;
 use punktfunk_core::config::{CompositorPref, GamepadPref, Mode};
 use std::cell::RefCell;
@@ -31,6 +31,14 @@ const RESOLUTIONS: &[(u32, u32)] = &[
     (3840, 2160),
 ];
 const REFRESH: &[u32] = &[0, 30, 60, 90, 120, 144, 165, 240];
+/// Decode backend presets: `(stored value, display label)`.
+const DECODERS: &[(&str, &str)] = &[
+    ("auto", "Automatic (GPU, fall back to CPU)"),
+    ("hardware", "Hardware (GPU / D3D11VA)"),
+    ("software", "Software (CPU)"),
+];
+/// Bitrate presets in Mb/s; `0` = host default.
+const BITRATES_MBPS: &[u32] = &[0, 10, 20, 30, 50, 80, 150];
 
 #[derive(Clone, PartialEq)]
 enum Screen {
@@ -189,10 +197,61 @@ fn page(children: Vec<Element>) -> Element {
     scroll_view(col).into()
 }
 
-/// A clickable host row: name + address/badge + chevron.
+/// A rounded square "monogram" for a host, the first letter on an accent fill — a clean leading
+/// visual that avoids depending on an icon font being installed.
+fn avatar(name: &str) -> Border {
+    let initial = name
+        .chars()
+        .find(|c| c.is_alphanumeric())
+        .map(|c| c.to_uppercase().to_string())
+        .unwrap_or_else(|| "?".into());
+    border(
+        text_block(initial)
+            .font_size(17.0)
+            .semibold()
+            .foreground(ThemeRef::AccentText)
+            .horizontal_alignment(HorizontalAlignment::Center)
+            .vertical_alignment(VerticalAlignment::Center),
+    )
+    .background(ThemeRef::Accent)
+    .corner_radius(10.0)
+    .width(40.0)
+    .height(40.0)
+}
+
+/// Pill chip colour intent.
+#[derive(Clone, Copy)]
+enum Pill {
+    Accent,
+    Good,
+    Neutral,
+}
+
+/// A small rounded status chip (paired/PIN/HDR/etc.).
+fn pill(text: &str, kind: Pill) -> Border {
+    let (bg, fg) = match kind {
+        Pill::Accent => (ThemeRef::Accent, ThemeRef::AccentText),
+        Pill::Good => (ThemeRef::SystemSuccessBackground, ThemeRef::SystemSuccess),
+        Pill::Neutral => (ThemeRef::SubtleFill, ThemeRef::SecondaryText),
+    };
+    border(text_block(text).font_size(11.0).semibold().foreground(fg))
+        .background(bg)
+        .corner_radius(10.0)
+        .padding(edges(9.0, 3.0, 9.0, 3.0))
+}
+
+/// A clickable host row: monogram + name/address + status pill + chevron.
 fn host_card(name: &str, sub: &str, badge: &str, on_tap: impl Fn() + 'static) -> Element {
+    let kind = match badge {
+        "Paired" => Pill::Good,
+        "Open" => Pill::Neutral,
+        _ => Pill::Accent, // Trusted / PIN
+    };
     card(
         grid((
+            avatar(name)
+                .grid_column(0)
+                .vertical_alignment(VerticalAlignment::Center),
             vstack((
                 text_block(name).font_size(15.0).semibold(),
                 text_block(sub)
@@ -200,21 +259,25 @@ fn host_card(name: &str, sub: &str, badge: &str, on_tap: impl Fn() + 'static) ->
                     .foreground(ThemeRef::SecondaryText),
             ))
             .spacing(2.0)
-            .grid_column(0)
-            .vertical_alignment(VerticalAlignment::Center),
-            text_block(badge)
-                .font_size(12.0)
-                .foreground(ThemeRef::SecondaryText)
             .grid_column(1)
             .vertical_alignment(VerticalAlignment::Center)
-                .margin(edges(0.0, 0.0, 12.0, 0.0)),
+            .margin(edges(12.0, 0.0, 0.0, 0.0)),
+            pill(badge, kind)
+                .grid_column(2)
+                .vertical_alignment(VerticalAlignment::Center)
+                .margin(edges(0.0, 0.0, 10.0, 0.0)),
             text_block("\u{203A}")
                 .font_size(18.0)
                 .foreground(ThemeRef::SecondaryText)
-                .grid_column(2)
+                .grid_column(3)
                 .vertical_alignment(VerticalAlignment::Center),
         ))
-        .columns([GridLength::Star(1.0), GridLength::Auto, GridLength::Auto]),
+        .columns([
+            GridLength::Auto,
+            GridLength::Star(1.0),
+            GridLength::Auto,
+            GridLength::Auto,
+        ]),
     )
     .on_tapped(on_tap)
     .into()
@@ -281,22 +344,35 @@ fn root(cx: &mut RenderCx, ctx: &Arc<AppCtx>) -> Element {
     };
     match screen {
         Screen::Hosts => component(hosts_page, HostsProps { svc, hosts, status }),
-        Screen::Connecting => vstack((
+        Screen::Connecting => {
+            let target_name = ctx.shared.target.lock().unwrap().name.clone();
+            let headline = if target_name.is_empty() {
+                "Connecting\u{2026}".to_string()
+            } else {
+                format!("Connecting to {target_name}\u{2026}")
+            };
+            vstack((
                 ProgressRing::indeterminate()
                     .width(48.0)
                     .height(48.0)
                     .horizontal_alignment(HorizontalAlignment::Center),
-            text_block("Connecting\u{2026}")
+                text_block(headline)
-                .font_size(16.0)
+                    .font_size(18.0)
+                    .semibold()
                     .horizontal_alignment(HorizontalAlignment::Center),
-            text_block(status.clone())
+                text_block(if status.is_empty() {
+                    "Negotiating the session and creating the virtual display\u{2026}".to_string()
+                } else {
+                    status.clone()
+                })
                 .foreground(ThemeRef::SecondaryText)
                 .horizontal_alignment(HorizontalAlignment::Center),
             ))
             .spacing(16.0)
             .horizontal_alignment(HorizontalAlignment::Center)
             .vertical_alignment(VerticalAlignment::Center)
-        .into(),
+            .into()
+        }
         // settings_page uses no hooks (it never touches `cx`), so calling it inline is sound.
         Screen::Settings => settings_page(ctx, &set_screen),
         Screen::Pair => component(pair_page, svc),
@@ -327,6 +403,7 @@ fn hosts_page(props: &HostsProps, cx: &mut RenderCx) -> Element {
             .grid_column(0)
             .vertical_alignment(VerticalAlignment::Center),
             button("Settings")
+                .icon(SymbolGlyph::Setting)
                 .on_click({
                     let ss = set_screen.clone();
                     move || ss.call(Screen::Settings)
@@ -340,7 +417,13 @@ fn hosts_page(props: &HostsProps, cx: &mut RenderCx) -> Element {
     );
 
     if !status.is_empty() {
-        body.push(card(text_block(status.to_string()).foreground(ThemeRef::SystemCritical)).into());
+        body.push(
+            InfoBar::new("Couldn't connect")
+                .message(status.to_string())
+                .error()
+                .is_closable(false)
+                .into(),
+        );
     }
 
     // Saved (trusted/paired) hosts.
@@ -439,6 +522,7 @@ fn hosts_page(props: &HostsProps, cx: &mut RenderCx) -> Element {
                     .vertical_alignment(VerticalAlignment::Center),
                 button("Connect")
                     .accent()
+                    .icon(SymbolGlyph::Forward)
                     .on_click(connect_manual)
                     .grid_column(1)
                     .margin(edges(8.0, 0.0, 0.0, 0.0)),
@@ -515,6 +599,8 @@ fn connect(
         gamepad: gamepad_pref,
         bitrate_kbps: s.bitrate_kbps,
         mic_enabled: s.mic_enabled,
+        hdr_enabled: s.hdr_enabled,
+        decoder: DecoderPref::from_name(&s.decoder),
         pin,
         identity: ctx.identity.clone(),
     });
@@ -594,9 +680,13 @@ fn pair_page(props: &Svc, cx: &mut RenderCx) -> Element {
             code.clone(),
             target.clone(),
         );
-        button("Pair & Connect").accent().on_click(move || {
+        button("Pair & Connect")
+            .accent()
+            .icon(SymbolGlyph::Accept)
+            .on_click(move || {
                 let pin = code2.trim().to_string();
-            let (ctx3, ss, st, target3) = (ctx2.clone(), ss.clone(), st.clone(), target2.clone());
+                let (ctx3, ss, st, target3) =
+                    (ctx2.clone(), ss.clone(), st.clone(), target2.clone());
                 std::thread::spawn(move || {
                     let name =
                         std::env::var("COMPUTERNAME").unwrap_or_else(|_| "windows-client".into());
@@ -630,28 +720,47 @@ fn pair_page(props: &Svc, cx: &mut RenderCx) -> Element {
     };
     let cancel_btn = {
         let ss = set_screen.clone();
-        button("Cancel").on_click(move || ss.call(Screen::Hosts))
+        button("Cancel")
+            .icon(SymbolGlyph::Cancel)
+            .on_click(move || ss.call(Screen::Hosts))
     };
 
     let content = card(vstack((
+        grid((
+            avatar(&target.name)
+                .grid_column(0)
+                .vertical_alignment(VerticalAlignment::Center),
+            vstack((
                 text_block(format!("Pair with {}", target.name))
                     .font_size(20.0)
                     .semibold(),
-        text_block(
+                text_block(format!("{}:{}", target.addr, target.port))
-            "Arm pairing on the host (its console or web console), then enter the 4-digit PIN it \
+                    .font_size(12.0)
-             shows.",
+                    .foreground(ThemeRef::SecondaryText),
+            ))
+            .spacing(2.0)
+            .grid_column(1)
+            .vertical_alignment(VerticalAlignment::Center)
+            .margin(edges(12.0, 0.0, 0.0, 0.0)),
+        ))
+        .columns([GridLength::Auto, GridLength::Star(1.0)]),
+        InfoBar::new("Arm pairing on the host")
+            .message(
+                "On the host's console or web console, start pairing — it shows a 4-digit PIN. \
+                 Enter it below within 90 seconds.",
             )
-        .foreground(ThemeRef::SecondaryText)
+            .informational()
-        .max_width(440.0),
+            .is_closable(false),
         text_box(code)
             .placeholder("PIN")
+            .font_size(28.0)
             .on_changed(move |s| set_code.call(s)),
         hstack((pair_btn, cancel_btn)).spacing(8.0),
     ))
-    .spacing(14.0))
+    .spacing(16.0))
     .max_width(480.0)
     .horizontal_alignment(HorizontalAlignment::Center)
-    .margin(edges(0.0, 80.0, 0.0, 0.0));
+    .margin(edges(0.0, 60.0, 0.0, 0.0));
 
     page(vec![content.into()])
 }
@@ -708,10 +817,69 @@ fn settings_page(ctx: &Arc<AppCtx>, set_screen: &AsyncSetState<Screen>) -> Eleme
                 s.save();
             })
     };
+    let dec_i = DECODERS
+        .iter()
+        .position(|&(v, _)| v == s.decoder)
+        .unwrap_or(0) as i32;
+    let dec_names: Vec<String> = DECODERS.iter().map(|&(_, l)| l.to_string()).collect();
+    let decoder_combo = {
+        let ctx = ctx.clone();
+        ComboBox::new(dec_names)
+            .header("Video decoder")
+            .selected_index(dec_i)
+            .on_selection_changed(move |i: i32| {
+                let (v, _) = DECODERS[(i.max(0) as usize).min(DECODERS.len() - 1)];
+                let mut s = ctx.settings.lock().unwrap();
+                s.decoder = v.to_string();
+                s.save();
+            })
+    };
+
+    let br_i = BITRATES_MBPS
+        .iter()
+        .position(|&m| m * 1000 == s.bitrate_kbps)
+        .unwrap_or(0) as i32;
+    let br_names: Vec<String> = BITRATES_MBPS
+        .iter()
+        .map(|&m| {
+            if m == 0 {
+                "Automatic".into()
+            } else {
+                format!("{m} Mb/s")
+            }
+        })
+        .collect();
+    let bitrate_combo = {
+        let ctx = ctx.clone();
+        ComboBox::new(br_names)
+            .header("Bitrate")
+            .selected_index(br_i)
+            .on_selection_changed(move |i: i32| {
+                let m = BITRATES_MBPS[(i.max(0) as usize).min(BITRATES_MBPS.len() - 1)];
+                let mut s = ctx.settings.lock().unwrap();
+                s.bitrate_kbps = m * 1000;
+                s.save();
+            })
+    };
+
+    let hdr_toggle = {
+        let ctx = ctx.clone();
+        ToggleSwitch::new(s.hdr_enabled)
+            .header("HDR (10-bit, BT.2020 PQ)")
+            .on_content("On")
+            .off_content("Off")
+            .on_changed(move |on: bool| {
+                let mut s = ctx.settings.lock().unwrap();
+                s.hdr_enabled = on;
+                s.save();
+            })
+    };
     let mic_toggle = {
         let ctx = ctx.clone();
-        check_box(s.mic_enabled)
+        ToggleSwitch::new(s.mic_enabled)
-            .label("Stream microphone to the host")
+            .header("Stream microphone to the host")
+            .on_content("On")
+            .off_content("Off")
             .on_changed(move |on: bool| {
                 let mut s = ctx.settings.lock().unwrap();
                 s.mic_enabled = on;
@@ -727,6 +895,7 @@ fn settings_page(ctx: &Arc<AppCtx>, set_screen: &AsyncSetState<Screen>) -> Eleme
             .vertical_alignment(VerticalAlignment::Center),
         button("Back")
             .accent()
+            .icon(SymbolGlyph::Back)
             .on_click({
                 let ss = set_screen.clone();
                 move || ss.call(Screen::Hosts)
@@ -739,7 +908,7 @@ fn settings_page(ctx: &Arc<AppCtx>, set_screen: &AsyncSetState<Screen>) -> Eleme
 
     let stream_card = card(
         vstack((
-            text_block("Stream").font_size(15.0).semibold(),
+            text_block("Display").font_size(15.0).semibold(),
             text_block("The host creates a virtual display at exactly this mode.")
                 .font_size(12.0)
                 .foreground(ThemeRef::SecondaryText),
@@ -749,13 +918,31 @@ fn settings_page(ctx: &Arc<AppCtx>, set_screen: &AsyncSetState<Screen>) -> Eleme
         .spacing(10.0),
     );
 
+    let video_card = card(
+        vstack((
+            text_block("Video").font_size(15.0).semibold(),
+            text_block(
+                "Hardware decode (D3D11VA) is zero-copy and far lighter than software — keep it on \
+                 Automatic unless debugging.",
+            )
+            .font_size(12.0)
+            .foreground(ThemeRef::SecondaryText),
+            decoder_combo,
+            bitrate_combo,
+            hdr_toggle,
+        ))
+        .spacing(10.0),
+    );
+
     let audio_card =
         card(vstack((text_block("Audio").font_size(15.0).semibold(), mic_toggle)).spacing(10.0));
 
     page(vec![
         header.into(),
-        section("STREAM"),
+        section("DISPLAY"),
         stream_card.into(),
+        section("VIDEO"),
+        video_card.into(),
         section("AUDIO"),
         audio_card.into(),
     ])
@@ -764,12 +951,13 @@ fn settings_page(ctx: &Arc<AppCtx>, set_screen: &AsyncSetState<Screen>) -> Eleme
 // --- stream page --------------------------------------------------------------------------
 
 fn present_newest(ctx: &mut PresentCtx) {
+    // Drain to the newest decoded frame (drop any backlog) and hand it to the presenter by value —
+    // the GPU zero-copy path retains the decoder surface across re-presents, so ownership matters.
     let mut newest = None;
     while let Ok(f) = ctx.frames.try_recv() {
         newest = Some(f);
     }
-    let cpu = newest.as_ref().map(|DecodedFrame::Cpu(c)| c);
+    ctx.presenter.present(newest);
-    ctx.presenter.present(cpu);
 }
 
 fn stream_page(props: &StreamProps, cx: &mut RenderCx) -> Element {
@@ -839,34 +1027,53 @@ fn stream_page(props: &StreamProps, cx: &mut RenderCx) -> Element {
     .into()
 }
 
-/// The streaming HUD overlay (top-right), mirroring the Apple client: mode + fps/throughput, the
+/// A small chip for the dark HUD: coloured text on a translucent dark fill.
-/// capture→client latency + decode time, and the release-cursor hint. Layered over the
+fn hud_chip(text: &str, color: Color) -> Border {
+    border(
+        text_block(text)
+            .font_size(11.0)
+            .semibold()
+            .foreground(color),
+    )
+    .background(Color::rgb(38, 38, 38))
+    .corner_radius(8.0)
+    .padding(edges(8.0, 2.0, 8.0, 2.0))
+}
+
+/// The streaming HUD overlay (top-right), mirroring the Apple client: a chip row (mode · decode
+/// path · HDR), the fps/throughput/latency line, and the release-cursor hint. Layered over the
 /// `SwapChainPanel` in the same grid cell.
 fn hud_overlay(stats: &Stats, mode: Option<Mode>) -> Element {
     let res = mode
         .map(|m| format!("{}\u{00D7}{}@{}", m.width, m.height, m.refresh_hz))
         .unwrap_or_else(|| "\u{2014}".into());
-    let line1 = format!("{res}   {:.0} fps   {:.1} Mb/s", stats.fps, stats.mbps);
+    let mut chips: Vec<Element> = vec![hud_chip(&res, Color::rgb(235, 235, 235)).into()];
-    let line2 = format!(
+    chips.push(if stats.hardware {
-        "capture\u{2192}client {:.1} ms p50 \u{00B7} decode {:.1} ms",
+        hud_chip("GPU decode", Color::rgb(120, 220, 150)).into()
-        stats.latency_ms, stats.decode_ms
+    } else {
+        hud_chip("CPU decode", Color::rgb(240, 190, 90)).into()
+    });
+    if stats.hdr {
+        chips.push(hud_chip("HDR", Color::rgb(255, 205, 90)).into());
+    }
+    let line = format!(
+        "{:.0} fps \u{00B7} {:.1} Mb/s \u{00B7} {:.1} ms p50 \u{00B7} decode {:.1} ms",
+        stats.fps, stats.mbps, stats.latency_ms, stats.decode_ms
     );
     border(
         vstack((
-            text_block(line1)
+            hstack(chips).spacing(6.0),
-                .font_size(12.0)
+            text_block(line)
-                .foreground(Color::rgb(255, 255, 255)),
-            text_block(line2)
                 .font_size(11.0)
-                .foreground(Color::rgb(200, 200, 200)),
+                .foreground(Color::rgb(210, 210, 210)),
             text_block("Ctrl+Alt+Shift+Q releases the mouse")
                 .font_size(11.0)
-                .foreground(Color::rgb(160, 160, 160)),
+                .foreground(Color::rgb(150, 150, 150)),
         ))
-        .spacing(2.0),
+        .spacing(6.0),
     )
     .background(Color::rgb(0, 0, 0))
-    .corner_radius(8.0)
+    .corner_radius(10.0)
     .padding(uniform(10.0))
     .opacity(0.82)
     .horizontal_alignment(HorizontalAlignment::Right)

clients/windows/src/gpu.rs

@@ -0,0 +1,121 @@
+//! The single Direct3D 11 device shared by the video decoder (D3D11VA hardware decode) and the
+//! presenter (the `SwapChainPanel` composition swapchain + the present draw).
+//!
+//! Zero-copy hardware decode requires FFmpeg to decode HEVC into `ID3D11Texture2D`s created by the
+//! **same** device the presenter binds as shader resources and draws with — a texture from one
+//! device can't be sampled by another. So the device is created once, here, and both subsystems
+//! pull it from a process-global `OnceLock` (initialised on whichever thread asks first: the
+//! session pump when it builds the decoder, or the UI thread when it builds the presenter).
+//!
+//! **Thread-safety.** windows-rs COM interfaces are deliberately `!Send`/`!Sync` — thread-safety
+//! is per-object, not universal. An `ID3D11Device` and its immediate context become free-threaded
+//! once `ID3D11Multithread::SetMultithreadProtected(TRUE)` is set, which FFmpeg's D3D11VA backend
+//! does inside `av_hwdevice_ctx_init` (it installs an `ID3D11Multithread`-based default lock when we
+//! leave `AVD3D11VADeviceContext.lock` null). The decoder then uses FFmpeg's separate
+//! `ID3D11VideoContext` for decode while the presenter uses the immediate context for draw; under
+//! multithread protection D3D serialises the two internally, and decode/draw touch disjoint context
+//! state. That makes the `unsafe impl Send + Sync` below sound for exactly this usage.
+
+use anyhow::{anyhow, Result};
+use std::sync::OnceLock;
+use windows::core::Interface;
+use windows::Win32::Graphics::Direct3D::{
+    D3D_DRIVER_TYPE_HARDWARE, D3D_DRIVER_TYPE_WARP, D3D_FEATURE_LEVEL_11_0, D3D_FEATURE_LEVEL_11_1,
+};
+use windows::Win32::Graphics::Direct3D11::{
+    D3D11CreateDevice, ID3D11Device, ID3D11DeviceContext, ID3D11Multithread,
+    D3D11_CREATE_DEVICE_BGRA_SUPPORT, D3D11_CREATE_DEVICE_VIDEO_SUPPORT, D3D11_SDK_VERSION,
+};
+
+pub struct SharedDevice {
+    pub device: ID3D11Device,
+    pub context: ID3D11DeviceContext,
+    /// True when this is a real GPU (hardware) adapter — a precondition for D3D11VA decode. WARP
+    /// (the GPU-less dev box) creates fine for present but cannot hardware-decode HEVC, so the
+    /// decoder skips straight to the software path there.
+    pub hardware: bool,
+}
+
+// Sound for our usage — see the module docs: the device + immediate context are free-threaded under
+// the multithread protection FFmpeg installs, and decode (video context) / present (immediate
+// context) never share mutable context state.
+unsafe impl Send for SharedDevice {}
+unsafe impl Sync for SharedDevice {}
+
+static SHARED: OnceLock<Option<SharedDevice>> = OnceLock::new();
+
+/// The process-wide shared D3D11 device, created on first call. `None` only if D3D11 device
+/// creation fails for both a hardware adapter and WARP (effectively never — WARP is always present).
+pub fn shared() -> Option<&'static SharedDevice> {
+    SHARED.get_or_init(create).as_ref()
+}
+
+fn create() -> Option<SharedDevice> {
+    match create_device() {
+        Ok(d) => Some(d),
+        Err(e) => {
+            tracing::error!(error = %e, "shared D3D11 device creation failed — no present/decode");
+            None
+        }
+    }
+}
+
+fn create_device() -> Result<SharedDevice> {
+    // Preference order: a hardware adapter with video support (enables D3D11VA); the same without
+    // the VIDEO flag (a driver that rejects it still presents + software-decodes); finally WARP for
+    // the GPU-less box. BGRA_SUPPORT is required for the composition swapchain in every case.
+    let attempts = [
+        (D3D_DRIVER_TYPE_HARDWARE, true, true),
+        (D3D_DRIVER_TYPE_HARDWARE, false, true),
+        (D3D_DRIVER_TYPE_WARP, false, false),
+    ];
+    for (driver, video, hardware) in attempts {
+        let flags = if video {
+            D3D11_CREATE_DEVICE_BGRA_SUPPORT | D3D11_CREATE_DEVICE_VIDEO_SUPPORT
+        } else {
+            D3D11_CREATE_DEVICE_BGRA_SUPPORT
+        };
+        let mut device = None;
+        let mut context = None;
+        let r = unsafe {
+            D3D11CreateDevice(
+                None,
+                driver,
+                None,
+                flags,
+                Some(&[D3D_FEATURE_LEVEL_11_1, D3D_FEATURE_LEVEL_11_0]),
+                D3D11_SDK_VERSION,
+                Some(&mut device),
+                None,
+                Some(&mut context),
+            )
+        };
+        if r.is_ok() {
+            let (device, context) = (device.unwrap(), context.unwrap());
+            // Make the device + immediate context free-threaded: the decoder (D3D11VA video context,
+            // pump thread) and the presenter (immediate context, UI thread) both touch this device.
+            // FFmpeg also sets this during hwdevice init, but doing it up front keeps the
+            // cross-thread `Send`/`Sync` sound from the moment the device exists.
+            if let Ok(mt) = context.cast::<ID3D11Multithread>() {
+                unsafe { mt.SetMultithreadProtected(true) };
+            }
+            tracing::info!(
+                driver = if hardware {
+                    "hardware"
+                } else {
+                    "WARP (software)"
+                },
+                video,
+                "shared D3D11 device created"
+            );
+            return Ok(SharedDevice {
+                device,
+                context,
+                hardware,
+            });
+        }
+    }
+    Err(anyhow!(
+        "D3D11CreateDevice failed for both hardware and WARP"
+    ))
+}

clients/windows/src/main.rs

@@ -10,7 +10,8 @@
 //!   punktfunk-client                          (open the WinUI 3 window: host list, settings, pairing)
 //!   punktfunk-client --discover               (list punktfunk hosts on the LAN)
 //!   punktfunk-client --headless --connect host[:port] [--pin HEX] [--pair PIN] [--mode WxHxHz]
-//!                    [--bitrate MBPS] [--mic]  (no window; count frames + print stats)
+//!                    [--bitrate MBPS] [--mic] [--decoder auto|hardware|software] [--no-hdr]
+//!                    (no window; count frames + print stats)
 
 // Link as a GUI (windows) subsystem binary so the default windowed launch (MSIX / double-click)
 // does NOT pop a console window. The CLI paths (--headless/--discover) reattach to the launching
@@ -26,6 +27,8 @@ mod discovery;
 #[cfg(windows)]
 mod gamepad;
 #[cfg(windows)]
+mod gpu;
+#[cfg(windows)]
 mod input;
 #[cfg(windows)]
 mod present;
@@ -162,7 +165,11 @@ fn run_headless_cli(args: &[String], identity: (String, String)) {
         }
     }
 
-    tracing::info!(%host, port, ?mode, tofu = pin.is_none(), "connecting (headless)");
+    let decoder = arg("--decoder")
+        .map(|d| crate::video::DecoderPref::from_name(&d))
+        .unwrap_or_default();
+
+    tracing::info!(%host, port, ?mode, tofu = pin.is_none(), ?decoder, "connecting (headless)");
     let handle = session::start(session::SessionParams {
         host,
         port,
@@ -171,6 +178,8 @@ fn run_headless_cli(args: &[String], identity: (String, String)) {
         gamepad: GamepadPref::Auto,
         bitrate_kbps,
         mic_enabled: flag("--mic"),
+        hdr_enabled: !flag("--no-hdr"),
+        decoder,
         pin,
         identity,
     });

clients/windows/src/present.rs

@@ -1,32 +1,41 @@
-//! Direct3D11 presenter for a WinUI 3 `SwapChainPanel`: upload a decoded `CpuFrame` (RGBA)
+//! Direct3D11 presenter for a WinUI 3 `SwapChainPanel`. It draws a decoded frame Contain-fit into a
-//! into a dynamic texture and draw it Contain-fit into a **composition** flip-model swapchain,
+//! **composition** flip-model swapchain, which the reactor stream page binds to the panel via
-//! which the reactor stream page binds to the panel via `SwapChainPanelHandle::set_swap_chain`.
+//! `SwapChainPanelHandle::set_swap_chain`.
 //!
-//! The device prefers a hardware adapter and falls back to **WARP** (the GPU-less dev box runs
+//! Two frame sources, one swapchain:
-//! the whole present path in software). The draw is a single full-screen triangle sampling the
-//! video texture; a letterbox is produced by clearing the back buffer black and setting the
-//! viewport to the Contain-fit rect (no per-frame vertex buffer).
 //!
-//! **HDR10**: when a frame is BT.2020 PQ (`CpuFrame::hdr`), the swapchain flips to
+//! * **GPU (zero-copy)** — [`crate::video::GpuFrame`] is a decoder-owned NV12/P010 `ID3D11Texture2D`
-//! `R10G10B10A2` + `DXGI_COLOR_SPACE_RGB_FULL_G2084_NONE_P2020` (+ HDR10 metadata) via
+//!   array slice (D3D11VA). We create per-plane shader-resource views over the slice and convert
-//! `ResizeBuffers`/`SetColorSpace1`; the decoded samples are already PQ-encoded so the shader is a
+//!   YUV→RGB in a pixel shader: NV12 via BT.709 (`ps_nv12`), P010 via BT.2020 with the PQ transfer
-//! plain passthrough and the compositor maps PQ→display. SDR stays 8-bit B8G8R8A8.
+//!   left intact (`ps_p010`). No CPU copy. The decoder uses the **same** shared device
+//!   ([`crate::gpu`]) so the texture is bindable here.
+//! * **CPU upload** — [`crate::video::CpuFrame`] is packed RGBA (SDR) or X2BGR10 (HDR) from the
+//!   software decoder; we upload it into a dynamic texture and draw it with a passthrough shader
+//!   (`ps_rgba`). The fallback path.
+//!
+//! **HDR10**: when a frame is BT.2020 PQ the swapchain flips to `R10G10B10A2` +
+//! `DXGI_COLOR_SPACE_RGB_FULL_G2084_NONE_P2020` (+ HDR10 metadata) via `ResizeBuffers`/
+//! `SetColorSpace1`; the shader output is already PQ-encoded so the compositor maps PQ→display. SDR
+//! stays 8-bit B8G8R8A8.
 //!
 //! All `windows` types here come from the same windows-rs commit as `windows-reactor`, so the
 //! `IDXGISwapChain1` handed to `set_swap_chain` satisfies reactor's `windows_core::Interface`.
 
-use crate::video::CpuFrame;
+use crate::video::{DecodedFrame, GpuFrame};
 use anyhow::{anyhow, Context, Result};
 use windows::core::{Interface, PCSTR};
 use windows::Win32::Graphics::Direct3D::Fxc::{D3DCompile, D3DCOMPILE_OPTIMIZATION_LEVEL3};
 use windows::Win32::Graphics::Direct3D::{
-    ID3DBlob, D3D_DRIVER_TYPE_HARDWARE, D3D_DRIVER_TYPE_WARP, D3D_FEATURE_LEVEL_11_0,
+    ID3DBlob, D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST, D3D_SRV_DIMENSION_TEXTURE2DARRAY,
-    D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST,
 };
 use windows::Win32::Graphics::Direct3D11::*;
 use windows::Win32::Graphics::Dxgi::Common::*;
 use windows::Win32::Graphics::Dxgi::*;
 
+// One vertex shader (fullscreen triangle) + three pixel shaders, selected per frame source. tex0 is
+// RGBA (passthrough) or the luma plane; tex1 is the chroma plane. The YUV→RGB matrices fold the
+// limited→full range scale into the coefficients; for P010 the R16 sample is rescaled (×65535/65472)
+// to undo the 10-bits-in-the-high-bits packing, then converted with BT.2020 NCL, PQ preserved.
 const SHADER_HLSL: &str = r#"
 struct VSOut { float4 pos : SV_Position; float2 uv : TEXCOORD0; };
 VSOut vs_main(uint vid : SV_VertexID) {
@@ -36,44 +45,104 @@ VSOut vs_main(uint vid : SV_VertexID) {
     o.uv = uv;
     return o;
 }
-Texture2D tex : register(t0);
+Texture2D tex0 : register(t0);
+Texture2D tex1 : register(t1);
 SamplerState smp : register(s0);
-float4 ps_main(VSOut i) : SV_Target { return tex.Sample(smp, i.uv); }
+
+float4 ps_rgba(VSOut i) : SV_Target { return tex0.Sample(smp, i.uv); }
+
+float4 ps_nv12(VSOut i) : SV_Target {
+    float  y  = tex0.Sample(smp, i.uv).r;
+    float2 uv = tex1.Sample(smp, i.uv).rg;
+    float yy = (y - 0.0627451) * 1.164384;   // (Y-16/255)*255/219
+    float u  = uv.x - 0.5;
+    float v  = uv.y - 0.5;                    // BT.709 limited, chroma scale folded
+    float r = yy + 1.792741 * v;
+    float g = yy - 0.213249 * u - 0.532909 * v;
+    float b = yy + 2.112402 * u;
+    return float4(saturate(float3(r, g, b)), 1.0);
+}
+
+float4 ps_p010(VSOut i) : SV_Target {
+    const float S = 65535.0 / 65472.0;       // undo P010 high-bit packing → exact 10-bit / 1023
+    float  y  = tex0.Sample(smp, i.uv).r  * S;
+    float2 uv = tex1.Sample(smp, i.uv).rg * S;
+    float yy = (y - 0.0625611) * 1.167808;   // (Y-64/1023)*1023/876
+    float u  = uv.x - 0.5;
+    float v  = uv.y - 0.5;                    // BT.2020 NCL limited, chroma scale folded; PQ kept
+    float r = yy + 1.683611 * v;
+    float g = yy - 0.187877 * u - 0.652337 * v;
+    float b = yy + 2.148072 * u;
+    return float4(saturate(float3(r, g, b)), 1.0);
+}
 "#;
 
+/// A bound GPU frame: per-plane SRVs over the decoder's texture-array slice, plus the `GpuFrame`
+/// itself kept alive so the decoder won't recycle the slice while we re-present it.
+struct GpuView {
+    y: ID3D11ShaderResourceView,
+    c: ID3D11ShaderResourceView,
+    frame: GpuFrame,
+}
+
+/// Current draw source.
+#[derive(Clone, Copy, PartialEq)]
+enum Mode {
+    Empty,
+    Rgba,
+    Nv12,
+    P010,
+}
+
 pub struct Presenter {
     device: ID3D11Device,
     context: ID3D11DeviceContext,
     vs: ID3D11VertexShader,
-    ps: ID3D11PixelShader,
+    ps_rgba: ID3D11PixelShader,
+    ps_nv12: ID3D11PixelShader,
+    ps_p010: ID3D11PixelShader,
     sampler: ID3D11SamplerState,
     swap: IDXGISwapChain1,
     rtv: Option<ID3D11RenderTargetView>,
-    /// Video texture + SRV + dimensions; recreated when the decoded size changes.
+    /// CPU-upload texture + SRV + dimensions; recreated when the decoded size/format changes.
-    tex: Option<(ID3D11Texture2D, ID3D11ShaderResourceView, u32, u32)>,
+    cpu_tex: Option<(ID3D11Texture2D, ID3D11ShaderResourceView, u32, u32)>,
+    /// Bound zero-copy GPU frame (held to keep its decoder surface alive).
+    gpu: Option<GpuView>,
+    mode: Mode,
+    /// Source frame dimensions, for the Contain-fit letterbox.
+    src_w: u32,
+    src_h: u32,
     /// Panel (swapchain) size in pixels, updated on resize.
     panel_w: u32,
     panel_h: u32,
-    /// Whether the swapchain is currently in 10-bit HDR10 (R10G10B10A2 + ST.2084) mode; flipped
+    /// Whether the swapchain is currently in 10-bit HDR10 (R10G10B10A2 + ST.2084) mode.
-    /// to match each frame's `hdr` flag.
     hdr: bool,
 }
 
 impl Presenter {
-    /// Create the D3D11 device + composition swapchain + shaders, sized to the panel.
+    /// Create the presenter on the process-wide shared D3D11 device (the one the decoder uses), plus
+    /// the composition swapchain + shaders, sized to the panel.
     pub fn new(width: u32, height: u32) -> Result<Presenter> {
-        let (device, context) = create_device()?;
+        let shared = crate::gpu::shared().ok_or_else(|| anyhow!("no shared D3D11 device"))?;
-        let (vs, ps, sampler) = build_pipeline(&device)?;
+        let device = shared.device.clone();
+        let context = shared.context.clone();
+        let (vs, ps_rgba, ps_nv12, ps_p010, sampler) = build_pipeline(&device)?;
         let swap = create_composition_swapchain(&device, width.max(1), height.max(1))?;
         Ok(Presenter {
             device,
             context,
             vs,
-            ps,
+            ps_rgba,
+            ps_nv12,
+            ps_p010,
             sampler,
             swap,
             rtv: None,
-            tex: None,
+            cpu_tex: None,
+            gpu: None,
+            mode: Mode::Empty,
+            src_w: 1,
+            src_h: 1,
             panel_w: width.max(1),
             panel_h: height.max(1),
             hdr: false,
@@ -104,31 +173,122 @@ impl Presenter {
         self.panel_h = height;
     }
 
-    /// Present one decoded frame (Contain-fit) — or, when `frame` is `None`, just re-present the
+    /// Present one decoded frame (Contain-fit) — or, when `frame` is `None`, re-present the last one
-    /// last texture (or black). Called from the reactor `on_rendering` per-frame callback.
+    /// (or black). Called from the reactor `on_rendering` per-frame callback on the UI thread. Takes
-    pub fn present(&mut self, frame: Option<&CpuFrame>) {
+    /// the frame by value so the GPU path can retain the decoder surface across re-presents.
-        if let Some(f) = frame {
+    pub fn present(&mut self, frame: Option<DecodedFrame>) {
-            if f.hdr != self.hdr {
+        match frame {
-                self.set_hdr(f.hdr);
+            Some(DecodedFrame::Cpu(c)) => {
+                if c.hdr != self.hdr {
+                    self.set_hdr(c.hdr);
                 }
-            if let Err(e) = self.upload(f) {
+                if let Err(e) = self.upload(&c) {
                     tracing::warn!(error = %e, "frame upload failed");
+                } else {
+                    self.mode = Mode::Rgba;
+                    self.src_w = c.width;
+                    self.src_h = c.height;
+                    self.gpu = None; // drop any held GPU frame
                 }
             }
+            Some(DecodedFrame::Gpu(g)) => {
+                if g.hdr != self.hdr {
+                    self.set_hdr(g.hdr);
+                }
+                match self.bind_gpu(g) {
+                    Ok(()) => {}
+                    Err(e) => tracing::warn!(error = %e, "GPU frame bind failed"),
+                }
+            }
+            None => {}
+        }
+        self.draw();
+    }
+
+    /// Build per-plane SRVs over the decoded texture-array slice and retain the frame.
+    fn bind_gpu(&mut self, g: GpuFrame) -> Result<()> {
+        let tex: ID3D11Texture2D = unsafe {
+            let raw = g.texture_ptr();
+            ID3D11Texture2D::from_raw_borrowed(&raw)
+                .ok_or_else(|| anyhow!("null D3D11 texture"))?
+                .clone()
+        };
+        // NV12: R8 luma + R8G8 chroma. P010: R16 luma + R16G16 chroma (10 bits in the high bits).
+        let (fy, fc) = if g.hdr {
+            (DXGI_FORMAT_R16_UNORM, DXGI_FORMAT_R16G16_UNORM)
+        } else {
+            (DXGI_FORMAT_R8_UNORM, DXGI_FORMAT_R8G8_UNORM)
+        };
+        let y = self.array_srv(&tex, fy, g.index)?;
+        let c = self.array_srv(&tex, fc, g.index)?;
+        self.mode = if g.hdr { Mode::P010 } else { Mode::Nv12 };
+        self.src_w = g.width;
+        self.src_h = g.height;
+        self.gpu = Some(GpuView { y, c, frame: g });
+        Ok(())
+    }
+
+    /// A shader-resource view over a single slice of a texture array, reinterpreting the plane
+    /// format (the NV12/P010 sub-format trick D3D11 allows on video textures).
+    fn array_srv(
+        &self,
+        tex: &ID3D11Texture2D,
+        format: DXGI_FORMAT,
+        slice: u32,
+    ) -> Result<ID3D11ShaderResourceView> {
+        let desc = D3D11_SHADER_RESOURCE_VIEW_DESC {
+            Format: format,
+            ViewDimension: D3D_SRV_DIMENSION_TEXTURE2DARRAY,
+            Anonymous: D3D11_SHADER_RESOURCE_VIEW_DESC_0 {
+                Texture2DArray: D3D11_TEX2D_ARRAY_SRV {
+                    MostDetailedMip: 0,
+                    MipLevels: 1,
+                    FirstArraySlice: slice,
+                    ArraySize: 1,
+                },
+            },
+        };
+        unsafe {
+            let mut srv = None;
+            self.device
+                .CreateShaderResourceView(tex, Some(&desc), Some(&mut srv))
+                .context("CreateShaderResourceView (array slice)")?;
+            srv.ok_or_else(|| anyhow!("null SRV"))
+        }
+    }
+
+    fn draw(&mut self) {
         let Ok(rtv) = self.rtv() else {
             return;
         };
         let (pw, ph) = (self.panel_w, self.panel_h);
+        // Resolve the current source's shader + the (up to two) SRVs to bind — cheap interface
+        // clones. Each arm yields `Option<(&pixel_shader, [Option<SRV>; 2])>`.
+        let binding = match self.mode {
+            Mode::Rgba => self
+                .cpu_tex
+                .as_ref()
+                .map(|(_, srv, _, _)| (&self.ps_rgba, [Some(srv.clone()), None])),
+            Mode::Nv12 => self
+                .gpu
+                .as_ref()
+                .map(|g| (&self.ps_nv12, [Some(g.y.clone()), Some(g.c.clone())])),
+            Mode::P010 => self
+                .gpu
+                .as_ref()
+                .map(|g| (&self.ps_p010, [Some(g.y.clone()), Some(g.c.clone())])),
+            Mode::Empty => None,
+        };
         unsafe {
             let c = &self.context;
             c.ClearRenderTargetView(&rtv, &[0.0, 0.0, 0.0, 1.0]);
-            if let Some((_, srv, vw, vh)) = &self.tex {
+            if let Some((ps, srvs)) = binding {
                 // Contain-fit viewport: scale to the smaller axis, centre, letterbox the rest.
                 let (ww, wh, vfw, vfh) = (
                     pw as f32,
                     ph as f32,
-                    (*vw).max(1) as f32,
+                    self.src_w.max(1) as f32,
-                    (*vh).max(1) as f32,
+                    self.src_h.max(1) as f32,
                 );
                 let scale = (ww / vfw).min(wh / vfh);
                 let (dw, dh) = (vfw * scale, vfh * scale);
@@ -146,8 +306,8 @@ impl Presenter {
                 c.IASetInputLayout(None);
                 c.IASetPrimitiveTopology(D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST);
                 c.VSSetShader(&self.vs, None);
-                c.PSSetShader(&self.ps, None);
+                c.PSSetShader(ps, None);
-                c.PSSetShaderResources(0, Some(&[Some(srv.clone())]));
+                c.PSSetShaderResources(0, Some(&srvs));
                 c.PSSetSamplers(0, Some(&[Some(self.sampler.clone())]));
                 c.Draw(3, 0);
             }
@@ -155,14 +315,13 @@ impl Presenter {
         }
     }
 
-    /// Switch the swapchain between 8-bit SDR (B8G8R8A8, sRGB/BT.709) and 10-bit HDR10
+    /// Switch the swapchain between 8-bit SDR (B8G8R8A8, BT.709) and 10-bit HDR10 (R10G10B10A2,
-    /// (R10G10B10A2, ST.2084 PQ BT.2020). `ResizeBuffers` can change the back-buffer format in
+    /// ST.2084 PQ BT.2020). `ResizeBuffers` changes the back-buffer format in place, so the panel
-    /// place, so the panel binding (`set_swap_chain`) stays valid — no rebind needed. The decoded
+    /// binding (`set_swap_chain`) stays valid — no rebind. Both frame sources already produce
-    /// samples are already PQ-encoded BT.2020 (see `video::convert`), so the colour space is all the
+    /// PQ-encoded BT.2020 for HDR, so the colour space is all the compositor needs.
-    /// compositor needs to map them to the display.
     fn set_hdr(&mut self, on: bool) {
         self.rtv = None; // release back-buffer refs before ResizeBuffers
-        self.tex = None; // texture format changes (R10G10B10A2 vs R8G8B8A8)
+        self.cpu_tex = None; // CPU texture format changes (R10G10B10A2 vs R8G8B8A8)
         let format = if on {
             DXGI_FORMAT_R10G10B10A2_UNORM
         } else {
@@ -208,9 +367,9 @@ impl Presenter {
         tracing::info!(hdr = on, "swapchain colour mode switched");
     }
 
-    fn upload(&mut self, frame: &CpuFrame) -> Result<()> {
+    fn upload(&mut self, frame: &crate::video::CpuFrame) -> Result<()> {
         let (w, h) = (frame.width, frame.height);
-        let need_new = !matches!(&self.tex, Some((_, _, tw, th)) if *tw == w && *th == h);
+        let need_new = !matches!(&self.cpu_tex, Some((_, _, tw, th)) if *tw == w && *th == h);
         if need_new {
             let format = if self.hdr {
                 DXGI_FORMAT_R10G10B10A2_UNORM
@@ -246,9 +405,9 @@ impl Presenter {
                     .context("CreateShaderResourceView")?;
                 s.unwrap()
             };
-            self.tex = Some((texture, srv, w, h));
+            self.cpu_tex = Some((texture, srv, w, h));
         }
-        let (texture, _, _, _) = self.tex.as_ref().unwrap();
+        let (texture, _, _, _) = self.cpu_tex.as_ref().unwrap();
         unsafe {
             let mut mapped = D3D11_MAPPED_SUBRESOURCE::default();
             self.context
@@ -286,38 +445,6 @@ impl Presenter {
     }
 }
 
-fn create_device() -> Result<(ID3D11Device, ID3D11DeviceContext)> {
-    for driver in [D3D_DRIVER_TYPE_HARDWARE, D3D_DRIVER_TYPE_WARP] {
-        let mut device = None;
-        let mut context = None;
-        let r = unsafe {
-            D3D11CreateDevice(
-                None,
-                driver,
-                None,
-                D3D11_CREATE_DEVICE_BGRA_SUPPORT,
-                Some(&[D3D_FEATURE_LEVEL_11_0]),
-                D3D11_SDK_VERSION,
-                Some(&mut device),
-                None,
-                Some(&mut context),
-            )
-        };
-        if r.is_ok() {
-            let name = if driver == D3D_DRIVER_TYPE_HARDWARE {
-                "hardware"
-            } else {
-                "WARP (software)"
-            };
-            tracing::info!(driver = name, "D3D11 device created");
-            return Ok((device.unwrap(), context.unwrap()));
-        }
-    }
-    Err(anyhow!(
-        "D3D11CreateDevice failed for both hardware and WARP"
-    ))
-}
-
 /// A composition flip-model swapchain (no HWND) for binding to a XAML `SwapChainPanel`.
 fn create_composition_swapchain(
     device: &ID3D11Device,
@@ -357,18 +484,34 @@ fn create_composition_swapchain(
 
 fn build_pipeline(
     device: &ID3D11Device,
-) -> Result<(ID3D11VertexShader, ID3D11PixelShader, ID3D11SamplerState)> {
+) -> Result<(
+    ID3D11VertexShader,
+    ID3D11PixelShader,
+    ID3D11PixelShader,
+    ID3D11PixelShader,
+    ID3D11SamplerState,
+)> {
     let vs_blob = compile(SHADER_HLSL, "vs_main", "vs_5_0")?;
-    let ps_blob = compile(SHADER_HLSL, "ps_main", "ps_5_0")?;
+    let rgba_blob = compile(SHADER_HLSL, "ps_rgba", "ps_5_0")?;
+    let nv12_blob = compile(SHADER_HLSL, "ps_nv12", "ps_5_0")?;
+    let p010_blob = compile(SHADER_HLSL, "ps_p010", "ps_5_0")?;
     unsafe {
         let mut vs = None;
         device
             .CreateVertexShader(blob_bytes(&vs_blob), None, Some(&mut vs))
             .context("CreateVertexShader")?;
-        let mut ps = None;
+        let mut ps_rgba = None;
         device
-            .CreatePixelShader(blob_bytes(&ps_blob), None, Some(&mut ps))
+            .CreatePixelShader(blob_bytes(&rgba_blob), None, Some(&mut ps_rgba))
-            .context("CreatePixelShader")?;
+            .context("CreatePixelShader (rgba)")?;
+        let mut ps_nv12 = None;
+        device
+            .CreatePixelShader(blob_bytes(&nv12_blob), None, Some(&mut ps_nv12))
+            .context("CreatePixelShader (nv12)")?;
+        let mut ps_p010 = None;
+        device
+            .CreatePixelShader(blob_bytes(&p010_blob), None, Some(&mut ps_p010))
+            .context("CreatePixelShader (p010)")?;
         let sdesc = D3D11_SAMPLER_DESC {
             Filter: D3D11_FILTER_MIN_MAG_MIP_LINEAR,
             AddressU: D3D11_TEXTURE_ADDRESS_CLAMP,
@@ -381,7 +524,13 @@ fn build_pipeline(
         device
             .CreateSamplerState(&sdesc, Some(&mut sampler))
             .context("CreateSamplerState")?;
-        Ok((vs.unwrap(), ps.unwrap(), sampler.unwrap()))
+        Ok((
+            vs.unwrap(),
+            ps_rgba.unwrap(),
+            ps_nv12.unwrap(),
+            ps_p010.unwrap(),
+            sampler.unwrap(),
+        ))
     }
 }
 
@@ -427,9 +576,9 @@ fn blob_bytes(blob: &ID3DBlob) -> &[u8] {
     }
 }
 
-/// Generic HDR10 mastering metadata: BT.2020 primaries + D65 white (0.00002 units), a 1000-nit
+/// Generic HDR10 mastering metadata: BT.2020 primaries + D65 white, a 1000-nit mastering display,
-/// mastering display, MaxCLL 1000 / MaxFALL 400. The protocol doesn't carry the stream's real
+/// MaxCLL 1000 / MaxFALL 400. The protocol doesn't carry the stream's real mastering metadata yet
-/// mastering metadata yet (host follow-up), so these are sane defaults the display tone-maps from.
+/// (host follow-up), so these are sane defaults the display tone-maps from.
 fn hdr10_metadata() -> DXGI_HDR_METADATA_HDR10 {
     DXGI_HDR_METADATA_HDR10 {
         RedPrimary: [35400, 14600],

clients/windows/src/session.rs

@@ -8,7 +8,7 @@
 //! (software-only here) and the audio backend (WASAPI). The pump body is identical.
 
 use crate::audio;
-use crate::video::{DecodedFrame, Decoder};
+use crate::video::{DecodedFrame, Decoder, DecoderPref};
 use punktfunk_core::client::NativeClient;
 use punktfunk_core::config::{CompositorPref, GamepadPref, Mode};
 use punktfunk_core::PunktfunkError;
@@ -25,6 +25,10 @@ pub struct SessionParams {
     pub bitrate_kbps: u32,
     /// Stream the default microphone to the host's virtual mic source.
     pub mic_enabled: bool,
+    /// Advertise 10-bit + HDR10 so the host may upgrade HDR content to a Main10/PQ stream.
+    pub hdr_enabled: bool,
+    /// Which video decode backend to use (auto/hardware/software).
+    pub decoder: DecoderPref,
     /// Pinned host fingerprint; `None` = trust on first use (caller persists the observed one).
     pub pin: Option<[u8; 32]>,
     pub identity: (String, String),
@@ -37,6 +41,10 @@ pub struct Stats {
     pub decode_ms: f32,
     /// Median capture→decoded latency over the last window (host-clock corrected).
     pub latency_ms: f32,
+    /// True when decoding on the GPU (D3D11VA zero-copy) vs. CPU (software).
+    pub hardware: bool,
+    /// True when the stream is BT.2020 PQ HDR10 (last decoded frame).
+    pub hdr: bool,
 }
 
 pub enum SessionEvent {
@@ -99,10 +107,15 @@ fn pump(
         params.compositor,
         params.gamepad,
         params.bitrate_kbps,
-        // Advertise 10-bit + HDR10: the presenter handles BT.2020 PQ (R10G10B10A2) frames, so the
+        // Advertise 10-bit + HDR10 (when enabled): the presenter handles BT.2020 PQ frames (P010 on
-        // host may upgrade HDR content to a Main10/PQ stream (it still only does so for actual HDR
+        // the GPU path, X2BGR10 on software), so the host may upgrade HDR content to a Main10/PQ
-        // content with its own 10-bit gate). 8-bit SDR is unaffected.
+        // stream — it still only does so for actual HDR content with its own 10-bit gate. 8-bit SDR
-        punktfunk_core::quic::VIDEO_CAP_10BIT | punktfunk_core::quic::VIDEO_CAP_HDR,
+        // is unaffected. A client that turns HDR off advertises `0` and always gets the 8-bit stream.
+        if params.hdr_enabled {
+            punktfunk_core::quic::VIDEO_CAP_10BIT | punktfunk_core::quic::VIDEO_CAP_HDR
+        } else {
+            0
+        },
         None, // launch: the Windows client has no library picker yet
         params.pin,
         Some(params.identity),
@@ -132,13 +145,15 @@ fn pump(
         fingerprint: connector.host_fingerprint,
     });
 
-    let mut decoder = match Decoder::new() {
+    let mut decoder = match Decoder::new(params.decoder) {
         Ok(d) => d,
         Err(e) => {
             let _ = ev_tx.send_blocking(SessionEvent::Ended(Some(format!("video decoder: {e}"))));
             return;
         }
     };
+    let mut hardware = decoder.is_hardware();
+    let mut hdr = false;
     // Audio is best-effort: a session without it still streams. Gamepads are the
     // app-lifetime service's job (the UI attaches it on Connected).
     let player = audio::AudioPlayer::spawn()
@@ -178,12 +193,16 @@ fn pump(
                 match decoder.decode(&frame.data) {
                     Ok(Some(decoded)) => {
                         total_frames += 1;
+                        hdr = decoded.hdr();
+                        // The backend can demote D3D11VA → software mid-session on a hardware error.
+                        hardware = decoder.is_hardware();
                         if total_frames == 1 {
-                            let DecodedFrame::Cpu(c) = &decoded;
+                            let (w, h) = decoded.dims();
                             tracing::info!(
-                                width = c.width,
+                                width = w,
-                                height = c.height,
+                                height = h,
-                                path = "software",
+                                path = if hardware { "d3d11va" } else { "software" },
+                                hdr,
                                 "first frame decoded"
                             );
                         }
@@ -253,6 +272,8 @@ fn pump(
                     0.0
                 },
                 latency_ms: p50 as f32 / 1000.0,
+                hardware,
+                hdr,
             }));
             window_start = Instant::now();
             frames_n = 0;

clients/windows/src/trust.rs

@@ -130,6 +130,11 @@ pub struct Settings {
     pub inhibit_shortcuts: bool,
     /// Stream the default microphone to the host's virtual mic source.
     pub mic_enabled: bool,
+    /// Advertise 10-bit + HDR10 so the host upgrades HDR content to a Main10/PQ stream (the client
+    /// presents it on a 10-bit ST.2084 swapchain). No effect on SDR content.
+    pub hdr_enabled: bool,
+    /// Video decode backend: `auto` (D3D11VA, fall back to software), `hardware`, or `software`.
+    pub decoder: String,
 }
 
 impl Default for Settings {
@@ -143,6 +148,8 @@ impl Default for Settings {
             compositor: "auto".into(),
             inhibit_shortcuts: true,
             mic_enabled: false,
+            hdr_enabled: true,
+            decoder: "auto".into(),
         }
     }
 }

clients/windows/src/video.rs

@@ -1,27 +1,76 @@
 //! Video decode: reassembled HEVC access units → frames for the D3D11 presenter.
 //!
-//! The dev box has no working GPU, so this ships the **software** backend first: libavcodec
+//! Two backends, picked at session start (override via [`DecoderPref`] / the Settings UI):
-//! on the CPU + swscale to RGBA, uploaded into a D3D11 texture by the presenter. It runs
-//! `AV_CODEC_FLAG_LOW_DELAY` with slice threading only — the host encodes zero-reorder
-//! streams (no B-frames, in-band parameter sets on every IDR), so decode is strictly
-//! one-in/one-out and frame threading would only add latency.
 //!
-//! `DecodedFrame` is an enum so the real-GPU **D3D11VA** path (decode → `NV12`/`P010`
+//! * **D3D11VA** (any GPU): libavcodec decodes on the GPU straight into `ID3D11Texture2D`s that
-//! `ID3D11Texture2D`, zero-copy into the swapchain) can be added as a second variant without
+//!   carry `D3D11_BIND_SHADER_RESOURCE`, so the presenter samples the decoded NV12/P010 surface
-//! touching the session pump or the presenter's frame contract.
+//!   directly — **zero copy** (no swscale, no CPU readback, no per-frame upload). The textures are
+//!   created by the process-wide shared device ([`crate::gpu`]) the presenter also draws with, which
+//!   is what makes them bindable there. This is the big latency/throughput win over software decode.
+//! * **Software**: libavcodec on the CPU + swscale to a packed 4-byte format the presenter uploads
+//!   (`RGBA` for SDR, `X2BGR10` for HDR). The fallback on a GPU-less box (WARP), when D3D11VA init
+//!   fails, or when a mid-session hardware error demotes us — the host's IDR/RFI recovery
+//!   resynchronizes on the next keyframe either way.
+//!
+//! Both run `AV_CODEC_FLAG_LOW_DELAY`; the host encodes zero-reorder streams (no B-frames, in-band
+//! parameter sets on every IDR), so decode is strictly one-in/one-out.
+//!
+//! HDR is detected in-band from the decoded frame's transfer characteristic (`SMPTE2084` / PQ in the
+//! HEVC VUI) — the same signal every other punktfunk client keys off — not from a protocol field.
 
-use anyhow::{anyhow, Context as _, Result};
+use anyhow::{anyhow, bail, Context as _, Result};
 use ffmpeg::format::Pixel;
 use ffmpeg::software::scaling;
 use ffmpeg::util::frame::Video as AvFrame;
 use ffmpeg_next as ffmpeg;
+use std::ffi::c_void;
+use std::ptr;
+use windows::core::Interface; // ID3D11Device::clone().into_raw() for the FFmpeg hwdevice ctx
+
+/// Which decode backend to use; the Settings UI persists this as a string.
+#[derive(Clone, Copy, PartialEq, Eq, Debug, Default)]
+pub enum DecoderPref {
+    /// Try D3D11VA, fall back to software.
+    #[default]
+    Auto,
+    /// Force D3D11VA (error out if unavailable, for debugging).
+    Hardware,
+    /// Force software decode.
+    Software,
+}
+
+impl DecoderPref {
+    pub fn from_name(s: &str) -> DecoderPref {
+        match s {
+            "hardware" => DecoderPref::Hardware,
+            "software" => DecoderPref::Software,
+            _ => DecoderPref::Auto,
+        }
+    }
+}
 
 pub enum DecodedFrame {
     Cpu(CpuFrame),
+    Gpu(GpuFrame),
 }
 
-/// Packed 4-byte-per-pixel frame for a D3D11 texture upload (which takes a row pitch). The bytes
+impl DecodedFrame {
-/// are `R8G8B8A8` for SDR and `X2BGR10` (== DXGI `R10G10B10A2`, R in the low 10 bits) for HDR.
+    pub fn dims(&self) -> (u32, u32) {
+        match self {
+            DecodedFrame::Cpu(c) => (c.width, c.height),
+            DecodedFrame::Gpu(g) => (g.width, g.height),
+        }
+    }
+    pub fn hdr(&self) -> bool {
+        match self {
+            DecodedFrame::Cpu(c) => c.hdr,
+            DecodedFrame::Gpu(g) => g.hdr,
+        }
+    }
+}
+
+/// Packed 4-byte-per-pixel frame for a D3D11 dynamic-texture upload (which takes a row pitch). The
+/// bytes are `R8G8B8A8` for SDR and `X2BGR10` (== DXGI `R10G10B10A2`, R in the low 10 bits) for HDR.
 pub struct CpuFrame {
     pub width: u32,
     pub height: u32,
@@ -33,25 +82,100 @@ pub struct CpuFrame {
     pub hdr: bool,
 }
 
+/// A decoded frame still on the GPU: a D3D11 texture **array** plus the slice index the decoder
+/// wrote this frame into. The presenter creates per-plane shader-resource views over the slice and
+/// converts YUV→RGB in a pixel shader. The underlying surface stays alive — and out of the decoder's
+/// reuse pool — for exactly as long as `guard` (an `av_frame_clone` of the decoded frame) lives.
+pub struct GpuFrame {
+    pub width: u32,
+    pub height: u32,
+    /// Texture-array slice this frame occupies (`AVFrame::data[1]`).
+    pub index: u32,
+    /// BT.2020 PQ HDR10 (P010, ST.2084) vs ordinary 8-bit BT.709 SDR (NV12).
+    pub hdr: bool,
+    /// 10-bit (P010, R16 planes) vs 8-bit (NV12, R8 planes) — kept for the first-frame log; the
+    /// present path keys colour/format off `hdr` (the host couples 10-bit ⟺ HDR).
+    pub ten_bit: bool,
+    guard: D3d11FrameGuard,
+}
+
+impl GpuFrame {
+    /// The decoder's D3D11 texture array holding this frame's slice, borrowed from the live cloned
+    /// `AVFrame`. Construct the windows-rs interface on the thread that will use it (the presenter /
+    /// UI thread): COM interfaces are `!Send`, but the raw pointer is fine to carry across threads.
+    pub fn texture_ptr(&self) -> *mut c_void {
+        unsafe { (*self.guard.0).data[0] as *mut c_void }
+    }
+}
+
+/// Owns a cloned decoded `AVFrame` (which refs the D3D11 surface in the decoder pool). Dropping it
+/// releases the surface back for reuse. The clone is plain refcounted data; freeing it from the
+/// presenter thread is fine.
+pub struct D3d11FrameGuard(*mut ffmpeg::ffi::AVFrame);
+unsafe impl Send for D3d11FrameGuard {}
+impl Drop for D3d11FrameGuard {
+    fn drop(&mut self) {
+        unsafe { ffmpeg::ffi::av_frame_free(&mut self.0) };
+    }
+}
+
+enum Backend {
+    D3d11va(D3d11vaDecoder),
+    Software(SoftwareDecoder),
+}
+
 pub struct Decoder {
-    inner: SoftwareDecoder,
+    backend: Backend,
 }
 
 impl Decoder {
-    pub fn new() -> Result<Decoder> {
+    pub fn new(pref: DecoderPref) -> Result<Decoder> {
         ffmpeg::init().context("ffmpeg init")?;
+        if pref != DecoderPref::Software {
+            match D3d11vaDecoder::new() {
+                Ok(d) => {
+                    tracing::info!("D3D11VA hardware decode active (zero-copy)");
+                    return Ok(Decoder {
+                        backend: Backend::D3d11va(d),
+                    });
+                }
+                Err(e) => {
+                    if pref == DecoderPref::Hardware {
+                        return Err(e.context("decoder=hardware but D3D11VA failed"));
+                    }
+                    tracing::info!(reason = %e, "D3D11VA unavailable — software decode");
+                }
+            }
+        }
         Ok(Decoder {
-            inner: SoftwareDecoder::new()?,
+            backend: Backend::Software(SoftwareDecoder::new()?),
         })
     }
 
-    /// Feed one access unit; returns the decoded frame (the host's streams are
+    /// True for the zero-copy hardware backend (shown in the stream HUD).
-    /// one-in/one-out). A decode error after packet loss is survivable — log upstream and
+    pub fn is_hardware(&self) -> bool {
-    /// keep feeding; the host's IDR/RFI recovery resynchronizes on the next keyframe.
+        matches!(self.backend, Backend::D3d11va(_))
+    }
+
+    /// Feed one access unit; returns the decoded frame (the host's streams are one-in/one-out). A
+    /// software decode error after packet loss is survivable — keep feeding. A D3D11VA error demotes
+    /// to software for the rest of the session (the next IDR resynchronizes).
     pub fn decode(&mut self, au: &[u8]) -> Result<Option<DecodedFrame>> {
-        Ok(self.inner.decode(au)?.map(DecodedFrame::Cpu))
+        match &mut self.backend {
+            Backend::D3d11va(d) => match d.decode(au) {
+                Ok(f) => Ok(f.map(DecodedFrame::Gpu)),
+                Err(e) => {
+                    tracing::warn!(error = %e, "D3D11VA decode failed — falling back to software");
+                    self.backend = Backend::Software(SoftwareDecoder::new()?);
+                    Ok(None)
+                }
+            },
+            Backend::Software(s) => Ok(s.decode(au)?.map(DecodedFrame::Cpu)),
         }
     }
+}
+
+// --- software backend ---------------------------------------------------------------
 
 struct SoftwareDecoder {
     decoder: ffmpeg::decoder::Video,
@@ -90,10 +214,9 @@ impl SoftwareDecoder {
     }
 
     /// Convert the decoded YUV frame to a packed 4-byte format the presenter uploads directly:
-    /// SDR → `RGBA` (BT.709), HDR (SMPTE ST.2084 / PQ transfer) → `X2BGR10` (10-bit, == DXGI
+    /// SDR → `RGBA` (BT.709), HDR (SMPTE ST.2084 / PQ transfer) → `X2BGR10` (== DXGI R10G10B10A2)
-    /// R10G10B10A2) using the BT.2020 matrix. For HDR the PQ-encoded values pass through unchanged
+    /// using the BT.2020 matrix. For HDR the PQ-encoded values pass through unchanged (swscale only
-    /// (swscale only applies the YUV→RGB matrix + range, never the transfer) — exactly what an
+    /// applies the YUV→RGB matrix + range, never the transfer) — exactly what an HDR10 swapchain wants.
-    /// HDR10/ST.2084 swapchain wants.
     fn convert(&mut self, frame: &AvFrame) -> Result<CpuFrame> {
         use ffmpeg::color::TransferCharacteristic;
         let (fmt, w, h) = (frame.format(), frame.width(), frame.height());
@@ -134,3 +257,255 @@ impl SoftwareDecoder {
         })
     }
 }
+
+// --- D3D11VA backend ------------------------------------------------------------------
+//
+// Raw FFI: ffmpeg-next has no hwaccel wrappers. The COM-typed hwcontext structs are declared here
+// (stable FFmpeg public ABI) rather than relied on from ffmpeg-sys bindgen — the generic
+// AVHWDeviceContext / AVHWFramesContext (whose payload is an opaque `void *hwctx`) come from
+// ffmpeg-sys, and we cast `hwctx` to the structs below. All owned pointers are freed in Drop;
+// decoded surfaces transfer out through D3d11FrameGuard.
+
+const AVERROR_EAGAIN: i32 = -11; // -EAGAIN
+const D3D11_BIND_SHADER_RESOURCE: u32 = 0x8; // <d3d11.h>; FFmpeg ORs D3D11_BIND_DECODER itself
+
+/// `hwcontext_d3d11va.h` — `AVHWDeviceContext::hwctx`. Leaving `lock` null makes FFmpeg install an
+/// `ID3D11Multithread` default lock + set multithread protection on `device_context` during init,
+/// which is what lets the presenter share this device's immediate context from the UI thread.
+#[repr(C)]
+struct AVD3D11VADeviceContext {
+    device: *mut c_void,         // ID3D11Device*
+    device_context: *mut c_void, // ID3D11DeviceContext*
+    video_device: *mut c_void,   // ID3D11VideoDevice*
+    video_context: *mut c_void,  // ID3D11VideoContext*
+    lock: *mut c_void,           // void (*)(void*)
+    unlock: *mut c_void,         // void (*)(void*)
+    lock_ctx: *mut c_void,
+}
+
+/// `hwcontext_d3d11va.h` — `AVHWFramesContext::hwctx`. `BindFlags` lets us add
+/// `D3D11_BIND_SHADER_RESOURCE` so the decoded array texture is sampleable (zero copy).
+#[repr(C)]
+struct AVD3D11VAFramesContext {
+    texture: *mut c_void, // ID3D11Texture2D* (null → FFmpeg allocates the pool)
+    bind_flags: u32,      // UINT BindFlags
+    misc_flags: u32,      // UINT MiscFlags
+}
+
+fn averr(what: &str, code: i32) -> anyhow::Error {
+    anyhow!("{what}: {}", ffmpeg::Error::from(code))
+}
+
+/// libavcodec's `get_format` callback: accept the D3D11 hw surface, building a frames context whose
+/// textures carry `BIND_SHADER_RESOURCE` (so the presenter can sample them). Returning anything but
+/// `AV_PIX_FMT_D3D11` aborts hardware decode → the session demotes to software.
+unsafe extern "C" fn get_format_d3d11(
+    avctx: *mut ffmpeg::ffi::AVCodecContext,
+    mut list: *const ffmpeg::ffi::AVPixelFormat,
+) -> ffmpeg::ffi::AVPixelFormat {
+    use ffmpeg::ffi::*;
+    unsafe {
+        let mut found = false;
+        while *list != AVPixelFormat::AV_PIX_FMT_NONE {
+            if *list == AVPixelFormat::AV_PIX_FMT_D3D11 {
+                found = true;
+                break;
+            }
+            list = list.add(1);
+        }
+        if !found {
+            return AVPixelFormat::AV_PIX_FMT_NONE;
+        }
+        let device_ref = (*avctx).hw_device_ctx;
+        if device_ref.is_null() {
+            return AVPixelFormat::AV_PIX_FMT_NONE;
+        }
+        let frames_ref = av_hwframe_ctx_alloc(device_ref);
+        if frames_ref.is_null() {
+            return AVPixelFormat::AV_PIX_FMT_NONE;
+        }
+        let frames = (*frames_ref).data as *mut AVHWFramesContext;
+        (*frames).format = AVPixelFormat::AV_PIX_FMT_D3D11;
+        let sw = if (*avctx).sw_pix_fmt != AVPixelFormat::AV_PIX_FMT_NONE {
+            (*avctx).sw_pix_fmt
+        } else {
+            AVPixelFormat::AV_PIX_FMT_NV12
+        };
+        (*frames).sw_format = sw;
+        (*frames).width = (*avctx).coded_width;
+        (*frames).height = (*avctx).coded_height;
+        // DPB + a few in-flight (decoded channel + the presenter's held frame); the host's
+        // zero-reorder stream needs only a small DPB, so 20 is comfortable headroom.
+        (*frames).initial_pool_size = 20;
+        let fhw = (*frames).hwctx as *mut AVD3D11VAFramesContext;
+        (*fhw).bind_flags = D3D11_BIND_SHADER_RESOURCE;
+        let r = av_hwframe_ctx_init(frames_ref);
+        if r < 0 {
+            let mut fr = frames_ref;
+            av_buffer_unref(&mut fr);
+            return AVPixelFormat::AV_PIX_FMT_NONE;
+        }
+        (*avctx).hw_frames_ctx = frames_ref; // decoder takes ownership
+        AVPixelFormat::AV_PIX_FMT_D3D11
+    }
+}
+
+struct D3d11vaDecoder {
+    ctx: *mut ffmpeg::ffi::AVCodecContext,
+    hw_device: *mut ffmpeg::ffi::AVBufferRef,
+    packet: *mut ffmpeg::ffi::AVPacket,
+    frame: *mut ffmpeg::ffi::AVFrame,
+}
+
+// Single-owner pointers, only touched from the session pump thread.
+unsafe impl Send for D3d11vaDecoder {}
+
+impl D3d11vaDecoder {
+    fn new() -> Result<D3d11vaDecoder> {
+        use ffmpeg::ffi;
+        let shared = crate::gpu::shared().ok_or_else(|| anyhow!("no shared D3D11 device"))?;
+        if !shared.hardware {
+            bail!("shared device is WARP (no hardware video decode)");
+        }
+        unsafe {
+            // Build a D3D11VA hwdevice context around the *shared* device, so decoded textures live
+            // on the same device the presenter samples + draws with.
+            let hw_device =
+                ffi::av_hwdevice_ctx_alloc(ffi::AVHWDeviceType::AV_HWDEVICE_TYPE_D3D11VA);
+            if hw_device.is_null() {
+                bail!("av_hwdevice_ctx_alloc(D3D11VA) failed");
+            }
+            let devctx = (*hw_device).data as *mut ffi::AVHWDeviceContext;
+            let d3dctx = (*devctx).hwctx as *mut AVD3D11VADeviceContext;
+            // Hand FFmpeg an owned ref to the device + immediate context (it Releases them when the
+            // hwdevice ctx is freed). `into_raw()` transfers a +1 ref without releasing.
+            (*d3dctx).device = shared.device.clone().into_raw();
+            (*d3dctx).device_context = shared.context.clone().into_raw();
+            // lock left null → FFmpeg installs the ID3D11Multithread default lock in init.
+            let r = ffi::av_hwdevice_ctx_init(hw_device);
+            if r < 0 {
+                let mut hw = hw_device;
+                ffi::av_buffer_unref(&mut hw);
+                bail!("av_hwdevice_ctx_init: {}", ffmpeg::Error::from(r));
+            }
+
+            let codec = ffi::avcodec_find_decoder(ffi::AVCodecID::AV_CODEC_ID_HEVC);
+            if codec.is_null() {
+                let mut hw = hw_device;
+                ffi::av_buffer_unref(&mut hw);
+                bail!("no HEVC decoder");
+            }
+            let ctx = ffi::avcodec_alloc_context3(codec);
+            (*ctx).hw_device_ctx = ffi::av_buffer_ref(hw_device);
+            (*ctx).get_format = Some(get_format_d3d11);
+            (*ctx).flags |= ffi::AV_CODEC_FLAG_LOW_DELAY as i32;
+            (*ctx).thread_count = 1; // hwaccel: threads only add latency
+            let r = ffi::avcodec_open2(ctx, codec, ptr::null_mut());
+            if r < 0 {
+                let mut ctx = ctx;
+                ffi::avcodec_free_context(&mut ctx);
+                let mut hw = hw_device;
+                ffi::av_buffer_unref(&mut hw);
+                bail!("avcodec_open2 (D3D11VA): {}", ffmpeg::Error::from(r));
+            }
+            Ok(D3d11vaDecoder {
+                ctx,
+                hw_device,
+                packet: ffi::av_packet_alloc(),
+                frame: ffi::av_frame_alloc(),
+            })
+        }
+    }
+
+    fn decode(&mut self, au: &[u8]) -> Result<Option<GpuFrame>> {
+        use ffmpeg::ffi;
+        unsafe {
+            let r = ffi::av_new_packet(self.packet, au.len() as i32);
+            if r < 0 {
+                return Err(averr("av_new_packet", r));
+            }
+            ptr::copy_nonoverlapping(au.as_ptr(), (*self.packet).data, au.len());
+            let r = ffi::avcodec_send_packet(self.ctx, self.packet);
+            ffi::av_packet_unref(self.packet);
+            if r < 0 {
+                return Err(averr("send_packet", r));
+            }
+            let mut out = None;
+            loop {
+                let r = ffi::avcodec_receive_frame(self.ctx, self.frame);
+                if r == AVERROR_EAGAIN {
+                    break;
+                }
+                if r < 0 {
+                    return Err(averr("receive_frame", r));
+                }
+                out = Some(self.lift()?); // newest wins; older guards drop here
+                ffi::av_frame_unref(self.frame);
+            }
+            Ok(out)
+        }
+    }
+
+    /// Lift the decoded D3D11 surface into a `GpuFrame`. `data[0]` is the texture array, `data[1]`
+    /// the slice index. We `av_frame_clone` so the surface stays referenced (kept out of the reuse
+    /// pool) until the presenter drops the guard.
+    unsafe fn lift(&mut self) -> Result<GpuFrame> {
+        use ffmpeg::ffi;
+        unsafe {
+            if (*self.frame).format != ffi::AVPixelFormat::AV_PIX_FMT_D3D11 as i32 {
+                bail!("decoder returned a software frame (no D3D11 surface)");
+            }
+            let hdr =
+                (*self.frame).color_trc == ffi::AVColorTransferCharacteristic::AVCOL_TRC_SMPTE2084;
+            let ten_bit = {
+                let hwfc = (*self.frame).hw_frames_ctx;
+                !hwfc.is_null()
+                    && (*((*hwfc).data as *const ffi::AVHWFramesContext)).sw_format
+                        == ffi::AVPixelFormat::AV_PIX_FMT_P010LE
+            };
+            let cloned = ffi::av_frame_clone(self.frame);
+            if cloned.is_null() {
+                bail!("av_frame_clone failed");
+            }
+            let frame = GpuFrame {
+                width: (*self.frame).width as u32,
+                height: (*self.frame).height as u32,
+                index: (*self.frame).data[1] as usize as u32,
+                hdr,
+                ten_bit,
+                guard: D3d11FrameGuard(cloned),
+            };
+            log_layout_once(frame.width, frame.height, frame.index, hdr, ten_bit);
+            Ok(frame)
+        }
+    }
+}
+
+impl Drop for D3d11vaDecoder {
+    fn drop(&mut self) {
+        use ffmpeg::ffi;
+        unsafe {
+            ffi::av_packet_free(&mut self.packet);
+            ffi::av_frame_free(&mut self.frame);
+            ffi::avcodec_free_context(&mut self.ctx);
+            ffi::av_buffer_unref(&mut self.hw_device);
+        }
+    }
+}
+
+/// One-time dump of the first decoded surface's layout — so a new GPU/driver combination's real
+/// format (slice index range, HDR/bit-depth) is visible in the logs without a debugger.
+fn log_layout_once(width: u32, height: u32, index: u32, hdr: bool, ten_bit: bool) {
+    use std::sync::atomic::{AtomicBool, Ordering};
+    static ONCE: AtomicBool = AtomicBool::new(true);
+    if ONCE.swap(false, Ordering::Relaxed) {
+        tracing::info!(
+            width,
+            height,
+            slice = index,
+            hdr,
+            ten_bit,
+            "D3D11VA first frame (zero-copy)"
+        );
+    }
+}