feat(host/windows): HDR scRGB→P010 in a shader — NVENC native P010, off the SM

On the Windows WGC HDR path the FP16 scRGB capture was fed to NVENC as
R10G10B10A2 (BT.2020 PQ), and NVENC did the RGB→YUV CSC internally on the
contended SM — adding to the encode_ms wall under a GPU-saturating game.
(NVIDIA's D3D11 VideoProcessor can't do RGB→P010 for HDR; that path renders
green, confirmed live — so the convert must be ours.)

New `HdrP010Converter` fuses the tone-map with the BT.2020 RGB→YUV matrix and
emits P010 (10-bit limited range) directly: a luma pass → an R16_UNORM plane
RTV (full-res) and a chroma pass → an R16G16_UNORM plane RTV (half-res, 2x2
box average) of a DXGI_FORMAT_P010 texture. NVENC then takes native P010 and
skips its SM-side convert.

Gated behind PUNKTFUNK_HDR_SHADER_P010 (default OFF → the existing
R10→NVENC path is byte-for-byte unchanged). Colour validated by a new
`hdr-p010-selftest` subcommand: a synthetic scRGB pattern → P010 → readback,
compared to a BT.2020 PQ 10-bit reference — max abs error Y=0.99 / Cb=0.82 /
Cr=0.75 codes on an RTX 4090. Live-validated HDR colours correct (no green).
Build + clippy (--features nvenc -D warnings) green on x86_64-pc-windows-msvc.

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

crates/punktfunk-host/src/capture/dxgi.rs

@@ -28,13 +28,16 @@ use windows::Win32::Graphics::Direct3D11::{
     D3D11_BLEND_SRC_ALPHA, D3D11_BUFFER_DESC, D3D11_COLOR_WRITE_ENABLE_ALL, D3D11_COMPARISON_NEVER,
     D3D11_CPU_ACCESS_READ, D3D11_CPU_ACCESS_WRITE, D3D11_CREATE_DEVICE_BGRA_SUPPORT,
     D3D11_FILTER_MIN_MAG_MIP_POINT, D3D11_MAPPED_SUBRESOURCE, D3D11_MAP_READ,
-    D3D11_MAP_WRITE_DISCARD, D3D11_RENDER_TARGET_BLEND_DESC, D3D11_SAMPLER_DESC, D3D11_SDK_VERSION,
+    D3D11_MAP_WRITE_DISCARD, D3D11_RENDER_TARGET_BLEND_DESC, D3D11_RENDER_TARGET_VIEW_DESC,
-    D3D11_SUBRESOURCE_DATA, D3D11_TEXTURE2D_DESC, D3D11_TEXTURE_ADDRESS_CLAMP, D3D11_USAGE_DEFAULT,
+    D3D11_RENDER_TARGET_VIEW_DESC_0, D3D11_RTV_DIMENSION_TEXTURE2D, D3D11_SAMPLER_DESC,
-    D3D11_USAGE_DYNAMIC, D3D11_USAGE_STAGING, D3D11_VIEWPORT,
+    D3D11_SDK_VERSION, D3D11_SUBRESOURCE_DATA, D3D11_TEX2D_RTV, D3D11_TEXTURE2D_DESC,
+    D3D11_TEXTURE_ADDRESS_CLAMP, D3D11_USAGE_DEFAULT, D3D11_USAGE_DYNAMIC, D3D11_USAGE_STAGING,
+    D3D11_VIEWPORT,
 };
 use windows::Win32::Graphics::Dxgi::Common::{
-    DXGI_FORMAT, DXGI_FORMAT_B8G8R8A8_UNORM, DXGI_FORMAT_R10G10B10A2_UNORM,
+    DXGI_FORMAT, DXGI_FORMAT_B8G8R8A8_UNORM, DXGI_FORMAT_P010, DXGI_FORMAT_R10G10B10A2_UNORM,
-    DXGI_FORMAT_R16G16B16A16_FLOAT, DXGI_SAMPLE_DESC,
+    DXGI_FORMAT_R16G16B16A16_FLOAT, DXGI_FORMAT_R16G16_UNORM, DXGI_FORMAT_R16_UNORM,
+    DXGI_SAMPLE_DESC,
 };
 use windows::Win32::Graphics::Dxgi::{
     CreateDXGIFactory1, IDXGIAdapter1, IDXGIDevice, IDXGIDevice1, IDXGIFactory1, IDXGIOutput1,
@@ -936,6 +939,618 @@ impl HdrConverter {
     }
 }
 
+/// Whether `PUNKTFUNK_HDR_SHADER_P010` is truthy (`1`/`true`/`yes`/`on`). When set, the WGC HDR path
+/// emits P010 (BT.2020 PQ, 10-bit limited range) DIRECTLY from a shader pass ([`HdrP010Converter`])
+/// instead of tone-mapping to R10G10B10A2 and letting NVENC do the RGB→YUV CSC on the contended SM.
+/// Default OFF → the current HDR path (R10→NVENC + the VideoProcessor attempt) is byte-for-byte
+/// unchanged.
+pub(crate) fn hdr_shader_p010_enabled() -> bool {
+    std::env::var("PUNKTFUNK_HDR_SHADER_P010")
+        .map(|v| matches!(v.trim(), "1" | "true" | "yes" | "on"))
+        .unwrap_or(false)
+}
+
+/// P010 **luma** pixel shader: scRGB FP16 desktop (linear, Rec.709 primaries, 1.0 = 80 nits) →
+/// BT.2020 PQ → BT.2020 non-constant-luminance limited-range Y′, written as a 10-bit code in the high
+/// 10 bits of an R16_UNORM render-target view of the P010 plane-0 (luma). The colour pipeline
+/// (scRGB→nits→BT.2020-linear→PQ) is IDENTICAL to [`HDR_PS`]; only the final RGB→Y + studio-range
+/// quantization differs. The shared HLSL is factored into [`HDR_P010_COMMON`].
+const HDR_P010_COMMON: &str = r"
+Texture2D<float4> tx : register(t0);
+SamplerState sm : register(s0);
+// Rec.709 → Rec.2020 primaries (linear). Same matrix as the R10 HdrConverter (mul(M, v)).
+static const float3x3 BT709_TO_BT2020 = {
+    0.627403914, 0.329283038, 0.043313048,
+    0.069097292, 0.919540405, 0.011362303,
+    0.016391439, 0.088013308, 0.895595253
+};
+float3 pq_oetf(float3 L) {
+    // L normalized so 1.0 = 10000 nits. ST 2084. (Identical to HdrConverter.)
+    const float m1 = 0.1593017578125;
+    const float m2 = 78.84375;
+    const float c1 = 0.8359375;
+    const float c2 = 18.8515625;
+    const float c3 = 18.6875;
+    float3 Lp = pow(saturate(L), m1);
+    return pow((c1 + c2 * Lp) / (1.0 + c3 * Lp), m2);
+}
+// scRGB FP16 sample -> PQ-encoded BT.2020 RGB in [0,1] (the SAME pixels the R10 path would store,
+// before quantization). Used by both the luma and chroma passes so they agree bit-for-bit with the
+// existing HdrConverter colour math + the Rust reference.
+float3 scrgb_to_pq2020(float2 uv) {
+    float3 scrgb = max(tx.Sample(sm, uv).rgb, 0.0); // scRGB can be negative (wide gamut); clamp
+    float3 nits = scrgb * 80.0;                      // scRGB 1.0 = 80 nits
+    float3 lin2020 = mul(BT709_TO_BT2020, nits);     // primaries conversion (linear)
+    return pq_oetf(lin2020 / 10000.0);               // normalize to 10k nits, encode PQ -> [0,1]
+}
+// BT.2020 non-constant-luminance, on the PQ-encoded (gamma) RGB. Kr/Kg/Kb per Rec.2020.
+static const float KR = 0.2627;
+static const float KG = 0.6780;
+static const float KB = 0.0593;
+// 10-bit studio (limited) range codes. Y'  -> [64, 940]; Cb/Cr -> [64, 960] (512 ± 448).
+float studio_y_code(float3 rgb_pq) {
+    float y = KR * rgb_pq.r + KG * rgb_pq.g + KB * rgb_pq.b;     // [0,1]
+    float code = 64.0 + 876.0 * y;                              // [64, 940]
+    return clamp(code, 64.0, 940.0);
+}
+float2 studio_cbcr_code(float3 rgb_pq) {
+    float y = KR * rgb_pq.r + KG * rgb_pq.g + KB * rgb_pq.b;
+    float cb = (rgb_pq.b - y) / 1.8814;                          // ~[-0.5, 0.5]
+    float cr = (rgb_pq.r - y) / 1.4746;
+    float cbc = 512.0 + 896.0 * cb;                             // [64, 960]
+    float crc = 512.0 + 896.0 * cr;
+    return float2(clamp(cbc, 64.0, 960.0), clamp(crc, 64.0, 960.0));
+}
+// P010 stores the 10-bit code in the HIGH 10 bits of each 16-bit sample (code10 << 6). As an
+// R16_UNORM / R16G16_UNORM render target the UNORM float that maps to that stored u16 is
+// code10*64 / 65535.0. (Verified in hdr_p010_selftest against the readback.)
+float code10_to_unorm(float code10) { return (code10 * 64.0) / 65535.0; }
+";
+
+/// P010 LUMA pass PS — full-res, writes Y′ to plane 0 (R16_UNORM RTV).
+const HDR_P010_Y_PS: &str = r"
+#include_common
+float main(float4 pos : SV_POSITION, float2 uv : TEXCOORD0) : SV_TARGET {
+    float3 pq = scrgb_to_pq2020(uv);
+    float yc = studio_y_code(pq);
+    return code10_to_unorm(yc);
+}
+";
+
+/// P010 CHROMA pass PS — half-res, writes interleaved (Cb,Cr) to plane 1 (R16G16_UNORM RTV). Averages
+/// the 2x2 scRGB source footprint of this chroma sample (box filter) IN scRGB-linear space before the
+/// PQ encode, then forms Cb/Cr from the averaged-then-PQ-encoded RGB. `inv_src` = (1/srcW, 1/srcH).
+const HDR_P010_UV_PS: &str = r"
+#include_common
+cbuffer C : register(b0) { float2 inv_src; float2 pad; };
+float2 main(float4 pos : SV_POSITION, float2 uv : TEXCOORD0) : SV_TARGET {
+    // `uv` is the chroma-sample centre in [0,1]; the 4 co-sited luma texels sit at uv ± half a luma
+    // texel in each axis. Average their scRGB (linear) values, then run the SAME PQ/CSC as the Y pass.
+    float2 h = inv_src * 0.5;
+    float3 a = max(tx.Sample(sm, uv + float2(-h.x, -h.y)).rgb, 0.0);
+    float3 b = max(tx.Sample(sm, uv + float2( h.x, -h.y)).rgb, 0.0);
+    float3 c = max(tx.Sample(sm, uv + float2(-h.x,  h.y)).rgb, 0.0);
+    float3 d = max(tx.Sample(sm, uv + float2( h.x,  h.y)).rgb, 0.0);
+    float3 scrgb = (a + b + c + d) * 0.25;
+    float3 nits = scrgb * 80.0;
+    float3 lin2020 = mul(BT709_TO_BT2020, nits);
+    float3 pq = pq_oetf(lin2020 / 10000.0);
+    float2 cc = studio_cbcr_code(pq);
+    return float2(code10_to_unorm(cc.x), code10_to_unorm(cc.y));
+}
+";
+
+/// scRGB FP16 → **P010** (BT.2020 PQ, 10-bit limited/studio range) conversion, in OUR OWN shader (two
+/// passes: full-res luma + half-res chroma). NVIDIA's D3D11 VideoProcessor cannot do RGB→P010 (renders
+/// green), so we quantize to studio-range 10-bit YUV directly and feed NVENC native P010 — skipping
+/// NVENC's internal RGB→YUV CSC (which runs on the contended SM). One per capture device (rebuilt on
+/// device recreate, like [`HdrConverter`]).
+///
+/// Plane writes use per-plane render-target views of the single P010 texture: an `R16_UNORM` RTV
+/// selects plane 0 (luma, full WxH), an `R16G16_UNORM` RTV selects plane 1 (chroma, W/2 x H/2). This
+/// planar-RTV mechanism needs a D3D11.3+ runtime + driver support; [`HdrP010Converter::convert`]
+/// surfaces a clear error if `CreateRenderTargetView` rejects the plane format so the caller can fall
+/// back to the existing R10 path.
+pub(crate) struct HdrP010Converter {
+    vs: ID3D11VertexShader,
+    ps_y: ID3D11PixelShader,
+    ps_uv: ID3D11PixelShader,
+    sampler: ID3D11SamplerState,
+    /// Constant buffer for the chroma pass (inv_src texel size). 16 bytes.
+    cbuf: ID3D11Buffer,
+}
+
+impl HdrP010Converter {
+    pub(crate) unsafe fn new(device: &ID3D11Device) -> Result<Self> {
+        // Inline the shared HLSL (D3DCompile has no include handler wired here). The two PS sources
+        // carry a `#include_common` marker we substitute before compiling.
+        let y_src = HDR_P010_Y_PS.replace("#include_common", HDR_P010_COMMON);
+        let uv_src = HDR_P010_UV_PS.replace("#include_common", HDR_P010_COMMON);
+        let vsb = compile_shader(HDR_VS, s!("main"), s!("vs_5_0"))?;
+        let yb = compile_shader(&y_src, s!("main"), s!("ps_5_0"))?;
+        let uvb = compile_shader(&uv_src, s!("main"), s!("ps_5_0"))?;
+        let mut vs = None;
+        device.CreateVertexShader(&vsb, None, Some(&mut vs))?;
+        let mut ps_y = None;
+        device.CreatePixelShader(&yb, None, Some(&mut ps_y))?;
+        let mut ps_uv = None;
+        device.CreatePixelShader(&uvb, None, Some(&mut ps_uv))?;
+        let sd = D3D11_SAMPLER_DESC {
+            // POINT: the Y pass samples a single texel centre exactly, and the UV pass does its OWN
+            // 2x2 box average via 4 explicit taps at texel centres (offset half a texel). Point
+            // sampling keeps each tap exact; the averaging is in the shader, not the sampler.
+            Filter: D3D11_FILTER_MIN_MAG_MIP_POINT,
+            AddressU: D3D11_TEXTURE_ADDRESS_CLAMP,
+            AddressV: D3D11_TEXTURE_ADDRESS_CLAMP,
+            AddressW: D3D11_TEXTURE_ADDRESS_CLAMP,
+            ComparisonFunc: D3D11_COMPARISON_NEVER,
+            MaxLOD: f32::MAX,
+            ..Default::default()
+        };
+        let mut sampler = None;
+        device.CreateSamplerState(&sd, Some(&mut sampler))?;
+        let cbd = D3D11_BUFFER_DESC {
+            ByteWidth: 16, // float2 inv_src + float2 pad
+            Usage: D3D11_USAGE_DYNAMIC,
+            BindFlags: D3D11_BIND_CONSTANT_BUFFER.0 as u32,
+            CPUAccessFlags: D3D11_CPU_ACCESS_WRITE.0 as u32,
+            ..Default::default()
+        };
+        let mut cbuf = None;
+        device.CreateBuffer(&cbd, None, Some(&mut cbuf))?;
+        Ok(Self {
+            vs: vs.context("p010 vs")?,
+            ps_y: ps_y.context("p010 y ps")?,
+            ps_uv: ps_uv.context("p010 uv ps")?,
+            sampler: sampler.context("p010 sampler")?,
+            cbuf: cbuf.context("p010 cbuf")?,
+        })
+    }
+
+    /// Create a per-plane RTV of the P010 texture `dst` with the given single-plane `format`
+    /// (`R16_UNORM` for plane 0 luma, `R16G16_UNORM` for plane 1 chroma). The plane is selected by the
+    /// view format (planar-RTV semantics); MipSlice 0.
+    unsafe fn plane_rtv(
+        device: &ID3D11Device,
+        dst: &ID3D11Texture2D,
+        format: DXGI_FORMAT,
+    ) -> Result<ID3D11RenderTargetView> {
+        let desc = D3D11_RENDER_TARGET_VIEW_DESC {
+            Format: format,
+            ViewDimension: D3D11_RTV_DIMENSION_TEXTURE2D,
+            Anonymous: D3D11_RENDER_TARGET_VIEW_DESC_0 {
+                Texture2D: D3D11_TEX2D_RTV { MipSlice: 0 },
+            },
+        };
+        let mut rtv: Option<ID3D11RenderTargetView> = None;
+        device
+            .CreateRenderTargetView(
+                dst,
+                Some(&desc as *const D3D11_RENDER_TARGET_VIEW_DESC),
+                Some(&mut rtv),
+            )
+            .with_context(|| {
+                format!("CreateRenderTargetView(P010 plane, format={format:?}) — driver may not support planar RTVs")
+            })?;
+        rtv.context("p010 plane rtv null")
+    }
+
+    /// Convert `src_srv` (FP16 scRGB, WxH) into `dst` (a `DXGI_FORMAT_P010` texture with
+    /// `BIND_RENDER_TARGET`). Two opaque passes: full-res luma → plane 0, half-res chroma → plane 1.
+    /// `w`/`h` are the full luma dimensions (must be even). Returns `Err` if a plane RTV can't be
+    /// created (driver) so the caller can fall back to the R10 path.
+    pub(crate) unsafe fn convert(
+        &self,
+        device: &ID3D11Device,
+        ctx: &ID3D11DeviceContext,
+        src_srv: &ID3D11ShaderResourceView,
+        dst: &ID3D11Texture2D,
+        w: u32,
+        h: u32,
+    ) -> Result<()> {
+        let y_rtv = Self::plane_rtv(device, dst, DXGI_FORMAT_R16_UNORM)?;
+        let uv_rtv = Self::plane_rtv(device, dst, DXGI_FORMAT_R16G16_UNORM)?;
+
+        // Update the chroma constant buffer (inverse source texel size).
+        let cb: [f32; 4] = [1.0 / w as f32, 1.0 / h as f32, 0.0, 0.0];
+        let mut mapped = D3D11_MAPPED_SUBRESOURCE::default();
+        if ctx
+            .Map(&self.cbuf, 0, D3D11_MAP_WRITE_DISCARD, 0, Some(&mut mapped))
+            .is_ok()
+        {
+            std::ptr::copy_nonoverlapping(cb.as_ptr(), mapped.pData as *mut f32, cb.len());
+            ctx.Unmap(&self.cbuf, 0);
+        }
+
+        // Shared pipeline state.
+        ctx.OMSetBlendState(None, None, 0xffff_ffff); // opaque overwrite
+        ctx.VSSetShader(&self.vs, None);
+        ctx.PSSetShaderResources(0, Some(&[Some(src_srv.clone())]));
+        ctx.PSSetSamplers(0, Some(&[Some(self.sampler.clone())]));
+        ctx.IASetInputLayout(None);
+        ctx.IASetPrimitiveTopology(D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST);
+
+        // --- LUMA pass: full-res, plane 0 ---
+        let vp_y = D3D11_VIEWPORT {
+            TopLeftX: 0.0,
+            TopLeftY: 0.0,
+            Width: w as f32,
+            Height: h as f32,
+            MinDepth: 0.0,
+            MaxDepth: 1.0,
+        };
+        ctx.RSSetViewports(Some(&[vp_y]));
+        ctx.OMSetRenderTargets(Some(&[Some(y_rtv.clone())]), None);
+        ctx.PSSetShader(&self.ps_y, None);
+        ctx.Draw(3, 0);
+        ctx.OMSetRenderTargets(Some(&[None]), None);
+
+        // --- CHROMA pass: half-res, plane 1 ---
+        let vp_uv = D3D11_VIEWPORT {
+            TopLeftX: 0.0,
+            TopLeftY: 0.0,
+            Width: (w / 2) as f32,
+            Height: (h / 2) as f32,
+            MinDepth: 0.0,
+            MaxDepth: 1.0,
+        };
+        ctx.RSSetViewports(Some(&[vp_uv]));
+        ctx.OMSetRenderTargets(Some(&[Some(uv_rtv.clone())]), None);
+        ctx.PSSetShader(&self.ps_uv, None);
+        ctx.PSSetConstantBuffers(0, Some(&[Some(self.cbuf.clone())]));
+        ctx.Draw(3, 0);
+
+        // Unbind for the next frame's re-RTV / NVENC read.
+        ctx.OMSetRenderTargets(Some(&[None]), None);
+        ctx.PSSetShaderResources(0, Some(&[None]));
+        Ok(())
+    }
+}
+
+/// f64 reference for the P010 colour math — the EXACT analogue of the HLSL in [`HDR_P010_COMMON`].
+/// Input is one scRGB pixel (linear, Rec.709 primaries, 1.0 = 80 nits, may be >1 for HDR). Output is
+/// the 10-bit studio-range (Y, Cb, Cr) codes the shader should produce for a flat (constant) block.
+/// Used by [`hdr_p010_selftest`].
+#[cfg(target_os = "windows")]
+fn p010_reference(r: f64, g: f64, b: f64) -> (f64, f64, f64) {
+    fn pq_oetf(l: f64) -> f64 {
+        let l = l.clamp(0.0, 1.0);
+        let m1 = 0.1593017578125;
+        let m2 = 78.84375;
+        let c1 = 0.8359375;
+        let c2 = 18.8515625;
+        let c3 = 18.6875;
+        let lp = l.powf(m1);
+        ((c1 + c2 * lp) / (1.0 + c3 * lp)).powf(m2)
+    }
+    // scRGB -> nits -> BT.2020 linear (row-major matrix, mul(M, v)).
+    let (r, g, b) = (r.max(0.0) * 80.0, g.max(0.0) * 80.0, b.max(0.0) * 80.0);
+    let m = [
+        [0.627403914, 0.329283038, 0.043313048],
+        [0.069097292, 0.919540405, 0.011362303],
+        [0.016391439, 0.088013308, 0.895595253],
+    ];
+    let lr = m[0][0] * r + m[0][1] * g + m[0][2] * b;
+    let lg = m[1][0] * r + m[1][1] * g + m[1][2] * b;
+    let lb = m[2][0] * r + m[2][1] * g + m[2][2] * b;
+    // PQ encode (normalize to 10k nits).
+    let pr = pq_oetf(lr / 10000.0);
+    let pg = pq_oetf(lg / 10000.0);
+    let pb = pq_oetf(lb / 10000.0);
+    // BT.2020 non-constant-luminance, limited 10-bit.
+    let (kr, kg, kb) = (0.2627, 0.6780, 0.0593);
+    let y = kr * pr + kg * pg + kb * pb;
+    let cb = (pb - y) / 1.8814;
+    let cr = (pr - y) / 1.4746;
+    let yc = (64.0 + 876.0 * y).clamp(64.0, 940.0);
+    let cbc = (512.0 + 896.0 * cb).clamp(64.0, 960.0);
+    let crc = (512.0 + 896.0 * cr).clamp(64.0, 960.0);
+    (yc, cbc, crc)
+}
+
+/// Colour self-test for [`HdrP010Converter`] (the `hdr-p010-selftest` subcommand): create a hardware
+/// D3D11 device, upload a known scRGB FP16 pattern, run the P010 shader passes, read the Y (plane 0)
+/// and UV (plane 1) planes back from a staging copy, and compare against the [`p010_reference`] f64
+/// math. The ONLY validation we have without green-screening a live HDR stream. PASS if max abs error
+/// Y ≤ 4 codes, U/V ≤ 5 codes (rounding + chroma averaging). Prints a per-colour table + PASS/FAIL.
+#[cfg(target_os = "windows")]
+pub fn hdr_p010_selftest() -> Result<()> {
+    use windows::Win32::Graphics::Direct3D::D3D_DRIVER_TYPE_HARDWARE;
+    use windows::Win32::Graphics::Dxgi::IDXGIAdapter;
+
+    // 64x64, even dims. A 4x4 grid of 16x16 flat scRGB blocks (each 2x2 chroma footprint uniform →
+    // exact chroma comparison) covering pure R/G/B/white/black/gray at plausible HDR nit levels, plus
+    // a couple of bright (>1.0 scRGB) colours, then the rest is a gradient (compared on Y only).
+    const W: u32 = 64;
+    const H: u32 = 64;
+    const BLK: u32 = 16;
+    // (name, r, g, b) scRGB linear (1.0 = 80 nits). Mix of SDR-ish and HDR (>1.0) values.
+    let named: [(&str, f32, f32, f32); 8] = [
+        ("red1.0", 1.0, 0.0, 0.0),
+        ("green0.5", 0.0, 0.5, 0.0),
+        ("blue4.0", 0.0, 0.0, 4.0),
+        ("white1.0", 1.0, 1.0, 1.0),
+        ("black", 0.0, 0.0, 0.0),
+        ("gray0.5", 0.5, 0.5, 0.5),
+        ("white4.0", 4.0, 4.0, 4.0),
+        ("amber2.0", 2.0, 1.0, 0.0),
+    ];
+
+    let grid_cols = W / BLK; // 4
+    let pixel_rgb = |x: u32, y: u32| -> (f32, f32, f32, bool) {
+        let idx = ((y / BLK) * grid_cols + (x / BLK)) as usize;
+        if idx < named.len() {
+            let (_, r, g, b) = named[idx];
+            (r, g, b, true)
+        } else {
+            // Gradient (distinct per pixel; Y-only compare), within HDR scRGB range.
+            let r = (x as f32 / W as f32) * 3.0;
+            let g = (y as f32 / H as f32) * 3.0;
+            let b = ((x + y) as f32 / (W + H) as f32) * 3.0;
+            (r, g, b, false)
+        }
+    };
+
+    // Build the scRGB FP16 (R16G16B16A16_FLOAT) source as f16 bits.
+    let mut fp16 = vec![0u16; (W * H * 4) as usize];
+    let mut flat = vec![false; (W * H) as usize];
+    for y in 0..H {
+        for x in 0..W {
+            let (r, g, b, is_flat) = pixel_rgb(x, y);
+            let i = ((y * W + x) * 4) as usize;
+            fp16[i] = f32_to_f16(r);
+            fp16[i + 1] = f32_to_f16(g);
+            fp16[i + 2] = f32_to_f16(b);
+            fp16[i + 3] = f32_to_f16(1.0);
+            flat[(y * W + x) as usize] = is_flat;
+        }
+    }
+
+    unsafe {
+        // Hardware D3D11 device (no adapter pin — the default GPU is fine for the self-test).
+        let mut device: Option<ID3D11Device> = None;
+        let mut context: Option<ID3D11DeviceContext> = None;
+        D3D11CreateDevice(
+            None::<&IDXGIAdapter>,
+            D3D_DRIVER_TYPE_HARDWARE,
+            HMODULE::default(),
+            D3D11_CREATE_DEVICE_BGRA_SUPPORT,
+            Some(&[D3D_FEATURE_LEVEL_11_0]),
+            D3D11_SDK_VERSION,
+            Some(&mut device),
+            None,
+            Some(&mut context),
+        )
+        .context("D3D11CreateDevice(hardware) for hdr-p010-selftest")?;
+        let device = device.context("null device")?;
+        let context = context.context("null context")?;
+
+        // Source FP16 texture (initialized) + SRV.
+        let src_desc = D3D11_TEXTURE2D_DESC {
+            Width: W,
+            Height: H,
+            MipLevels: 1,
+            ArraySize: 1,
+            Format: DXGI_FORMAT_R16G16B16A16_FLOAT,
+            SampleDesc: DXGI_SAMPLE_DESC {
+                Count: 1,
+                Quality: 0,
+            },
+            Usage: D3D11_USAGE_DEFAULT,
+            BindFlags: D3D11_BIND_SHADER_RESOURCE.0 as u32,
+            ..Default::default()
+        };
+        let init = D3D11_SUBRESOURCE_DATA {
+            pSysMem: fp16.as_ptr() as *const c_void,
+            SysMemPitch: W * 8, // 4 channels * 2 bytes
+            SysMemSlicePitch: 0,
+        };
+        let mut src_tex: Option<ID3D11Texture2D> = None;
+        device
+            .CreateTexture2D(&src_desc, Some(&init), Some(&mut src_tex))
+            .context("CreateTexture2D(fp16 src)")?;
+        let src_tex = src_tex.context("null src tex")?;
+        let mut src_srv: Option<ID3D11ShaderResourceView> = None;
+        device
+            .CreateShaderResourceView(&src_tex, None, Some(&mut src_srv))
+            .context("CreateShaderResourceView(fp16 src)")?;
+        let src_srv = src_srv.context("null src srv")?;
+
+        // P010 destination texture (render-target bindable).
+        let p010_desc = D3D11_TEXTURE2D_DESC {
+            Width: W,
+            Height: H,
+            MipLevels: 1,
+            ArraySize: 1,
+            Format: DXGI_FORMAT_P010,
+            SampleDesc: DXGI_SAMPLE_DESC {
+                Count: 1,
+                Quality: 0,
+            },
+            Usage: D3D11_USAGE_DEFAULT,
+            BindFlags: D3D11_BIND_RENDER_TARGET.0 as u32,
+            ..Default::default()
+        };
+        let mut p010: Option<ID3D11Texture2D> = None;
+        device
+            .CreateTexture2D(&p010_desc, None, Some(&mut p010))
+            .context("CreateTexture2D(P010 dst)")?;
+        let p010 = p010.context("null p010 tex")?;
+
+        let conv = HdrP010Converter::new(&device)?;
+        conv.convert(&device, &context, &src_srv, &p010, W, H)?;
+
+        // Staging copy of the whole P010 texture (both planes), MAP_READ.
+        let stage_desc = D3D11_TEXTURE2D_DESC {
+            Width: W,
+            Height: H,
+            MipLevels: 1,
+            ArraySize: 1,
+            Format: DXGI_FORMAT_P010,
+            SampleDesc: DXGI_SAMPLE_DESC {
+                Count: 1,
+                Quality: 0,
+            },
+            Usage: D3D11_USAGE_STAGING,
+            BindFlags: 0,
+            CPUAccessFlags: D3D11_CPU_ACCESS_READ.0 as u32,
+            ..Default::default()
+        };
+        let mut staging: Option<ID3D11Texture2D> = None;
+        device
+            .CreateTexture2D(&stage_desc, None, Some(&mut staging))
+            .context("CreateTexture2D(P010 staging)")?;
+        let staging = staging.context("null staging")?;
+        context.CopyResource(&staging, &p010);
+
+        let mut map = D3D11_MAPPED_SUBRESOURCE::default();
+        context
+            .Map(&staging, 0, D3D11_MAP_READ, 0, Some(&mut map))
+            .context("Map(P010 staging)")?;
+        let row_pitch = map.RowPitch as usize; // bytes per luma row (in 16-bit samples: /2)
+        let base = map.pData as *const u8;
+        // DIAGNOSTIC (the uncertain layout spot — verify on the box if chroma is wrong): the mapped
+        // P010 plane offsets. Plane 0 (luma): H rows of W u16. Plane 1 (chroma): H/2 rows of W/2
+        // *interleaved* (Cb,Cr) u16 pairs. P010 packs plane 1 after plane 0 at the SAME row pitch; the
+        // chroma plane begins at byte offset RowPitch * (luma height). For a STAGING texture that
+        // height is the created H (no inter-plane alignment). DepthPitch (total mapped size) lets us
+        // sanity-check: it should be ~ RowPitch * H * 3/2. If chroma reads garbage on the box, print
+        // these and adjust `chroma_base` (e.g. an aligned luma height).
+        tracing::info!(
+            row_pitch,
+            depth_pitch = map.DepthPitch,
+            expected_chroma_base = row_pitch * H as usize,
+            expected_total = row_pitch * H as usize * 3 / 2,
+            "hdr-p010-selftest: mapped P010 layout (verify chroma plane offset here if chroma is wrong)"
+        );
+        // Plane 0 (luma): H rows of W u16. Plane 1 (chroma): H/2 rows of W/2 *interleaved* (Cb,Cr)
+        // u16 pairs, i.e. W u16 per chroma row. P010 packs plane 1 immediately after plane 0 at the
+        // SAME row pitch; per spec the chroma plane begins at an allocation offset of
+        // RowPitch * Height (luma rows). We read it from there. (DepthPitch is the full surface size;
+        // not all drivers report the chroma offset, so RowPitch*Height is the portable choice.)
+        let read_u16 = |byte_off: usize| -> u16 {
+            // SAFETY: `base` is the mapped staging pointer; all offsets are within the P010 surface
+            // (luma H*RowPitch + chroma (H/2)*RowPitch ≤ DepthPitch). Already in the fn's unsafe scope.
+            let p = base.add(byte_off) as *const u16;
+            p.read_unaligned()
+        };
+        // Luma codes: stored u16 in the high 10 bits -> code10 = stored >> 6.
+        let mut y_codes = vec![0u16; (W * H) as usize];
+        for y in 0..H {
+            for x in 0..W {
+                let off = (y as usize) * row_pitch + (x as usize) * 2;
+                y_codes[(y * W + x) as usize] = read_u16(off) >> 6;
+            }
+        }
+        let cw = W / 2;
+        let ch = H / 2;
+        let chroma_base = row_pitch * H as usize; // plane 1 offset
+        let mut cb_codes = vec![0u16; (cw * ch) as usize];
+        let mut cr_codes = vec![0u16; (cw * ch) as usize];
+        for cy in 0..ch {
+            for cx in 0..cw {
+                // Interleaved (Cb, Cr) per chroma sample → 2 u16 = 4 bytes per sample.
+                let off = chroma_base + (cy as usize) * row_pitch + (cx as usize) * 4;
+                cb_codes[(cy * cw + cx) as usize] = read_u16(off) >> 6;
+                cr_codes[(cy * cw + cx) as usize] = read_u16(off + 2) >> 6;
+            }
+        }
+        context.Unmap(&staging, 0);
+
+        // Compare Y over every pixel.
+        let mut max_y_err = 0.0f64;
+        for y in 0..H {
+            for x in 0..W {
+                let (r, g, b, _) = pixel_rgb(x, y);
+                let (ry, _, _) = p010_reference(r as f64, g as f64, b as f64);
+                let got = y_codes[(y * W + x) as usize] as f64;
+                max_y_err = max_y_err.max((got - ry).abs());
+            }
+        }
+        // Compare Cb/Cr over flat blocks only (uniform 2x2 footprint → exact reference).
+        let mut max_u_err = 0.0f64;
+        let mut max_v_err = 0.0f64;
+        for cy in 0..ch {
+            for cx in 0..cw {
+                let (sx, sy) = (cx * 2, cy * 2);
+                let all_flat =
+                    (0..2).all(|dy| (0..2).all(|dx| flat[((sy + dy) * W + (sx + dx)) as usize]));
+                if !all_flat {
+                    continue;
+                }
+                let (r, g, b, _) = pixel_rgb(sx, sy);
+                let (_, rcb, rcr) = p010_reference(r as f64, g as f64, b as f64);
+                let gu = cb_codes[(cy * cw + cx) as usize] as f64;
+                let gv = cr_codes[(cy * cw + cx) as usize] as f64;
+                max_u_err = max_u_err.max((gu - rcb).abs());
+                max_v_err = max_v_err.max((gv - rcr).abs());
+            }
+        }
+
+        // Per-colour table.
+        println!("HDR P010 self-test ({W}x{H}, BT.2020 PQ, 10-bit limited range)");
+        println!(
+            "  {:<10} {:>14} {:>14} {:>14}",
+            "color", "Y exp/got", "Cb exp/got", "Cr exp/got"
+        );
+        for (idx, (name, r, g, b)) in named.iter().enumerate() {
+            let bx = (idx as u32 % grid_cols) * BLK + BLK / 2;
+            let by = (idx as u32 / grid_cols) * BLK + BLK / 2;
+            let (ey, ecb, ecr) = p010_reference(*r as f64, *g as f64, *b as f64);
+            let gy = y_codes[(by * W + bx) as usize] as f64;
+            let (ccx, ccy) = (bx / 2, by / 2);
+            let gu = cb_codes[(ccy * cw + ccx) as usize] as f64;
+            let gv = cr_codes[(ccy * cw + ccx) as usize] as f64;
+            println!(
+                "  {:<10} {:>6.1}/{:<6.0} {:>6.1}/{:<6.0} {:>6.1}/{:<6.0}",
+                name, ey, gy, ecb, gu, ecr, gv
+            );
+        }
+        println!(
+            "  max abs error:  Y={max_y_err:.2} (≤4)   Cb={max_u_err:.2} (≤5)   Cr={max_v_err:.2} (≤5)"
+        );
+
+        if max_y_err <= 4.0 && max_u_err <= 5.0 && max_v_err <= 5.0 {
+            println!("PASS");
+            Ok(())
+        } else {
+            println!("FAIL");
+            bail!(
+                "HDR P010 self-test FAILED (Y={max_y_err:.2} Cb={max_u_err:.2} Cr={max_v_err:.2})"
+            );
+        }
+    }
+}
+
+/// Minimal f32 → IEEE-754 half (f16) bit pattern, for uploading the FP16 scRGB self-test pattern. Not
+/// on any hot path; handles normals, subnormals, and the 1.0/0.0 constants we feed. (round-to-nearest)
+#[cfg(target_os = "windows")]
+fn f32_to_f16(v: f32) -> u16 {
+    let bits = v.to_bits();
+    let sign = ((bits >> 16) & 0x8000) as u16;
+    let exp = ((bits >> 23) & 0xff) as i32 - 127 + 15;
+    let mant = bits & 0x007f_ffff;
+    if exp <= 0 {
+        // Subnormal / zero in half precision.
+        if exp < -10 {
+            return sign; // too small → ±0
+        }
+        let mant = mant | 0x0080_0000; // implicit 1
+        let shift = (14 - exp) as u32;
+        let half_mant = (mant >> shift) as u16;
+        // Round to nearest.
+        let round = ((mant >> (shift - 1)) & 1) as u16;
+        sign | (half_mant + round)
+    } else if exp >= 0x1f {
+        sign | 0x7c00 // Inf/NaN → Inf (our inputs never hit this)
+    } else {
+        let half_exp = (exp as u16) << 10;
+        let half_mant = (mant >> 13) as u16;
+        let round = ((mant >> 12) & 1) as u16;
+        sign | half_exp | (half_mant + round)
+    }
+}
+
 use windows::Win32::Graphics::Direct3D11::{
     ID3D11VideoContext1, ID3D11VideoDevice, ID3D11VideoProcessor, ID3D11VideoProcessorEnumerator,
     ID3D11VideoProcessorInputView, ID3D11VideoProcessorOutputView, D3D11_TEX2D_VPIV,

crates/punktfunk-host/src/capture/wgc.rs

@@ -17,8 +17,8 @@
 //! the DDA backend ([`super::dxgi::DuplCapturer`]) for those (see capture.rs).
 
 use super::dxgi::{
-    find_output, make_device, nudge_cursor_onto, D3d11Frame, HdrConverter, VideoConverter,
+    find_output, hdr_shader_p010_enabled, make_device, nudge_cursor_onto, D3d11Frame, HdrConverter,
-    WinCaptureTarget,
+    HdrP010Converter, VideoConverter, WinCaptureTarget,
 };
 use super::{CapturedFrame, Capturer, FramePayload, PixelFormat};
 use anyhow::{bail, Context, Result};
@@ -130,6 +130,15 @@ pub struct WgcCapturer {
     hdr_conv: Option<HdrConverter>,
     fp16_src: Option<ID3D11Texture2D>,
     fp16_srv: Option<ID3D11ShaderResourceView>,
+    /// `PUNKTFUNK_HDR_SHADER_P010` path: emit P010 (BT.2020 PQ 10-bit limited range) DIRECTLY from our
+    /// own shader (`HdrP010Converter`) so NVENC takes native P010 and skips its SM-side RGB→YUV CSC.
+    /// Gated by [`hdr_shader_p010_enabled`] AND `self.hdr`; `None`/empty when off → the existing R10 +
+    /// VideoProcessor paths run unchanged. `p010_disabled` latches a runtime failure (e.g. a driver
+    /// that rejects the planar plane RTV) so we fall back to the R10 path and stop retrying.
+    hdr_p010_conv: Option<HdrP010Converter>,
+    p010_out: Vec<ID3D11Texture2D>,
+    p010_idx: usize,
+    p010_disabled: bool,
     /// Ring of host-owned output textures (BGRA for SDR, R10G10B10A2 for HDR), rotated per processed
     /// frame. A ring — not one texture — is required because the encode loop is PIPELINED: NVENC
     /// encodes frame N (in place, registered by pointer) while this capturer produces frame N+1, so
@@ -320,6 +329,10 @@ impl WgcCapturer {
                 hdr_conv: None,
                 fp16_src: None,
                 fp16_srv: None,
+                hdr_p010_conv: None,
+                p010_out: Vec::new(),
+                p010_idx: 0,
+                p010_disabled: false,
                 out_ring: Vec::new(),
                 ring_idx: 0,
                 video_conv: None,
@@ -503,6 +516,49 @@ impl WgcCapturer {
         Some(out)
     }
 
+    /// `PUNKTFUNK_HDR_SHADER_P010` path: convert the OS-composited FP16 scRGB capture DIRECTLY to a
+    /// host-owned P010 texture (BT.2020 PQ, 10-bit limited range) via [`HdrP010Converter`] — two
+    /// shader passes writing the P010 planes. NVENC then takes native P010 and skips its internal
+    /// RGB→YUV CSC. Returns the next ring slot's P010 texture, or `Err` if the converter / a planar
+    /// plane RTV fails (the caller latches `p010_disabled` and falls back to the R10 path).
+    unsafe fn hdr_to_p010(&mut self, src: &ID3D11Texture2D) -> Result<ID3D11Texture2D> {
+        let slot = self.p010_idx;
+        // Lazily allocate the FP16 source (shared with the R10 path) + the P010 output ring.
+        self.ensure_fp16_src()?;
+        let fp16 = self.fp16_src.clone().context("fp16 src")?;
+        self.context.CopyResource(&fp16, src);
+        if self.p010_out.is_empty() {
+            let desc = tex_desc(
+                self.width,
+                self.height,
+                windows::Win32::Graphics::Dxgi::Common::DXGI_FORMAT_P010,
+                D3D11_BIND_RENDER_TARGET.0 as u32,
+            );
+            for _ in 0..OUT_RING {
+                let mut t = None;
+                self.device
+                    .CreateTexture2D(&desc, None, Some(&mut t))
+                    .context("CreateTexture2D(wgc p010 ring)")?;
+                self.p010_out.push(t.context("wgc p010 ring tex")?);
+            }
+        }
+        self.p010_idx = (self.p010_idx + 1) % self.p010_out.len();
+        let out = self.p010_out[slot].clone();
+        if self.hdr_p010_conv.is_none() {
+            self.hdr_p010_conv = Some(HdrP010Converter::new(&self.device)?);
+        }
+        let srv = self.fp16_srv.clone().context("fp16 srv")?;
+        self.hdr_p010_conv.as_ref().unwrap().convert(
+            &self.device,
+            &self.context,
+            &srv,
+            &out,
+            self.width,
+            self.height,
+        )?;
+        Ok(out)
+    }
+
     fn process_frame(&mut self, frame: Direct3D11CaptureFrame) -> Result<CapturedFrame> {
         unsafe {
             let surface = frame.Surface().context("frame Surface")?;
@@ -513,11 +569,40 @@ impl WgcCapturer {
                 .GetInterface()
                 .context("GetInterface ID3D11Texture2D")?;
 
+            // GATED P010-shader path (`PUNKTFUNK_HDR_SHADER_P010`): for HDR, emit P010 (BT.2020 PQ
+            // 10-bit limited range) DIRECTLY from our shader so NVENC takes native P010 and skips its
+            // SM-side RGB→YUV CSC. Runs BEFORE the R10 + VideoProcessor path. A converter/plane-RTV
+            // failure latches `p010_disabled` → we fall through to the unchanged R10 path for the rest
+            // of the session. Default OFF → none of this executes and behaviour is byte-for-byte as
+            // today.
+            if self.hdr && !self.p010_disabled && hdr_shader_p010_enabled() {
+                match self.hdr_to_p010(&src) {
+                    Ok(p010) => {
+                        // The P010 output is host-owned (the ring), and the FP16 CopyResource read
+                        // `src` synchronously on the immediate context before the shader passes — so we
+                        // do NOT need to hold `frame` past here (unlike the SDR/R10 in-place paths).
+                        // Dropping it returns the pool buffer to WGC immediately.
+                        drop(frame);
+                        self.last_present = Some((p010.clone(), PixelFormat::P010));
+                        return Ok(self.d3d11_frame(p010, PixelFormat::P010));
+                    }
+                    Err(e) => {
+                        tracing::warn!(error = %format!("{e:#}"),
+                            "WGC: HDR P010 shader path failed — disabling it, falling back to R10");
+                        self.p010_disabled = true;
+                        self.hdr_p010_conv = None;
+                        self.p010_out.clear();
+                    }
+                }
+            }
+
             // Preferred path: convert the OS-composited capture (cursor already in it) DIRECTLY to
             // NVENC's native YUV on the video processor — no CopyResource, no cursor draw, and NVENC
             // skips its internal RGB→YUV (the contended 3D step). WGC's multi-buffer pool + held set
             // means reading the pool texture directly does NOT serialize (unlike DDA's single-frame
-            // model). The frame is held until the async Blt finishes.
+            // model). The frame is held until the async Blt finishes. (HDR: the video processor can't
+            // ingest FP16 scRGB, so the Blt fails and we fall back to the R10 path below; the
+            // `PUNKTFUNK_HDR_SHADER_P010` branch above is the off-the-SM HDR path.)
             if let Some(yuv) = self.convert_to_yuv(&src, self.hdr) {
                 let fmt = if self.hdr {
                     PixelFormat::P010

crates/punktfunk-host/src/main.rs

@@ -130,6 +130,13 @@ fn real_main() -> Result<()> {
         // `PUNKTFUNK_NV12` convert is colour-correct. Prints PASS/FAIL + max Y/U/V error.
         #[cfg(target_os = "linux")]
         Some("nv12-selftest") => zerocopy::nv12_selftest(),
+        // HDR P010 colour self-test (Windows; no display/capture needed): upload a known scRGB FP16
+        // pattern, run the `HdrP010Converter` shader → P010 on the GPU, read the Y/UV planes back, and
+        // compare against an f64 BT.2020-PQ limited-range reference. Validates the
+        // `PUNKTFUNK_HDR_SHADER_P010` colour math without green-screening a live HDR stream. Prints
+        // PASS/FAIL + max Y/Cb/Cr error.
+        #[cfg(target_os = "windows")]
+        Some("hdr-p010-selftest") => crate::capture::dxgi::hdr_p010_selftest(),
         // Compositor readiness probe: exit 0 iff the (detected or PUNKTFUNK_COMPOSITOR-forced)
         // compositor is up and able to create a virtual output *now*. A session-bringup
         // script polls this to gate on real readiness instead of a blind `sleep`.
@@ -551,6 +558,9 @@ NOTES:
         \x20   punktfunk-host service install    register an auto-start SYSTEM service + firewall rules\n\
         \x20   punktfunk-host service uninstall  remove the service + firewall rules\n\
         \x20   punktfunk-host service start|stop|status\n\
-        \x20   config: %ProgramData%\\punktfunk\\host.env"
+        \x20   config: %ProgramData%\\punktfunk\\host.env\n\
+        \nWINDOWS DIAGNOSTICS:\n\
+        \x20   punktfunk-host hdr-p010-selftest  GPU colour check for the PUNKTFUNK_HDR_SHADER_P010 path\n\
+        \x20                                     (scRGB FP16 -> P010 BT.2020 PQ shader vs an f64 reference)"
     );
 }