fix(windows/capture): match the HDR-composited cursor to the target's SDR white level

sRGB→linear alone lands cursor-white at 80 nits (scRGB 1.0) while DWM
composes the surrounding SDR desktop at the user's SDR-brightness
setting (~200+ nits by default) — the cursor read visibly dark on HDR.
Scale by DISPLAYCONFIG_SDR_WHITE_LEVEL (new
win_display::sdr_white_level_scale, queried on blend-scratch rebuilds —
the CCD query stays off the per-frame path).

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

crates/pf-capture/src/windows/idd_push.rs

@@ -429,6 +429,10 @@ pub struct IddPushCapturer {
     last_blend_key: Option<(u64, i32, i32, bool)>,
     /// The ring slot of the last FRESH publish — the regen source.
     last_slot: Option<usize>,
+    /// The target's SDR-white scale (vs 80 nits) for HDR cursor compositing — refreshed on
+    /// each blend-scratch rebuild (first use + ring geometry changes). 2.5 ≈ the Windows
+    /// SDR-brightness default; without it the composited cursor renders visibly dark on HDR.
+    sdr_white_scale: f32,
     width: u32,
     height: u32,
     slots: Vec<HostSlot>,
@@ -1090,6 +1094,7 @@ impl IddPushCapturer {
                 blend_scratch: None,
                 last_blend_key: None,
                 last_slot: None,
+                sdr_white_scale: 1.0,
                 // Held from BEFORE the first-frame gate (the display must not idle off while we
                 // wait for the first compose) until the capturer drops with the session.
                 _display_wake: pf_frame::session_tuning::DisplayWakeRequest::new(),
@@ -1819,9 +1824,14 @@ impl IddPushCapturer {
                 }
             }
             if let Some(pass) = self.cursor_blend.as_mut() {
-                // FP16 ring = scRGB linear composition (HDR): linearize the sRGB shape.
-                let is_linear = self.display_hdr;
-                if let Err(e) = pass.blend(&self.device, &self.context, &tex, &ov, is_linear) {
+                // FP16 ring = scRGB linear composition (HDR): linearize the sRGB shape and
+                // scale it to the target's SDR white so it matches the desktop around it.
+                let scale = if self.display_hdr {
+                    self.sdr_white_scale
+                } else {
+                    0.0
+                };
+                if let Err(e) = pass.blend(&self.device, &self.context, &tex, &ov, scale) {
                     if !self.cursor_blend_failed {
                         self.cursor_blend_failed = true;
                         tracing::warn!("cursor blend draw failed — pointer-less frames: {e:#}");

crates/pf-capture/src/windows/idd_push/cursor_blend.rs

@@ -26,16 +26,18 @@ use windows::Win32::Graphics::Direct3D11::{
 };
 use windows::Win32::Graphics::Dxgi::Common::DXGI_FORMAT_R8G8B8A8_UNORM;
 
-/// Straight-alpha sample of the cursor bitmap; `to_linear` ≠ 0 linearizes sRGB→scRGB for an
-/// FP16 (HDR-composed) frame so the cursor doesn't glow at 8× SDR white.
+/// Straight-alpha sample of the cursor bitmap. `linear_scale` = 0 passes sRGB through (SDR
+/// ring); non-zero linearizes sRGB→scRGB AND multiplies by the target's SDR-white scale
+/// (`sdr_white_level_scale` — 1.0 would put cursor-white at 80 nits, visibly DARKER than the
+/// surrounding SDR desktop content DWM composes at the user's SDR-brightness setting).
 const CURSOR_PS: &str = r"
 Texture2D<float4> tx : register(t0);
 SamplerState sm : register(s0);
-cbuffer C : register(b0) { float to_linear; float3 pad; };
+cbuffer C : register(b0) { float linear_scale; float3 pad; };
 float4 main(float4 pos : SV_POSITION, float2 uv : TEXCOORD0) : SV_Target {
     float4 c = tx.Sample(sm, uv);
-    if (to_linear != 0.0) {
-        c.rgb = pow(abs(c.rgb), 2.2);
+    if (linear_scale != 0.0) {
+        c.rgb = pow(abs(c.rgb), 2.2) * linear_scale;
     }
     return c;
 }
@@ -48,7 +50,7 @@ pub(super) struct CursorBlendPass {
     sampler: ID3D11SamplerState,
     blend: ID3D11BlendState,
     cbuf: ID3D11Buffer,
-    cbuf_is_linear: Option<bool>,
+    cbuf_scale: Option<f32>,
     /// The uploaded shape (serial-keyed): SRV + dims in host pixels.
     shape: Option<(u64, ID3D11ShaderResourceView, u32, u32)>,
 }
@@ -103,7 +105,7 @@ impl CursorBlendPass {
             sampler: sampler.context("cursor blend sampler")?,
             blend: blend.context("cursor blend state")?,
             cbuf: cbuf.context("cursor blend cbuf")?,
-            cbuf_is_linear: None,
+            cbuf_scale: None,
             shape: None,
         })
     }
@@ -153,21 +155,22 @@ impl CursorBlendPass {
     }
 
     /// Alpha-blend `ov` onto `dst` (frame-sized, RENDER_TARGET-capable — the blend scratch).
-    /// `is_linear` = the frame is FP16 scRGB (HDR composition). The quad is placed purely via
-    /// the viewport (the fullscreen-triangle VS fills whatever viewport is set), clipped by the
-    /// target automatically.
+    /// `linear_scale`: 0 = SDR passthrough; non-zero = the frame is FP16 scRGB (HDR
+    /// composition) — linearize and scale to the target's SDR white. The quad is placed purely
+    /// via the viewport (the fullscreen-triangle VS fills whatever viewport is set), clipped by
+    /// the target automatically.
     pub(super) unsafe fn blend(
         &mut self,
         device: &ID3D11Device,
         ctx: &ID3D11DeviceContext,
         dst: &ID3D11Texture2D,
         ov: &pf_frame::CursorOverlay,
-        is_linear: bool,
+        linear_scale: f32,
     ) -> Result<()> {
         self.ensure_shape(device, ov)?;
         let (_, srv, w, h) = self.shape.as_ref().expect("shape just ensured");
-        if self.cbuf_is_linear != Some(is_linear) {
-            let cb: [f32; 4] = [if is_linear { 1.0 } else { 0.0 }, 0.0, 0.0, 0.0];
+        if self.cbuf_scale != Some(linear_scale) {
+            let cb: [f32; 4] = [linear_scale, 0.0, 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))
@@ -176,7 +179,7 @@ impl CursorBlendPass {
                 std::ptr::copy_nonoverlapping(cb.as_ptr(), mapped.pData as *mut f32, cb.len());
                 ctx.Unmap(&self.cbuf, 0);
             }
-            self.cbuf_is_linear = Some(is_linear);
+            self.cbuf_scale = Some(linear_scale);
         }
         let mut rtv: Option<ID3D11RenderTargetView> = None;
         device

crates/pf-win-display/src/win_display.rs

@@ -23,10 +23,10 @@ use windows::core::PCWSTR;
 use windows::Win32::Devices::Display::{
     DisplayConfigGetDeviceInfo, DisplayConfigSetDeviceInfo, GetDisplayConfigBufferSizes,
     QueryDisplayConfig, SetDisplayConfig, DISPLAYCONFIG_DEVICE_INFO_GET_ADVANCED_COLOR_INFO,
-    DISPLAYCONFIG_DEVICE_INFO_GET_SOURCE_NAME, DISPLAYCONFIG_DEVICE_INFO_GET_TARGET_NAME,
-    DISPLAYCONFIG_DEVICE_INFO_SET_ADVANCED_COLOR_STATE, DISPLAYCONFIG_GET_ADVANCED_COLOR_INFO,
-    DISPLAYCONFIG_MODE_INFO, DISPLAYCONFIG_MODE_INFO_TYPE_SOURCE,
-    DISPLAYCONFIG_OUTPUT_TECHNOLOGY_COMPONENT_VIDEO,
+    DISPLAYCONFIG_DEVICE_INFO_GET_SDR_WHITE_LEVEL, DISPLAYCONFIG_DEVICE_INFO_GET_SOURCE_NAME,
+    DISPLAYCONFIG_DEVICE_INFO_GET_TARGET_NAME, DISPLAYCONFIG_DEVICE_INFO_SET_ADVANCED_COLOR_STATE,
+    DISPLAYCONFIG_GET_ADVANCED_COLOR_INFO, DISPLAYCONFIG_MODE_INFO,
+    DISPLAYCONFIG_MODE_INFO_TYPE_SOURCE, DISPLAYCONFIG_OUTPUT_TECHNOLOGY_COMPONENT_VIDEO,
     DISPLAYCONFIG_OUTPUT_TECHNOLOGY_COMPOSITE_VIDEO,
     DISPLAYCONFIG_OUTPUT_TECHNOLOGY_DISPLAYPORT_EMBEDDED,
     DISPLAYCONFIG_OUTPUT_TECHNOLOGY_DISPLAYPORT_EXTERNAL, DISPLAYCONFIG_OUTPUT_TECHNOLOGY_DVI,
@@ -35,10 +35,11 @@ use windows::Win32::Devices::Display::{
     DISPLAYCONFIG_OUTPUT_TECHNOLOGY_SDI, DISPLAYCONFIG_OUTPUT_TECHNOLOGY_SDTVDONGLE,
     DISPLAYCONFIG_OUTPUT_TECHNOLOGY_SVIDEO, DISPLAYCONFIG_OUTPUT_TECHNOLOGY_UDI_EMBEDDED,
     DISPLAYCONFIG_OUTPUT_TECHNOLOGY_UDI_EXTERNAL, DISPLAYCONFIG_PATH_INFO,
-    DISPLAYCONFIG_SET_ADVANCED_COLOR_STATE, DISPLAYCONFIG_SOURCE_DEVICE_NAME,
-    DISPLAYCONFIG_TARGET_DEVICE_NAME, DISPLAYCONFIG_VIDEO_OUTPUT_TECHNOLOGY, QDC_ALL_PATHS,
-    QDC_ONLY_ACTIVE_PATHS, SDC_ALLOW_CHANGES, SDC_APPLY, SDC_FORCE_MODE_ENUMERATION,
-    SDC_SAVE_TO_DATABASE, SDC_TOPOLOGY_EXTEND, SDC_USE_SUPPLIED_DISPLAY_CONFIG,
+    DISPLAYCONFIG_SDR_WHITE_LEVEL, DISPLAYCONFIG_SET_ADVANCED_COLOR_STATE,
+    DISPLAYCONFIG_SOURCE_DEVICE_NAME, DISPLAYCONFIG_TARGET_DEVICE_NAME,
+    DISPLAYCONFIG_VIDEO_OUTPUT_TECHNOLOGY, QDC_ALL_PATHS, QDC_ONLY_ACTIVE_PATHS, SDC_ALLOW_CHANGES,
+    SDC_APPLY, SDC_FORCE_MODE_ENUMERATION, SDC_SAVE_TO_DATABASE, SDC_TOPOLOGY_EXTEND,
+    SDC_USE_SUPPLIED_DISPLAY_CONFIG,
 };
 use windows::Win32::Foundation::POINTL;
 use windows::Win32::Graphics::Gdi::{
@@ -481,6 +482,52 @@ pub unsafe fn advanced_color_enabled(target_id: u32) -> Option<bool> {
     None
 }
 
+/// The target's SDR white level as a SCALE relative to 80 nits (`1.0` = 80 nits): where DWM
+/// places SDR-white when composing SDR content onto this HDR desktop. An SDR-authored overlay
+/// (the composited cursor) must be multiplied by this in scRGB space or it renders visibly
+/// darker than the surrounding SDR desktop content (the Windows "SDR content brightness"
+/// slider default alone is ~2.5x). `None` = query failed / target not active (callers keep
+/// their last value or 1.0).
+///
+/// # Safety
+/// Runs the read-only CCD query FFI over owned locals (same shape as [`advanced_color_enabled`]).
+pub unsafe fn sdr_white_level_scale(target_id: u32) -> Option<f32> {
+    let mut np = 0u32;
+    let mut nm = 0u32;
+    if GetDisplayConfigBufferSizes(QDC_ONLY_ACTIVE_PATHS, &mut np, &mut nm).is_err() {
+        return None;
+    }
+    let mut paths = vec![DISPLAYCONFIG_PATH_INFO::default(); np as usize];
+    let mut modes = vec![DISPLAYCONFIG_MODE_INFO::default(); nm as usize];
+    if QueryDisplayConfig(
+        QDC_ONLY_ACTIVE_PATHS,
+        &mut np,
+        paths.as_mut_ptr(),
+        &mut nm,
+        modes.as_mut_ptr(),
+        None,
+    )
+    .is_err()
+    {
+        return None;
+    }
+    for p in paths.iter().take(np as usize) {
+        if p.targetInfo.id == target_id {
+            let mut info = DISPLAYCONFIG_SDR_WHITE_LEVEL::default();
+            info.header.r#type = DISPLAYCONFIG_DEVICE_INFO_GET_SDR_WHITE_LEVEL;
+            info.header.size = size_of::<DISPLAYCONFIG_SDR_WHITE_LEVEL>() as u32;
+            info.header.adapterId = p.targetInfo.adapterId;
+            info.header.id = p.targetInfo.id;
+            if DisplayConfigGetDeviceInfo(&mut info.header) == 0 && info.SDRWhiteLevel > 0 {
+                // Contract: SDRWhiteLevel/1000 * 80 = nits, i.e. the /1000 IS the 80-nit scale.
+                return Some(info.SDRWhiteLevel as f32 / 1000.0);
+            }
+            return None;
+        }
+    }
+    None
+}
+
 /// Force the freshly-added virtual monitor to the client's exact `WxH@Hz`. The ADD IOCTL only
 /// ADVERTISES the mode; Windows otherwise activates an IDD target at a 1280x720 default, so the
 /// ACTIVE mode (what DXGI Desktop Duplication captures) must be set explicitly. CDS_TEST first so a