[client/idd] hdr: fix incorrect tone mapping knee bug

This reverts commit 05d8bc5311.

client/displayservers/Wayland/gl.c

@@ -280,22 +280,21 @@ void waylandSetHDRImageDescription(const uint16_t displayPrimary[3][2],
       hdrPQ ? WP_COLOR_MANAGER_V1_TRANSFER_FUNCTION_ST2084_PQ
             : WP_COLOR_MANAGER_V1_TRANSFER_FUNCTION_EXT_LINEAR);
 
-  // Set luminance range from metadata.
-  // min_lum is in 0.0001 cd/m² units (multiplied by 10000 as per protocol).
-  // max_lum and reference_lum are in unscaled cd/m² units.
+  // Set the primary colour volume luminances.
+  //   min_lum       : 0.0001 cd/m² (source already scaled) -> pass through
+  //   max_lum       : cd/m² (used only for scRGB; ignored for PQ, where the
+  //                   compositor forces it to min_lum + 10000 cd/m²)
+  //   reference_lum : cd/m² -> BT.2408 diffuse white (203 PQ / 80 scRGB)
   if (wlWm.cmHasLuminances)
     wp_image_description_creator_params_v1_set_luminances(
         wlWm.hdrImageCreator,
         minDisplayLuminance > 0 ? minDisplayLuminance : 50,
-        maxDisplayLuminance > 0 ? maxDisplayLuminance / 10000 : 1000,
-        hdrPQ                   ? 203                         : 80);
+        maxDisplayLuminance > 0 ? maxDisplayLuminance : 1000,
+        hdrPQ                   ? 203                 : 80);
 
-  // Set mastering display primaries from frame HDR metadata.
-  // Always set when the compositor supports it, falling back to the
-  // primaries from the metadata (even if zero, in which case the
-  // compositor uses its own defaults).
+  // Advertise the content primaries (BT.2020 for PQ). These describe the real
+  // colour gamut of the signal and are safe to forward.
   if (wlWm.cmHasMasteringPrimaries)
-  {
     wp_image_description_creator_params_v1_set_mastering_display_primaries(
         wlWm.hdrImageCreator,
         displayPrimary[0][0], displayPrimary[0][1],
@@ -303,19 +302,16 @@ void waylandSetHDRImageDescription(const uint16_t displayPrimary[3][2],
         displayPrimary[2][0], displayPrimary[2][1],
         whitePoint[0], whitePoint[1]);
 
-    // Set mastering luminance range so the compositor knows the target
-    // color volume for tone mapping.  min_lum is in ×10000 units (0.0001 cd/m²),
-    // max_lum is in unscaled cd/m².
-    wp_image_description_creator_params_v1_set_mastering_luminance(
-        wlWm.hdrImageCreator,
-        minDisplayLuminance > 0 ? minDisplayLuminance : 50,
-        maxDisplayLuminance > 0 ? maxDisplayLuminance / 10000 : 1000);
-  }
-
-  if (maxCLL > 0)
-    wp_image_description_creator_params_v1_set_max_cll(wlWm.hdrImageCreator, maxCLL);
-  if (maxFALL > 0)
-    wp_image_description_creator_params_v1_set_max_fall(wlWm.hdrImageCreator, maxFALL);
+  // NOTE: we deliberately do NOT forward the host's mastering luminance range,
+  // MaxCLL or MaxFALL. Those values describe the *virtual display* the IDD
+  // advertises to Windows (~1000 cd/m² from our EDID), not the content. LG
+  // captures the full-range scRGB desktop composition and passes it through to
+  // PQ without tone mapping, so the signal spans the entire PQ range (scRGB
+  // 125.0 == 10000 cd/m²). Advertising the virtual display's peak as the
+  // content peak makes the compositor place its tone-mapping knee there and
+  // hard-clip everything brighter, crushing highlights. Leaving these unset
+  // lets the content be treated as standard full-range PQ so the compositor
+  // tone-maps it to the actual physical display.
 
   wlWm.hdrImageDesc =
     wp_image_description_creator_params_v1_create(wlWm.hdrImageCreator);
@@ -342,8 +338,10 @@ void waylandSetHDRImageDescription(const uint16_t displayPrimary[3][2],
       WP_COLOR_MANAGER_V1_RENDER_INTENT_PERCEPTUAL);
 
   wlWm.hdrActive = true;
-  DEBUG_INFO("HDR image description set on surface (%s, %s, %u nits)",
-      hdrPQ ? "PQ" : "scRGB", hdrPQ ? "BT.2020" : "sRGB", maxDisplayLuminance);
+  DEBUG_INFO("HDR image description set on surface (%s, %s, "
+      "maxLum:%u cd/m² minLum:%u (0.0001 cd/m²) maxCLL:%u maxFALL:%u)",
+      hdrPQ ? "PQ" : "scRGB", hdrPQ ? "BT.2020" : "sRGB",
+      maxDisplayLuminance, minDisplayLuminance, maxCLL, maxFALL);
 }
 
 bool waylandRequestHDR(const uint16_t displayPrimary[3][2],

common/include/common/KVMFR.h

@@ -167,7 +167,11 @@ typedef struct KVMFRFrame
   // Display color primaries in 0.00002 units (SMPTE ST 2086 format)
   uint16_t hdrDisplayPrimary[3][2];      // Rx,Ry, Gx,Gy, Bx,By
   uint16_t hdrWhitePoint[2];             // Wx, Wy
-  uint32_t hdrMaxDisplayLuminance;       // Max mastering display luminance (0.0001 cd/m²)
+  // Mastering display luminances follow SMPTE ST 2086 units: the maximum is
+  // in whole cd/m², the minimum in 0.0001 cd/m². (Note: the DXGI docs
+  // describe MaxMasteringLuminance as 0.0001 cd/m², but IddCx/ST 2086 provide
+  // it in whole cd/m².)
+  uint32_t hdrMaxDisplayLuminance;       // Max mastering display luminance (cd/m²)
   uint32_t hdrMinDisplayLuminance;       // Min mastering display luminance (0.0001 cd/m²)
   uint32_t hdrMaxContentLightLevel;      // MaxCLL (cd/m²)
   uint32_t hdrMaxFrameAverageLightLevel; // MaxFALL (cd/m²)

idd/LGIdd/CSwapChainProcessor.cpp

@@ -396,8 +396,9 @@ bool CSwapChainProcessor::SwapChainNewFrame(ComPtr<IDXGIResource> acquiredBuffer
       // D65 white point (in 0.00002 units):
       srcFormat.whitePoint[0] = 15635;
       srcFormat.whitePoint[1] = 16450;
-      // 80 cd/m² display, 0.005 cd/m² black (in 0.0001 cd/m² units):
-      srcFormat.maxDisplayLuminance = 800000;
+      // Mastering luminances follow SMPTE ST 2086 units: max in whole cd/m²,
+      // min in 0.0001 cd/m². 80 cd/m² display, 0.005 cd/m² black:
+      srcFormat.maxDisplayLuminance = 80;
       srcFormat.minDisplayLuminance = 50;
       // Content light levels unknown:
       srcFormat.maxContentLightLevel      = 0;

idd/LGIdd/effect/CHDR16to10Effect.h

@@ -18,8 +18,8 @@ class CHDR16to10Effect : public CComputeEffect
 private:
   struct Consts
   {
-    float ReferenceWhiteNits; // scRGB reference white in nits (BT.2408 recommends 203)
-  } m_consts = { 203.0f };
+    float ReferenceWhiteNits; // scRGB reference white in nits (typically 80)
+  } m_consts = { 80.0f };
   ComPtr<ID3D12Resource> m_constBuffer;
 
 public: