From e8dcbddea03900174694a1d19d4d7ec0f08c4e1d Mon Sep 17 00:00:00 2001
From: Geoffrey McRae <geoff@hostfission.com>
Date: Thu, 16 Jul 2026 09:28:01 +1000
Subject: [PATCH] [idd/client] correct HDR luminance when PQ conversion is in
 use

---
 client/displayservers/Wayland/gl.c  | 12 +++++++++++-
 idd/LGIdd/effect/CHDR16to10Effect.h |  4 ++--
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/client/displayservers/Wayland/gl.c b/client/displayservers/Wayland/gl.c
index 22c8d541..e103d57f 100644
--- a/client/displayservers/Wayland/gl.c
+++ b/client/displayservers/Wayland/gl.c
@@ -286,7 +286,7 @@ void waylandSetHDRImageDescription(const uint16_t displayPrimary[3][2],
   if (wlWm.cmHasLuminances)
     wp_image_description_creator_params_v1_set_luminances(
         wlWm.hdrImageCreator,
-        minDisplayLuminance > 0 ? minDisplayLuminance / 10000 : 50,
+        minDisplayLuminance > 0 ? minDisplayLuminance : 50,
         maxDisplayLuminance > 0 ? maxDisplayLuminance / 10000 : 1000,
         hdrPQ                   ? 203                         : 80);
 
@@ -295,6 +295,7 @@ void waylandSetHDRImageDescription(const uint16_t displayPrimary[3][2],
   // primaries from the metadata (even if zero, in which case the
   // compositor uses its own defaults).
   if (wlWm.cmHasMasteringPrimaries)
+  {
     wp_image_description_creator_params_v1_set_mastering_display_primaries(
         wlWm.hdrImageCreator,
         displayPrimary[0][0], displayPrimary[0][1],
@@ -302,6 +303,15 @@ 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)
diff --git a/idd/LGIdd/effect/CHDR16to10Effect.h b/idd/LGIdd/effect/CHDR16to10Effect.h
index cf701f73..4da4b0b4 100644
--- a/idd/LGIdd/effect/CHDR16to10Effect.h
+++ b/idd/LGIdd/effect/CHDR16to10Effect.h
@@ -18,8 +18,8 @@ class CHDR16to10Effect : public CComputeEffect
 private:
   struct Consts
   {
-    float ReferenceWhiteNits; // scRGB reference white in nits (typically 80)
-  } m_consts = { 80.0f };
+    float ReferenceWhiteNits; // scRGB reference white in nits (BT.2408 recommends 203)
+  } m_consts = { 203.0f };
   ComPtr<ID3D12Resource> m_constBuffer;
 
 public: