From aa25b0cfe34da6c6bc271e2c9fc3cf46af929a08 Mon Sep 17 00:00:00 2001
From: Geoffrey McRae <geoff@hostfission.com>
Date: Thu, 16 Jul 2026 23:03:13 +1000
Subject: [PATCH] [idd] hdr: correct the BT.709 to BT.2020 conversion

---
 idd/LGIdd/effect/CHDR16to10Effect.cpp | 22 ++++++++++--
 obs/effect/hdrpq.effect               | 52 +++++++++++++++++++++++++++
 resources/{icondata.h => icondata.c}  |  0
 3 files changed, 72 insertions(+), 2 deletions(-)
 create mode 100644 obs/effect/hdrpq.effect
 rename resources/{icondata.h => icondata.c} (100%)

diff --git a/idd/LGIdd/effect/CHDR16to10Effect.cpp b/idd/LGIdd/effect/CHDR16to10Effect.cpp
index b431009f..9acc38cb 100644
--- a/idd/LGIdd/effect/CHDR16to10Effect.cpp
+++ b/idd/LGIdd/effect/CHDR16to10Effect.cpp
@@ -49,8 +49,17 @@ bool CHDR16to10Effect::Init(const ComPtr<ID3D12Device3>& device)
     "void main(uint3 dt : SV_DispatchThreadID)\n"
     "{\n"
     "  float3 linearValue = src[dt.xy].rgb * ReferenceWhiteNits;\n"
+    "  // scRGB uses BT.709 primaries whereas HDR10/PQ output uses BT.2020.\n"
+    "  // Rotate the gamut in linear light (BT.2087) BEFORE applying the PQ\n"
+    "  // curve, otherwise the BT.709 values are later reinterpreted as BT.2020\n"
+    "  // and saturated colours (most visibly red) are pushed outside their\n"
+    "  // intended gamut.\n"
+    "  float3 rec2020 = float3(\n"
+    "    dot(linearValue, float3(0.6274039, 0.3292830, 0.0433131)),\n"
+    "    dot(linearValue, float3(0.0690973, 0.9195404, 0.0113623)),\n"
+    "    dot(linearValue, float3(0.0163914, 0.0880133, 0.8955953)));\n"
     "  // scRGB to PQ (ST.2084)\n"
-    "  float3 Y = linearValue / 10000.0;\n"
+    "  float3 Y = rec2020 / 10000.0;\n"
     "  float3 Ym1 = pow(max(Y, 0.0), PQ_m1);\n"
     "  float3 pq = pow((PQ_c1 + PQ_c2 * Ym1) / (1.0 + PQ_c3 * Ym1), PQ_m2);\n"
     "  dst[dt.xy] = float4(pq, src[dt.xy].a);\n"
@@ -116,7 +125,16 @@ PostProcessStatus CHDR16to10Effect::SetFormat(
 
   // Explicitly propagate HDR static metadata so it survives post-processing.
   // Do not rely on the caller's copy semantics in CPostProcessor::Configure().
-  memcpy(dst.displayPrimary, src.displayPrimary, sizeof(dst.displayPrimary));
+  // The shader rotates the gamut from BT.709 to BT.2020, so advertise BT.2020
+  // mastering primaries here rather than forwarding the source's BT.709 set
+  // (in 0.00002 units) - otherwise consumers see PQ/BT.2020 content tagged
+  // with BT.709 primaries.
+  dst.displayPrimary[0][0] = 35400; // Rx
+  dst.displayPrimary[0][1] = 14600; // Ry
+  dst.displayPrimary[1][0] =  8500; // Gx
+  dst.displayPrimary[1][1] = 39850; // Gy
+  dst.displayPrimary[2][0] =  6550; // Bx
+  dst.displayPrimary[2][1] =  2300; // By
   memcpy(dst.whitePoint    , src.whitePoint    , sizeof(dst.whitePoint    ));
   dst.maxDisplayLuminance       = src.maxDisplayLuminance;
   dst.minDisplayLuminance       = src.minDisplayLuminance;
diff --git a/obs/effect/hdrpq.effect b/obs/effect/hdrpq.effect
new file mode 100644
index 00000000..9ec6e104
--- /dev/null
+++ b/obs/effect/hdrpq.effect
@@ -0,0 +1,52 @@
+uniform float4x4  ViewProj;
+uniform texture2d image;
+uniform float2    outputSize;
+uniform float3    colorMatrix0;
+uniform float3    colorMatrix1;
+uniform float3    colorMatrix2;
+uniform float     scale;
+
+struct VertData
+{
+  float4 pos : POSITION;
+  float2 uv  : TEXCOORD0;
+};
+
+VertData VSDefault(VertData vert_in)
+{
+  VertData vert_out;
+  vert_out.pos = mul(float4(vert_in.pos.xyz, 1.0), ViewProj);
+  vert_out.uv  = vert_in.uv;
+  return vert_out;
+}
+
+float4 PSHDRPQ(VertData vert_in) : TARGET
+{
+  uvec2 outputPos = uvec2(vert_in.uv * outputSize);
+  vec4  pixel     = texelFetch(image, ivec2(outputPos), 0);
+
+  /* SMPTE ST.2084 (PQ) EOTF: recover normalised linear light where 1.0
+   * corresponds to 10000 cd/m² */
+  vec3 e   = pow(max(pixel.rgb, 0.0), vec3(1.0 / 78.84375));
+  vec3 num = max(e - 0.8359375, 0.0);
+  vec3 den = 18.8515625 - 18.6875 * e;
+  vec3 lin = pow(num / den, vec3(1.0 / 0.1593017578125));
+
+  /* convert from the source display primaries to BT.709 in linear light */
+  vec3 rgb709 = vec3(
+    dot(colorMatrix0, lin),
+    dot(colorMatrix1, lin),
+    dot(colorMatrix2, lin));
+
+  /* scale to the scRGB reference white convention (1.0 = 80 cd/m²) */
+  return float4(rgb709 * scale, pixel.a);
+}
+
+technique Draw
+{
+  pass
+  {
+    vertex_shader = VSDefault(vert_in);
+    pixel_shader  = PSHDRPQ(vert_in);
+  }
+}
diff --git a/resources/icondata.h b/resources/icondata.c
similarity index 100%
rename from resources/icondata.h
rename to resources/icondata.c