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[idd] hdr: correct the BT.709 to BT.2020 conversion

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This commit is contained in:
Geoffrey McRae
2026-07-16 23:03:13 +10:00
parent e332dc77f2
commit aa25b0cfe3
3 changed files with 72 additions and 2 deletions
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22
idd/LGIdd/effect/CHDR16to10Effect.cpp
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@@ -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;