From dce516437518f4a6cc6c894887284fb23ed7949e Mon Sep 17 00:00:00 2001
From: Allan Sandfeld Jensen <allan.jensen@qt.io>
Date: Mon, 10 Jun 2024 15:38:35 +0200
Subject: [PATCH] Fix BT.2100PQ

From and to algorithms were accidentally reversed before submitting
final patch.

Change-Id: I16e0afda26b77047facf4f65065224c8f3dce6c6
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
(cherry picked from commit 1b1f613b56fce643a7565e9280acc90e171caf7c)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
---
 src/gui/painting/qcolortransfergeneric_p.h    | 18 +++++++--------
 .../painting/qcolorspace/tst_qcolorspace.cpp  | 23 +++++++++++++++++++
 2 files changed, 32 insertions(+), 9 deletions(-)

diff --git a/src/gui/painting/qcolortransfergeneric_p.h b/src/gui/painting/qcolortransfergeneric_p.h
index b4e6e8dec7..c46f00e9dc 100644
--- a/src/gui/painting/qcolortransfergeneric_p.h
+++ b/src/gui/painting/qcolortransfergeneric_p.h
@@ -82,18 +82,18 @@ private:
     constexpr static float m_hlg_b = 1.f - (4.f * m_hlg_a);
     constexpr static float m_hlg_c = 0.55991073f; // 0.5 - a * ln(4 * a)
 
+    // PQ to linear [0-1] -> [0-64]
+    static float pqToLinear(float x)
+    {
+        x = std::pow(x, 1.f / m_pq_m2);
+        return std::pow((m_pq_c1 - x) / (m_pq_c3 * x - m_pq_c2), (1.f / m_pq_m1)) * m_pq_f;
+    }
+
     // PQ from linear [0-64] -> [0-1]
     static float pqFromLinear(float x)
     {
-        x = std::pow(x * (1.f / m_pq_f), (1.f / m_pq_m1));
-        return std::pow((m_pq_c1 - x) / (m_pq_c3 * x - m_pq_c2), (1.f / m_pq_m2));
-    }
-
-    // PQ from linear [0-1] -> [0-64]
-    static float pqToLinear(float x)
-    {
-        x = std::pow(x, m_pq_m1);
-        return std::pow((m_pq_c1 + m_pq_c2 * x) / (1.f + m_pq_c3 * x), m_pq_m2) * m_pq_f;
+        x = std::pow(x * (1.f / m_pq_f), m_pq_m1);
+        return std::pow((m_pq_c1 + m_pq_c2 * x) / (1.f + m_pq_c3 * x), m_pq_m2);
     }
 
     constexpr static float m_pq_c1 =  107.f / 128.f; // c3 - c2 + 1
diff --git a/tests/auto/gui/painting/qcolorspace/tst_qcolorspace.cpp b/tests/auto/gui/painting/qcolorspace/tst_qcolorspace.cpp
index 6c103ce4f3..04887a3350 100644
--- a/tests/auto/gui/painting/qcolorspace/tst_qcolorspace.cpp
+++ b/tests/auto/gui/painting/qcolorspace/tst_qcolorspace.cpp
@@ -7,6 +7,7 @@
 #include <qcolorspace.h>
 #include <qimage.h>
 #include <qimagereader.h>
+#include <qrgbafloat.h>
 
 #include <private/qcolorspace_p.h>
 
@@ -73,6 +74,7 @@ private slots:
     void grayColorSpaceEffectivelySRgb();
 
     void scaleAlphaValue();
+    void hdrColorSpaces();
 };
 
 tst_QColorSpace::tst_QColorSpace()
@@ -1064,5 +1066,26 @@ void tst_QColorSpace::scaleAlphaValue()
     QCOMPARE(reinterpret_cast<const QRgba64 *>(image.constBits())->alpha(), 257 * 125);
 }
 
+void tst_QColorSpace::hdrColorSpaces()
+{
+    QColorSpace bt2020linear = QColorSpace::Bt2020;
+    bt2020linear.setTransferFunction(QColorSpace::TransferFunction::Linear);
+
+    QColorTransform pqToLinear = QColorSpace(QColorSpace::Bt2100Pq).transformationToColorSpace(bt2020linear);
+    QColorTransform hlgToLinear = QColorSpace(QColorSpace::Bt2100Hlg).transformationToColorSpace(bt2020linear);
+
+    QColor maxWhite = QColor::fromRgbF(1.0f, 1.0f, 1.0f);
+    QColor hlgWhite = QColor::fromRgbF(0.5f, 0.5f, 0.5f);
+    QCOMPARE(hlgToLinear.map(maxWhite).redF(), 12.f);
+    QCOMPARE(hlgToLinear.map(maxWhite).greenF(), 12.f);
+    QCOMPARE(hlgToLinear.map(maxWhite).blueF(), 12.f);
+    QCOMPARE(hlgToLinear.map(hlgWhite).redF(), 1.f);
+    QCOMPARE(hlgToLinear.map(hlgWhite).greenF(), 1.f);
+    QCOMPARE(hlgToLinear.map(hlgWhite).blueF(), 1.f);
+    QCOMPARE(pqToLinear.map(maxWhite).redF(), 64.f);
+    QCOMPARE(pqToLinear.map(maxWhite).greenF(), 64.f);
+    QCOMPARE(pqToLinear.map(maxWhite).blueF(), 64.f);
+}
+
 QTEST_MAIN(tst_QColorSpace)
 #include "tst_qcolorspace.moc"