core/colorpipeline: refactor tone mapping to be about dimming instead of addnig headroom

No functional change, just so that the logic is a bit easier to follow

src/core/colorpipeline.cpp

@@ -37,7 +37,7 @@ ColorPipeline ColorPipeline::create(const ColorDescription &from, const ColorDes
     // that's not necessarily true, and figuring out the actual range could be complicated..
     ret.addMatrix(from.toOther(to, intent), ret.currentOutputRange() * (to.referenceLuminance() / from.referenceLuminance()));
     if (!s_disableTonemapping && ret.currentOutputRange().max > maxOutputLuminance * 1.01 && intent == RenderingIntent::Perceptual) {
-        ret.addTonemapper(to.containerColorimetry(), to.referenceLuminance(), ret.currentOutputRange().max, maxOutputLuminance, 1.5);
+        ret.addTonemapper(to.containerColorimetry(), to.referenceLuminance(), ret.currentOutputRange().max, maxOutputLuminance);
     }
 
     ret.addInverseTransferFunction(to.transferFunction());
@@ -240,7 +240,7 @@ static const QMatrix4x4 s_toICtCp = QMatrix4x4(
     0.0,               0.0,               0.0,             1.0).transposed();
 static const QMatrix4x4 s_fromICtCp = s_toICtCp.inverted();
 
-void ColorPipeline::addTonemapper(const Colorimetry &containerColorimetry, double referenceLuminance, double maxInputLuminance, double maxOutputLuminance, double maxAddedHeadroom)
+void ColorPipeline::addTonemapper(const Colorimetry &containerColorimetry, double referenceLuminance, double maxInputLuminance, double maxOutputLuminance)
 {
     // convert from rgb to ICtCp
     addMatrix(containerColorimetry.toLMS(), currentOutputRange());
@@ -249,8 +249,8 @@ void ColorPipeline::addTonemapper(const Colorimetry &containerColorimetry, doubl
     // apply the tone mapping to the intensity component
     ops.push_back(ColorOp{
         .input = currentOutputRange(),
-        .operation = ColorTonemapper(referenceLuminance, maxInputLuminance, maxOutputLuminance, maxAddedHeadroom),
-        .output = ValueRange {
+        .operation = ColorTonemapper(referenceLuminance, maxInputLuminance, maxOutputLuminance),
+        .output = ValueRange{
             .min = currentOutputRange().min,
             .max = maxOutputLuminance,
         },
@@ -335,23 +335,26 @@ ColorMultiplier::ColorMultiplier(double factor)
 {
 }
 
-ColorTonemapper::ColorTonemapper(double referenceLuminance, double maxInputLuminance, double maxOutputLuminance, double maxAddedHeadroom)
+ColorTonemapper::ColorTonemapper(double referenceLuminance, double maxInputLuminance, double maxOutputLuminance)
     : m_inputReferenceLuminance(referenceLuminance)
     , m_maxInputLuminance(maxInputLuminance)
     , m_maxOutputLuminance(maxOutputLuminance)
 {
     m_inputRange = maxInputLuminance / referenceLuminance;
     const double outputRange = maxOutputLuminance / referenceLuminance;
-    // = how much dynamic range this algorithm adds, by reducing the reference luminance
-    m_addedRange = std::min(m_inputRange, std::clamp(maxAddedHeadroom / outputRange, 1.0, maxAddedHeadroom));
-    m_outputReferenceLuminance = referenceLuminance / m_addedRange;
+    // how much range we need to at least decently present the content
+    // 50% HDR headroom should be enough for the tone mapper to do a good enough job, without dimming the image too much
+    const double minDecentRange = std::min(m_inputRange, 1.5);
+    // if the output doesn't provide enough HDR headroom for the tone mapper to do a good job, dim the image to create some
+    m_referenceDimming = 1.0 / std::clamp(outputRange / minDecentRange, 1.0, minDecentRange);
+    m_outputReferenceLuminance = referenceLuminance * m_referenceDimming;
 }
 
 double ColorTonemapper::map(double pqEncodedLuminance) const
 {
     const double luminance = TransferFunction(TransferFunction::PerceptualQuantizer).encodedToNits(pqEncodedLuminance);
     // keep things linear up to the reference luminance
-    const double low = std::min(luminance / m_addedRange, m_outputReferenceLuminance);
+    const double low = std::min(luminance * m_referenceDimming, m_outputReferenceLuminance);
     // and apply a nonlinear curve above, to reduce the luminance without completely removing differences
     const double relativeHighlight = std::clamp((luminance / m_inputReferenceLuminance - 1.0) / (m_inputRange - 1.0), 0.0, 1.0);
     const double high = std::log(relativeHighlight * (std::numbers::e - 1) + 1) * (m_maxOutputLuminance - m_outputReferenceLuminance);

src/core/colorpipeline.h

@@ -67,7 +67,7 @@ public:
 class KWIN_EXPORT ColorTonemapper
 {
 public:
-    explicit ColorTonemapper(double referenceLuminance, double maxInputLuminance, double maxOutputLuminance, double maxAddedHeadroom);
+    explicit ColorTonemapper(double referenceLuminance, double maxInputLuminance, double maxOutputLuminance);
 
     double map(double pqEncodedLuminance) const;
     bool operator==(const ColorTonemapper &) const = default;
@@ -77,7 +77,7 @@ public:
     double m_maxOutputLuminance;
 private:
     double m_inputRange;
-    double m_addedRange;
+    double m_referenceDimming;
     double m_outputReferenceLuminance;
 };
 
@@ -118,7 +118,7 @@ public:
     void addTransferFunction(TransferFunction tf);
     void addInverseTransferFunction(TransferFunction tf);
     void addMatrix(const QMatrix4x4 &mat, const ValueRange &output);
-    void addTonemapper(const Colorimetry &containerColorimetry, double referenceLuminance, double maxInputLuminance, double maxOutputLuminance, double maxAddedHeadroom);
+    void addTonemapper(const Colorimetry &containerColorimetry, double referenceLuminance, double maxInputLuminance, double maxOutputLuminance);
     void add(const ColorOp &op);
 
     ValueRange inputRange;

src/opengl/colormanagement.glsl

@@ -114,14 +114,16 @@ vec3 doTonemapping(vec3 color) {
     vec3 ICtCp = toICtCp * lms_PQ;
     float luminance = singlePqToLinear(ICtCp.r) * 10000.0;
 
-    // if the reference is too close to the maximum luminance, reduce it to get up to 50% headroom
     float inputRange = maxTonemappingLuminance / destinationReferenceLuminance;
     float outputRange = maxDestinationLuminance / destinationReferenceLuminance;
-    float addedRange = min(inputRange, clamp(1.5 / outputRange, 1.0, 1.5));
-    float outputReferenceLuminance = destinationReferenceLuminance / addedRange;
+    // how much dynamic range we need to decently present the content
+    float minDecentRange = min(inputRange, 1.5);
+    // if the output doesn't provide enough HDR headroom for the tone mapper to do a good job, dim the image to create some
+    float referenceDimming = 1.0 / clamp(outputRange / minDecentRange, 1.0, minDecentRange);
+    float outputReferenceLuminance = destinationReferenceLuminance * referenceDimming;
 
     // keep it linear up to the reference luminance
-    float low = min(luminance / addedRange, outputReferenceLuminance);
+    float low = min(luminance * referenceDimming, outputReferenceLuminance);
     // and apply a nonlinear curve above, to reduce the luminance without completely removing differences
     float relativeHighlight = clamp((luminance / destinationReferenceLuminance - 1.0) / (inputRange - 1.0), 0.0, 1.0);
     const float e = 2.718281828459045;