core/colorspace: implement black point compensation

src/core/colorpipeline.cpp

@@ -11,6 +11,14 @@
 namespace KWin
 {
 
+ValueRange ValueRange::operator*(double mult) const
+{
+    return ValueRange{
+        .min = min * mult,
+        .max = max * mult,
+    };
+}
+
 ColorPipeline ColorPipeline::create(const ColorDescription &from, const ColorDescription &to, RenderingIntent intent)
 {
     const auto range1 = ValueRange(from.minLuminance(), from.maxHdrLuminance().value_or(from.referenceLuminance()));
@@ -19,11 +27,10 @@ ColorPipeline ColorPipeline::create(const ColorDescription &from, const ColorDes
         .max = from.transferFunction().nitsToEncoded(range1.max),
     });
     ret.addTransferFunction(from.transferFunction());
-    ret.addMultiplier(to.referenceLuminance() / from.referenceLuminance());
 
     // FIXME this assumes that the range stays the same with matrix multiplication
     // that's not necessarily true, and figuring out the actual range could be complicated..
-    ret.addMatrix(from.toOther(to, intent), ret.currentOutputRange());
+    ret.addMatrix(from.toOther(to, intent), ret.currentOutputRange() * (to.referenceLuminance() / from.referenceLuminance()));
 
     ret.addInverseTransferFunction(to.transferFunction());
     return ret;

src/core/colorpipeline.h

@@ -20,6 +20,7 @@ public:
     double max = 1;
 
     bool operator==(const ValueRange &) const = default;
+    ValueRange operator*(double mult) const;
 };
 
 class KWIN_EXPORT ColorTransferFunction

src/core/colorspace.cpp

@@ -326,15 +326,36 @@ std::optional<double> ColorDescription::maxHdrLuminance() const
 
 QMatrix4x4 ColorDescription::toOther(const ColorDescription &other, RenderingIntent intent) const
 {
+    QMatrix4x4 luminanceBefore;
+    QMatrix4x4 luminanceAfter;
+    if (intent == RenderingIntent::Perceptual || intent == RenderingIntent::RelativeColorimetricWithBPC) {
+        // add black point compensation: black and reference white from the source color space
+        // should both be mapped to black and reference white in the destination color space
+
+        // before color conversions, map [src min, src ref] to [0, 1]
+        luminanceBefore.scale(1.0 / (referenceLuminance() - minLuminance()));
+        luminanceBefore.translate(-minLuminance(), -minLuminance(), -minLuminance());
+        // afterwards, map [0, 1] again to [dst min, dst ref]
+        luminanceAfter.translate(other.minLuminance(), other.minLuminance(), other.minLuminance());
+        luminanceAfter.scale(other.referenceLuminance() - other.minLuminance());
+    } else {
+        // map only the reference luminance
+        luminanceBefore.scale(other.referenceLuminance() / referenceLuminance());
+    }
     switch (intent) {
     case RenderingIntent::Perceptual: {
         const Colorimetry &srcContainer = containerColorimetry() == NamedColorimetry::BT709 ? other.sdrColorimetry() : containerColorimetry();
-        return other.containerColorimetry().fromXYZ() * Colorimetry::chromaticAdaptationMatrix(srcContainer.white(), other.containerColorimetry().white()) * srcContainer.toXYZ();
+        return luminanceAfter * other.containerColorimetry().fromXYZ() * Colorimetry::chromaticAdaptationMatrix(srcContainer.white(), other.containerColorimetry().white()) * srcContainer.toXYZ() * luminanceBefore;
+    }
+    case RenderingIntent::RelativeColorimetric: {
+        return luminanceAfter * other.containerColorimetry().fromXYZ() * Colorimetry::chromaticAdaptationMatrix(containerColorimetry().white(), other.containerColorimetry().white()) * containerColorimetry().toXYZ() * luminanceBefore;
+    }
+    case RenderingIntent::RelativeColorimetricWithBPC: {
+        return luminanceAfter * other.containerColorimetry().fromXYZ() * Colorimetry::chromaticAdaptationMatrix(containerColorimetry().white(), other.containerColorimetry().white()) * containerColorimetry().toXYZ() * luminanceBefore;
+    }
+    case RenderingIntent::AbsoluteColorimetric: {
+        return luminanceAfter * other.containerColorimetry().fromXYZ() * containerColorimetry().toXYZ() * luminanceBefore;
     }
-    case RenderingIntent::RelativeColorimetric:
-        return other.containerColorimetry().fromXYZ() * Colorimetry::chromaticAdaptationMatrix(containerColorimetry().white(), other.containerColorimetry().white()) * containerColorimetry().toXYZ();
-    case RenderingIntent::AbsoluteColorimetric:
-        return other.containerColorimetry().fromXYZ() * containerColorimetry().toXYZ();
     }
     Q_UNREACHABLE();
 }
@@ -342,7 +363,6 @@ QMatrix4x4 ColorDescription::toOther(const ColorDescription &other, RenderingInt
 QVector3D ColorDescription::mapTo(QVector3D rgb, const ColorDescription &dst, RenderingIntent intent) const
 {
     rgb = m_transferFunction.encodedToNits(rgb);
-    rgb *= dst.referenceLuminance() / m_referenceLuminance;
     rgb = toOther(dst, intent) * rgb;
     return dst.transferFunction().nitsToEncoded(rgb);
 }

src/core/colorspace.h

@@ -27,7 +27,7 @@ enum class RenderingIntent {
     /* colorimetric mapping between color spaces, without whitepoint adaptation */
     AbsoluteColorimetric,
     /* colorimetric mapping between color spaces, with whitepoint adaptation and black point compensation */
-    // TODO RelativeColorimetricWithBPC,
+    RelativeColorimetricWithBPC,
 };
 
 enum class NamedColorimetry {

src/opengl/colormanagement.glsl

@@ -94,7 +94,6 @@ vec4 encodingToNits(vec4 color, int sourceTransferFunction, float luminanceOffse
 
 vec4 sourceEncodingToNitsInDestinationColorspace(vec4 color) {
     color = encodingToNits(color, sourceNamedTransferFunction, sourceTransferFunctionParams.x, sourceTransferFunctionParams.y);
-    color.rgb = color.rgb * (destinationReferenceLuminance / sourceReferenceLuminance);
     color.rgb = (colorimetryTransform * vec4(color.rgb, 1.0)).rgb;
     return vec4(doTonemapping(color.rgb, maxDestinationLuminance), color.a);
 }

src/wayland/xx_colormanagement_v4.cpp

@@ -46,7 +46,8 @@ void XXColorManagerV4::xx_color_manager_v4_bind_resource(Resource *resource)
     send_supported_intent(resource->handle, render_intent::render_intent_perceptual);
     send_supported_intent(resource->handle, render_intent::render_intent_relative);
     send_supported_intent(resource->handle, render_intent::render_intent_absolute);
-    // TODO implement saturation and relative bpc intents
+    send_supported_intent(resource->handle, render_intent::render_intent_relative_bpc);
+    // TODO implement saturation intent
 }
 
 void XXColorManagerV4::xx_color_manager_v4_destroy(Resource *resource)