feat: HuePreservingReinhard

package/Shaders/Common/DICETonemapper.hlsli

@@ -156,3 +156,38 @@ float3 HuePreservingHejlBurgessDawson(float3 col)
 
 	return col;
 }
+
+float3 HuePreservingReinhard(float3 col)
+{
+	float3 ictcp = RGBToICtCp(col);
+
+	// Hue-preserving range compression requires desaturation in order to achieve a natural look. We adaptively desaturate the input based on its luminance.
+	float saturationAmount = pow(smoothstep(1.0, 0.3, ictcp.x), 1.3);
+	col = ICtCpToRGB(ictcp * float3(1, saturationAmount.xx));
+
+	// Hue preserving mapping
+	float maxCol = Color::RGBToLuminance(col);
+	float mappedMax = GetTonemapFactorReinhard(maxCol);
+	float3 compressedHuePreserving = col * mappedMax / maxCol;
+
+	// Non-hue preserving mapping
+	float3 perChannelCompressed = GetTonemapFactorReinhard(col);
+	// Compensate for modified tonemapping desaturating colors
+	perChannelCompressed = lerp(Color::RGBToLuminance(perChannelCompressed), perChannelCompressed, 1.5);
+
+	col = lerp(perChannelCompressed, compressedHuePreserving, 0.6);
+
+	float3 ictcpMapped = RGBToICtCp(col);
+
+	// Smoothly ramp off saturation as brightness increases, but keep some even for very bright input
+	float postCompressionSaturationBoost = 0.3 * smoothstep(1.0, 0.5, ictcp.x);
+
+	// Re-introduce some hue from the pre-compression color. Something similar could be accomplished by delaying the luma-dependent desaturation before range compression.
+	// Doing it here however does a better job of preserving perceptual luminance of highly saturated colors. Because in the hue-preserving path we only range-compress the max channel,
+	// saturated colors lose luminance. By desaturating them more aggressively first, compressing, and then re-adding some saturation, we can preserve their brightness to a greater extent.
+	ictcpMapped.yz = lerp(ictcpMapped.yz, ictcp.yz * ictcpMapped.x / max(1e-3, ictcp.x), postCompressionSaturationBoost);
+
+	col = ICtCpToRGB(ictcpMapped);
+
+	return col;
+}

package/Shaders/ISHDR.hlsl

@@ -111,7 +111,7 @@ PS_OUTPUT main(PS_INPUT input)
 		if (Param.z > 0.5) {		
 			blendedColor = HuePreservingHejlBurgessDawson(inputColor);
 		} else {
-			blendedColor = GetTonemapFactorReinhard(inputColor);
+			blendedColor = HuePreservingReinhard(inputColor);
 		}
 
 		blendedColor += saturate(Param.x - blendedColor) * bloomColor;