renderer: implement low light restoration for when tone mapping is enabled

Author: illwieckz

src/engine/renderer/gl_shader.cpp

@@ -799,6 +799,11 @@ static std::string GenEngineConstants() {
 		AddDefine( str, "r_highPrecisionRendering", 1 );
 	}
 
+	if ( r_toneMappingLowLightRestorationSteps.Get() ) {
+		AddDefine( str, "r_toneMappingLowLightRestorationSteps", r_toneMappingLowLightRestorationSteps.Get() );
+		AddDefine( str, "r_toneMappingLowLightRestorationThreshold", r_toneMappingLowLightRestorationThreshold.Get() );
+	}
+
 	if ( r_dithering.Get() ) {
 		AddDefine( str, "r_dithering", 1 );
 	}

src/engine/renderer/glsl_source/cameraEffects_fp.glsl

@@ -88,7 +88,58 @@ void main()
 
 #if defined(r_highPrecisionRendering) && defined(HAVE_ARB_texture_float)
 	if( u_Tonemap ) {
-		color.rgb = TonemapLottes( color.rgb );
+		#if defined(r_toneMappingLowLightRestorationSteps)
+			vec3 mapped = TonemapLottes( color.rgb );
+
+			/* The threshold is the color channel value under which we blend
+			the tone mapped color with the original color to restore low
+			light in dark shadows clipped by the tone mapper.
+
+			The threshold is in sRGB space for convenience. For example,
+			a threshold of 5 would make sure we start restoring low light
+			when the sRGB-converted tone mapped color goes below #050505,
+			a threshold of 10 would do the same for #0A0A0A and 16 would
+			target #101010.
+
+			We need to convert the threshold back to linear because we're
+			still operating in linear space at this point.
+
+			We don't exactly restore up to the threshold, we blend half
+			the tone-mapped color with half the color, to avoid producing
+			color artifacts that would happen by bumping the RGB channels
+			directly. Each step does the half blend again, so we can say
+			that the restoration _tends to the threshold_ with each step.
+
+			Each step is done with a small bias to lower the threshold a bit
+			every step to not not always use the exact same frontier between
+			the area having light restored and the area being kept unmodified,
+			then smoothly blending the restoratiin with a gradient.
+
+			Since we blend with half the colors, a threshold smaller than 2 would
+			do nothing. */
+			const float threshold =
+				pow( float( r_toneMappingLowLightRestorationThreshold ) / 255.0f, 2.2f );
+
+			for ( int i = 0; i < r_toneMappingLowLightRestorationSteps; i++ )
+			{
+				float t = threshold - ( i * ( threshold / 10 ) );
+
+				bvec3 cutoff = lessThan(mapped, vec3(t));
+
+				#if __VERSION__ > 120
+					vec3 interpolation = vec3(0.5f) * vec3(cutoff);
+				#else
+					vec3 interpolation = 0.5f * cutoff;
+				#endif
+
+				mapped = mix( mapped, vec3(0.0f), interpolation )
+					+ mix( vec3(0.0f), color.rgb, interpolation );
+			}
+
+			color.rgb = mapped;
+		#else
+			color.rgb = TonemapLottes( color.rgb );
+		#endif
 	}
 #endif
 

src/engine/renderer/tr_init.cpp

@@ -205,6 +205,12 @@ Cvar::Cvar<int> r_rendererAPI( "r_rendererAPI", "Renderer API: 0: OpenGL, 1: Vul
 	Cvar::Range<Cvar::Cvar<float>> r_toneMappingDarkAreaPointLDR(
 		"r_toneMappingDarkAreaPointLDR", "Convert to this brightness at dark area cut-off",
 		Cvar::NONE, 0.268f, 0.0f, 1.0f );
+	Cvar::Range<Cvar::Cvar<int>> r_toneMappingLowLightRestorationSteps(
+		"r_toneMappingLowLightRestorationSteps", "Amount of steps to restore the low lights",
+		Cvar::NONE, 1, 0, 5 );
+	Cvar::Range<Cvar::Cvar<int>> r_toneMappingLowLightRestorationThreshold(
+		"r_toneMappingLowLightRestorationThreshold", "Color channel sRGB value under which low light is restored",
+		Cvar::NONE, 10, 2, 20 );
 
 	Cvar::Cvar<bool> r_dithering(
 		"r_dithering", "Use dithering", Cvar::NONE, true );
@@ -1270,6 +1276,8 @@ ScreenshotCmd screenshotPNGRegistration("screenshotPNG", ssFormat_t::SSF_PNG, "p
 
 		Cvar::Latch( r_highPrecisionRendering );
 		Cvar::Latch( r_accurateSRGB );
+		Cvar::Latch( r_toneMappingLowLightRestorationSteps );
+		Cvar::Latch( r_toneMappingLowLightRestorationThreshold );
 		Cvar::Latch( r_dithering );
 
 		r_drawBuffer = Cvar_Get( "r_drawBuffer", "GL_BACK", CVAR_CHEAT );

src/engine/renderer/tr_local.h

@@ -2716,6 +2716,8 @@ enum
 
 	extern Cvar::Cvar<bool> r_highPrecisionRendering;
 	extern Cvar::Cvar<bool> r_accurateSRGB;
+	extern Cvar::Range<Cvar::Cvar<int>> r_toneMappingLowLightRestorationSteps;
+	extern Cvar::Range<Cvar::Cvar<int>> r_toneMappingLowLightRestorationThreshold;
 	extern Cvar::Cvar<bool> r_dithering;
 
 	extern Cvar::Range<Cvar::Cvar<int>> r_shadows;