renderer: implement low light dithering

Author: illwieckz

src/engine/renderer/gl_shader.cpp

@@ -799,6 +799,18 @@ static std::string GenEngineConstants() {
 		AddDefine( str, "r_highPrecisionRendering", 1 );
 	}
 
+	if ( r_lowLightDithering.Get() )
+	{
+		AddDefine( str, "r_lowLightDithering", 1 );
+
+		AddDefine( str, "r_lowLightDitheringThreshold", r_lowLightDitheringThreshold.Get() );
+
+		if ( r_showLowLightDithering.Get() )
+		{
+			AddDefine( str, "r_showLowLightDithering", 1 );
+		}
+	}
+
 	return str;
 }
 
@@ -3066,4 +3078,4 @@ GlobalUBOProxy::GlobalUBOProxy() :
 	u_Tonemap( this ),
 	u_TonemapParms( this ),
 	u_Exposure( this ) {
-}
+}

src/engine/renderer/glsl_source/cameraEffects_fp.glsl

@@ -92,6 +92,46 @@ void main()
 	}
 #endif
 
+	#if defined(r_lowLightDithering)
+	{
+		vec3 dithered = color.rgb;
+
+		const float bayer[ 16 ] = {
+			 0.0f,  8.0f,  2.0f, 10.0f,
+			12.0f,  4.0f, 14.0f,  6.0f,
+			 3.0f, 11.0f,  1.0f,  9.0f,
+			15.0f,  7.0f, 13.0f,  5.0f
+		};
+
+		float dither = bayer[
+				int( mod( gl_FragCoord.x, 4.0 ) ) +
+				int( mod( gl_FragCoord.y, 4.0 ) ) * 4
+			] / 16.0;
+
+		dithered += vec3( ( dither - 0.50 ) / 255.0 );
+
+		float threshold;
+
+		if ( u_SRGB )
+		{
+			threshold = pow( float( r_lowLightDitheringThreshold ) / 255.0f, 2.2f );
+		}
+		else
+		{
+			threshold = float( r_lowLightDitheringThreshold ) / 255.0f;
+		}
+
+		if ( ( color.r + color.g + color.b ) < ( 3 * threshold ) )
+		{
+			color.rgb = dithered;
+
+			#if defined(r_showLowLightDithering)
+				color.rgb = vec3(1.0f, 0.0f, 0.0f);
+			#endif
+		}
+	}
+	#endif
+
 	color.rgb = clamp( color.rgb, vec3( 0.0f ), vec3( 1.0f ) );
 
 	if ( u_SRGB )

src/engine/renderer/tr_init.cpp

@@ -206,6 +206,14 @@ Cvar::Cvar<int> r_rendererAPI( "r_rendererAPI", "Renderer API: 0: OpenGL, 1: Vul
 		"r_toneMappingDarkAreaPointLDR", "Convert to this brightness at dark area cut-off",
 		Cvar::NONE, 0.268f, 0.0f, 1.0f );
 
+	Cvar::Cvar<bool> r_lowLightDithering(
+		"r_lowLightDithering", "Use dithering in low light areas", Cvar::NONE, true );
+	Cvar::Range<Cvar::Cvar<int>> r_lowLightDitheringThreshold(
+		"r_lowLightDitheringThreshold", "Color channel sRGB value under which low light is dithered",
+		Cvar::NONE, 6, 2, 20 );
+	Cvar::Cvar<bool> r_showLowLightDithering(
+		"r_showLowLightDithering", "Show dithering applied on low light areas", Cvar::CHEAT, false );
+
 	cvar_t      *r_lockpvs;
 	cvar_t      *r_noportals;
 
@@ -1268,6 +1276,10 @@ ScreenshotCmd screenshotPNGRegistration("screenshotPNG", ssFormat_t::SSF_PNG, "p
 		Cvar::Latch( r_highPrecisionRendering );
 		Cvar::Latch( r_accurateSRGB );
 
+		Cvar::Latch( r_lowLightDithering );
+		Cvar::Latch( r_lowLightDitheringThreshold );
+		Cvar::Latch( r_showLowLightDithering );
+
 		r_drawBuffer = Cvar_Get( "r_drawBuffer", "GL_BACK", CVAR_CHEAT );
 		r_lockpvs = Cvar_Get( "r_lockpvs", "0", CVAR_CHEAT );
 		r_noportals = Cvar_Get( "r_noportals", "0", CVAR_CHEAT );

src/engine/renderer/tr_local.h

@@ -2717,6 +2717,10 @@ enum
 	extern Cvar::Cvar<bool> r_highPrecisionRendering;
 	extern Cvar::Cvar<bool> r_accurateSRGB;
 
+	extern Cvar::Cvar<bool> r_lowLightDithering;
+	extern Cvar::Range<Cvar::Cvar<int>> r_lowLightDitheringThreshold;
+	extern Cvar::Cvar<bool> r_showLowLightDithering;
+
 	extern Cvar::Range<Cvar::Cvar<int>> r_shadows;
 
 	extern cvar_t *r_lockpvs;