diff --git a/src/am_map.cpp b/src/am_map.cpp
index aa7ed83b6a..b6ccce91b9 100644
--- a/src/am_map.cpp
+++ b/src/am_map.cpp
@@ -2141,7 +2141,7 @@ void DAutomap::drawSubsectors()
 			// is necessary in order to best reproduce Doom's original lighting.
 			double fadelevel;
 
-			if (!V_IsHardwareRenderer() || primaryLevel->lightMode == ELightMode::Doom || primaryLevel->lightMode == ELightMode::ZDoomSoftware || primaryLevel->lightMode == ELightMode::DoomSoftware)
+			if (!V_IsHardwareRenderer() || primaryLevel->lightMode == ELightMode::DoomDark || primaryLevel->lightMode == ELightMode::Doom || primaryLevel->lightMode == ELightMode::ZDoomSoftware || primaryLevel->lightMode == ELightMode::DoomSoftware)
 			{
 				double map = (NUMCOLORMAPS * 2.) - ((floorlight + 12) * (NUMCOLORMAPS / 128.));
 				fadelevel = clamp((map - 12) / NUMCOLORMAPS, 0.0, 1.0);
diff --git a/src/doomtype.h b/src/doomtype.h
index 5b1819d085..2a3fecbd0f 100644
--- a/src/doomtype.h
+++ b/src/doomtype.h
@@ -48,6 +48,7 @@ enum class ELightMode : int8_t
 	Doom = 2,
 	DoomDark = 3,
 	DoomLegacy = 4,
+	Build = 5,
 	ZDoomSoftware = 8,
 	DoomSoftware = 16
 };
diff --git a/src/g_level.cpp b/src/g_level.cpp
index 38e172687d..21010137ed 100644
--- a/src/g_level.cpp
+++ b/src/g_level.cpp
@@ -141,8 +141,8 @@ CUSTOM_CVAR(Bool, gl_notexturefill, false, CVAR_NOINITCALL)
 CUSTOM_CVAR(Int, gl_lightmode, 3, CVAR_ARCHIVE | CVAR_NOINITCALL)
 {
 	int newself = self;
-	if (newself > 8) newself = 16;	// use 8 and 16 for software lighting to avoid conflicts with the bit mask
-	else if (newself > 4) newself = 8;
+	if (newself > 8) newself = 16;	// use 8 and 16 for software lighting to avoid conflicts with the bit mask ( in hindsight a bad idea.)
+	else if (newself > 5) newself = 8;
 	else if (newself < 0) newself = 0;
 	if (self != newself) self = newself;
 	else for (auto Level : AllLevels())
diff --git a/src/rendering/hwrenderer/scene/hw_drawinfo.h b/src/rendering/hwrenderer/scene/hw_drawinfo.h
index e603716742..262fee3f56 100644
--- a/src/rendering/hwrenderer/scene/hw_drawinfo.h
+++ b/src/rendering/hwrenderer/scene/hw_drawinfo.h
@@ -314,12 +314,22 @@ public:
 
 	bool isSoftwareLighting() const
 	{
-		return lightmode >= ELightMode::ZDoomSoftware;
+		return lightmode == ELightMode::ZDoomSoftware || lightmode == ELightMode::DoomSoftware || lightmode == ELightMode::Build;
+	}
+
+	bool isBuildSoftwareLighting() const
+	{
+		return lightmode == ELightMode::Build;
+	}
+
+	bool isDoomSoftwareLighting() const
+	{
+		return lightmode == ELightMode::ZDoomSoftware || lightmode == ELightMode::DoomSoftware;
 	}
 
 	bool isDarkLightMode() const
 	{
-		return !!((int)lightmode & (int)ELightMode::Doom);
+		return lightmode == ELightMode::Doom || lightmode == ELightMode::DoomDark;
 	}
 
 	void SetFallbackLightMode()
diff --git a/wadsrc/static/shaders/glsl/main.fp b/wadsrc/static/shaders/glsl/main.fp
index 048579eeea..611dbe0b21 100644
--- a/wadsrc/static/shaders/glsl/main.fp
+++ b/wadsrc/static/shaders/glsl/main.fp
@@ -308,6 +308,16 @@ float R_DoomLightingEquation(float light)
 	{
 		z = pixelpos.w;
 	}
+	
+	if ((uPalLightLevels >> 16) == 5) // gl_lightmode 5: Build software lighting emulation.
+	{
+		// This is a lot more primitive than Doom's lighting...
+		float numShades = float(uPalLightLevels & 255);
+		float curshade = (1.0 - light) / (numShades - 1.0);
+		float visibility = max(uGlobVis * uLightFactor * z, 0.0);
+		float shade = clamp((curshade + visibility), 0.0, numShades - 1.0);
+		return clamp(shade * uLightDist, 0.0, 1.0);
+	}
 
 	float colormap = R_DoomColormap(light, z);