diff --git a/src/hardware/hw_main.c b/src/hardware/hw_main.c
index a5fae4fad..bd0c874ad 100644
--- a/src/hardware/hw_main.c
+++ b/src/hardware/hw_main.c
@@ -289,7 +289,7 @@ static FUINT HWR_CalcWallLight(FUINT lightnum, seg_t *seg)
 {
 	INT16 finallight = lightnum;
 
-	if (seg != NULL && P_ApplyLightOffset(lightnum))
+	if (seg != NULL && P_ApplyLightOffsetFine(lightnum))
 	{
 		finallight += seg->hwLightOffset;
 
@@ -304,7 +304,7 @@ static FUINT HWR_CalcSlopeLight(FUINT lightnum, pslope_t *slope)
 {
 	INT16 finallight = lightnum;
 
-	if (slope != NULL && P_ApplyLightOffset(lightnum))
+	if (slope != NULL && P_ApplyLightOffsetFine(lightnum))
 	{
 		finallight += slope->hwLightOffset;
 
diff --git a/src/p_setup.c b/src/p_setup.c
index 9cb2ceabf..a47077ba5 100644
--- a/src/p_setup.c
+++ b/src/p_setup.c
@@ -2352,9 +2352,17 @@ void P_UpdateSegLightOffset(seg_t *li)
 #endif
 }
 
-boolean P_ApplyLightOffset(UINT8 baselightlevel)
+boolean P_ApplyLightOffset(UINT8 baselightnum)
 {
 	// Don't apply light offsets at full bright or full dark.
+	// Is in steps of light num .
+	return (baselightnum < LIGHTLEVELS-1 && baselightnum > 0);
+}
+
+boolean P_ApplyLightOffsetFine(UINT8 baselightlevel)
+{
+	// Don't apply light offsets at full bright or full dark.
+	// Uses exact light levels for more smoothness.
 	return (baselightlevel < 255 && baselightlevel > 0);
 }
 
diff --git a/src/p_setup.h b/src/p_setup.h
index 128ddd8dc..7a8a8c18f 100644
--- a/src/p_setup.h
+++ b/src/p_setup.h
@@ -113,7 +113,8 @@ void P_LoadSoundsRange(UINT16 wadnum, UINT16 first, UINT16 num);
 void P_LoadMusicsRange(UINT16 wadnum, UINT16 first, UINT16 num);
 void P_WriteThings(void);
 void P_UpdateSegLightOffset(seg_t *li);
-boolean P_ApplyLightOffset(UINT8 baselightlevel);
+boolean P_ApplyLightOffset(UINT8 baselightnum);
+boolean P_ApplyLightOffsetFine(UINT8 baselightlevel);
 size_t P_PrecacheLevelFlats(void);
 void P_AllocMapHeader(INT16 i);
 
diff --git a/src/p_slopes.c b/src/p_slopes.c
index 3ed61b283..20652a08a 100644
--- a/src/p_slopes.c
+++ b/src/p_slopes.c
@@ -49,17 +49,31 @@ void P_UpdateSlopeLightOffset(pslope_t *slope)
 
 	if (maplighting.directional == true)
 	{
-		fixed_t dX = slope->d.x;
-		fixed_t dY = slope->d.y;
+		fixed_t nX = -slope->normal.x;
+		fixed_t nY = -slope->normal.y;
+		fixed_t nLen = FixedHypot(nX, nY);
 
-		if (slope->zdelta < 0)
+		if (nLen == 0)
 		{
-			dX = -dX;
-			dY = -dY;
+			slope->lightOffset = slope->hwLightOffset = 0;
+			return;
 		}
 
-		light = FixedMul(dX, FINECOSINE(maplighting.angle >> ANGLETOFINESHIFT))
-			+ FixedMul(dY, FINESINE(maplighting.angle >> ANGLETOFINESHIFT));
+		nX = FixedDiv(nX, nLen);
+		nY = FixedDiv(nY, nLen);
+
+		/*
+		if (slope is ceiling)
+		{
+			// There is no good way to calculate this condition here.
+			// We reverse it in R_FindPlane now.
+			nX = -nX;
+			nY = -nY;
+		}
+		*/
+
+		light = FixedMul(nX, FINECOSINE(maplighting.angle >> ANGLETOFINESHIFT))
+			+ FixedMul(nY, FINESINE(maplighting.angle >> ANGLETOFINESHIFT));
 		light = (light + FRACUNIT) / 2;
 	}
 	else
@@ -177,7 +191,7 @@ void P_ReconfigureViaVertexes (pslope_t *slope, const vector3_t v1, const vector
 
 		// Get angles
 		slope->xydirection = R_PointToAngle2(0, 0, slope->d.x, slope->d.y)+ANGLE_180;
-		slope->zangle = InvAngle(R_PointToAngle2(0, 0, FRACUNIT, slope->zdelta));
+		slope->zangle = R_PointToAngle2(0, 0, FRACUNIT, -slope->zdelta);
 	}
 
 	P_UpdateSlopeLightOffset(slope);
@@ -210,7 +224,7 @@ static void ReconfigureViaConstants (pslope_t *slope, const fixed_t a, const fix
 
 	// Get angles
 	slope->xydirection = R_PointToAngle2(0, 0, slope->d.x, slope->d.y)+ANGLE_180;
-	slope->zangle = InvAngle(R_PointToAngle2(0, 0, FRACUNIT, slope->zdelta));
+	slope->zangle = R_PointToAngle2(0, 0, FRACUNIT, -slope->zdelta);
 
 	P_UpdateSlopeLightOffset(slope);
 }
@@ -597,6 +611,7 @@ static void line_SpawnViaMapthingVertexes(const int linenum, const boolean spawn
 	UINT16 tag2 = line->args[2];
 	UINT16 tag3 = line->args[3];
 	UINT8 flags = 0; // Slope flags
+
 	if (line->args[4] & TMSL_NOPHYSICS)
 		flags |= SL_NOPHYSICS;
 	if (line->args[4] & TMSL_DYNAMIC)
diff --git a/src/r_bsp.c b/src/r_bsp.c
index a8236be61..7253fa930 100644
--- a/src/r_bsp.c
+++ b/src/r_bsp.c
@@ -938,9 +938,14 @@ static void R_Subsector(size_t num)
 		|| frontsector->floorpic == skyflatnum
 		|| (frontsector->heightsec != -1 && sectors[frontsector->heightsec].ceilingpic == skyflatnum))
 	{
-		floorplane = R_FindPlane(frontsector->floorheight, frontsector->floorpic, floorlightlevel,
-			frontsector->floor_xoffs, frontsector->floor_yoffs, frontsector->floorpic_angle, floorcolormap, NULL, NULL, frontsector->f_slope,
-			R_NoEncore(frontsector, false), R_IsRipplePlane(frontsector, NULL, false));
+		floorplane = R_FindPlane(
+			frontsector->floorheight, frontsector->floorpic, floorlightlevel,
+			frontsector->floor_xoffs, frontsector->floor_yoffs, frontsector->floorpic_angle,
+			floorcolormap, NULL, NULL, frontsector->f_slope,
+			R_NoEncore(frontsector, false),
+			R_IsRipplePlane(frontsector, NULL, false),
+			false
+		);
 	}
 	else
 		floorplane = NULL;
@@ -949,10 +954,14 @@ static void R_Subsector(size_t num)
 		|| frontsector->ceilingpic == skyflatnum
 		|| (frontsector->heightsec != -1 && sectors[frontsector->heightsec].floorpic == skyflatnum))
 	{
-		ceilingplane = R_FindPlane(frontsector->ceilingheight, frontsector->ceilingpic,
-			ceilinglightlevel, frontsector->ceiling_xoffs, frontsector->ceiling_yoffs, frontsector->ceilingpic_angle,
+		ceilingplane = R_FindPlane(
+			frontsector->ceilingheight, frontsector->ceilingpic, ceilinglightlevel,
+			frontsector->ceiling_xoffs, frontsector->ceiling_yoffs, frontsector->ceilingpic_angle,
 			ceilingcolormap, NULL, NULL, frontsector->c_slope,
-			R_NoEncore(frontsector, true), R_IsRipplePlane(frontsector, NULL, true));
+			R_NoEncore(frontsector, true),
+			R_IsRipplePlane(frontsector, NULL, true),
+			true
+		);
 	}
 	else
 		ceilingplane = NULL;
@@ -993,11 +1002,14 @@ static void R_Subsector(size_t num)
 				light = R_GetPlaneLight(frontsector, planecenterz,
 					viewz < heightcheck);
 
-				ffloor[numffloors].plane = R_FindPlane(*rover->bottomheight, *rover->bottompic,
+				ffloor[numffloors].plane = R_FindPlane(
+					*rover->bottomheight, *rover->bottompic,
 					*frontsector->lightlist[light].lightlevel, *rover->bottomxoffs,
 					*rover->bottomyoffs, *rover->bottomangle, *frontsector->lightlist[light].extra_colormap, rover, NULL, *rover->b_slope,
 					R_NoEncore(rover->master->frontsector, true),
-					R_IsRipplePlane(rover->master->frontsector, rover, true));
+					R_IsRipplePlane(rover->master->frontsector, rover, true),
+					true
+				);
 
 				ffloor[numffloors].slope = *rover->b_slope;
 
@@ -1024,11 +1036,14 @@ static void R_Subsector(size_t num)
 			{
 				light = R_GetPlaneLight(frontsector, planecenterz, viewz < heightcheck);
 
-				ffloor[numffloors].plane = R_FindPlane(*rover->topheight, *rover->toppic,
+				ffloor[numffloors].plane = R_FindPlane(
+					*rover->topheight, *rover->toppic,
 					*frontsector->lightlist[light].lightlevel, *rover->topxoffs, *rover->topyoffs, *rover->topangle,
 					*frontsector->lightlist[light].extra_colormap, rover, NULL, *rover->t_slope,
 					R_NoEncore(rover->master->frontsector, false),
-					R_IsRipplePlane(rover->master->frontsector, rover, false));
+					R_IsRipplePlane(rover->master->frontsector, rover, false),
+					false
+				);
 
 				ffloor[numffloors].slope = *rover->t_slope;
 
@@ -1069,12 +1084,16 @@ static void R_Subsector(size_t num)
 			{
 				light = R_GetPlaneLight(frontsector, polysec->floorheight, viewz < polysec->floorheight);
 
-				ffloor[numffloors].plane = R_FindPlane(polysec->floorheight, polysec->floorpic,
+				ffloor[numffloors].plane = R_FindPlane(
+					polysec->floorheight, polysec->floorpic,
 					(light == -1 ? frontsector->lightlevel : *frontsector->lightlist[light].lightlevel), polysec->floor_xoffs, polysec->floor_yoffs,
 					polysec->floorpic_angle-po->angle,
 					(light == -1 ? frontsector->extra_colormap : *frontsector->lightlist[light].extra_colormap), NULL, po,
 					NULL, // will ffloors be slopable eventually?
-					R_NoEncore(polysec, false), false);/* TODO: wet polyobjects? */
+					R_NoEncore(polysec, false),
+					false, /* TODO: wet polyobjects? */
+					true
+				);
 
 				ffloor[numffloors].height = polysec->floorheight;
 				ffloor[numffloors].polyobj = po;
@@ -1095,11 +1114,15 @@ static void R_Subsector(size_t num)
 			{
 				light = R_GetPlaneLight(frontsector, polysec->floorheight, viewz < polysec->floorheight);
 
-				ffloor[numffloors].plane = R_FindPlane(polysec->ceilingheight, polysec->ceilingpic,
+				ffloor[numffloors].plane = R_FindPlane(
+					polysec->ceilingheight, polysec->ceilingpic,
 					(light == -1 ? frontsector->lightlevel : *frontsector->lightlist[light].lightlevel), polysec->ceiling_xoffs, polysec->ceiling_yoffs, polysec->ceilingpic_angle-po->angle,
 					(light == -1 ? frontsector->extra_colormap : *frontsector->lightlist[light].extra_colormap), NULL, po,
 					NULL, // will ffloors be slopable eventually?
-					R_NoEncore(polysec, true), false);/* TODO: wet polyobjects? */
+					R_NoEncore(polysec, true),
+					false, /* TODO: wet polyobjects? */
+					false
+				);
 
 				ffloor[numffloors].polyobj = po;
 				ffloor[numffloors].height = polysec->ceilingheight;
diff --git a/src/r_plane.c b/src/r_plane.c
index 9d127c762..4018ed49b 100644
--- a/src/r_plane.c
+++ b/src/r_plane.c
@@ -349,7 +349,7 @@ static visplane_t *new_visplane(unsigned hash)
 visplane_t *R_FindPlane(fixed_t height, INT32 picnum, INT32 lightlevel,
 	fixed_t xoff, fixed_t yoff, angle_t plangle, extracolormap_t *planecolormap,
 	ffloor_t *pfloor, polyobj_t *polyobj, pslope_t *slope, boolean noencore,
-	boolean ripple)
+	boolean ripple, boolean reverseLight)
 {
 	visplane_t *check;
 	unsigned hash;
@@ -386,9 +386,16 @@ visplane_t *R_FindPlane(fixed_t height, INT32 picnum, INT32 lightlevel,
 		}
 	}
 
-	if (slope != NULL && P_ApplyLightOffset(lightlevel))
+	if (slope != NULL && P_ApplyLightOffset(lightlevel >> LIGHTSEGSHIFT))
 	{
-		lightlevel += slope->lightOffset * 8;
+		if (reverseLight)
+		{
+			lightlevel -= slope->lightOffset * 8;
+		}
+		else
+		{
+			lightlevel += slope->lightOffset * 8;
+		}
 	}
 
 	// This appears to fix the Nimbus Ruins sky bug.
diff --git a/src/r_plane.h b/src/r_plane.h
index 81757f515..61bac1968 100644
--- a/src/r_plane.h
+++ b/src/r_plane.h
@@ -84,7 +84,7 @@ void R_ClearFFloorClips (void);
 void R_DrawPlanes(void);
 visplane_t *R_FindPlane(fixed_t height, INT32 picnum, INT32 lightlevel, fixed_t xoff, fixed_t yoff, angle_t plangle,
 	extracolormap_t *planecolormap, ffloor_t *ffloor, polyobj_t *polyobj, pslope_t *slope, boolean noencore,
-	boolean ripple);
+	boolean ripple, boolean reverseLight);
 visplane_t *R_CheckPlane(visplane_t *pl, INT32 start, INT32 stop);
 void R_ExpandPlane(visplane_t *pl, INT32 start, INT32 stop);
 void R_PlaneBounds(visplane_t *plane);
diff --git a/src/r_segs.c b/src/r_segs.c
index 111fb94a2..d8d9adfd5 100644
--- a/src/r_segs.c
+++ b/src/r_segs.c
@@ -286,7 +286,7 @@ void R_RenderMaskedSegRange(drawseg_t *ds, INT32 x1, INT32 x2)
 
 			if (rlight->extra_colormap && (rlight->extra_colormap->flags & CMF_FOG))
 				;
-			else if (P_ApplyLightOffset(lightnum << LIGHTSEGSHIFT))
+			else if (P_ApplyLightOffset(lightnum))
 				lightnum += curline->lightOffset;
 
 			rlight->lightnum = lightnum;
@@ -303,7 +303,7 @@ void R_RenderMaskedSegRange(drawseg_t *ds, INT32 x1, INT32 x2)
 		if ((R_CheckColumnFunc(COLDRAWFUNC_FOG) == true)
 			|| (frontsector->extra_colormap && (frontsector->extra_colormap->flags & CMF_FOG)))
 			;
-		else if (P_ApplyLightOffset(lightnum << LIGHTSEGSHIFT))
+		else if (P_ApplyLightOffset(lightnum))
 			lightnum += curline->lightOffset;
 
 		if (lightnum < 0)
@@ -770,7 +770,7 @@ void R_RenderThickSideRange(drawseg_t *ds, INT32 x1, INT32 x2, ffloor_t *pfloor)
 
 			if (pfloor->flags & FF_FOG || rlight->flags & FF_FOG || (rlight->extra_colormap && (rlight->extra_colormap->flags & CMF_FOG)))
 				;
-			else if (P_ApplyLightOffset(rlight->lightnum << LIGHTSEGSHIFT))
+			else if (P_ApplyLightOffset(rlight->lightnum))
 				rlight->lightnum += curline->lightOffset;
 
 			p++;
@@ -793,7 +793,7 @@ void R_RenderThickSideRange(drawseg_t *ds, INT32 x1, INT32 x2, ffloor_t *pfloor)
 
 		if (pfloor->flags & FF_FOG || (frontsector->extra_colormap && (frontsector->extra_colormap->flags & CMF_FOG)))
 			;
-		else if (P_ApplyLightOffset(lightnum << LIGHTSEGSHIFT))
+		else if (P_ApplyLightOffset(lightnum))
 			lightnum += curline->lightOffset;
 
 		if (lightnum < 0)
@@ -1383,7 +1383,7 @@ static void R_RenderSegLoop (void)
 
 				if (dc_lightlist[i].extra_colormap)
 					;
-				else if (P_ApplyLightOffset(lightnum << LIGHTSEGSHIFT))
+				else if (P_ApplyLightOffset(lightnum))
 					lightnum += curline->lightOffset;
 
 				if (lightnum < 0)
@@ -2436,7 +2436,7 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 		// OPTIMIZE: get rid of LIGHTSEGSHIFT globally
 		lightnum = (frontsector->lightlevel >> LIGHTSEGSHIFT);
 
-		if (P_ApplyLightOffset(lightnum << LIGHTSEGSHIFT))
+		if (P_ApplyLightOffset(lightnum))
 			lightnum += curline->lightOffset;
 
 		if (lightnum < 0)