diff --git a/src/hardware/hw_main.c b/src/hardware/hw_main.c
index 44da2b9f2..597990a62 100644
--- a/src/hardware/hw_main.c
+++ b/src/hardware/hw_main.c
@@ -2433,6 +2433,12 @@ static boolean CheckClip(seg_t * seg, sector_t * afrontsector, sector_t * abacks
 {
 	fixed_t frontf1,frontf2, frontc1, frontc2; // front floor/ceiling ends
 	fixed_t backf1, backf2, backc1, backc2; // back floor ceiling ends
+	boolean bothceilingssky = false, bothfloorssky = false;
+
+	if (abacksector->ceilingpic == skyflatnum && afrontsector->ceilingpic == skyflatnum)
+		bothceilingssky = true;
+	if (abacksector->floorpic == skyflatnum && afrontsector->floorpic == skyflatnum)
+		bothfloorssky = true;
 
 	// GZDoom method of sloped line clipping
 
@@ -2467,8 +2473,7 @@ static boolean CheckClip(seg_t * seg, sector_t * afrontsector, sector_t * abacks
 	}
 	// properly render skies (consider door "open" if both ceilings are sky)
 	// same for floors
-	if ((abacksector->ceilingpic != skyflatnum || afrontsector->ceilingpic != skyflatnum)
-	 && (abacksector->floorpic != skyflatnum   || afrontsector->floorpic != skyflatnum))
+	if (!bothceilingssky && !bothfloorssky)
 	{
 		// now check for closed sectors!
 		if ((backc1 <= frontf1 && backc2 <= frontf2)
@@ -2490,12 +2495,21 @@ static boolean CheckClip(seg_t * seg, sector_t * afrontsector, sector_t * abacks
 		}
 	}
 
-	if (backc1 != frontc1 || backc2 != frontc2
-		|| backf1 != frontf1 || backf2 != frontf2)
+	if (!bothceilingssky) {
+		if (backc1 != frontc1 || backc2 != frontc2)
 		{
 			checkforemptylines = false;
 			return false;
 		}
+	}
+
+	if (!bothfloorssky) {
+		if (backf1 != frontf1 || backf2 != frontf2)
+		{
+			checkforemptylines = false;
+			return false;
+		}
+	}
 
 	return false;
 }
@@ -2798,6 +2812,7 @@ static void HWR_AddLine(seg_t * line)
 #ifndef NEWCLIP
 	INT32 x1, x2;
 	angle_t span, tspan;
+	boolean bothceilingssky = false, bothfloorssky = false;
 #endif
 
 	// SoM: Backsector needs to be run through R_FakeFlat
@@ -2923,7 +2938,30 @@ static void HWR_AddLine(seg_t * line)
     }
     else
     {
+		boolean bothceilingssky = false, bothfloorssky = false;
+
 		gr_backsector = R_FakeFlat(gr_backsector, &tempsec, NULL, NULL, true);
+
+		if (gr_backsector->ceilingpic == skyflatnum && gr_frontsector->ceilingpic == skyflatnum)
+			bothceilingssky = true;
+		if (gr_backsector->floorpic == skyflatnum && gr_frontsector->floorpic == skyflatnum)
+			bothfloorssky = true;
+
+		if (bothceilingssky && bothfloorssky) // everything's sky? let's save us a bit of time then
+		{
+			if (
+#ifdef POLYOBJECTS
+			!line->polyseg &&
+#endif
+			!line->sidedef->midtexture
+			&& ((!gr_frontsector->ffloors && !gr_backsector->ffloors)
+			|| (gr_frontsector->tag == gr_backsector->tag)))
+				return; // line is empty, don't even bother
+			// treat like wide open window instead
+			HWR_ProcessSeg(); // Doesn't need arguments because they're defined globally :D
+			return;
+		}
+
 		if (CheckClip(line, gr_frontsector, gr_backsector))
 		{
 			gld_clipper_SafeAddClipRange(angle2, angle1);
@@ -2946,6 +2984,25 @@ static void HWR_AddLine(seg_t * line)
 
 	gr_backsector = R_FakeFlat(gr_backsector, &tempsec, NULL, NULL, true);
 
+	if (gr_backsector->ceilingpic == skyflatnum && gr_frontsector->ceilingpic == skyflatnum)
+		bothceilingssky = true;
+	if (gr_backsector->floorpic == skyflatnum && gr_frontsector->floorpic == skyflatnum)
+		bothfloorssky = true;
+
+	if (bothceilingssky && bothfloorssky) // everything's sky? let's save us a bit of time then
+	{
+		if (
+#ifdef POLYOBJECTS
+		!line->polyseg &&
+#endif
+		!line->sidedef->midtexture
+		&& ((!gr_frontsector->ffloors && !gr_backsector->ffloors)
+		|| (gr_frontsector->tag == gr_backsector->tag)))
+			return; // line is empty, don't even bother
+
+		goto clippass; // treat like wide open window instead
+	}
+
 #ifdef ESLOPE
 	if (gr_frontsector->f_slope || gr_frontsector->c_slope || gr_backsector->f_slope || gr_backsector->c_slope)
 	{
@@ -2966,8 +3023,7 @@ static void HWR_AddLine(seg_t * line)
 #undef SLOPEPARAMS
 		// if both ceilings are skies, consider it always "open"
 		// same for floors
-		if ((gr_backsector->ceilingpic != skyflatnum || gr_frontsector->ceilingpic != skyflatnum)
-		 && (gr_backsector->floorpic != skyflatnum   || gr_frontsector->floorpic != skyflatnum))
+		if (!bothceilingssky && !bothfloorssky)
 		{
 			// Closed door.
 			if ((backc1 <= frontf1 && backc2 <= frontf2)
@@ -2984,19 +3040,19 @@ static void HWR_AddLine(seg_t * line)
 		}
 
 		// Window.
-		if (backc1 != frontc1 || backc2 != frontc2
-			|| backf1 != frontf1 || backf2 != frontf2)
-		{
-			goto clippass;
-		}
+		if (!bothceilingssky) // ceilings are always the "same" when sky
+			if (backc1 != frontc1 || backc2 != frontc2)
+				goto clippass;
+		if (!bothfloorssky)	// floors are always the "same" when sky
+			if (backf1 != frontf1 || backf2 != frontf2)
+				goto clippass;
 	}
 	else
 #endif
 	{
 		// if both ceilings are skies, consider it always "open"
 		// same for floors
-		if ((gr_backsector->ceilingpic != skyflatnum || gr_frontsector->ceilingpic != skyflatnum)
-		 && (gr_backsector->floorpic != skyflatnum   || gr_frontsector->floorpic != skyflatnum))
+		if (!bothceilingssky && !bothfloorssky)
 		{
 			// Closed door.
 			if (gr_backsector->ceilingheight <= gr_frontsector->floorheight ||
@@ -3011,9 +3067,12 @@ static void HWR_AddLine(seg_t * line)
 		}
 
 		// Window.
-		if (gr_backsector->ceilingheight != gr_frontsector->ceilingheight ||
-			gr_backsector->floorheight != gr_frontsector->floorheight)
-			goto clippass;
+		if (!bothceilingssky) // ceilings are always the "same" when sky
+			if (gr_backsector->ceilingheight != gr_frontsector->ceilingheight)
+				goto clippass;
+		if (!bothfloorssky)	// floors are always the "same" when sky
+			if (gr_backsector->floorheight != gr_frontsector->floorheight)
+				goto clippass;
 	}
 
 	// Reject empty lines used for triggers and special events.
diff --git a/src/r_bsp.c b/src/r_bsp.c
index 5b0a4ca36..183def25a 100644
--- a/src/r_bsp.c
+++ b/src/r_bsp.c
@@ -401,6 +401,7 @@ static void R_AddLine(seg_t *line)
 	INT32 x1, x2;
 	angle_t angle1, angle2, span, tspan;
 	static sector_t tempsec; // ceiling/water hack
+	boolean bothceilingssky = false, bothfloorssky = false;
 
 	if (line->polyseg && !(line->polyseg->flags & POF_RENDERSIDES))
 		return;
@@ -487,6 +488,25 @@ static void R_AddLine(seg_t *line)
 
 	doorclosed = 0;
 
+	if (backsector->ceilingpic == skyflatnum && frontsector->ceilingpic == skyflatnum)
+		bothceilingssky = true;
+	if (backsector->floorpic == skyflatnum && frontsector->floorpic == skyflatnum)
+		bothfloorssky = true;
+
+	if (bothceilingssky && bothfloorssky) // everything's sky? let's save us a bit of time then
+	{
+		if (
+#ifdef POLYOBJECTS
+		!line->polyseg &&
+#endif
+		!line->sidedef->midtexture
+		&& ((!frontsector->ffloors && !backsector->ffloors)
+		|| (frontsector->tag == backsector->tag)))
+			return; // line is empty, don't even bother
+
+		goto clippass; // treat like wide open window instead
+	}
+
 	// Closed door.
 #ifdef ESLOPE
 	if (frontsector->f_slope || frontsector->c_slope || backsector->f_slope || backsector->c_slope)
@@ -507,8 +527,7 @@ static void R_AddLine(seg_t *line)
 #undef SLOPEPARAMS
 		// if both ceilings are skies, consider it always "open"
 		// same for floors
-		if ((backsector->ceilingpic != skyflatnum || frontsector->ceilingpic != skyflatnum)
-		 && (backsector->floorpic != skyflatnum   || frontsector->floorpic != skyflatnum))
+		if (!bothceilingssky && !bothfloorssky)
 		{
 			if ((backc1 <= frontf1 && backc2 <= frontf2)
 				|| (backf1 >= frontc1 && backf2 >= frontc2))
@@ -526,19 +545,19 @@ static void R_AddLine(seg_t *line)
 		}
 
 		// Window.
-		if (backc1 != frontc1 || backc2 != frontc2
-			|| backf1 != frontf1 || backf2 != frontf2)
-		{
-			goto clippass;
-		}
+		if (!bothceilingssky) // ceilings are always the "same" when sky
+			if (backc1 != frontc1 || backc2 != frontc2)
+				goto clippass;
+		if (!bothfloorssky)	// floors are always the "same" when sky
+			if (backf1 != frontf1 || backf2 != frontf2)
+				goto clippass;
 	}
 	else
 #endif
 	{
 		// if both ceilings are skies, consider it always "open"
 		// same for floors
-		if ((backsector->ceilingpic != skyflatnum || frontsector->ceilingpic != skyflatnum)
-		 && (backsector->floorpic != skyflatnum   || frontsector->floorpic != skyflatnum))
+		if (!bothceilingssky && !bothfloorssky)
 		{
 			if (backsector->ceilingheight <= frontsector->floorheight
 				|| backsector->floorheight >= frontsector->ceilingheight)
@@ -553,11 +572,12 @@ static void R_AddLine(seg_t *line)
 		}
 
 		// Window.
-		if (backsector->ceilingheight != frontsector->ceilingheight
-			|| backsector->floorheight != frontsector->floorheight)
-		{
-			goto clippass;
-		}
+		if (!bothceilingssky) // ceilings are always the "same" when sky
+			if (backsector->ceilingheight != frontsector->ceilingheight)
+				goto clippass;
+		if (!bothfloorssky)	// floors are always the "same" when sky
+			if (backsector->floorheight != frontsector->floorheight)
+				goto clippass;
 	}
 
 	// Reject empty lines used for triggers and special events.
diff --git a/src/r_segs.c b/src/r_segs.c
index ff3a956ad..5395f414a 100644
--- a/src/r_segs.c
+++ b/src/r_segs.c
@@ -1321,6 +1321,18 @@ void R_RenderThickSideRange(drawseg_t *ds, INT32 x1, INT32 x2, ffloor_t *pfloor)
 #undef CLAMPMIN
 }
 
+// R_ExpandPlaneY
+//
+// A simple function to modify a vsplane's top and bottom for a particular column
+// Sort of like R_ExpandPlane in r_plane.c, except this is vertical expansion
+static inline void R_ExpandPlaneY(visplane_t *pl, INT32 x, INT16 top, INT16 bottom)
+{
+	// Expand the plane, don't shrink it!
+	// note: top and bottom default to 0xFFFF and 0x0000 respectively, which is totally compatible with this
+	if (pl->top[x] > top)       pl->top[x] = top;
+	if (pl->bottom[x] < bottom) pl->bottom[x] = bottom;
+}
+
 //
 // R_RenderSegLoop
 // Draws zero, one, or two textures (and possibly a masked
@@ -1344,7 +1356,6 @@ UINT32 nombre = 100000;
 #endif
 //profile stuff ---------------------------------------------------------
 
-
 static void R_RenderSegLoop (void)
 {
 	angle_t angle;
@@ -1374,13 +1385,19 @@ static void R_RenderSegLoop (void)
 
 		if (markceiling)
 		{
+#if 0
+			bottom = yl-1;
+
+			if (bottom >= floorclip[rw_x])
+				bottom = floorclip[rw_x]-1;
+
+			if (top <= bottom)
+#else
 			bottom = yl > floorclip[rw_x] ? floorclip[rw_x] : yl;
 
-			if (top <= --bottom)
-			{
-				ceilingplane->top[rw_x] = (INT16)top;
-				ceilingplane->bottom[rw_x] = (INT16)bottom;
-			}
+			if (top <= --bottom && ceilingplane)
+#endif
+				R_ExpandPlaneY(ceilingplane, rw_x, top, bottom);
 		}
 
 
@@ -1396,10 +1413,7 @@ static void R_RenderSegLoop (void)
 			top = yh < ceilingclip[rw_x] ? ceilingclip[rw_x] : yh;
 
 			if (++top <= bottom && floorplane)
-			{
-				floorplane->top[rw_x] = (INT16)top;
-				floorplane->bottom[rw_x] = (INT16)bottom;
-			}
+				R_ExpandPlaneY(floorplane, rw_x, top, bottom);
 		}
 
 		if (numffloors)
@@ -1413,15 +1427,6 @@ static void R_RenderSegLoop (void)
 #ifdef POLYOBJECTS_PLANES
 				if (ffloor[i].polyobj && (!curline->polyseg || ffloor[i].polyobj != curline->polyseg))
 					continue;
-/*
-				// FIXME hack to fix planes disappearing when a seg goes behind the camera. This NEEDS to be changed to be done properly. -Red
-				if (curline->polyseg) {
-					if (ffloor[i].plane->minx > rw_x)
-						ffloor[i].plane->minx = rw_x;
-					else if (ffloor[i].plane->maxx < rw_x)
-						ffloor[i].plane->maxx = rw_x;
-				}
-*/
 #endif
 
 				if (ffloor[i].height < viewz)