diff --git a/src/hardware/hw_main.c b/src/hardware/hw_main.c
index 864cd04b6..597990a62 100644
--- a/src/hardware/hw_main.c
+++ b/src/hardware/hw_main.c
@@ -1409,9 +1409,9 @@ static void HWR_SplitFog(sector_t *sector, wallVert3D *wallVerts, FSurfaceInfo*
 	HWR_AddTransparentWall(wallVerts, Surf, 0, PF_Translucent|PF_NoTexture, true, lightnum, colormap);
 }
 
-// HWR_DrawSkyWalls
+// HWR_DrawSkyWall
 // Draw walls into the depth buffer so that anything behind is culled properly
-static void HWR_DrawSkyWall(wallVert3D *wallVerts, FSurfaceInfo *Surf, fixed_t bottom, fixed_t top)
+static void HWR_DrawSkyWall(wallVert3D *wallVerts, FSurfaceInfo *Surf)
 {
 	HWD.pfnSetTexture(NULL);
 	// no texture
@@ -1419,9 +1419,7 @@ static void HWR_DrawSkyWall(wallVert3D *wallVerts, FSurfaceInfo *Surf, fixed_t b
 	wallVerts[0].t = wallVerts[1].t = 0;
 	wallVerts[0].s = wallVerts[3].s = 0;
 	wallVerts[2].s = wallVerts[1].s = 0;
-	// set top/bottom coords
-	wallVerts[2].y = wallVerts[3].y = FIXED_TO_FLOAT(top); // No real way to find the correct height of this
-	wallVerts[0].y = wallVerts[1].y = FIXED_TO_FLOAT(bottom); // worldlow/bottom because it needs to cover up the lower thok barrier wall
+	// this no longer sets top/bottom coords, this should be done before caling the function
 	HWR_ProjectWall(wallVerts, Surf, PF_Invisible|PF_Clip|PF_NoTexture, 255, NULL);
 	// PF_Invisible so it's not drawn into the colour buffer
 	// PF_NoTexture for no texture
@@ -1542,8 +1540,10 @@ static void HWR_StoreWallRange(double startfrac, double endfrac)
 
 	if (gr_backsector)
 	{
-		INT32 gr_toptexture, gr_bottomtexture;
+		INT32 gr_toptexture = 0, gr_bottomtexture = 0;
 		// two sided line
+		boolean bothceilingssky = false; // turned on if both back and front ceilings are sky
+		boolean bothfloorssky = false; // likewise, but for floors
 
 #ifdef ESLOPE
 		SLOPEPARAMS(gr_backsector->c_slope, worldhigh, worldhighslope, gr_backsector->ceilingheight)
@@ -1556,17 +1556,23 @@ static void HWR_StoreWallRange(double startfrac, double endfrac)
 
 		// hack to allow height changes in outdoor areas
 		// This is what gets rid of the upper textures if there should be sky
-		if (gr_frontsector->ceilingpic == skyflatnum &&
-			gr_backsector->ceilingpic  == skyflatnum)
+		if (gr_frontsector->ceilingpic == skyflatnum
+			&& gr_backsector->ceilingpic  == skyflatnum)
 		{
-			worldtop = worldhigh;
-#ifdef ESLOPE
-			worldtopslope = worldhighslope;
-#endif
+			bothceilingssky = true;
 		}
 
-		gr_toptexture = R_GetTextureNum(gr_sidedef->toptexture);
-		gr_bottomtexture = R_GetTextureNum(gr_sidedef->bottomtexture);
+		// likewise, but for floors and upper textures
+		if (gr_frontsector->floorpic == skyflatnum
+			&& gr_backsector->floorpic == skyflatnum)
+		{
+			bothfloorssky = true;
+		}
+
+		if (!bothceilingssky)
+			gr_toptexture = R_GetTextureNum(gr_sidedef->toptexture);
+		if (!bothfloorssky)
+			gr_bottomtexture = R_GetTextureNum(gr_sidedef->bottomtexture);
 
 		// check TOP TEXTURE
 		if ((
@@ -2008,84 +2014,42 @@ static void HWR_StoreWallRange(double startfrac, double endfrac)
 			}*/
 		}
 
-		// Isn't this just the most lovely mess
+#if 1
+		// Sky culling
+		// No longer so much a mess as before!
 		if (!gr_curline->polyseg) // Don't do it for polyobjects
 		{
-			if (gr_frontsector->ceilingpic == skyflatnum || gr_backsector->ceilingpic == skyflatnum)
+			if (gr_frontsector->ceilingpic == skyflatnum)
 			{
-				fixed_t depthwallheight;
-
-				if (!gr_sidedef->toptexture || (gr_frontsector->ceilingpic == skyflatnum && gr_backsector->ceilingpic == skyflatnum)) // when both sectors are sky, the top texture isn't drawn
-					depthwallheight = gr_frontsector->ceilingheight < gr_backsector->ceilingheight ? gr_frontsector->ceilingheight : gr_backsector->ceilingheight;
-				else
-					depthwallheight = gr_frontsector->ceilingheight > gr_backsector->ceilingheight ? gr_frontsector->ceilingheight : gr_backsector->ceilingheight;
-
-				if (gr_frontsector->ceilingheight-gr_frontsector->floorheight <= 0) // current sector is a thok barrier
+				if (gr_backsector->ceilingpic != skyflatnum) // don't cull if back sector is also sky
 				{
-					if (gr_backsector->ceilingheight-gr_backsector->floorheight <= 0) // behind sector is also a thok barrier
-					{
-						if (!gr_sidedef->bottomtexture) // Only extend further down if there's no texture
-							HWR_DrawSkyWall(wallVerts, &Surf, worldbottom < worldlow ? worldbottom : worldlow, INT32_MAX);
-						else
-							HWR_DrawSkyWall(wallVerts, &Surf, worldbottom > worldlow ? worldbottom : worldlow, INT32_MAX);
-					}
-					// behind sector is not a thok barrier
-					else if (gr_backsector->ceilingheight <= gr_frontsector->ceilingheight) // behind sector ceiling is lower or equal to current sector
-						HWR_DrawSkyWall(wallVerts, &Surf, depthwallheight, INT32_MAX);
-						// gr_front/backsector heights need to be used here because of the worldtop being set to worldhigh earlier on
-				}
-				else if (gr_backsector->ceilingheight-gr_backsector->floorheight <= 0) // behind sector is a thok barrier, current sector is not
-				{
-					if (gr_backsector->ceilingheight >= gr_frontsector->ceilingheight // thok barrier ceiling height is equal to or greater than current sector ceiling height
-						|| gr_backsector->floorheight <= gr_frontsector->floorheight // thok barrier ceiling height is equal to or less than current sector floor height
-						|| gr_backsector->ceilingpic != skyflatnum) // thok barrier is not a sky
-						HWR_DrawSkyWall(wallVerts, &Surf, depthwallheight, INT32_MAX);
-				}
-				else // neither sectors are thok barriers
-				{
-					if ((gr_backsector->ceilingheight < gr_frontsector->ceilingheight && !gr_sidedef->toptexture) // no top texture and sector behind is lower
-						|| gr_backsector->ceilingpic != skyflatnum) // behind sector is not a sky
-						HWR_DrawSkyWall(wallVerts, &Surf, depthwallheight, INT32_MAX);
+					wallVerts[2].y = wallVerts[3].y = FIXED_TO_FLOAT(INT32_MAX); // draw to top of map space
+#ifdef ESLOPE
+					wallVerts[0].y = FIXED_TO_FLOAT(worldtop);
+					wallVerts[1].y = FIXED_TO_FLOAT(worldtopslope);
+#else
+					wallVerts[0].y = wallVerts[1].y = FIXED_TO_FLOAT(worldtop);
+#endif
+					HWR_DrawSkyWall(wallVerts, &Surf);
 				}
 			}
-			// And now for sky floors!
-			if (gr_frontsector->floorpic == skyflatnum || gr_backsector->floorpic == skyflatnum)
+
+			if (gr_frontsector->floorpic == skyflatnum)
 			{
-				fixed_t depthwallheight;
-
-				if (!gr_sidedef->bottomtexture)
-					depthwallheight = worldbottom > worldlow ? worldbottom : worldlow;
-				else
-					depthwallheight = worldbottom < worldlow ? worldbottom : worldlow;
-
-				if (gr_frontsector->ceilingheight-gr_frontsector->floorheight <= 0) // current sector is a thok barrier
+				if (gr_backsector->floorpic != skyflatnum) // don't cull if back sector is also sky
 				{
-					if (gr_backsector->ceilingheight-gr_backsector->floorheight <= 0) // behind sector is also a thok barrier
-					{
-						if (!gr_sidedef->toptexture) // Only extend up if there's no texture
-							HWR_DrawSkyWall(wallVerts, &Surf, INT32_MIN, worldtop > worldhigh ? worldtop : worldhigh);
-						else
-							HWR_DrawSkyWall(wallVerts, &Surf, INT32_MIN, worldtop < worldhigh ? worldtop : worldhigh);
-					}
-					// behind sector is not a thok barrier
-					else if (gr_backsector->floorheight >= gr_frontsector->floorheight) // behind sector floor is greater or equal to current sector
-						HWR_DrawSkyWall(wallVerts, &Surf, INT32_MIN, depthwallheight);
-				}
-				else if (gr_backsector->ceilingheight-gr_backsector->floorheight <= 0) // behind sector is a thok barrier, current sector is not
-				{
-					if (gr_backsector->floorheight <= gr_frontsector->floorheight // thok barrier floor height is equal to or less than current sector floor height
-						|| gr_backsector->ceilingheight >= gr_frontsector->ceilingheight // thok barrier floor height is equal to or greater than current sector ceiling height
-						|| gr_backsector->floorpic != skyflatnum) // thok barrier is not a sky
-						HWR_DrawSkyWall(wallVerts, &Surf, INT32_MIN, depthwallheight);
-				}
-				else // neither sectors are thok barriers
-				{
-					if ((gr_backsector->floorheight > gr_frontsector->floorheight && !gr_sidedef->bottomtexture) // no bottom texture and sector behind is higher
-						|| gr_backsector->floorpic != skyflatnum) // behind sector is not a sky
-						HWR_DrawSkyWall(wallVerts, &Surf, INT32_MIN, depthwallheight);
+#ifdef ESLOPE
+					wallVerts[3].y = FIXED_TO_FLOAT(worldbottom);
+					wallVerts[2].y = FIXED_TO_FLOAT(worldbottomslope);
+#else
+					wallVerts[2].y = wallVerts[3].y = FIXED_TO_FLOAT(worldbottom);
+#endif
+					wallVerts[0].y = wallVerts[1].y = FIXED_TO_FLOAT(INT32_MIN); // draw to bottom of map space
+					HWR_DrawSkyWall(wallVerts, &Surf);
 				}
 			}
 		}
+#endif
 	}
 	else
 	{
@@ -2156,9 +2120,27 @@ static void HWR_StoreWallRange(double startfrac, double endfrac)
 		if (!gr_curline->polyseg)
 		{
 			if (gr_frontsector->ceilingpic == skyflatnum) // It's a single-sided line with sky for its sector
-				HWR_DrawSkyWall(wallVerts, &Surf, worldtop, INT32_MAX);
+			{
+				wallVerts[2].y = wallVerts[3].y = FIXED_TO_FLOAT(INT32_MAX); // draw to top of map space
+#ifdef ESLOPE
+				wallVerts[0].y = FIXED_TO_FLOAT(worldtop);
+				wallVerts[1].y = FIXED_TO_FLOAT(worldtopslope);
+#else
+				wallVerts[0].y = wallVerts[1].y = FIXED_TO_FLOAT(worldtop);
+#endif
+				HWR_DrawSkyWall(wallVerts, &Surf);
+			}
 			if (gr_frontsector->floorpic == skyflatnum)
-				HWR_DrawSkyWall(wallVerts, &Surf, INT32_MIN, worldbottom);
+			{
+#ifdef ESLOPE
+				wallVerts[3].y = FIXED_TO_FLOAT(worldbottom);
+				wallVerts[2].y = FIXED_TO_FLOAT(worldbottomslope);
+#else
+				wallVerts[2].y = wallVerts[3].y = FIXED_TO_FLOAT(worldbottom);
+#endif
+				wallVerts[0].y = wallVerts[1].y = FIXED_TO_FLOAT(INT32_MIN); // draw to bottom of map space
+				HWR_DrawSkyWall(wallVerts, &Surf);
+			}
 		}
 	}
 
@@ -2451,6 +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
 
@@ -2483,62 +2471,45 @@ static boolean CheckClip(seg_t * seg, sector_t * afrontsector, sector_t * abacks
 		backf1 = backf2 = abacksector->floorheight;
 		backc1 = backc2 = abacksector->ceilingheight;
 	}
-
-	// now check for closed sectors!
-	if (backc1 <= frontf1 && backc2 <= frontf2)
+	// properly render skies (consider door "open" if both ceilings are sky)
+	// same for floors
+	if (!bothceilingssky && !bothfloorssky)
 	{
-		checkforemptylines = false;
-		if (!seg->sidedef->toptexture)
-			return false;
-
-		if (abacksector->ceilingpic == skyflatnum && afrontsector->ceilingpic == skyflatnum)
-			return false;
-
-		return true;
-	}
-
-	if (backf1 >= frontc1 && backf2 >= frontc2)
-	{
-		checkforemptylines = false;
-		if (!seg->sidedef->bottomtexture)
-			return false;
-
-		// properly render skies (consider door "open" if both floors are sky):
-		if (abacksector->ceilingpic == skyflatnum && afrontsector->ceilingpic == skyflatnum)
-			return false;
-
-		return true;
-	}
-
-	if (backc1 <= backf1 && backc2 <= backf2)
-	{
-		checkforemptylines = false;
-		// preserve a kind of transparent door/lift special effect:
-		if (backc1 < frontc1 || backc2 < frontc2)
+		// now check for closed sectors!
+		if ((backc1 <= frontf1 && backc2 <= frontf2)
+			|| (backf1 >= frontc1 && backf2 >= frontc2))
 		{
-			if (!seg->sidedef->toptexture)
-				return false;
+			checkforemptylines = false;
+			return true;
 		}
-		if (backf1 > frontf1 || backf2 > frontf2)
+
+		if (backc1 <= backf1 && backc2 <= backf2)
 		{
-			if (!seg->sidedef->bottomtexture)
-				return false;
+			// preserve a kind of transparent door/lift special effect:
+			if (((backc1 >= frontc1 && backc2 >= frontc2) || seg->sidedef->toptexture)
+			&& ((backf1 <= frontf1 && backf2 <= frontf2) || seg->sidedef->bottomtexture))
+			{
+				checkforemptylines = false;
+				return true;
+			}
 		}
-		if (abacksector->ceilingpic == skyflatnum && afrontsector->ceilingpic == skyflatnum)
-			return false;
-
-		if (abacksector->floorpic == skyflatnum && afrontsector->floorpic == skyflatnum)
-			return false;
-
-		return true;
 	}
 
-	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;
 }
@@ -2841,6 +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
@@ -2966,7 +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);
@@ -2989,6 +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)
 	{
@@ -3007,47 +3021,58 @@ static void HWR_AddLine(seg_t * line)
 		SLOPEPARAMS( gr_backsector->f_slope, backf1,  backf2,  gr_backsector->floorheight)
 		SLOPEPARAMS( gr_backsector->c_slope, backc1,  backc2,  gr_backsector->ceilingheight)
 #undef SLOPEPARAMS
-
-		// Closed door.
-		if ((backc1 <= frontf1 && backc2 <= frontf2)
-			|| (backf1 >= frontc1 && backf2 >= frontc2))
+		// if both ceilings are skies, consider it always "open"
+		// same for floors
+		if (!bothceilingssky && !bothfloorssky)
 		{
-			goto clipsolid;
-		}
+			// Closed door.
+			if ((backc1 <= frontf1 && backc2 <= frontf2)
+				|| (backf1 >= frontc1 && backf2 >= frontc2))
+			{
+				goto clipsolid;
+			}
 
-		// Check for automap fix.
-		if (backc1 <= backf1 && backc2 <= backf2
-		&& ((backc1 >= frontc1 && backc2 >= frontc2) || gr_curline->sidedef->toptexture)
-		&& ((backf1 <= frontf1 && backf2 >= frontf2) || gr_curline->sidedef->bottomtexture)
-		&& (gr_backsector->ceilingpic != skyflatnum || gr_frontsector->ceilingpic != skyflatnum))
-			goto clipsolid;
+			// Check for automap fix.
+			if (backc1 <= backf1 && backc2 <= backf2
+			&& ((backc1 >= frontc1 && backc2 >= frontc2) || gr_curline->sidedef->toptexture)
+			&& ((backf1 <= frontf1 && backf2 >= frontf2) || gr_curline->sidedef->bottomtexture))
+				goto clipsolid;
+		}
 
 		// 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
 	{
-		// Closed door.
-		if (gr_backsector->ceilingheight <= gr_frontsector->floorheight ||
-			gr_backsector->floorheight >= gr_frontsector->ceilingheight)
-			goto clipsolid;
+		// if both ceilings are skies, consider it always "open"
+		// same for floors
+		if (!bothceilingssky && !bothfloorssky)
+		{
+			// Closed door.
+			if (gr_backsector->ceilingheight <= gr_frontsector->floorheight ||
+				gr_backsector->floorheight >= gr_frontsector->ceilingheight)
+				goto clipsolid;
 
-		// Check for automap fix.
-		if (gr_backsector->ceilingheight <= gr_backsector->floorheight
-		&& ((gr_backsector->ceilingheight >= gr_frontsector->ceilingheight) || gr_curline->sidedef->toptexture)
-		&& ((gr_backsector->floorheight <= gr_backsector->floorheight) || gr_curline->sidedef->bottomtexture)
-		&& (gr_backsector->ceilingpic != skyflatnum || gr_frontsector->ceilingpic != skyflatnum))
-			goto clipsolid;
+			// Check for automap fix.
+			if (gr_backsector->ceilingheight <= gr_backsector->floorheight
+			&& ((gr_backsector->ceilingheight >= gr_frontsector->ceilingheight) || gr_curline->sidedef->toptexture)
+			&& ((gr_backsector->floorheight <= gr_backsector->floorheight) || gr_curline->sidedef->bottomtexture))
+				goto clipsolid;
+		}
 
 		// 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.
@@ -5521,8 +5546,9 @@ static void HWR_DrawSkyBackground(player_t *player)
 
 	dimensionmultiply = ((float)textures[texturetranslation[skytexture]]->width/256.0f);
 
-	v[0].sow = v[3].sow = ((float) angle / ((ANGLE_90-1)*dimensionmultiply));
-	v[2].sow = v[1].sow = (-1.0f/dimensionmultiply)+((float) angle / ((ANGLE_90-1)*dimensionmultiply));
+	v[0].sow = v[3].sow = ((float) (-angle) / ((ANGLE_90-1)*dimensionmultiply)); // left
+	v[2].sow = v[1].sow = v[0].sow + (1.0f/dimensionmultiply); // right (or left + 1.0f)
+	// use +angle and -1.0f above instead if you wanted old backwards behavior
 
 	// Y
 	angle = aimingangle;
@@ -5536,13 +5562,13 @@ static void HWR_DrawSkyBackground(player_t *player)
 	if (atransform.flip)
 	{
 		// During vertical flip the sky should be flipped and it's y movement should also be flipped obviously
-		v[3].tow = v[2].tow = -(0.5f-(0.5f/dimensionmultiply));
-		v[0].tow = v[1].tow = (-1.0f/dimensionmultiply)-(0.5f-(0.5f/dimensionmultiply));
+		v[3].tow = v[2].tow = -(0.5f-(0.5f/dimensionmultiply)); // top
+		v[0].tow = v[1].tow = v[3].tow - (1.0f/dimensionmultiply); // bottom (or top - 1.0f)
 	}
 	else
 	{
-		v[3].tow = v[2].tow = (-1.0f/dimensionmultiply)-(0.5f-(0.5f/dimensionmultiply));
-		v[0].tow = v[1].tow = -(0.5f-(0.5f/dimensionmultiply));
+		v[0].tow = v[1].tow = -(0.5f-(0.5f/dimensionmultiply)); // bottom
+		v[3].tow = v[2].tow = v[0].tow - (1.0f/dimensionmultiply); // top (or bottom - 1.0f)
 	}
 
 	if (angle > ANGLE_180) // Do this because we don't want the sky to suddenly teleport when crossing over 0 to 360 and vice versa
diff --git a/src/r_bsp.c b/src/r_bsp.c
index ad4975cde..183def25a 100644
--- a/src/r_bsp.c
+++ b/src/r_bsp.c
@@ -220,10 +220,7 @@ static INT32 R_DoorClosed(void)
 	// preserve a kind of transparent door/lift special effect:
 	&& (backsector->ceilingheight >= frontsector->ceilingheight || curline->sidedef->toptexture)
 
-	&& (backsector->floorheight <= frontsector->floorheight || curline->sidedef->bottomtexture)
-
-	// properly render skies (consider door "open" if both ceilings are sky):
-	&& (backsector->ceilingpic != skyflatnum || frontsector->ceilingpic != skyflatnum);
+	&& (backsector->floorheight <= frontsector->floorheight || curline->sidedef->bottomtexture);
 }
 
 //
@@ -404,6 +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;
@@ -490,6 +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)
@@ -508,48 +525,59 @@ static void R_AddLine(seg_t *line)
 		SLOPEPARAMS( backsector->f_slope, backf1,  backf2,  backsector->floorheight)
 		SLOPEPARAMS( backsector->c_slope, backc1,  backc2,  backsector->ceilingheight)
 #undef SLOPEPARAMS
-		if ((backc1 <= frontf1 && backc2 <= frontf2)
-			|| (backf1 >= frontc1 && backf2 >= frontc2))
+		// if both ceilings are skies, consider it always "open"
+		// same for floors
+		if (!bothceilingssky && !bothfloorssky)
 		{
-			goto clipsolid;
+			if ((backc1 <= frontf1 && backc2 <= frontf2)
+				|| (backf1 >= frontc1 && backf2 >= frontc2))
+			{
+				goto clipsolid;
+			}
+
+			// Check for automap fix. Store in doorclosed for r_segs.c
+			doorclosed = (backc1 <= backf1 && backc2 <= backf2
+			&& ((backc1 >= frontc1 && backc2 >= frontc2) || curline->sidedef->toptexture)
+			&& ((backf1 <= frontf1 && backf2 >= frontf2) || curline->sidedef->bottomtexture));
+
+			if (doorclosed)
+				goto clipsolid;
 		}
 
-		// Check for automap fix. Store in doorclosed for r_segs.c
-		doorclosed = (backc1 <= backf1 && backc2 <= backf2
-		&& ((backc1 >= frontc1 && backc2 >= frontc2) || curline->sidedef->toptexture)
-		&& ((backf1 <= frontf1 && backf2 >= frontf2) || curline->sidedef->bottomtexture)
-		&& (backsector->ceilingpic != skyflatnum || frontsector->ceilingpic != skyflatnum));
-
-		if (doorclosed)
-			goto clipsolid;
-
 		// 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 (backsector->ceilingheight <= frontsector->floorheight
-			|| backsector->floorheight >= frontsector->ceilingheight)
+		// if both ceilings are skies, consider it always "open"
+		// same for floors
+		if (!bothceilingssky && !bothfloorssky)
 		{
-			goto clipsolid;
-		}
+			if (backsector->ceilingheight <= frontsector->floorheight
+				|| backsector->floorheight >= frontsector->ceilingheight)
+			{
+				goto clipsolid;
+			}
 
-		// Check for automap fix. Store in doorclosed for r_segs.c
-		doorclosed = R_DoorClosed();
-		if (doorclosed)
-			goto clipsolid;
+			// Check for automap fix. Store in doorclosed for r_segs.c
+			doorclosed = R_DoorClosed();
+			if (doorclosed)
+				goto clipsolid;
+		}
 
 		// 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.
@@ -922,7 +950,8 @@ static void R_Subsector(size_t num)
 #ifdef ESLOPE
 			frontsector->f_slope ? P_GetZAt(frontsector->f_slope, viewx, viewy) :
 #endif
-		frontsector->floorheight) < viewz || (frontsector->heightsec != -1
+		frontsector->floorheight) < viewz || frontsector->floorpic == skyflatnum
+		|| (frontsector->heightsec != -1
 		&& sectors[frontsector->heightsec].ceilingpic == skyflatnum)))
 	{
 		floorplane = R_FindPlane(frontsector->floorheight, frontsector->floorpic, floorlightlevel,
diff --git a/src/r_plane.c b/src/r_plane.c
index 670152eda..bc510bbb5 100644
--- a/src/r_plane.c
+++ b/src/r_plane.c
@@ -625,7 +625,7 @@ visplane_t *R_CheckPlane(visplane_t *pl, INT32 start, INT32 stop)
 // overlap.
 void R_ExpandPlane(visplane_t *pl, INT32 start, INT32 stop)
 {
-	INT32 unionl, unionh;
+//	INT32 unionl, unionh;
 //	INT32 x;
 
 #ifdef POLYOBJECTS_PLANES
@@ -634,6 +634,9 @@ void R_ExpandPlane(visplane_t *pl, INT32 start, INT32 stop)
 		return;
 #endif
 
+	if (pl->minx > start) pl->minx = start;
+	if (pl->maxx < stop)  pl->maxx = stop;
+/*
 	if (start < pl->minx)
 	{
 		unionl = start;
@@ -651,15 +654,16 @@ void R_ExpandPlane(visplane_t *pl, INT32 start, INT32 stop)
 	{
 		unionh = pl->maxx;
 	}
-/*
 	for (x = start; x <= stop; x++)
 		if (pl->top[x] != 0xffff || pl->bottom[x] != 0x0000)
 			break;
 
 	if (x <= stop)
 		I_Error("R_ExpandPlane: planes in same subsector overlap?!\nminx: %d, maxx: %d, start: %d, stop: %d\n", pl->minx, pl->maxx, start, stop);
-*/
+
 	pl->minx = unionl, pl->maxx = unionh;
+*/
+
 }
 
 //
@@ -694,10 +698,10 @@ void R_MakeSpans(INT32 x, INT32 t1, INT32 b1, INT32 t2, INT32 b2)
 void R_DrawPlanes(void)
 {
 	visplane_t *pl;
-	INT32 x;
-	INT32 angle;
 	INT32 i;
 
+	// Note: are these two lines really needed?
+	// R_DrawSinglePlane and R_DrawSkyPlane do span/column drawer resets themselves anyway
 	spanfunc = basespanfunc;
 	wallcolfunc = walldrawerfunc;
 
@@ -705,45 +709,6 @@ void R_DrawPlanes(void)
 	{
 		for (pl = visplanes[i]; pl; pl = pl->next)
 		{
-			// sky flat
-			if (pl->picnum == skyflatnum)
-			{
-				if (!viewsky)
-				{
-					skyVisible = true;
-					continue;
-				}
-
-				// use correct aspect ratio scale
-				dc_iscale = skyscale;
-
-				// Sky is always drawn full bright,
-				//  i.e. colormaps[0] is used.
-				// Because of this hack, sky is not affected
-				//  by INVUL inverse mapping.
-				dc_colormap = colormaps;
-				dc_texturemid = skytexturemid;
-				dc_texheight = textureheight[skytexture]
-					>>FRACBITS;
-				for (x = pl->minx; x <= pl->maxx; x++)
-				{
-					dc_yl = pl->top[x];
-					dc_yh = pl->bottom[x];
-
-					if (dc_yl <= dc_yh)
-					{
-						angle = (pl->viewangle + xtoviewangle[x])>>ANGLETOSKYSHIFT;
-						dc_iscale = FixedMul(skyscale, FINECOSINE(xtoviewangle[x]>>ANGLETOFINESHIFT));
-						dc_x = x;
-						dc_source =
-							R_GetColumn(texturetranslation[skytexture],
-								angle);
-						wallcolfunc();
-					}
-				}
-				continue;
-			}
-
 			if (pl->ffloor != NULL
 #ifdef POLYOBJECTS_PLANES
 			|| pl->polyobj != NULL
@@ -760,6 +725,59 @@ void R_DrawPlanes(void)
 #endif
 }
 
+// R_DrawSkyPlane
+//
+// Draws the sky within the plane's top/bottom bounds
+// Note: this uses column drawers instead of span drawers, since the sky is always a texture
+//
+static void R_DrawSkyPlane(visplane_t *pl)
+{
+	INT32 x;
+	INT32 angle;
+
+	// If we're not supposed to draw the sky (e.g. for skyboxes), don't do anything!
+	// This probably utterly ruins sky rendering for FOFs and polyobjects, unfortunately
+	if (!viewsky)
+	{
+		// Mark that the sky was visible here for next tic
+		// (note: this is a hack and it sometimes can cause HOMs to appear for a tic IIRC)
+		skyVisible = true;
+		return;
+	}
+
+	// Reset column drawer function (note: couldn't we just call walldrawerfunc directly?)
+	// (that is, unless we'll need to switch drawers in future for some reason)
+	wallcolfunc = walldrawerfunc;
+
+	// use correct aspect ratio scale
+	dc_iscale = skyscale;
+
+	// Sky is always drawn full bright,
+	//  i.e. colormaps[0] is used.
+	// Because of this hack, sky is not affected
+	//  by sector colormaps (INVUL inverse mapping is not implemented in SRB2 so is irrelevant).
+	dc_colormap = colormaps;
+	dc_texturemid = skytexturemid;
+	dc_texheight = textureheight[skytexture]
+		>>FRACBITS;
+	for (x = pl->minx; x <= pl->maxx; x++)
+	{
+		dc_yl = pl->top[x];
+		dc_yh = pl->bottom[x];
+
+		if (dc_yl <= dc_yh)
+		{
+			angle = (pl->viewangle + xtoviewangle[x])>>ANGLETOSKYSHIFT;
+			dc_iscale = FixedMul(skyscale, FINECOSINE(xtoviewangle[x]>>ANGLETOFINESHIFT));
+			dc_x = x;
+			dc_source =
+				R_GetColumn(texturetranslation[skytexture],
+					-angle); // get negative of angle for each column to display sky correct way round! --Monster Iestyn 27/01/18
+			wallcolfunc();
+		}
+	}
+}
+
 void R_DrawSinglePlane(visplane_t *pl)
 {
 	INT32 light = 0;
@@ -771,6 +789,13 @@ void R_DrawSinglePlane(visplane_t *pl)
 	if (!(pl->minx <= pl->maxx))
 		return;
 
+	// sky flat
+	if (pl->picnum == skyflatnum)
+	{
+		R_DrawSkyPlane(pl);
+		return;
+	}
+
 #ifndef NOWATER
 	itswater = false;
 #endif
diff --git a/src/r_segs.c b/src/r_segs.c
index ecf707e0f..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;
@@ -1366,7 +1377,6 @@ static void R_RenderSegLoop (void)
 		// mark floor / ceiling areas
 		yl = (topfrac+HEIGHTUNIT-1)>>HEIGHTBITS;
 
-		// no space above wall?
 		top = ceilingclip[rw_x]+1;
 
 		// no space above wall?
@@ -1375,16 +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)
-			{
-				ceilingplane->top[rw_x] = (INT16)top;
-				ceilingplane->bottom[rw_x] = (INT16)bottom;
-			}
+#else
+			bottom = yl > floorclip[rw_x] ? floorclip[rw_x] : yl;
+
+			if (top <= --bottom && ceilingplane)
+#endif
+				R_ExpandPlaneY(ceilingplane, rw_x, top, bottom);
 		}
 
 
@@ -1397,24 +1410,10 @@ static void R_RenderSegLoop (void)
 
 		if (markfloor)
 		{
-#if 0 // Old Doom Legacy code
-			bottom = floorclip[rw_x]-1;
-			if (top <= ceilingclip[rw_x])
-				top = ceilingclip[rw_x]+1;
-			if (top <= bottom && floorplane)
-			{
-				floorplane->top[rw_x] = (INT16)top;
-				floorplane->bottom[rw_x] = (INT16)bottom;
-			}
-#else // Spiffy new PRBoom code
-			top  = yh < ceilingclip[rw_x] ? ceilingclip[rw_x] : yh;
+			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;
-			}
-#endif
+				R_ExpandPlaneY(floorplane, rw_x, top, bottom);
 		}
 
 		if (numffloors)
@@ -1428,14 +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)
@@ -1451,7 +1442,7 @@ static void R_RenderSegLoop (void)
 
 #ifdef POLYOBJECTS_PLANES
 					// Polyobject-specific hack to fix plane leaking -Red
-					if (curline->polyseg && ffloor[i].polyobj && ffloor[i].polyobj == curline->polyseg && top_w >= bottom_w) {
+					if (ffloor[i].polyobj && top_w >= bottom_w) {
 						ffloor[i].plane->top[rw_x] = ffloor[i].plane->bottom[rw_x] = 0xFFFF;
 					} else
 #endif
@@ -1475,7 +1466,7 @@ static void R_RenderSegLoop (void)
 
 #ifdef POLYOBJECTS_PLANES
 					// Polyobject-specific hack to fix plane leaking -Red
-					if (curline->polyseg && ffloor[i].polyobj && ffloor[i].polyobj == curline->polyseg && top_w >= bottom_w) {
+					if (ffloor[i].polyobj && top_w >= bottom_w) {
 						ffloor[i].plane->top[rw_x] = ffloor[i].plane->bottom[rw_x] = 0xFFFF;
 					} else
 #endif
@@ -1908,29 +1899,26 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 		}
 	}
 
-	if (frontsector->c_slope) {
-		worldtop = P_GetZAt(frontsector->c_slope, segleft.x, segleft.y) - viewz;
-		worldtopslope = P_GetZAt(frontsector->c_slope, segright.x, segright.y) - viewz;
-	} else {
-		worldtopslope =
-#else
-	{
-#endif
-		worldtop = frontsector->ceilingheight - viewz;
-	}
 
+#define SLOPEPARAMS(slope, end1, end2, normalheight) \
+	if (slope) { \
+		end1 = P_GetZAt(slope, segleft.x, segleft.y); \
+		end2 = P_GetZAt(slope, segright.x, segright.y); \
+	} else \
+		end1 = end2 = normalheight;
 
-#ifdef ESLOPE
-	if (frontsector->f_slope) {
-		worldbottom = P_GetZAt(frontsector->f_slope, segleft.x, segleft.y) - viewz;
-		worldbottomslope = P_GetZAt(frontsector->f_slope, segright.x, segright.y) - viewz;
-	} else {
-		worldbottomslope =
+	SLOPEPARAMS(frontsector->c_slope, worldtop,    worldtopslope,    frontsector->ceilingheight)
+	SLOPEPARAMS(frontsector->f_slope, worldbottom, worldbottomslope, frontsector->floorheight)
+	// subtract viewz from these to turn them into
+	// positions relative to the camera's z position
+	worldtop -= viewz;
+	worldtopslope -= viewz;
+	worldbottom -= viewz;
+	worldbottomslope -= viewz;
 #else
-	{
+	worldtop = frontsector->ceilingheight - viewz;
+	worldbottom = frontsector->floorheight - viewz;
 #endif
-		worldbottom = frontsector->floorheight - viewz;
-	}
 
 	midtexture = toptexture = bottomtexture = maskedtexture = 0;
 	ds_p->maskedtexturecol = NULL;
@@ -2032,125 +2020,128 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 	else
 	{
 		// two sided line
+		boolean bothceilingssky = false; // turned on if both back and front ceilings are sky
+		boolean bothfloorssky = false; // likewise, but for floors
 
 #ifdef ESLOPE
-		if (backsector->c_slope) {
-			worldhigh = P_GetZAt(backsector->c_slope, segleft.x, segleft.y) - viewz;
-			worldhighslope = P_GetZAt(backsector->c_slope, segright.x, segright.y) - viewz;
-		} else {
-			worldhighslope =
+		SLOPEPARAMS(backsector->c_slope, worldhigh, worldhighslope, backsector->ceilingheight)
+		SLOPEPARAMS(backsector->f_slope, worldlow,  worldlowslope,  backsector->floorheight)
+		worldhigh -= viewz;
+		worldhighslope -= viewz;
+		worldlow -= viewz;
+		worldlowslope -= viewz;
 #else
-		{
+		worldhigh = backsector->ceilingheight - viewz;
+		worldlow = backsector->floorheight - viewz;
 #endif
-			worldhigh = backsector->ceilingheight - viewz;
-		}
-
-
-#ifdef ESLOPE
-		if (backsector->f_slope) {
-			worldlow = P_GetZAt(backsector->f_slope, segleft.x, segleft.y) - viewz;
-			worldlowslope = P_GetZAt(backsector->f_slope, segright.x, segright.y) - viewz;
-		} else {
-			worldlowslope =
-#else
-		{
-#endif
-			worldlow = backsector->floorheight - viewz;
-		}
-
 
 		// hack to allow height changes in outdoor areas
+		// This is what gets rid of the upper textures if there should be sky
 		if (frontsector->ceilingpic == skyflatnum
 			&& backsector->ceilingpic == skyflatnum)
 		{
-#ifdef ESLOPE
-			worldtopslope = worldhighslope =
-#endif
-			worldtop = worldhigh;
+			bothceilingssky = true;
+		}
+
+		// likewise, but for floors and upper textures
+		if (frontsector->floorpic == skyflatnum
+			&& backsector->floorpic == skyflatnum)
+		{
+			bothfloorssky = true;
 		}
 
 		ds_p->sprtopclip = ds_p->sprbottomclip = NULL;
 		ds_p->silhouette = 0;
 
-		if (
-#ifdef ESLOPE
-			worldbottomslope > worldlowslope ||
-#endif
-			worldbottom > worldlow)
+		if (!bothfloorssky)
 		{
-			ds_p->silhouette = SIL_BOTTOM;
+			if (
 #ifdef ESLOPE
-			if ((backsector->f_slope ? P_GetZAt(backsector->f_slope, viewx, viewy) : backsector->floorheight) > viewz)
+				worldbottomslope > worldlowslope ||
+#endif
+				worldbottom > worldlow)
+			{
+				ds_p->silhouette = SIL_BOTTOM;
+#ifdef ESLOPE
+				if ((backsector->f_slope ? P_GetZAt(backsector->f_slope, viewx, viewy) : backsector->floorheight) > viewz)
+					ds_p->bsilheight = INT32_MAX;
+				else
+					ds_p->bsilheight = (frontsector->f_slope ? INT32_MAX : frontsector->floorheight);
+#else
+				ds_p->bsilheight = frontsector->floorheight;
+#endif
+			}
+#ifdef ESLOPE
+			else if ((backsector->f_slope ? P_GetZAt(backsector->f_slope, viewx, viewy) : backsector->floorheight) > viewz)
+#else
+			else if (backsector->floorheight > viewz)
+#endif
+			{
+				ds_p->silhouette = SIL_BOTTOM;
 				ds_p->bsilheight = INT32_MAX;
-			else
-				ds_p->bsilheight = (frontsector->f_slope ? INT32_MAX : frontsector->floorheight);
-#else
-			ds_p->bsilheight = frontsector->floorheight;
-#endif
-		}
-#ifdef ESLOPE
-		else if ((backsector->f_slope ? P_GetZAt(backsector->f_slope, viewx, viewy) : backsector->floorheight) > viewz)
-#else
-		else if (backsector->floorheight > viewz)
-#endif
-		{
-			ds_p->silhouette = SIL_BOTTOM;
-			ds_p->bsilheight = INT32_MAX;
-			// ds_p->sprbottomclip = negonearray;
+				// ds_p->sprbottomclip = negonearray;
+			}
 		}
 
-		if (
-#ifdef ESLOPE
-			worldtopslope < worldhighslope ||
-#endif
-			worldtop < worldhigh)
+		if (!bothceilingssky)
 		{
-			ds_p->silhouette |= SIL_TOP;
+			if (
 #ifdef ESLOPE
-			if ((backsector->c_slope ? P_GetZAt(backsector->c_slope, viewx, viewy) : backsector->ceilingheight) < viewz)
+				worldtopslope < worldhighslope ||
+#endif
+				worldtop < worldhigh)
+			{
+				ds_p->silhouette |= SIL_TOP;
+#ifdef ESLOPE
+				if ((backsector->c_slope ? P_GetZAt(backsector->c_slope, viewx, viewy) : backsector->ceilingheight) < viewz)
+					ds_p->tsilheight = INT32_MIN;
+				else
+					ds_p->tsilheight = (frontsector->c_slope ? INT32_MIN : frontsector->ceilingheight);
+#else
+				ds_p->tsilheight = frontsector->ceilingheight;
+#endif
+			}
+#ifdef ESLOPE
+			else if ((backsector->c_slope ? P_GetZAt(backsector->c_slope, viewx, viewy) : backsector->ceilingheight) < viewz)
+#else
+			else if (backsector->ceilingheight < viewz)
+#endif
+			{
+				ds_p->silhouette |= SIL_TOP;
 				ds_p->tsilheight = INT32_MIN;
-			else
-				ds_p->tsilheight = (frontsector->c_slope ? INT32_MIN : frontsector->ceilingheight);
-#else
-			ds_p->tsilheight = frontsector->ceilingheight;
-#endif
-		}
-#ifdef ESLOPE
-		else if ((backsector->c_slope ? P_GetZAt(backsector->c_slope, viewx, viewy) : backsector->ceilingheight) < viewz)
-#else
-		else if (backsector->ceilingheight < viewz)
-#endif
-		{
-			ds_p->silhouette |= SIL_TOP;
-			ds_p->tsilheight = INT32_MIN;
-			// ds_p->sprtopclip = screenheightarray;
+				// ds_p->sprtopclip = screenheightarray;
+			}
 		}
 
-#ifdef ESLOPE
-		if (worldhigh <= worldbottom && worldhighslope <= worldbottomslope)
-#else
-		if (worldhigh <= worldbottom)
-#endif
+		if (!bothceilingssky && !bothfloorssky)
 		{
-			ds_p->sprbottomclip = negonearray;
-			ds_p->bsilheight = INT32_MAX;
-			ds_p->silhouette |= SIL_BOTTOM;
-		}
+#ifdef ESLOPE
+			if (worldhigh <= worldbottom && worldhighslope <= worldbottomslope)
+#else
+			if (worldhigh <= worldbottom)
+#endif
+			{
+				ds_p->sprbottomclip = negonearray;
+				ds_p->bsilheight = INT32_MAX;
+				ds_p->silhouette |= SIL_BOTTOM;
+			}
 
 #ifdef ESLOPE
-		if (worldlow >= worldtop && worldlowslope >= worldtopslope)
+			if (worldlow >= worldtop && worldlowslope >= worldtopslope)
 #else
-		if (worldlow >= worldtop)
+			if (worldlow >= worldtop)
 #endif
-		{
-			ds_p->sprtopclip = screenheightarray;
-			ds_p->tsilheight = INT32_MIN;
-			ds_p->silhouette |= SIL_TOP;
+			{
+				ds_p->sprtopclip = screenheightarray;
+				ds_p->tsilheight = INT32_MIN;
+				ds_p->silhouette |= SIL_TOP;
+			}
 		}
 
 		//SoM: 3/25/2000: This code fixes an automap bug that didn't check
 		// frontsector->ceiling and backsector->floor to see if a door was closed.
 		// Without the following code, sprites get displayed behind closed doors.
+		if (!bothceilingssky && !bothfloorssky)
 		{
 #ifdef ESLOPE
 			if (doorclosed || (worldhigh <= worldbottom && worldhighslope <= worldbottomslope))
@@ -2174,7 +2165,13 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 			}
 		}
 
-		if (worldlow != worldbottom
+		if (bothfloorssky)
+		{
+			// see double ceiling skies comment
+			// this is the same but for upside down thok barriers where the floor is sky and the ceiling is normal
+			markfloor = false;
+		}
+		else if (worldlow != worldbottom
 #ifdef ESLOPE
 			|| worldlowslope != worldbottomslope
 			|| backsector->f_slope != frontsector->f_slope
@@ -2186,7 +2183,7 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 		    || backsector->floor_yoffs != frontsector->floor_yoffs
 		    || backsector->floorpic_angle != frontsector->floorpic_angle
 		    //SoM: 3/22/2000: Prevents bleeding.
-		    || frontsector->heightsec != -1
+		    || (frontsector->heightsec != -1 && frontsector->floorpic != skyflatnum)
 		    || backsector->floorlightsec != frontsector->floorlightsec
 		    //SoM: 4/3/2000: Check for colormaps
 		    || frontsector->extra_colormap != backsector->extra_colormap
@@ -2200,7 +2197,14 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 			markfloor = false;
 		}
 
-		if (worldhigh != worldtop
+		if (bothceilingssky)
+		{
+			// double ceiling skies are special
+			// we don't want to lower the ceiling clipping, (no new plane is drawn anyway)
+			// so we can see the floor of thok barriers always regardless of sector properties
+			markceiling = false;
+		}
+		else if (worldhigh != worldtop
 #ifdef ESLOPE
 			|| worldhighslope != worldtopslope
 			|| backsector->c_slope != frontsector->c_slope
@@ -2226,19 +2230,28 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 			markceiling = false;
 		}
 
-		if (backsector->ceilingheight <= frontsector->floorheight ||
-		    backsector->floorheight >= frontsector->ceilingheight)
+		if (!bothceilingssky && !bothfloorssky)
 		{
-			// closed door
-			markceiling = markfloor = true;
+#ifdef ESLOPE
+			if ((worldhigh <= worldbottom && worldhighslope <= worldbottomslope)
+			 || (worldlow >= worldtop && worldlowslope >= worldtopslope))
+#else
+			if (backsector->ceilingheight <= frontsector->floorheight
+			 || backsector->floorheight >= frontsector->ceilingheight)
+#endif
+			{
+				// closed door
+				markceiling = markfloor = true;
+			}
 		}
 
 		// check TOP TEXTURE
-		if (worldhigh < worldtop
+		if (!bothceilingssky // never draw the top texture if on
+			&& (worldhigh < worldtop
 #ifdef ESLOPE
 				|| worldhighslope < worldtopslope
 #endif
-			)
+			))
 		{
 			fixed_t texheight;
 			// top texture
@@ -2287,11 +2300,12 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 			}
 		}
 		// check BOTTOM TEXTURE
-		if (worldlow > worldbottom
+		if (!bothfloorssky // never draw the bottom texture if on
+			&& (worldlow > worldbottom
 #ifdef ESLOPE
 				|| worldlowslope > worldbottomslope
 #endif
-			)     //seulement si VISIBLE!!!
+			))     //seulement si VISIBLE!!!
 		{
 			// bottom texture
 			bottomtexture = R_GetTextureNum(sidedef->bottomtexture);
@@ -2364,16 +2378,8 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 						continue;
 
 #ifdef ESLOPE
-					if (*rover->t_slope) {
-						high1 = P_GetZAt(*rover->t_slope, segleft.x, segleft.y);
-						highslope1 = P_GetZAt(*rover->t_slope, segright.x, segright.y);
-					} else
-						high1 = highslope1 = *rover->topheight;
-					if (*rover->b_slope) {
-						low1 = P_GetZAt(*rover->b_slope, segleft.x, segleft.y);
-						lowslope1 = P_GetZAt(*rover->b_slope, segright.x, segright.y);
-					} else
-						low1 = lowslope1 = *rover->bottomheight;
+					SLOPEPARAMS(*rover->t_slope, high1, highslope1, *rover->topheight)
+					SLOPEPARAMS(*rover->b_slope, low1,  lowslope1,  *rover->bottomheight)
 
 					if ((high1 < lowcut && highslope1 < lowcutslope) || (low1 > highcut && lowslope1 > highcutslope))
 						continue;
@@ -2405,16 +2411,8 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 						}
 
 #ifdef ESLOPE
-						if (*r2->t_slope) {
-							high2 = P_GetZAt(*r2->t_slope, segleft.x, segleft.y);
-							highslope2 = P_GetZAt(*r2->t_slope, segright.x, segright.y);
-						} else
-							high2 = highslope2 = *r2->topheight;
-						if (*r2->b_slope) {
-							low2 = P_GetZAt(*r2->b_slope, segleft.x, segleft.y);
-							lowslope2 = P_GetZAt(*r2->b_slope, segright.x, segright.y);
-						} else
-							low2 = lowslope2 = *r2->bottomheight;
+						SLOPEPARAMS(*r2->t_slope, high2, highslope2, *r2->topheight)
+						SLOPEPARAMS(*r2->b_slope, low2,  lowslope2,  *r2->bottomheight)
 
 						if ((high2 < lowcut || highslope2 < lowcutslope) || (low2 > highcut || lowslope2 > highcutslope))
 							continue;
@@ -2447,16 +2445,8 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 						continue;
 
 #ifdef ESLOPE
-					if (*rover->t_slope) {
-						high1 = P_GetZAt(*rover->t_slope, segleft.x, segleft.y);
-						highslope1 = P_GetZAt(*rover->t_slope, segright.x, segright.y);
-					} else
-						high1 = highslope1 = *rover->topheight;
-					if (*rover->b_slope) {
-						low1 = P_GetZAt(*rover->b_slope, segleft.x, segleft.y);
-						lowslope1 = P_GetZAt(*rover->b_slope, segright.x, segright.y);
-					} else
-						low1 = lowslope1 = *rover->bottomheight;
+					SLOPEPARAMS(*rover->t_slope, high1, highslope1, *rover->topheight)
+					SLOPEPARAMS(*rover->b_slope, low1,  lowslope1,  *rover->bottomheight)
 
 					if ((high1 < lowcut && highslope1 < lowcutslope) || (low1 > highcut && lowslope1 > highcutslope))
 						continue;
@@ -2488,17 +2478,9 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 						}
 
 #ifdef ESLOPE
-						if (*r2->t_slope) {
-							high2 = P_GetZAt(*r2->t_slope, segleft.x, segleft.y);
-							highslope2 = P_GetZAt(*r2->t_slope, segright.x, segright.y);
-						} else
-							high2 = highslope2 = *r2->topheight;
-						if (*r2->b_slope) {
-							low2 = P_GetZAt(*r2->b_slope, segleft.x, segleft.y);
-							lowslope2 = P_GetZAt(*r2->b_slope, segright.x, segright.y);
-						} else
-							low2 = lowslope2 = *r2->bottomheight;
-
+						SLOPEPARAMS(*r2->t_slope, high2, highslope2, *r2->topheight)
+						SLOPEPARAMS(*r2->b_slope, low2,  lowslope2,  *r2->bottomheight)
+#undef SLOPEPARAMS
 						if ((high2 < lowcut || highslope2 < lowcutslope) || (low2 > highcut || lowslope2 > highcutslope))
 							continue;
 						if ((high1 > high2 || highslope1 > highslope2) || (low1 < low2 || lowslope1 < lowslope2))
@@ -2677,7 +2659,8 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 	//  and doesn't need to be marked.
 	if (frontsector->heightsec == -1)
 	{
-		if ((
+		if (frontsector->floorpic != skyflatnum
+		&& (
 #ifdef ESLOPE
 			frontsector->f_slope ? P_GetZAt(frontsector->f_slope, viewx, viewy) :
 #endif
@@ -2687,12 +2670,12 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 			markfloor = false;
 		}
 
-		if ((
+		if (frontsector->ceilingpic != skyflatnum
+		&& (
 #ifdef ESLOPE
 			frontsector->c_slope ? P_GetZAt(frontsector->c_slope, viewx, viewy) :
 #endif
-			frontsector->ceilingheight) <= viewz &&
-		    frontsector->ceilingpic != skyflatnum)
+			frontsector->ceilingheight) <= viewz)
 		{
 			// below view plane
 			markceiling = false;
@@ -3150,6 +3133,22 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 			for (i = 0; i < numffloors; i++)
 				R_ExpandPlane(ffloor[i].plane, rw_x, rw_stopx - 1);
 		}
+#ifdef POLYOBJECTS_PLANES
+		// 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)
+		{
+			for (i = 0; i < numffloors; i++)
+			{
+				if (!ffloor[i].polyobj || ffloor[i].polyobj != curline->polyseg)
+					continue;
+				if (ffloor[i].plane->minx > rw_x)
+					ffloor[i].plane->minx = rw_x;
+
+				if (ffloor[i].plane->maxx < rw_stopx - 1)
+					ffloor[i].plane->maxx = rw_stopx - 1;
+			}
+		}
+#endif
 	}
 
 #ifdef WALLSPLATS