diff --git a/src/r_bsp.cpp b/src/r_bsp.cpp
index 93efb8d30..84987a32d 100644
--- a/src/r_bsp.cpp
+++ b/src/r_bsp.cpp
@@ -439,7 +439,7 @@ static void R_AddLine(seg_t *line)
 	static sector_t tempsec;
 	boolean bothceilingssky = false, bothfloorssky = false;
 
-	portalline = false;
+	g_portal = NULL;
 
 	if (line->polyseg && !(line->polyseg->flags & POF_RENDERSIDES))
 		return;
diff --git a/src/r_bsp.h b/src/r_bsp.h
index 051253a5e..66d1ff9cc 100644
--- a/src/r_bsp.h
+++ b/src/r_bsp.h
@@ -14,6 +14,8 @@
 #ifndef __R_BSP__
 #define __R_BSP__
 
+#include "typedef.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -27,7 +29,7 @@ extern side_t *sidedef;
 extern line_t *linedef;
 extern sector_t *frontsector;
 extern sector_t *backsector;
-extern boolean portalline; // is curline a portal seg?
+extern portal_t *g_portal; // is curline a portal seg?
 
 // drawsegs are allocated on the fly... see r_segs.c
 
diff --git a/src/r_portal.c b/src/r_portal.c
index aa0164eb6..e66d4343c 100644
--- a/src/r_portal.c
+++ b/src/r_portal.c
@@ -30,7 +30,7 @@ line_t *portalclipline;
 sector_t *portalcullsector;
 INT32 portalclipstart, portalclipend;
 
-boolean portalline; // is curline a portal seg?
+portal_t *g_portal; // is curline a portal seg?
 
 void Portal_InitList (void)
 {
@@ -38,32 +38,6 @@ void Portal_InitList (void)
 	portal_base = portal_cap = NULL;
 }
 
-/** Store the clipping window for a portal in its given range.
- *
- * The window is copied from the current window at the time
- * the function is called, so it is useful for converting one-sided
- * lines into portals.
- */
-void Portal_ClipRange (portal_t* portal)
-{
-	INT32 start	= portal->start;
-	INT32 end	= portal->end;
-	INT16 *ceil		= portal->ceilingclip;
-	INT16 *floor	= portal->floorclip;
-	fixed_t *scale	= portal->frontscale;
-
-	INT32 i;
-	for (i = 0; i < end-start; i++)
-	{
-		*ceil = ceilingclip[start+i];
-		ceil++;
-		*floor = floorclip[start+i];
-		floor++;
-		*scale = frontscale[start+i];
-		scale++;
-	}
-}
-
 /** Apply the clipping window from a portal.
  */
 void Portal_ClipApply (const portal_t* portal)
@@ -187,9 +161,7 @@ void Portal_Add2Lines (const INT32 line1, const INT32 line2, const INT32 x1, con
 
 	portal->clipline = line2;
 
-	Portal_ClipRange(portal);
-
-	portalline = true; // this tells R_StoreWallRange that curline is a portal seg
+	g_portal = portal; // this tells R_StoreWallRange that curline is a portal seg
 }
 
 /** Store the clipping window for a portal using a visplane.
diff --git a/src/r_portal.h b/src/r_portal.h
index 460c8a036..4798030f0 100644
--- a/src/r_portal.h
+++ b/src/r_portal.h
@@ -60,7 +60,6 @@ void Portal_Remove		(portal_t* portal);
 void Portal_Add2Lines	(const INT32 line1, const INT32 line2, const INT32 x1, const INT32 x2);
 void Portal_AddSkybox	(const player_t* player, const visplane_t* plane);
 
-void Portal_ClipRange (portal_t* portal);
 void Portal_ClipApply (const portal_t* portal);
 
 void Portal_AddSkyboxPortals (const player_t* player);
diff --git a/src/r_segs.c b/src/r_segs.c
index 44784b087..2b6f471d6 100644
--- a/src/r_segs.c
+++ b/src/r_segs.c
@@ -1526,6 +1526,21 @@ static void R_RenderSegLoop (void)
 
 		frontscale[rw_x] = rw_scale;
 
+		const INT16 topclip = (yl >= 0) ? ((yl > viewheight) ? (INT16)viewheight : (INT16)((INT16)yl - 1)) : -1;
+		const INT16 bottomclip = (yh < viewheight) ? ((yh < -1) ? -1 : (INT16)((INT16)yh + 1)) : (INT16)viewheight;
+
+		// Portal line
+		// Spans the entire height of a single-sided line or
+		// the "window" of a double-sided line.
+		if (g_portal)
+		{
+			I_Assert(rw_x >= g_portal->start && rw_x < g_portal->end);
+			i = rw_x - g_portal->start;
+			g_portal->frontscale[i] = rw_scale;
+			g_portal->ceilingclip[i] = topclip;
+			g_portal->floorclip[i] = bottomclip;
+		}
+
 		// draw the wall tiers
 		if (midtexture)
 		{
@@ -1567,16 +1582,13 @@ static void R_RenderSegLoop (void)
 			{
 				// note: don't use min/max macros, since casting from INT32 to INT16 is involved here
 				if (markceiling && (!rw_ceilingmarked))
-					ceilingclip[rw_x] = (yl >= 0) ? ((yl > viewheight) ? (INT16)viewheight : (INT16)((INT16)yl - 1)) : -1;
+					ceilingclip[rw_x] = topclip;
 				if (markfloor && (!rw_floormarked))
-					floorclip[rw_x] = (yh < viewheight) ? ((yh < -1) ? -1 : (INT16)((INT16)yh + 1)) : (INT16)viewheight;
+					floorclip[rw_x] = bottomclip;
 			}
 		}
 		else
 		{
-			INT16 topclip = (yl >= 0) ? ((yl > viewheight) ? (INT16)viewheight : (INT16)((INT16)yl - 1)) : -1;
-			INT16 bottomclip = (yh < viewheight) ? ((yh < -1) ? -1 : (INT16)((INT16)yh + 1)) : (INT16)viewheight;
-
 			// two sided line
 			if (toptexture)
 			{
@@ -2146,7 +2158,9 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 			|| backsector->floorlightsec != frontsector->floorlightsec
 			//SoM: 4/3/2000: Check for colormaps
 			|| frontsector->extra_colormap != backsector->extra_colormap
-			|| (frontsector->ffloors != backsector->ffloors && !Tag_Compare(&frontsector->tags, &backsector->tags)))
+			|| (frontsector->ffloors != backsector->ffloors && !Tag_Compare(&frontsector->tags, &backsector->tags))
+			// Portals block traversal behind them
+			|| g_portal)
 		{
 			markfloor = true;
 		}
@@ -2179,7 +2193,9 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 			|| backsector->ceilinglightsec != frontsector->ceilinglightsec
 			//SoM: 4/3/2000: Check for colormaps
 			|| frontsector->extra_colormap != backsector->extra_colormap
-			|| (frontsector->ffloors != backsector->ffloors && !Tag_Compare(&frontsector->tags, &backsector->tags)))
+			|| (frontsector->ffloors != backsector->ffloors && !Tag_Compare(&frontsector->tags, &backsector->tags))
+			// Portals block traversal behind them
+			|| g_portal)
 		{
 				markceiling = true;
 		}
@@ -2974,7 +2990,7 @@ void R_StoreWallRange(INT32 start, INT32 stop)
 	R_RenderSegLoop();
 	R_SetColumnFunc(BASEDRAWFUNC, false);
 
-	if (portalline) // if curline is a portal, set portalrender for drawseg
+	if (g_portal) // if curline is a portal, set portalrender for drawseg
 		ds_p->portalpass = portalrender+1;
 	else
 		ds_p->portalpass = 0;