diff --git a/src/r_main.c b/src/r_main.c
index 244b96cae..70c271f5d 100644
--- a/src/r_main.c
+++ b/src/r_main.c
@@ -1081,6 +1081,7 @@ void R_RenderPlayerView(player_t *player)
 	// Portal rendering. Hijacks the BSP traversal.
 	if (portal_base)
 	{
+		INT32 i, p;
 		portal_t *portal;
 
 		for(portal = portal_base; portal; portal = portal_base)
@@ -1113,6 +1114,16 @@ void R_RenderPlayerView(player_t *player)
 			masks[nummasks - 1].viewsector = viewsector;
 			curdrawsegs = ds_p;
 
+			// opening / clipping determination
+			for (i = 0; i < viewwidth; i++)
+			{
+				for (p = 0; p < MAXFFLOORS; p++)
+				{
+					ffloor[p].f_clip[i] = (INT16)viewheight;
+					ffloor[p].c_clip[i] = -1;
+				}
+			}
+
 			R_RenderBSPNode((INT32)numnodes - 1);
 			masks[nummasks - 1].drawsegs[1]		= ds_p - drawsegs;
 			masks[nummasks - 1].vissprites[1]	= visspritecount;
diff --git a/src/r_plane.c b/src/r_plane.c
index 18e5fda52..2c2fe3432 100644
--- a/src/r_plane.c
+++ b/src/r_plane.c
@@ -45,9 +45,8 @@
 //#define QUINCUNX
 
 //SoM: 3/23/2000: Use Boom visplane hashing.
-#define MAXVISPLANES 512
 
-static visplane_t *visplanes[MAXVISPLANES];
+visplane_t *visplanes[MAXVISPLANES];
 static visplane_t *freetail;
 static visplane_t **freehead = &freetail;
 
@@ -1178,31 +1177,3 @@ void R_PlaneBounds(visplane_t *plane)
 	plane->high = hi;
 	plane->low = low;
 }
-
-/** Creates portals for the currently existing sky visplanes.
- * The visplanes are also removed and cleared from the list.
- */
-void Portal_AddSkyboxPortals (void)
-{
-	visplane_t *pl;
-	INT32 i;
-	UINT16 count = 0;
-
-	for (i = 0; i < MAXVISPLANES; i++, pl++)
-	{
-		for (pl = visplanes[i]; pl; pl = pl->next)
-		{
-			if (pl->picnum == skyflatnum)
-			{
-				Portal_AddSkybox(pl);
-
-				pl->minx = 0;
-				pl->maxx = -1;
-
-				count++;
-			}
-		}
-	}
-
-	CONS_Debug(DBG_RENDER, "Skybox portals: %d\n", count);
-}
diff --git a/src/r_plane.h b/src/r_plane.h
index c101e3218..a269fed6a 100644
--- a/src/r_plane.h
+++ b/src/r_plane.h
@@ -18,6 +18,8 @@
 #include "r_data.h"
 #include "p_polyobj.h"
 
+#define MAXVISPLANES 512
+
 //
 // Now what is a visplane, anyway?
 // Simple: kinda floor/ceiling polygon optimised for SRB2 rendering.
@@ -53,6 +55,7 @@ typedef struct visplane_s
 #endif
 } visplane_t;
 
+extern visplane_t *visplanes[MAXVISPLANES];
 extern visplane_t *floorplane;
 extern visplane_t *ceilingplane;
 
diff --git a/src/r_portal.c b/src/r_portal.c
index 0c0a34b7d..ad5d32e70 100644
--- a/src/r_portal.c
+++ b/src/r_portal.c
@@ -18,6 +18,7 @@
 #include "p_spec.h" // Skybox viewpoints
 #include "z_zone.h"
 #include "r_things.h"
+#include "r_sky.h"
 
 UINT8 portalrender;			/**< When rendering a portal, it establishes the depth of the current BSP traversal. */
 sector_t *portalcullsector;
@@ -215,25 +216,19 @@ static void Portal_ClipVisplane (const visplane_t* plane, portal_t* portal)
 
 extern INT32 viewwidth;
 
-/** Creates a skybox portal out of a visplane.
- *
- * Applies the necessary offsets and rotation to give
- * a depth illusion to the skybox.
- */
-void Portal_AddSkybox (const visplane_t* plane)
+static boolean TrimVisplaneBounds (const visplane_t* plane, INT16* start, INT16* end)
 {
-	INT16 start = plane->minx;
-	INT16 end = plane->maxx + 1;
-	mapheader_t *mh;
-	portal_t* portal;
+	*start = plane->minx;
+	*end = plane->maxx + 1;
 
 	// Visplanes have 1-px pads on their sides (extra columns).
 	// Trim them, else it may render out of bounds.
-	if (end > viewwidth)
-		end = viewwidth;
+	if (*end > viewwidth)
+		*end = viewwidth;
+
+	if (!(*start < *end))
+		return true;
 
-	if (!(start < end))
-		return;
 
 	/** Trims a visplane's horizontal gap to match its render area.
 	 *
@@ -242,17 +237,33 @@ void Portal_AddSkybox (const visplane_t* plane)
 	 * valid area.
 	 */
 
-	while (plane->bottom[start] == 0 && plane->top[start] == 65535 && start < end)
+	while (plane->bottom[*start] == 0 && plane->top[*start] == 65535 && *start < *end)
 	{
-		start++;
+		(*start)++;
 	}
 
 
-	while (plane->bottom[end - 1] == 0 && plane->top[start] == 65535 && end > start)
+	while (plane->bottom[*end - 1] == 0 && plane->top[*start] == 65535 && *end > *start)
 	{
-		end--;
+		(*end)--;
 	}
 
+	return false;
+}
+
+/** Creates a skybox portal out of a visplane.
+ *
+ * Applies the necessary offsets and rotation to give
+ * a depth illusion to the skybox.
+ */
+void Portal_AddSkybox (const visplane_t* plane)
+{
+	INT16 start, end;
+	mapheader_t *mh;
+	portal_t* portal;
+
+	if (TrimVisplaneBounds(plane, &start, &end))
+		return;
 
 	portal = Portal_Add(start, end);
 
@@ -291,3 +302,31 @@ void Portal_AddSkybox (const visplane_t* plane)
 
 	portal->clipline = -1;
 }
+
+/** Creates portals for the currently existing sky visplanes.
+ * The visplanes are also removed and cleared from the list.
+ */
+void Portal_AddSkyboxPortals (void)
+{
+	visplane_t *pl;
+	INT32 i;
+	UINT16 count = 0;
+
+	for (i = 0; i < MAXVISPLANES; i++, pl++)
+	{
+		for (pl = visplanes[i]; pl; pl = pl->next)
+		{
+			if (pl->picnum == skyflatnum)
+			{
+				Portal_AddSkybox(pl);
+
+				pl->minx = 0;
+				pl->maxx = -1;
+
+				count++;
+			}
+		}
+	}
+
+	CONS_Debug(DBG_RENDER, "Skybox portals: %d\n", count);
+}
diff --git a/src/r_portal.h b/src/r_portal.h
index f7d2fb2fe..e5ff0fb9f 100644
--- a/src/r_portal.h
+++ b/src/r_portal.h
@@ -56,4 +56,5 @@ void Portal_AddSkybox	(const visplane_t* plane);
 void Portal_ClipRange (portal_t* portal);
 void Portal_ClipApply (const portal_t* portal);
 
+void Portal_AddSkyboxPortals (void);
 #endif