diff --git a/docs/rh-log.txt b/docs/rh-log.txt
index 733aef90d..0c96f07a5 100644
--- a/docs/rh-log.txt
+++ b/docs/rh-log.txt
@@ -1,3 +1,7 @@
+December 27, 2009  (Changes by Graf Zahl)
+- added a linedef based method to define portals. Portals defined this way
+  still have the same limitations as those defines with the portal things.
+
 December 25, 2009  (Changes by Graf Zahl)
 - Fixed: Decals could spread to walls which had a decal-less texture or
   were flagged not to have decals.
diff --git a/src/actionspecials.h b/src/actionspecials.h
index 98c899f5b..417c823b1 100644
--- a/src/actionspecials.h
+++ b/src/actionspecials.h
@@ -55,6 +55,7 @@ DEFINE_SPECIAL(Line_SetTextureOffset, 53, 5, 5, 5)
 DEFINE_SPECIAL(Sector_ChangeFlags, 54, 3, 3, 3)
 DEFINE_SPECIAL(Line_SetBlocking, 55, 3, 3, 3)
 DEFINE_SPECIAL(Line_SetTextureScale, 56, 5, 5, 5)
+DEFINE_SPECIAL(Sector_SetPortal, 57, -1, -1, 5)
 
 DEFINE_SPECIAL(Plat_PerpetualRaise, 60, 3, 3, 3)
 DEFINE_SPECIAL(Plat_Stop, 61, 1, 1, 1)
diff --git a/src/g_shared/a_sharedglobal.h b/src/g_shared/a_sharedglobal.h
index 798f314e3..c152bf44d 100644
--- a/src/g_shared/a_sharedglobal.h
+++ b/src/g_shared/a_sharedglobal.h
@@ -89,16 +89,24 @@ public:
 class ASkyViewpoint : public AActor
 {
 	DECLARE_CLASS (ASkyViewpoint, AActor)
+	HAS_OBJECT_POINTERS
 public:
 	void Serialize (FArchive &arc);
 	void BeginPlay ();
 	void Destroy ();
 	bool bInSkybox;
 	bool bAlways;
-	ASkyViewpoint *Mate;
+	TObjPtr<ASkyViewpoint> Mate;
 	fixed_t PlaneAlpha;
 };
 
+class AStackPoint : public ASkyViewpoint
+{
+	DECLARE_CLASS (AStackPoint, ASkyViewpoint)
+public:
+	void BeginPlay ();
+};
+
 class DFlashFader : public DThinker
 {
 	DECLARE_CLASS (DFlashFader, DThinker)
diff --git a/src/g_shared/a_skies.cpp b/src/g_shared/a_skies.cpp
index 77704dd50..cfe024b48 100644
--- a/src/g_shared/a_skies.cpp
+++ b/src/g_shared/a_skies.cpp
@@ -38,7 +38,9 @@
 
 // arg0 = Visibility*4 for this skybox
 
-IMPLEMENT_CLASS (ASkyViewpoint)
+IMPLEMENT_POINTY_CLASS (ASkyViewpoint)
+	DECLARE_POINTER(Mate)
+END_POINTERS
 
 // If this actor has no TID, make it the default sky box
 void ASkyViewpoint::BeginPlay ()
@@ -144,13 +146,6 @@ void ASkyPicker::PostBeginPlay ()
 
 // arg0 = opacity of plane; 0 = invisible, 255 = fully opaque
 
-class AStackPoint : public ASkyViewpoint
-{
-	DECLARE_CLASS (AStackPoint, ASkyViewpoint)
-public:
-	void BeginPlay ();
-};
-
 IMPLEMENT_CLASS (AStackPoint)
 
 void AStackPoint::BeginPlay ()
diff --git a/src/p_spec.cpp b/src/p_spec.cpp
index bce0bcb30..1571ef31b 100644
--- a/src/p_spec.cpp
+++ b/src/p_spec.cpp
@@ -59,6 +59,7 @@
 #include "statnums.h"
 #include "g_level.h"
 #include "v_font.h"
+#include "a_sharedglobal.h"
 
 // State.
 #include "r_state.h"
@@ -827,6 +828,50 @@ void DWallLightTransfer::DoTransfer (BYTE lightlevel, int target, BYTE flags)
 	}
 }
 
+void P_SpawnPortal(line_t *line, int sectortag, INTBOOL ceiling, int alpha)
+{
+	for (int i=0;i<numlines;i++)
+	{
+		// We must look for the reference line with a linear search unless we want to waste the line ID for it
+		// which is not a good idea.
+		if (lines[i].special == Sector_SetPortal &&
+			lines[i].args[0] == sectortag &&
+			lines[i].args[1] == 0 &&
+			lines[i].args[2] == ceiling &&
+			lines[i].args[3] == 1)
+		{
+			fixed_t x1 = (line->v1->x + line->v2->x) >> 1;
+			fixed_t y1 = (line->v1->y + line->v2->y) >> 1;
+			fixed_t x2 = (lines[i].v1->x + lines[i].v2->x) >> 1;
+			fixed_t y2 = (lines[i].v1->y + lines[i].v2->y) >> 1;
+
+			AStackPoint *anchor = Spawn<AStackPoint>(x1, y1, 0, NO_REPLACE);
+			AStackPoint *reference = Spawn<AStackPoint>(x2, y2, 0, NO_REPLACE);
+
+			reference->Mate = anchor;
+			anchor->Mate = reference;
+
+			// This is so that the renderer can distinguish these portals from
+			// the ones spawned with the '*StackLookOnly' things.
+			reference->flags |= MF_JUSTATTACKED;
+			anchor->flags |= MF_JUSTATTACKED;
+
+		    for (int s=-1; (s = P_FindSectorFromTag(sectortag,s)) >= 0;)
+			{
+				if (ceiling)
+				{
+					if (sectors[s].CeilingSkyBox == NULL) sectors[s].CeilingSkyBox = reference;
+				}
+				else
+				{
+					if (sectors[s].FloorSkyBox == NULL) sectors[s].FloorSkyBox = reference;
+				}
+			}
+			return;
+		}
+	}
+}
+
 
 //
 // P_SpawnSpecials
@@ -1049,6 +1094,18 @@ void P_SpawnSpecials (void)
 			}
 			break;
 
+		case Sector_SetPortal:
+			// arg 0 = sector tag
+			// arg 1 = type (must be 0, other values reserved for later use)
+			// arg 2 = 0:floor, 1:ceiling
+			// arg 3 = 0: anchor, 1: reference line
+			// arg 4 = for the anchor only: alpha
+			if (lines[i].args[1] == 0 && lines[i].args[3] == 0)
+			{
+				P_SpawnPortal(&lines[i], lines[i].args[0], lines[i].args[2], lines[i].args[4]);
+			}
+			break;
+
 		// [RH] ZDoom Static_Init settings
 		case Static_Init:
 			switch (lines[i].args[1])