diff --git a/src/actionspecials.h b/src/actionspecials.h
index 752ef77ebd..879f16a113 100644
--- a/src/actionspecials.h
+++ b/src/actionspecials.h
@@ -140,7 +140,7 @@ DEFINE_SPECIAL(Teleport_NoStop, 154, 2, 3, 3)
 // Although ZDoom doesn't support them it's better to have them defined so that
 // WADs using them somewhere can at least be started without aborting due
 // to an error message.
-DEFINE_SPECIAL(SetGlobalFogParameter, 157, 2, 2, 2)
+DEFINE_SPECIAL(SetGlobalFogParameter, 157, 2, 2, 2) // vavoom? interferes with Line_SetVisualPortal
 DEFINE_SPECIAL(FS_Execute, 158, 1, 4, 4)
 DEFINE_SPECIAL(Sector_SetPlaneReflection, 159, 3, 3, 3)
 DEFINE_SPECIAL(Sector_Set3DFloor, 160, -1, -1, 5)
@@ -236,4 +236,7 @@ DEFINE_SPECIAL(Ceiling_LowerToLowest, 253, 2, 2, 2)
 DEFINE_SPECIAL(Ceiling_LowerToFloor, 254, 2, 2, 2)
 DEFINE_SPECIAL(Ceiling_CrushRaiseAndStaySilA, 255, 4, 5, 5)
 
+DEFINE_SPECIAL(Line_SetVisualPortal, 155, -1, -1, 3)
+DEFINE_SPECIAL(Line_SetPortal, 156, -1, -1, 3)
+
 #undef DEFINE_SPECIAL
diff --git a/src/p_acs.cpp b/src/p_acs.cpp
index bc4cbdb758..e0eb524f1b 100644
--- a/src/p_acs.cpp
+++ b/src/p_acs.cpp
@@ -75,6 +75,7 @@
 #include "actorptrselect.h"
 #include "farchive.h"
 #include "decallib.h"
+#include "portal.h"
 
 #include "g_shared/a_pickups.h"
 
@@ -7902,6 +7903,9 @@ scriptwait:
 					line->args[4] = STACK(1);
 					DPrintf("Set special on line %d (id %d) to %d(%d,%d,%d,%d,%d)\n",
 						linenum, STACK(7), specnum, arg0, STACK(4), STACK(3), STACK(2), STACK(1));
+
+					// [ZZ] re-link with portals (in case this was something related to portal specials)
+					P_CheckPortal(line);
 				}
 				sp -= 7;
 			}
diff --git a/src/p_buildmap.cpp b/src/p_buildmap.cpp
index 2ee2ae83a8..d9f2857bab 100644
--- a/src/p_buildmap.cpp
+++ b/src/p_buildmap.cpp
@@ -17,6 +17,7 @@
 #include "g_level.h"
 #include "r_data/colormaps.h"
 #include "gi.h"
+#include "portal.h"
 
 // MACROS ------------------------------------------------------------------
 
@@ -628,6 +629,11 @@ static void LoadWalls (walltype *walls, int numwalls, sectortype *bsec)
 		}
 	}
 
+	// [ZZ] set initial line portal link
+	//      (even though this is rather hard to happen... build doesn't have portals in our sense and it's walls don't get translated into anything like this)
+	for (int i = 0; i < numlines; i++)
+		P_CheckPortal(&lines[i]);
+
 	// Finish setting sector properties that depend on walls
 	for (i = 0; i < numsectors; ++i, ++bsec)
 	{
diff --git a/src/p_setup.cpp b/src/p_setup.cpp
index 6b5d9403f0..8076e19a99 100644
--- a/src/p_setup.cpp
+++ b/src/p_setup.cpp
@@ -68,6 +68,7 @@
 #include "po_man.h"
 #include "r_renderer.h"
 #include "r_data/colormaps.h"
+#include "portal.h"
 
 #include "fragglescript/t_fs.h"
 
@@ -2161,6 +2162,11 @@ void P_LoadLineDefs (MapData * map)
 		if (level.flags2 & LEVEL2_WRAPMIDTEX) ld->flags |= ML_WRAP_MIDTEX;
 		if (level.flags2 & LEVEL2_CHECKSWITCHRANGE) ld->flags |= ML_CHECKSWITCHRANGE;
 	}
+
+	// [ZZ] check initial portal link
+	for (int i = 0; i < numlines; i++)
+		P_CheckPortal(&lines[i]);
+
 	delete[] mldf;
 }
 
diff --git a/src/p_udmf.cpp b/src/p_udmf.cpp
index 473cdcd814..cb9ed19f16 100644
--- a/src/p_udmf.cpp
+++ b/src/p_udmf.cpp
@@ -46,6 +46,7 @@
 #include "r_state.h"
 #include "r_data/colormaps.h"
 #include "w_wad.h"
+#include "portal.h"
 
 //===========================================================================
 //
@@ -1704,6 +1705,11 @@ public:
 			P_AdjustLine(&lines[line]);
 			P_FinishLoadingLineDef(&lines[line], tempalpha[0]);
 		}
+
+		// [ZZ] check initial portal link
+		for (int i = 0; i < numlines; i++)
+			P_CheckPortal(&lines[i]);
+
 		assert(side <= numsides);
 		if (side < numsides)
 		{
diff --git a/src/r_bsp.cpp b/src/r_bsp.cpp
index c562558880..e35c95f451 100644
--- a/src/r_bsp.cpp
+++ b/src/r_bsp.cpp
@@ -56,6 +56,7 @@
 #include "r_sky.h"
 #include "po_man.h"
 #include "r_data/colormaps.h"
+#include "portal.h"
 
 seg_t*			curline;
 side_t* 		sidedef;
@@ -98,8 +99,8 @@ static BYTE		FakeSide;
 
 int WindowLeft, WindowRight;
 WORD MirrorFlags;
-seg_t *ActiveWallMirror;
-TArray<size_t> WallMirrors;
+TArray<PortalDrawseg> WallPortals;
+
 
 static subsector_t *InSubsector;
 
@@ -553,6 +554,10 @@ void R_AddLine (seg_t *line)
 		return;
 	}
 
+	// reject lines that aren't seen from the portal (if any)
+	if (CurrentPortal && P_ClipLineToPortal(line->linedef, CurrentPortal->dst, viewx, viewy))
+		return;
+
 	vertex_t *v1, *v2;
 
 	v1 = line->linedef->v1;
@@ -583,6 +588,8 @@ void R_AddLine (seg_t *line)
 	rw_mustmarkfloor = rw_mustmarkceiling = false;
 	rw_havehigh = rw_havelow = false;
 
+	bool is_portal = (line->linedef && line->linedef->special == Line_Mirror);
+
 	// Single sided line?
 	if (backsector == NULL)
 	{
@@ -649,7 +656,9 @@ void R_AddLine (seg_t *line)
 		// Window.
 			solid = false;
 		}
-		else if (backsector->lightlevel != frontsector->lightlevel
+		else if (is_portal
+			
+			|| backsector->lightlevel != frontsector->lightlevel
 			|| backsector->GetTexture(sector_t::floor) != frontsector->GetTexture(sector_t::floor)
 			|| backsector->GetTexture(sector_t::ceiling) != frontsector->GetTexture(sector_t::ceiling)
 			|| curline->sidedef->GetTexture(side_t::mid).isValid()
diff --git a/src/r_bsp.h b/src/r_bsp.h
index 1b5af9805d..59a2753987 100644
--- a/src/r_bsp.h
+++ b/src/r_bsp.h
@@ -25,6 +25,7 @@
 
 #include "tarray.h"
 #include <stddef.h>
+#include "portal.h"
 
 // The 3072 below is just an arbitrary value picked to avoid
 // drawing lines the player is too close to that would overflow
@@ -88,6 +89,7 @@ struct drawseg_t
 // backups
 	ptrdiff_t	bkup;	// sprtopclip backup, for mid and fake textures
 	FWallTmapVals tmapvals;
+	int			CurrentPortalUniq; // [ZZ] to identify the portal that this drawseg is in. used for sprite clipping.
 };
 
 
@@ -106,9 +108,8 @@ extern size_t			FirstInterestingDrawseg;
 
 extern int			WindowLeft, WindowRight;
 extern WORD			MirrorFlags;
-extern seg_t*		ActiveWallMirror;
 
-extern TArray<size_t>	WallMirrors;
+extern TArray<PortalDrawseg> WallPortals;
 
 typedef void (*drawfunc_t) (int start, int stop);
 
diff --git a/src/r_defs.h b/src/r_defs.h
index dd21365087..8f94db3f2e 100644
--- a/src/r_defs.h
+++ b/src/r_defs.h
@@ -899,6 +899,10 @@ struct line_t
 	sector_t	*frontsector, *backsector;
 	int 		validcount;	// if == validcount, already checked
 	int			locknumber;	// [Dusk] lock number for special
+	bool		portal;
+	bool		portal_mirror;
+	bool		portal_passive;
+	line_t		*portal_dst;
 };
 
 // phares 3/14/98
diff --git a/src/r_main.cpp b/src/r_main.cpp
index bc3c4c7c02..c6af51533a 100644
--- a/src/r_main.cpp
+++ b/src/r_main.cpp
@@ -57,6 +57,7 @@
 #include "v_font.h"
 #include "r_data/colormaps.h"
 #include "farchive.h"
+#include "portal.h"
 
 // MACROS ------------------------------------------------------------------
 
@@ -610,52 +611,91 @@ void R_SetupFreelook()
 
 //==========================================================================
 //
-// R_EnterMirror
+// R_EnterPortal
 //
 // [RH] Draw the reflection inside a mirror
+// [ZZ] Merged with portal code, originally called R_EnterMirror
 //
 //==========================================================================
 
-void R_EnterMirror (drawseg_t *ds, int depth)
+void R_EnterPortal (PortalDrawseg* pds, int depth)
 {
+	// [ZZ] check depth. fill portal with black if it's exceeding the visual recursion limit, and continue like nothing happened.
+	if (depth > 4)
+	{
+		BYTE color = (BYTE)BestColor((DWORD *)GPalette.BaseColors, 0, 0, 0, 0, 255);
+		int spacing = RenderTarget->GetPitch();
+		for (int x = pds->x1; x <= pds->x2; x++)
+		{
+			int Ytop = pds->ceilingclip[x-pds->x1];
+			int Ybottom = pds->floorclip[x-pds->x1];
+
+			BYTE *dest = RenderTarget->GetBuffer() + x + Ytop * spacing;
+
+			if (Ytop < 0) Ytop = 0;
+			if (Ybottom >= RenderTarget->GetHeight())
+				Ybottom = RenderTarget->GetHeight()-1;
+
+			for (int y = Ytop; y <= Ybottom; y++)
+			{
+				*dest = color;
+				dest += spacing;
+			}
+		}
+
+		return;
+	}
+
 	angle_t startang = viewangle;
 	fixed_t startx = viewx;
 	fixed_t starty = viewy;
 
-	CurrentMirror++;
+	int prevuniq = CurrentPortalUniq;
+	CurrentPortalUniq++;
 
-	unsigned int mirrorsAtStart = WallMirrors.Size ();
+	unsigned int portalsAtStart = WallPortals.Size ();
 
-	vertex_t *v1 = ds->curline->v1;
+	if (pds->mirror)
+	{
+		//vertex_t *v1 = ds->curline->v1;
+		vertex_t *v1 = pds->src->v1;
+
+		// Reflect the current view behind the mirror.
+		if (pds->src->dx == 0)
+		{ // vertical mirror
+			viewx = v1->x - startx + v1->x;
+		}
+		else if (pds->src->dy == 0)
+		{ // horizontal mirror
+			viewy = v1->y - starty + v1->y;
+		}
+		else
+		{ // any mirror--use floats to avoid integer overflow
+			vertex_t *v2 = pds->src->v2;
+
+			float dx = FIXED2FLOAT(v2->x - v1->x);
+			float dy = FIXED2FLOAT(v2->y - v1->y);
+			float x1 = FIXED2FLOAT(v1->x);
+			float y1 = FIXED2FLOAT(v1->y);
+			float x = FIXED2FLOAT(startx);
+			float y = FIXED2FLOAT(starty);
+
+			// the above two cases catch len == 0
+			float r = ((x - x1)*dx + (y - y1)*dy) / (dx*dx + dy*dy);
+
+			viewx = FLOAT2FIXED((x1 + r * dx)*2 - x);
+			viewy = FLOAT2FIXED((y1 + r * dy)*2 - y);
+		}
+		viewangle = 2*R_PointToAngle2 (pds->src->v1->x, pds->src->v1->y,
+									   pds->src->v2->x, pds->src->v2->y) - startang;
 
-	// Reflect the current view behind the mirror.
-	if (ds->curline->linedef->dx == 0)
-	{ // vertical mirror
-		viewx = v1->x - startx + v1->x;
-	}
-	else if (ds->curline->linedef->dy == 0)
-	{ // horizontal mirror
-		viewy = v1->y - starty + v1->y;
 	}
 	else
-	{ // any mirror--use floats to avoid integer overflow
-		vertex_t *v2 = ds->curline->v2;
-
-		float dx = FIXED2FLOAT(v2->x - v1->x);
-		float dy = FIXED2FLOAT(v2->y - v1->y);
-		float x1 = FIXED2FLOAT(v1->x);
-		float y1 = FIXED2FLOAT(v1->y);
-		float x = FIXED2FLOAT(startx);
-		float y = FIXED2FLOAT(starty);
-
-		// the above two cases catch len == 0
-		float r = ((x - x1)*dx + (y - y1)*dy) / (dx*dx + dy*dy);
-
-		viewx = FLOAT2FIXED((x1 + r * dx)*2 - x);
-		viewy = FLOAT2FIXED((y1 + r * dy)*2 - y);
+	{
+		P_TranslatePortalXY(pds->src, pds->dst, viewx, viewy);
+		P_TranslatePortalZ(pds->src, pds->dst, viewz);
+		P_TranslatePortalAngle(pds->src, pds->dst, viewangle);
 	}
-	viewangle = 2*R_PointToAngle2 (ds->curline->v1->x, ds->curline->v1->y,
-								   ds->curline->v2->x, ds->curline->v2->y) - startang;
 
 	viewsin = finesine[viewangle>>ANGLETOFINESHIFT];
 	viewcos = finecosine[viewangle>>ANGLETOFINESHIFT];
@@ -666,35 +706,57 @@ void R_EnterMirror (drawseg_t *ds, int depth)
 	R_CopyStackedViewParameters();
 
 	validcount++;
-	ActiveWallMirror = ds->curline;
+	PortalDrawseg* prevpds = CurrentPortal;
+	CurrentPortal = pds;
 
 	R_ClearPlanes (false);
-	R_ClearClipSegs (ds->x1, ds->x2 + 1);
+	R_ClearClipSegs (pds->x1, pds->x2 + 1); // todo: check if this "+1" is actually needed
 
-	memcpy (ceilingclip + ds->x1, openings + ds->sprtopclip, (ds->x2 - ds->x1 + 1)*sizeof(*ceilingclip));
-	memcpy (floorclip + ds->x1, openings + ds->sprbottomclip, (ds->x2 - ds->x1 + 1)*sizeof(*floorclip));
+	// some portals have height differences, account for this here
+	R_3D_EnterSkybox(); // push 3D floor height map
 
-	WindowLeft = ds->x1;
-	WindowRight = ds->x2;
-	MirrorFlags = (depth + 1) & 1;
+	memcpy (ceilingclip + pds->x1, &pds->ceilingclip[0], pds->ceilingclip.Size()*sizeof(*ceilingclip));
+	memcpy (floorclip + pds->x1, &pds->floorclip[0], pds->floorclip.Size()*sizeof(*floorclip));
+
+	WindowLeft = pds->x1;
+	WindowRight = pds->x2;
+
+	// RF_XFLIP should be removed before calling the root function
+	int prevmf = MirrorFlags;
+	if (pds->mirror)
+	{
+		if (MirrorFlags & RF_XFLIP)
+			MirrorFlags &= ~RF_XFLIP;
+		else MirrorFlags |= RF_XFLIP;
+	}
 
 	R_RenderBSPNode (nodes + numnodes - 1);
 	R_3D_ResetClip(); // reset clips (floor/ceiling)
 
+	PlaneCycles.Clock();
 	R_DrawPlanes ();
 	R_DrawSkyBoxes ();
+	PlaneCycles.Unclock();
 
-	// Allow up to 4 recursions through a mirror
-	if (depth < 4)
+	// depth check is in another place right now
+	unsigned int portalsAtEnd = WallPortals.Size ();
+	for (; portalsAtStart < portalsAtEnd; portalsAtStart++)
 	{
-		unsigned int mirrorsAtEnd = WallMirrors.Size ();
-
-		for (; mirrorsAtStart < mirrorsAtEnd; mirrorsAtStart++)
-		{
-			R_EnterMirror (drawsegs + WallMirrors[mirrorsAtStart], depth + 1);
-		}
+		R_EnterPortal (&WallPortals[portalsAtStart], depth + 1);
 	}
 
+	NetUpdate();
+
+	MaskedCycles.Clock(); // [ZZ] count sprites in portals/mirrors along with normal ones.
+	R_DrawMasked ();	  //      this is required since with portals there often will be cases when more than 80% of the view is inside a portal.
+	MaskedCycles.Unclock();
+
+	NetUpdate();
+
+	R_3D_LeaveSkybox(); // pop 3D floor height map
+	CurrentPortal = prevpds;
+	MirrorFlags = prevmf;
+	CurrentPortalUniq = prevuniq;
 	viewangle = startang;
 	viewx = startx;
 	viewy = starty;
@@ -781,7 +843,7 @@ void R_RenderActorView (AActor *actor, bool dontmaplines)
 	WindowLeft = 0;
 	WindowRight = viewwidth - 1;
 	MirrorFlags = 0;
-	ActiveWallMirror = NULL;
+	CurrentPortal = NULL;
 
 	r_dontmaplines = dontmaplines;
 	
@@ -815,10 +877,11 @@ void R_RenderActorView (AActor *actor, bool dontmaplines)
 		PlaneCycles.Unclock();
 
 		// [RH] Walk through mirrors
-		size_t lastmirror = WallMirrors.Size ();
-		for (unsigned int i = 0; i < lastmirror; i++)
+		// [ZZ] Merged with portals
+		size_t lastportal = WallPortals.Size();
+		for (unsigned int i = 0; i < lastportal; i++)
 		{
-			R_EnterMirror (drawsegs + WallMirrors[i], 0);
+			R_EnterPortal(&WallPortals[i], 0);
 		}
 
 		NetUpdate ();
@@ -829,7 +892,7 @@ void R_RenderActorView (AActor *actor, bool dontmaplines)
 
 		NetUpdate ();
 	}
-	WallMirrors.Clear ();
+	WallPortals.Clear ();
 	interpolator.RestoreInterpolations ();
 	R_SetupBuffer ();
 
diff --git a/src/r_plane.cpp b/src/r_plane.cpp
index 6572c1f0b1..dc13e95a3c 100644
--- a/src/r_plane.cpp
+++ b/src/r_plane.cpp
@@ -58,6 +58,7 @@
 #include "r_3dfloors.h"
 #include "v_palette.h"
 #include "r_data/colormaps.h"
+#include "portal.h"
 
 #ifdef _MSC_VER
 #pragma warning(disable:4244)
@@ -687,7 +688,7 @@ visplane_t *R_FindPlane (const secplane_t &height, FTextureID picnum, int lightl
 			yscale == check->yscale &&
 			angle == check->angle && 
 			sky == check->sky &&
-			CurrentMirror == check->CurrentMirror &&
+			CurrentPortalUniq == check->CurrentPortalUniq &&
 			MirrorFlags == check->MirrorFlags &&
 			CurrentSkybox == check->CurrentSkybox
 			)
@@ -719,7 +720,7 @@ visplane_t *R_FindPlane (const secplane_t &height, FTextureID picnum, int lightl
 	check->viewangle = stacked_angle;
 	check->Alpha = alpha;
 	check->Additive = additive;
-	check->CurrentMirror = CurrentMirror;
+	check->CurrentPortalUniq = CurrentPortalUniq;
 	check->MirrorFlags = MirrorFlags;
 	check->CurrentSkybox = CurrentSkybox;
 
@@ -808,7 +809,7 @@ visplane_t *R_CheckPlane (visplane_t *pl, int start, int stop)
 		new_pl->sky = pl->sky;
 		new_pl->Alpha = pl->Alpha;
 		new_pl->Additive = pl->Additive;
-		new_pl->CurrentMirror = pl->CurrentMirror;
+		new_pl->CurrentPortalUniq = pl->CurrentPortalUniq;
 		new_pl->MirrorFlags = pl->MirrorFlags;
 		new_pl->CurrentSkybox = pl->CurrentSkybox;
 		pl = new_pl;
@@ -1044,7 +1045,7 @@ int R_DrawPlanes ()
 		for (pl = visplanes[i]; pl; pl = pl->next)
 		{
 			// kg3D - draw only correct planes
-			if(pl->CurrentMirror != CurrentMirror || pl->CurrentSkybox != CurrentSkybox)
+			if(pl->CurrentPortalUniq != CurrentPortalUniq || pl->CurrentSkybox != CurrentSkybox)
 				continue;
 			// kg3D - draw only real planes now
 			if(pl->sky >= 0) {
@@ -1270,6 +1271,7 @@ void R_DrawSkyBoxes ()
 
 		// Create a drawseg to clip sprites to the sky plane
 		R_CheckDrawSegs ();
+		ds_p->CurrentPortalUniq = CurrentPortalUniq;
 		ds_p->siz1 = INT_MAX;
 		ds_p->siz2 = INT_MAX;
 		ds_p->sz1 = 0;
diff --git a/src/r_plane.h b/src/r_plane.h
index 231fa3ad4c..5fd9b8eebf 100644
--- a/src/r_plane.h
+++ b/src/r_plane.h
@@ -58,7 +58,7 @@ struct visplane_s
 
 	// kg3D - keep track of mirror and skybox owner
 	int CurrentSkybox;
-	int CurrentMirror; // mirror counter, counts all of them
+	int CurrentPortalUniq; // mirror counter, counts all of them
 	int MirrorFlags; // this is not related to CurrentMirror
 
 	unsigned short *bottom;			// [RH] bottom and top arrays are dynamically
diff --git a/src/r_segs.cpp b/src/r_segs.cpp
index 15f736c932..9e20a3535a 100644
--- a/src/r_segs.cpp
+++ b/src/r_segs.cpp
@@ -52,6 +52,7 @@
 #include "r_3dfloors.h"
 #include "v_palette.h"
 #include "r_data/colormaps.h"
+#include "portal.h"
 
 #define WALLYREPEAT 8
 
@@ -103,7 +104,7 @@ extern fixed_t	rw_frontfz1, rw_frontfz2;
 int				rw_ceilstat, rw_floorstat;
 bool			rw_mustmarkfloor, rw_mustmarkceiling;
 bool			rw_prepped;
-bool			rw_markmirror;
+bool			rw_markportal;
 bool			rw_havehigh;
 bool			rw_havelow;
 
@@ -1948,7 +1949,7 @@ void R_NewWall (bool needlights)
 {
 	fixed_t rowoffset, yrepeat;
 
-	rw_markmirror = false;
+	rw_markportal = false;
 
 	sidedef = curline->sidedef;
 	linedef = curline->linedef;
@@ -1958,65 +1959,64 @@ void R_NewWall (bool needlights)
 
 	midtexture = toptexture = bottomtexture = 0;
 
-	if (backsector == NULL)
+	if (sidedef == linedef->sidedef[0] &&
+		linedef->portal &&
+		(!linedef->portal_mirror || r_drawmirrors)) // [ZZ] compatibility with r_drawmirrors cvar that existed way before portals
+	{
+		markfloor = markceiling = true; // act like an one-sided wall here (todo: check how does this work with transparency)
+		rw_markportal = true;
+	}
+	else if (backsector == NULL)
 	{
 		// single sided line
 		// a single sided line is terminal, so it must mark ends
 		markfloor = markceiling = true;
-		// [RH] Render mirrors later, but mark them now.
-		if (linedef->special != Line_Mirror || !r_drawmirrors)
+		// [RH] Horizon lines do not need to be textured
+		if (linedef->special != Line_Horizon)
 		{
-			// [RH] Horizon lines do not need to be textured
-			if (linedef->special != Line_Horizon)
-			{
-				midtexture = TexMan(sidedef->GetTexture(side_t::mid), true);
-				rw_offset_mid = sidedef->GetTextureXOffset(side_t::mid);
-				rowoffset = sidedef->GetTextureYOffset(side_t::mid);
-				rw_midtexturescalex = sidedef->GetTextureXScale(side_t::mid);
-				rw_midtexturescaley = sidedef->GetTextureYScale(side_t::mid);
-				yrepeat = FixedMul(midtexture->yScale, rw_midtexturescaley);
-				if (yrepeat >= 0)
-				{ // normal orientation
-					if (linedef->flags & ML_DONTPEGBOTTOM)
-					{ // bottom of texture at bottom
-						rw_midtexturemid = MulScale16(frontsector->GetPlaneTexZ(sector_t::floor) - viewz, yrepeat) + (midtexture->GetHeight() << FRACBITS);
-					}
-					else
-					{ // top of texture at top
-						rw_midtexturemid = MulScale16(frontsector->GetPlaneTexZ(sector_t::ceiling) - viewz, yrepeat);
-						if (rowoffset < 0 && midtexture != NULL)
-						{
-							rowoffset += midtexture->GetHeight() << FRACBITS;
-						}
-					}
+			midtexture = TexMan(sidedef->GetTexture(side_t::mid), true);
+			rw_offset_mid = sidedef->GetTextureXOffset(side_t::mid);
+			rowoffset = sidedef->GetTextureYOffset(side_t::mid);
+			rw_midtexturescalex = sidedef->GetTextureXScale(side_t::mid);
+			rw_midtexturescaley = sidedef->GetTextureYScale(side_t::mid);
+			yrepeat = FixedMul(midtexture->yScale, rw_midtexturescaley);
+			if (yrepeat >= 0)
+			{ // normal orientation
+				if (linedef->flags & ML_DONTPEGBOTTOM)
+				{ // bottom of texture at bottom
+					rw_midtexturemid = MulScale16(frontsector->GetPlaneTexZ(sector_t::floor) - viewz, yrepeat) + (midtexture->GetHeight() << FRACBITS);
 				}
 				else
-				{ // upside down
-					rowoffset = -rowoffset;
-					if (linedef->flags & ML_DONTPEGBOTTOM)
-					{ // top of texture at bottom
-						rw_midtexturemid = MulScale16(frontsector->GetPlaneTexZ(sector_t::floor) - viewz, yrepeat);
+				{ // top of texture at top
+					rw_midtexturemid = MulScale16(frontsector->GetPlaneTexZ(sector_t::ceiling) - viewz, yrepeat);
+					if (rowoffset < 0 && midtexture != NULL)
+					{
+						rowoffset += midtexture->GetHeight() << FRACBITS;
 					}
-					else
-					{ // bottom of texture at top
-						rw_midtexturemid = MulScale16(frontsector->GetPlaneTexZ(sector_t::ceiling) - viewz, yrepeat) + (midtexture->GetHeight() << FRACBITS);
-					}
-				}
-				if (midtexture->bWorldPanning)
-				{
-					rw_midtexturemid += MulScale16(rowoffset, yrepeat);
-				}
-				else
-				{
-					// rowoffset is added outside the multiply so that it positions the texture
-					// by texels instead of world units.
-					rw_midtexturemid += rowoffset;
 				}
 			}
-		}
-		else
-		{
-			rw_markmirror = true;
+			else
+			{ // upside down
+				rowoffset = -rowoffset;
+				if (linedef->flags & ML_DONTPEGBOTTOM)
+				{ // top of texture at bottom
+					rw_midtexturemid = MulScale16(frontsector->GetPlaneTexZ(sector_t::floor) - viewz, yrepeat);
+				}
+				else
+				{ // bottom of texture at top
+					rw_midtexturemid = MulScale16(frontsector->GetPlaneTexZ(sector_t::ceiling) - viewz, yrepeat) + (midtexture->GetHeight() << FRACBITS);
+				}
+			}
+			if (midtexture->bWorldPanning)
+			{
+				rw_midtexturemid += MulScale16(rowoffset, yrepeat);
+			}
+			else
+			{
+				// rowoffset is added outside the multiply so that it positions the texture
+				// by texels instead of world units.
+				rw_midtexturemid += rowoffset;
+			}
 		}
 	}
 	else
@@ -2339,6 +2339,7 @@ void R_StoreWallRange (int start, int stop)
 	rw_offset = sidedef->GetTextureXOffset(side_t::mid);
 	rw_light = rw_lightleft + rw_lightstep * (start - WallC.sx1);
 
+	ds_p->CurrentPortalUniq = CurrentPortalUniq;
 	ds_p->sx1 = WallC.sx1;
 	ds_p->sx2 = WallC.sx2;
 	ds_p->sz1 = WallC.sz1;
@@ -2362,10 +2363,8 @@ void R_StoreWallRange (int start, int stop)
 	// killough 1/6/98, 2/1/98: remove limit on openings
 	ds_p->sprtopclip = ds_p->sprbottomclip = ds_p->maskedtexturecol = ds_p->bkup = ds_p->swall = -1;
 
-	if (rw_markmirror)
+	if (rw_markportal)
 	{
-		size_t drawsegnum = ds_p - drawsegs;
-		WallMirrors.Push (drawsegnum);
 		ds_p->silhouette = SIL_BOTH;
 	}
 	else if (backsector == NULL)
@@ -2576,9 +2575,28 @@ void R_StoreWallRange (int start, int stop)
 	}
 
 	// [RH] Draw any decals bound to the seg
-	for (DBaseDecal *decal = curline->sidedef->AttachedDecals; decal != NULL; decal = decal->WallNext)
+	// [ZZ] Only if not an active mirror
+	if (!rw_markportal)
 	{
-		R_RenderDecal (curline->sidedef, decal, ds_p, 0);
+		for (DBaseDecal *decal = curline->sidedef->AttachedDecals; decal != NULL; decal = decal->WallNext)
+		{
+			R_RenderDecal (curline->sidedef, decal, ds_p, 0);
+		}
+	}
+
+	if (rw_markportal)
+	{
+		PortalDrawseg pds;
+		pds.src = curline->linedef;
+		pds.dst = curline->linedef->portal_dst;
+		pds.x1 = ds_p->x1;
+		pds.x2 = ds_p->x2;
+		pds.ceilingclip.Resize((pds.x2-pds.x1)+1);
+		memcpy(&pds.ceilingclip[0], openings + ds_p->sprtopclip, pds.ceilingclip.Size()*sizeof(*openings));
+		pds.floorclip.Resize((pds.x2-pds.x1)+1);
+		memcpy(&pds.floorclip[0], openings + ds_p->sprbottomclip, pds.floorclip.Size()*sizeof(*openings));
+		pds.mirror = curline->linedef->portal_mirror;
+		WallPortals.Push(pds);
 	}
 
 	ds_p++;
diff --git a/src/r_things.cpp b/src/r_things.cpp
index c0475ee759..96afaa2166 100644
--- a/src/r_things.cpp
+++ b/src/r_things.cpp
@@ -316,6 +316,44 @@ nextpost:
 	}
 }
 
+// [ZZ]
+// R_ClipSpriteColumnWithPortals
+//
+bool R_ClipSpriteColumnWithPortals (fixed_t x, fixed_t y, vissprite_t* spr)
+{
+	size_t numdrawsegs = ds_p-firstdrawseg;
+	for (int i = (int)numdrawsegs-1; i >= 0; i--)
+	{
+		drawseg_t* seg = &firstdrawseg[i];
+
+		// ignore segs from other portals
+		if (seg->CurrentPortalUniq != CurrentPortalUniq)
+			continue;
+
+		line_t* line = seg->curline->linedef;
+		// divline? wtf, anyway, divlines aren't supposed to be drawn. But I definitely saw NULL linedefs in drawsegs.
+		if (!line) continue;
+
+		// check if this line will clip sprites to itself
+		if (!line->portal)
+			continue;
+
+		// don't clip sprites with portal's back side (it's transparent)
+		if (seg->curline->sidedef != line->sidedef[0])
+			continue;
+
+		// don't clip if the sprite is in front of the portal
+		if (!P_PointOnLineSide(x, y, line))
+			continue;
+
+		// now if current column is covered by this drawseg, we clip it away
+		if ((dc_x >= seg->x1) && (dc_x <= seg->x2))
+			return true;
+	}
+
+	return false;
+}
+
 //
 // R_DrawVisSprite
 //	mfloorclip and mceilingclip should also be set.
@@ -329,6 +367,7 @@ void R_DrawVisSprite (vissprite_t *vis)
 	int				x2, stop4;
 	fixed_t			xiscale;
 	ESPSResult		mode;
+	bool			ispsprite = (!vis->sector && !vis->gx && !vis->gy && !vis->gz);
 
 	dc_colormap = vis->Style.colormap;
 
@@ -372,7 +411,8 @@ void R_DrawVisSprite (vissprite_t *vis)
 			while ((dc_x < stop4) && (dc_x & 3))
 			{
 				pixels = tex->GetColumn (frac >> FRACBITS, &spans);
-				R_DrawMaskedColumn (pixels, spans);
+				if (ispsprite || !R_ClipSpriteColumnWithPortals(vis->gx, vis->gy, vis))
+					R_DrawMaskedColumn (pixels, spans);
 				dc_x++;
 				frac += xiscale;
 			}
@@ -383,7 +423,8 @@ void R_DrawVisSprite (vissprite_t *vis)
 				for (int zz = 4; zz; --zz)
 				{
 					pixels = tex->GetColumn (frac >> FRACBITS, &spans);
-					R_DrawMaskedColumnHoriz (pixels, spans);
+					if (ispsprite || !R_ClipSpriteColumnWithPortals(vis->gx, vis->gy, vis))
+						R_DrawMaskedColumnHoriz (pixels, spans);
 					dc_x++;
 					frac += xiscale;
 				}
@@ -393,7 +434,8 @@ void R_DrawVisSprite (vissprite_t *vis)
 			while (dc_x < x2)
 			{
 				pixels = tex->GetColumn (frac >> FRACBITS, &spans);
-				R_DrawMaskedColumn (pixels, spans);
+				if (ispsprite || !R_ClipSpriteColumnWithPortals(vis->gx, vis->gy, vis))
+					R_DrawMaskedColumn (pixels, spans);
 				dc_x++;
 				frac += xiscale;
 			}
@@ -503,7 +545,8 @@ void R_DrawWallSprite(vissprite_t *spr)
 			{ // calculate lighting
 				dc_colormap = usecolormap->Maps + (GETPALOOKUP (rw_light, shade) << COLORMAPSHIFT);
 			}
-			R_WallSpriteColumn(R_DrawMaskedColumn);
+			if (!R_ClipSpriteColumnWithPortals(spr->gx, spr->gy, spr))
+				R_WallSpriteColumn(R_DrawMaskedColumn);
 			dc_x++;
 		}
 
@@ -516,7 +559,8 @@ void R_DrawWallSprite(vissprite_t *spr)
 			rt_initcols();
 			for (int zz = 4; zz; --zz)
 			{
-				R_WallSpriteColumn(R_DrawMaskedColumnHoriz);
+				if (!R_ClipSpriteColumnWithPortals(spr->gx, spr->gy, spr))
+					R_WallSpriteColumn(R_DrawMaskedColumnHoriz);
 				dc_x++;
 			}
 			rt_draw4cols(dc_x - 4);
@@ -528,7 +572,8 @@ void R_DrawWallSprite(vissprite_t *spr)
 			{ // calculate lighting
 				dc_colormap = usecolormap->Maps + (GETPALOOKUP (rw_light, shade) << COLORMAPSHIFT);
 			}
-			R_WallSpriteColumn(R_DrawMaskedColumn);
+			if (!R_ClipSpriteColumnWithPortals(spr->gx, spr->gy, spr))
+				R_WallSpriteColumn(R_DrawMaskedColumn);
 			dc_x++;
 		}
 	}
@@ -668,6 +713,10 @@ void R_ProjectSprite (AActor *thing, int fakeside, F3DFloor *fakefloor, F3DFloor
 		return;
 	}
 
+	// [ZZ] Or less definitely not visible (hue)
+	if (CurrentPortal && !!P_PointOnLineSide(thing->x, thing->y, CurrentPortal->dst))
+		return;
+
 	// [RH] Interpolate the sprite's position to make it look smooth
 	fx = thing->PrevX + FixedMul (r_TicFrac, thing->x - thing->PrevX);
 	fy = thing->PrevY + FixedMul (r_TicFrac, thing->y - thing->PrevY);
@@ -885,6 +934,7 @@ void R_ProjectSprite (AActor *thing, int fakeside, F3DFloor *fakefloor, F3DFloor
 		// store information in a vissprite
 		vis = R_NewVisSprite();
 
+		vis->CurrentPortalUniq = CurrentPortalUniq;
 		vis->xscale = xscale;
 		vis->yscale = Scale(InvZtoScale, yscale, tz << 4);
 		vis->idepth = (unsigned)DivScale32(1, tz) >> 1;	// tz is 20.12, so idepth ought to be 12.20, but signed math makes it 13.19
@@ -912,6 +962,7 @@ void R_ProjectSprite (AActor *thing, int fakeside, F3DFloor *fakefloor, F3DFloor
 	{
 		vis = R_NewVisSprite();
 
+		vis->CurrentPortalUniq = CurrentPortalUniq;
 		vis->xscale = xscale;
 		vis->yscale = yscale;
 		vis->x1 = WindowLeft;
@@ -1073,6 +1124,7 @@ static void R_ProjectWallSprite(AActor *thing, fixed_t fx, fixed_t fy, fixed_t f
 	gzb = fz + yscale * scaled_bo;
 
 	vis = R_NewVisSprite();
+	vis->CurrentPortalUniq = CurrentPortalUniq;
 	vis->x1 = wallc.sx1 < WindowLeft ? WindowLeft : wallc.sx1;
 	vis->x2 = wallc.sx2 >= WindowRight ? WindowRight : wallc.sx2-1;
 	vis->yscale = yscale;
@@ -1767,7 +1819,6 @@ void R_SortVisSprites (bool (*compare)(vissprite_t *, vissprite_t *), size_t fir
 	std::stable_sort(&spritesorter[0], &spritesorter[vsprcount], compare);
 }
 
-
 //
 // R_DrawSprite
 //
@@ -2062,8 +2113,11 @@ void R_DrawSprite (vissprite_t *spr)
 
 	for (ds = ds_p; ds-- > firstdrawseg; )  // new -- killough
 	{
+		// [ZZ] portal handling here
+		if (ds->CurrentPortalUniq != spr->CurrentPortalUniq)
+			continue;
 		// kg3D - no clipping on fake segs
-		if(ds->fake) continue;
+		if (ds->fake) continue;
 		// determine if the drawseg obscures the sprite
 		if (ds->x1 > x2 || ds->x2 < x1 ||
 			(!(ds->silhouette & SIL_BOTH) && ds->maskedtexturecol == -1 &&
@@ -2198,6 +2252,8 @@ void R_DrawMaskedSingle (bool renew)
 
 	for (i = vsprcount; i > 0; i--)
 	{
+		if (spritesorter[i-1]->CurrentPortalUniq != CurrentPortalUniq)
+			continue; // probably another time
 		R_DrawSprite (spritesorter[i-1]);
 	}
 
@@ -2215,6 +2271,9 @@ void R_DrawMaskedSingle (bool renew)
 	}
 	for (ds = ds_p; ds-- > firstdrawseg; )	// new -- killough
 	{
+		// [ZZ] the same as above
+		if (ds->CurrentPortalUniq != CurrentPortalUniq)
+			continue;
 		// kg3D - no fake segs
 		if (ds->fake) continue;
 		if (ds->maskedtexturecol != -1 || ds->bFogBoundary)
@@ -2294,6 +2353,10 @@ void R_ProjectParticle (particle_t *particle, const sector_t *sector, int shade,
 	sector_t*			heightsec = NULL;
 	BYTE*				map;
 
+	// [ZZ] Particle not visible through the portal plane
+	if (CurrentPortal && !!P_PointOnLineSide(particle->x, particle->y, CurrentPortal->dst))
+		return;
+
 	// transform the origin point
 	tr_x = particle->x - viewx;
 	tr_y = particle->y - viewy;
@@ -2397,6 +2460,7 @@ void R_ProjectParticle (particle_t *particle, const sector_t *sector, int shade,
 
 	// store information in a vissprite
 	vis = R_NewVisSprite ();
+	vis->CurrentPortalUniq = CurrentPortalUniq;
 	vis->heightsec = heightsec;
 	vis->xscale = xscale;
 //	vis->yscale = FixedMul (xscale, InvZtoScale);
@@ -2449,8 +2513,11 @@ static void R_DrawMaskedSegsBehindParticle (const vissprite_t *vis)
 	for (unsigned int p = InterestingDrawsegs.Size(); p-- > FirstInterestingDrawseg; )
 	{
 		drawseg_t *ds = &drawsegs[InterestingDrawsegs[p]];
+		// [ZZ] only draw stuff that's inside the same portal as the particle, other portals will care for themselves
+		if (ds->CurrentPortalUniq != vis->CurrentPortalUniq)
+			continue;
 		// kg3D - no fake segs
-		if(ds->fake) continue;
+		if (ds->fake) continue;
 		if (ds->x1 >= x2 || ds->x2 < x1)
 		{
 			continue;
@@ -2488,6 +2555,8 @@ void R_DrawParticle (vissprite_t *vis)
 		fg = fg2rgb[color];
 	}
 
+	/*
+
 	spacing = RenderTarget->GetPitch() - countbase;
 	dest = ylookup[yl] + x1 + dc_destorg;
 
@@ -2501,7 +2570,28 @@ void R_DrawParticle (vissprite_t *vis)
 			*dest++ = RGB32k[0][0][bg & (bg>>15)];
 		} while (--count);
 		dest += spacing;
-	} while (--ycount);
+	} while (--ycount);*/
+
+	// original was row-wise
+	// width = countbase
+	// height = ycount
+
+	spacing = RenderTarget->GetPitch();
+
+	for (int x = x1; x < (x1+countbase); x++)
+	{
+		dc_x = x;
+		if (R_ClipSpriteColumnWithPortals(vis->gx, vis->gy, vis))
+			continue;
+		dest = ylookup[yl] + x + dc_destorg;
+		for (int y = 0; y < ycount; y++)
+		{
+			DWORD bg = bg2rgb[*dest];
+			bg = (fg+bg) | 0x1f07c1f;
+			*dest = RGB32k[0][0][bg & (bg>>15)];
+			dest += spacing;
+		}
+	}
 }
 
 extern fixed_t baseyaspectmul;
diff --git a/src/r_things.h b/src/r_things.h
index 3ce1a4d4dd..732ef95e3e 100644
--- a/src/r_things.h
+++ b/src/r_things.h
@@ -80,6 +80,7 @@ struct vissprite_t
 	short 			renderflags;
 	DWORD			Translation;	// [RH] for color translation
 	visstyle_t		Style;
+	int				CurrentPortalUniq; // [ZZ] to identify the portal that this thing is in. used for clipping.
 };
 
 struct particle_t;
diff --git a/zdoom.vcproj b/zdoom.vcproj
index cabc6c84f1..15895c0a35 100644
--- a/zdoom.vcproj
+++ b/zdoom.vcproj
@@ -6632,6 +6632,18 @@
 				>
 			</File>
 		</Filter>
+		<Filter
+			Name="Portals"
+			>
+			<File
+				RelativePath=".\src\portal.cpp"
+				>
+			</File>
+			<File
+				RelativePath=".\src\portal.h"
+				>
+			</File>
+		</Filter>
 	</Files>
 	<Globals>
 	</Globals>