diff --git a/src/rendering/hwrenderer/scene/hw_bsp.cpp b/src/rendering/hwrenderer/scene/hw_bsp.cpp
index 5e64c8fdd..3f834fcf6 100644
--- a/src/rendering/hwrenderer/scene/hw_bsp.cpp
+++ b/src/rendering/hwrenderer/scene/hw_bsp.cpp
@@ -718,6 +718,9 @@ void HWDrawInfo::DoSubsector(subsector_t * sub)
 		bool anglevisible = false;
 		bool pitchvisible = !(Viewpoint.IsAllowedOoB()); // No vertical clipping if viewpoint is not allowed out of bounds
 		bool radarvisible = !(Viewpoint.IsAllowedOoB()) || !r_radarclipper || (Level->flags3 & LEVEL3_NOFOGOFWAR) || ((sub->flags & SSECMF_DRAWN) && !deathmatch);
+		bool ceilreflect = (mCurrentPortal && strcmp(mCurrentPortal->GetName(), "Planemirror ceiling"));
+		bool floorreflect = (mCurrentPortal && strcmp(mCurrentPortal->GetName(), "Planemirror floor"));
+		double planez = (ceilreflect ? sector->ceilingplane.ZatPoint(Viewpoint.Pos) : sector->floorplane.ZatPoint(Viewpoint.Pos));
 		angle_t pitchtemp;
 		angle_t pitchmin = ANGLE_90;
 		angle_t pitchmax = 0;
@@ -741,16 +744,28 @@ void HWDrawInfo::DoSubsector(subsector_t * sub)
 
 				if (!pitchvisible)
 				{
-					pitchmin = clipperv.PointToPseudoPitch(seg->v1->fX(), seg->v1->fY(), sector->floorplane.ZatPoint(seg->v1));
-					pitchmax = clipperv.PointToPseudoPitch(seg->v1->fX(), seg->v1->fY(), sector->ceilingplane.ZatPoint(seg->v1));
+					pitchmin = clipperv.PointToPseudoPitch(seg->v1->fX(), seg->v1->fY(),
+														   (ceilreflect || floorreflect) ?
+														   2 * planez - sector->floorplane.ZatPoint(seg->v1) :
+														   sector->floorplane.ZatPoint(seg->v1));
+					pitchmax = clipperv.PointToPseudoPitch(seg->v1->fX(), seg->v1->fY(),
+														   (ceilreflect || floorreflect) ?
+														   2 * planez - sector->ceilingplane.ZatPoint(seg->v1) :
+														   sector->ceilingplane.ZatPoint(seg->v1));
 					pitchvisible |= clipperv.SafeCheckRange(pitchmin, pitchmax);
 				}
 				if (pitchvisible && anglevisible && radarvisible) break;
 				if (!pitchvisible)
 				{
-					pitchtemp = clipperv.PointToPseudoPitch(seg->v2->fX(), seg->v2->fY(), sector->floorplane.ZatPoint(seg->v2));
+					pitchtemp = clipperv.PointToPseudoPitch(seg->v2->fX(), seg->v2->fY(),
+															(ceilreflect || floorreflect) ?
+															2 * planez - sector->floorplane.ZatPoint(seg->v2) :
+															sector->floorplane.ZatPoint(seg->v2));
 					if (int(pitchmin) > int(pitchtemp)) pitchmin = pitchtemp;
-					pitchtemp = clipperv.PointToPseudoPitch(seg->v2->fX(), seg->v2->fY(), sector->ceilingplane.ZatPoint(seg->v2));
+					pitchtemp = clipperv.PointToPseudoPitch(seg->v2->fX(), seg->v2->fY(),
+															(ceilreflect || floorreflect) ?
+															2 * planez - sector->ceilingplane.ZatPoint(seg->v2) :
+															sector->ceilingplane.ZatPoint(seg->v2));
 					if (int(pitchmax) < int(pitchtemp)) pitchmax = pitchtemp;
 					pitchvisible |= clipperv.SafeCheckRange(pitchmin, pitchmax);
 				}
@@ -819,7 +834,7 @@ void HWDrawInfo::DoSubsector(subsector_t * sub)
 				SetupSprite.Unclock();
 			}
 		}
-		if (r_dithertransparency && Viewpoint.IsAllowedOoB() && (RTnum < MAXDITHERACTORS))
+		if (r_dithertransparency && Viewpoint.IsAllowedOoB() && (RTnum < MAXDITHERACTORS) && mCurrentPortal == nullptr)
 		{
 			// [DVR] Not parallelizable due to variables RTnum and RenderedTargets[]
 			for (auto p = sector->touching_renderthings; p != nullptr; p = p->m_snext)
diff --git a/src/rendering/hwrenderer/scene/hw_drawinfo.cpp b/src/rendering/hwrenderer/scene/hw_drawinfo.cpp
index 0fc7bc896..404203933 100644
--- a/src/rendering/hwrenderer/scene/hw_drawinfo.cpp
+++ b/src/rendering/hwrenderer/scene/hw_drawinfo.cpp
@@ -717,7 +717,7 @@ static ETraceStatus TraceCallbackForDitherTransparency(FTraceResults& res, void*
 		}
 		break;
 	case TRACE_HitFloor:
-		if (res.Sector->subsectorcount > 0 && (*CurMapSections)[res.Sector->subsectors[0]->mapsection])
+		if (res.Sector->subsectorcount > 0 && (*CurMapSections)[res.Sector->subsectors[0]->mapsection] && res.HitVector.dot(res.Sector->floorplane.Normal()) < 0.0)
 		{
 			if (res.HitPos.Z == res.Sector->floorplane.ZatPoint(res.HitPos))
 			{
@@ -743,7 +743,7 @@ static ETraceStatus TraceCallbackForDitherTransparency(FTraceResults& res, void*
 		}
 		break;
 	case TRACE_HitCeiling:
-		if (res.Sector->subsectorcount > 0 && (*CurMapSections)[res.Sector->subsectors[0]->mapsection])
+		if (res.Sector->subsectorcount > 0 && (*CurMapSections)[res.Sector->subsectors[0]->mapsection] && res.HitVector.dot(res.Sector->ceilingplane.Normal()) < 0.0)
 		{
 			if (res.HitPos.Z == res.Sector->ceilingplane.ZatPoint(res.HitPos))
 			{
@@ -779,6 +779,7 @@ static ETraceStatus TraceCallbackForDitherTransparency(FTraceResults& res, void*
 
 void HWDrawInfo::SetDitherTransFlags(AActor* actor)
 {
+	// This should really be moved to a shader and have the GPU do some shape-tracing.
 	if (actor && actor->Sector)
 	{
 		FTraceResults results;
diff --git a/src/rendering/hwrenderer/scene/hw_flats.cpp b/src/rendering/hwrenderer/scene/hw_flats.cpp
index f14757ade..317ba9399 100644
--- a/src/rendering/hwrenderer/scene/hw_flats.cpp
+++ b/src/rendering/hwrenderer/scene/hw_flats.cpp
@@ -47,6 +47,7 @@
 #include "hw_drawstructs.h"
 #include "hw_renderstate.h"
 #include "texturemanager.h"
+#include "hw_viewpointbuffer.h"
 
 #ifdef _DEBUG
 CVAR(Int, gl_breaksec, -1, 0)
@@ -314,6 +315,7 @@ void HWFlat::DrawFlat(HWDrawInfo *di, FRenderState &state, bool translucent)
 	int rel = getExtraLight();
 
 	state.SetNormal(plane.plane.Normal().X, plane.plane.Normal().Z, plane.plane.Normal().Y);
+	double zshift = (plane.plane.Normal().Z > 0.0 ? 0.01f : -0.01f); // The HWPlaneMirrorPortal::DrawPortalStencil() z-fights with flats
 
 	SetColor(state, di->Level, di->lightmode, lightlevel, rel, di->isFullbrightScene(), Colormap, alpha);
 	SetFog(state, di->Level, di->lightmode, lightlevel, rel, di->isFullbrightScene(), &Colormap, false);
@@ -364,7 +366,11 @@ void HWFlat::DrawFlat(HWDrawInfo *di, FRenderState &state, bool translucent)
 			else state.AlphaFunc(Alpha_GEqual, 0.f);
 			state.SetMaterial(texture, UF_Texture, 0, CLAMP_NONE, NO_TRANSLATION, -1);
 			SetPlaneTextureRotation(state, &plane, texture);
+			di->VPUniforms.mViewMatrix.translate(0.0, zshift, 0.0);
+			screen->mViewpoints->SetViewpoint(state, &di->VPUniforms);
 			DrawSubsectors(di, state);
+			di->VPUniforms.mViewMatrix.translate(0.0, -zshift, 0.0);
+			screen->mViewpoints->SetViewpoint(state, &di->VPUniforms);
 			state.EnableTextureMatrix(false);
 		}
 		state.SetRenderStyle(DefaultRenderStyle());
diff --git a/src/rendering/hwrenderer/scene/hw_portal.cpp b/src/rendering/hwrenderer/scene/hw_portal.cpp
index b103f0f81..01ab23b8a 100644
--- a/src/rendering/hwrenderer/scene/hw_portal.cpp
+++ b/src/rendering/hwrenderer/scene/hw_portal.cpp
@@ -158,13 +158,14 @@ bool FPortalSceneState::RenderFirstSkyPortal(int recursion, HWDrawInfo *outer_di
 	if (best)
 	{
 		portals.Delete(bestindex);
-		if (usestencil)
+		if (usestencil && ((strcmp(best->GetName(), "Sky") == 0) || (strcmp(best->GetName(), "Skybox") == 0)))
 		{
 			tempmatrix = outer_di->VPUniforms.mProjectionMatrix; // ensure perspective projection matrix for skies
 			outer_di->VPUniforms.mProjectionMatrix = outer_di->ProjectionMatrix2;
 		}
 		RenderPortal(best, state, usestencil, outer_di);
-		if (usestencil) outer_di->VPUniforms.mProjectionMatrix = tempmatrix;
+		if (usestencil && ((strcmp(best->GetName(), "Sky") == 0) || (strcmp(best->GetName(), "Skybox") == 0)))
+			outer_di->VPUniforms.mProjectionMatrix = tempmatrix;
 		delete best;
 		return true;
 	}
@@ -332,6 +333,7 @@ void HWPortal::RemoveStencil(HWDrawInfo *di, FRenderState &state, bool usestenci
 	bool needdepth = NeedDepthBuffer();
 
 	// Restore the old view
+
 	auto &vp = di->Viewpoint;
 	if (vp.camera != nullptr) vp.camera->renderflags = (vp.camera->renderflags & ~RF_MAYBEINVISIBLE) | savedvisibility;
 
@@ -870,12 +872,31 @@ bool HWPlaneMirrorPortal::Setup(HWDrawInfo *di, FRenderState &rstate, Clipper *c
 	state->PlaneMirrorFlag++;
 	di->SetClipHeight(planez, state->PlaneMirrorMode < 0 ? -1.f : 1.f);
 	di->SetupView(rstate, vp.Pos.X, vp.Pos.Y, vp.Pos.Z, !!(state->MirrorFlag & 1), !!(state->PlaneMirrorFlag & 1));
+	vp.ViewVector3D.Z = - vp.ViewVector3D.Z;
+	vp.OffPos.Z = 2 * planez - vp.OffPos.Z;
 	ClearClipper(di, clipper);
 
 	di->UpdateCurrentMapSection();
 	return true;
 }
 
+
+void HWPlaneMirrorPortal::DrawPortalStencil(FRenderState &state, int pass)
+{
+	bool isceiling = planesused & (1 << sector_t::ceiling);
+	for (unsigned int i = 0; i < lines.Size(); i++)
+	{
+		flat.section = lines[i].sub->section;
+		flat.iboindex = lines[i].sub->sector->iboindex[isceiling ? sector_t::ceiling : sector_t::floor];
+		flat.plane.GetFromSector(lines[i].sub->sector, isceiling ? sector_t::ceiling : sector_t::floor);
+		// if (isceiling) flat.plane.plane.FlipVert(); // Doesn't do anything. Stencil is a screen-space projection
+
+		state.SetNormal(flat.plane.plane.Normal().X, flat.plane.plane.Normal().Z, flat.plane.plane.Normal().Y);
+		state.DrawIndexed(DT_Triangles, flat.iboindex + flat.section->vertexindex, flat.section->vertexcount, i == 0);
+	}
+}
+
+
 void HWPlaneMirrorPortal::Shutdown(HWDrawInfo *di, FRenderState &rstate)
 {
 	auto state = mState;
diff --git a/src/rendering/hwrenderer/scene/hw_portal.h b/src/rendering/hwrenderer/scene/hw_portal.h
index 760b8c435..ab9fec945 100644
--- a/src/rendering/hwrenderer/scene/hw_portal.h
+++ b/src/rendering/hwrenderer/scene/hw_portal.h
@@ -59,13 +59,14 @@ class HWPortal
 	TArray<unsigned int> mPrimIndices;
 	unsigned int mTopCap = ~0u, mBottomCap = ~0u;
 
-	void DrawPortalStencil(FRenderState &state, int pass);
+	virtual void DrawPortalStencil(FRenderState &state, int pass);
 
 public:
 	FPortalSceneState * mState;
 	TArray<HWWall> lines;
 	BoundingRect boundingBox;
 	int planesused = 0;
+	HWFlat flat;
 
     HWPortal(FPortalSceneState *s, bool local = false) : mState(s), boundingBox(false)
     {
@@ -295,6 +296,7 @@ struct HWPlaneMirrorPortal : public HWScenePortalBase
 protected:
 	bool Setup(HWDrawInfo *di, FRenderState &rstate, Clipper *clipper) override;
 	void Shutdown(HWDrawInfo *di, FRenderState &rstate) override;
+	void DrawPortalStencil(FRenderState &state, int pass) override;
 	virtual void * GetSource() const { return origin; }
 	virtual const char *GetName();
 	secplane_t * origin;
diff --git a/src/rendering/hwrenderer/scene/hw_sky.cpp b/src/rendering/hwrenderer/scene/hw_sky.cpp
index 93fc827cc..4610f00da 100644
--- a/src/rendering/hwrenderer/scene/hw_sky.cpp
+++ b/src/rendering/hwrenderer/scene/hw_sky.cpp
@@ -154,7 +154,7 @@ void HWWall::SkyPlane(HWWallDispatcher *di, sector_t *sector, int plane, bool al
 		case PORTS_PORTAL:
 		case PORTS_LINKEDPORTAL:
 		{
-			if (di->di && di->di->Viewpoint.IsAllowedOoB()) return;
+			if (di->di && di->di->Viewpoint.IsAllowedOoB()) return; // Almost works (with planemirrorportal stencil), but no quite
 			auto glport = sector->GetPortalGroup(plane);
 			if (glport != NULL)
 			{
diff --git a/src/rendering/hwrenderer/scene/hw_walls.cpp b/src/rendering/hwrenderer/scene/hw_walls.cpp
index 47e651bd4..d5271d05f 100644
--- a/src/rendering/hwrenderer/scene/hw_walls.cpp
+++ b/src/rendering/hwrenderer/scene/hw_walls.cpp
@@ -644,7 +644,8 @@ void HWWall::PutPortal(HWWallDispatcher *di, int ptype, int plane)
 			break;
 
 		case PORTALTYPE_PLANEMIRROR:
-			if (portalState.PlaneMirrorMode * planemirror->fC() <= 0)
+			if (ddi->Viewpoint.IsOrtho() ? (ddi->Viewpoint.ViewVector3D.dot(planemirror->Normal()) < 0)
+				: (portalState.PlaneMirrorMode * planemirror->fC() <= 0))
 			{
 				planemirror = portalState.UniquePlaneMirrors.Get(planemirror);
 				portal = ddi->FindPortal(planemirror);