diff --git a/src/d_dehacked.cpp b/src/d_dehacked.cpp
index 9e068b596..dd71fdf75 100644
--- a/src/d_dehacked.cpp
+++ b/src/d_dehacked.cpp
@@ -2985,9 +2985,11 @@ static bool LoadDehSupp ()
 void FinishDehPatch ()
 {
 	unsigned int touchedIndex;
+	unsigned int nameindex = 0;
 
 	for (touchedIndex = 0; touchedIndex < TouchedActors.Size(); ++touchedIndex)
 	{
+		PClassActor *subclass;
 		PClassActor *type = TouchedActors[touchedIndex];
 		AActor *defaults1 = GetDefaultByType (type);
 		if (!(defaults1->flags & MF_SPECIAL))
@@ -2997,9 +2999,16 @@ void FinishDehPatch ()
 
 		// Create a new class that will serve as the actual pickup
 		char typeNameBuilder[32];
-		mysnprintf (typeNameBuilder, countof(typeNameBuilder), "DehackedPickup%d", touchedIndex);
-		PClassActor *subclass = static_cast<PClassActor *>(RUNTIME_CLASS(ADehackedPickup)->
-			CreateDerivedClass(typeNameBuilder, sizeof(ADehackedPickup)));
+		// 
+		do
+		{
+			// Retry until we find a free name. This is unlikely to happen but not impossible.
+			mysnprintf(typeNameBuilder, countof(typeNameBuilder), "DehackedPickup%d", nameindex++);
+			subclass = static_cast<PClassActor *>(RUNTIME_CLASS(ADehackedPickup)->
+				CreateDerivedClass(typeNameBuilder, sizeof(ADehackedPickup)));
+		} 
+		while (subclass == nullptr);
+		
 		AActor *defaults2 = GetDefaultByType (subclass);
 		memcpy ((void *)defaults2, (void *)defaults1, sizeof(AActor));
 
diff --git a/src/dobjtype.cpp b/src/dobjtype.cpp
index 9693da137..37ff26fe3 100644
--- a/src/dobjtype.cpp
+++ b/src/dobjtype.cpp
@@ -670,6 +670,20 @@ void PNamedType::GetTypeIDs(intptr_t &id1, intptr_t &id2) const
 	id2 = TypeName;
 }
 
+//==========================================================================
+//
+// PNamedType :: QualifiedName
+//
+//==========================================================================
+
+FString PNamedType::QualifiedName() const
+{
+	FString out;
+	if (Outer != nullptr) out = Outer->QualifiedName();
+	out << "::" << TypeName;
+	return out;
+}
+
 /* PInt *******************************************************************/
 
 IMPLEMENT_CLASS(PInt)
@@ -1752,7 +1766,7 @@ PEnum::PEnum()
 //
 //==========================================================================
 
-PEnum::PEnum(FName name, DObject *outer)
+PEnum::PEnum(FName name, PTypeBase *outer)
 : PNamedType(name, outer), ValueType(NULL)
 {
 }
@@ -1766,7 +1780,7 @@ PEnum::PEnum(FName name, DObject *outer)
 //
 //==========================================================================
 
-PEnum *NewEnum(FName name, DObject *outer)
+PEnum *NewEnum(FName name, PTypeBase *outer)
 {
 	size_t bucket;
 	PType *etype = TypeTable.FindType(RUNTIME_CLASS(PEnum), (intptr_t)outer, (intptr_t)name, &bucket);
@@ -2149,7 +2163,7 @@ PStruct::PStruct()
 //
 //==========================================================================
 
-PStruct::PStruct(FName name, DObject *outer)
+PStruct::PStruct(FName name, PTypeBase *outer)
 : PNamedType(name, outer)
 {
 }
@@ -2310,7 +2324,7 @@ size_t PStruct::PropagateMark()
 //
 //==========================================================================
 
-PStruct *NewStruct(FName name, DObject *outer)
+PStruct *NewStruct(FName name, PTypeBase *outer)
 {
 	size_t bucket;
 	PType *stype = TypeTable.FindType(RUNTIME_CLASS(PStruct), (intptr_t)outer, (intptr_t)name, &bucket);
@@ -2847,10 +2861,7 @@ void PClass::InsertIntoHash ()
 	found = TypeTable.FindType(RUNTIME_CLASS(PClass), (intptr_t)Outer, TypeName, &bucket);
 	if (found != NULL)
 	{ // This type has already been inserted
-	  // ... but there is no need whatsoever to make it a fatal error!
-		if (!strictdecorate) Printf (TEXTCOLOR_RED"Tried to register class '%s' more than once.\n", TypeName.GetChars());
-		else I_Error("Tried to register class '%s' more than once.\n", TypeName.GetChars());
-		TypeTable.ReplaceType(this, found, bucket);
+		I_Error("Tried to register class '%s' more than once.\n", TypeName.GetChars());
 	}
 	else
 	{
@@ -3015,15 +3026,23 @@ PClass *PClass::CreateDerivedClass(FName name, unsigned int size)
 	PClass *existclass = static_cast<PClass *>(TypeTable.FindType(RUNTIME_CLASS(PClass), /*FIXME:Outer*/0, name, &bucket));
 
 	// This is a placeholder so fill it in
-	if (existclass != NULL && (existclass->Size == TentativeClass))
+	if (existclass != nullptr)
 	{
-		type = const_cast<PClass*>(existclass);
-		if (!IsDescendantOf(type->ParentClass))
+		if (existclass->Size == TentativeClass)
 		{
-			I_Error("%s must inherit from %s but doesn't.", name.GetChars(), type->ParentClass->TypeName.GetChars());
+			type = const_cast<PClass*>(existclass);
+			if (!IsDescendantOf(type->ParentClass))
+			{
+				I_Error("%s must inherit from %s but doesn't.", name.GetChars(), type->ParentClass->TypeName.GetChars());
+			}
+			DPrintf(DMSG_SPAMMY, "Defining placeholder class %s\n", name.GetChars());
+			notnew = true;
+		}
+		else
+		{
+			// a different class with the same name already exists. Let the calling code deal with this.
+			return nullptr;
 		}
-		DPrintf(DMSG_SPAMMY, "Defining placeholder class %s\n", name.GetChars());
-		notnew = true;
 	}
 	else
 	{
@@ -3360,6 +3379,7 @@ CCMD(typetable)
 
 // Symbol tables ------------------------------------------------------------
 
+IMPLEMENT_ABSTRACT_CLASS(PTypeBase);
 IMPLEMENT_ABSTRACT_CLASS(PSymbol);
 IMPLEMENT_CLASS(PSymbolConst);
 IMPLEMENT_CLASS(PSymbolConstNumeric);
diff --git a/src/dobjtype.h b/src/dobjtype.h
index 62cfa6753..338c51f98 100644
--- a/src/dobjtype.h
+++ b/src/dobjtype.h
@@ -22,12 +22,28 @@ typedef std::pair<const class PType *, unsigned> FTypeAndOffset;
 
 // Symbol information -------------------------------------------------------
 
-class PSymbol : public DObject
+class PTypeBase : public DObject
 {
-	DECLARE_ABSTRACT_CLASS(PSymbol, DObject);
+	DECLARE_ABSTRACT_CLASS(PTypeBase, DObject)
+
+public:
+	virtual FString QualifiedName() const
+	{
+		return "";
+	}
+};
+
+class PSymbol : public PTypeBase
+{
+	DECLARE_ABSTRACT_CLASS(PSymbol, PTypeBase);
 public:
 	virtual ~PSymbol();
 
+	virtual FString QualifiedName() const
+	{
+		return SymbolName.GetChars();
+	}
+
 	FName SymbolName;
 
 protected:
@@ -153,13 +169,13 @@ extern PSymbolTable		 GlobalSymbols;
 
 struct ZCC_ExprConstant;
 class PClassType;
-class PType : public DObject
+class PType : public PTypeBase
 {
 	//DECLARE_ABSTRACT_CLASS_WITH_META(PType, DObject, PClassType);
 	// We need to unravel the _WITH_META macro, since PClassType isn't defined yet,
 	// and we can't define it until we've defined PClass. But we can't define that
 	// without defining PType.
-	DECLARE_ABSTRACT_CLASS(PType, DObject)
+	DECLARE_ABSTRACT_CLASS(PType, PTypeBase)
 	HAS_OBJECT_POINTERS;
 protected:
 	enum { MetaClassNum = CLASSREG_PClassType };
@@ -332,14 +348,15 @@ class PNamedType : public PCompoundType
 	DECLARE_ABSTRACT_CLASS(PNamedType, PCompoundType);
 	HAS_OBJECT_POINTERS;
 public:
-	DObject			*Outer;			// object this type is contained within
+	PTypeBase		*Outer;			// object this type is contained within
 	FName			TypeName;		// this type's name
 
 	PNamedType() : Outer(NULL) {}
-	PNamedType(FName name, DObject *outer) : Outer(outer), TypeName(name) {}
+	PNamedType(FName name, PTypeBase *outer) : Outer(outer), TypeName(name) {}
 
 	virtual bool IsMatch(intptr_t id1, intptr_t id2) const;
 	virtual void GetTypeIDs(intptr_t &id1, intptr_t &id2) const;
+	virtual FString QualifiedName() const;
 };
 
 // Basic types --------------------------------------------------------------
@@ -533,7 +550,7 @@ class PEnum : public PNamedType
 	DECLARE_CLASS(PEnum, PNamedType);
 	HAS_OBJECT_POINTERS;
 public:
-	PEnum(FName name, DObject *outer);
+	PEnum(FName name, PTypeBase *outer);
 
 	PType *ValueType;
 	TMap<FName, int> Values;
@@ -610,7 +627,7 @@ class PStruct : public PNamedType
 {
 	DECLARE_CLASS(PStruct, PNamedType);
 public:
-	PStruct(FName name, DObject *outer);
+	PStruct(FName name, PTypeBase *outer);
 
 	TArray<PField *> Fields;
 
@@ -822,8 +839,8 @@ PDynArray *NewDynArray(PType *type);
 PPointer *NewPointer(PType *type);
 PClassPointer *NewClassPointer(PClass *restrict);
 PClassWaitingForParent *NewUnknownClass(FName myname, FName parentname);
-PEnum *NewEnum(FName name, DObject *outer);
-PStruct *NewStruct(FName name, DObject *outer);
+PEnum *NewEnum(FName name, PTypeBase *outer);
+PStruct *NewStruct(FName name, PTypeBase *outer);
 PPrototype *NewPrototype(const TArray<PType *> &rettypes, const TArray<PType *> &argtypes);
 
 // Built-in types -----------------------------------------------------------
diff --git a/src/g_game.cpp b/src/g_game.cpp
index 36f5e52ba..4072f0b90 100644
--- a/src/g_game.cpp
+++ b/src/g_game.cpp
@@ -2000,7 +2000,7 @@ void G_DoLoadGame ()
 	{
 		// delete the resource file if anything goes wrong in here.
 		if (resfile != nullptr) delete resfile;
-		return;
+		throw;
 	}
 }
 
diff --git a/src/g_level.cpp b/src/g_level.cpp
index 90b31138d..119cde378 100644
--- a/src/g_level.cpp
+++ b/src/g_level.cpp
@@ -1512,7 +1512,11 @@ void G_UnSnapshotLevel (bool hubLoad)
 	if (level.info->isValid())
 	{
 		FSerializer arc;
-		if (!arc.OpenReader(&level.info->Snapshot)) return;
+		if (!arc.OpenReader(&level.info->Snapshot))
+		{
+			I_Error("Failed to load savegame");
+			return;
+		}
 
 		G_SerializeLevel (arc, hubLoad);
 		level.FromSnapshot = true;
diff --git a/src/g_shared/sbarinfo_commands.cpp b/src/g_shared/sbarinfo_commands.cpp
index 13c98d833..d105ab818 100644
--- a/src/g_shared/sbarinfo_commands.cpp
+++ b/src/g_shared/sbarinfo_commands.cpp
@@ -1187,12 +1187,12 @@ class CommandDrawNumber : public CommandDrawString
 
 							if (!(cvartype == CVAR_Bool || cvartype == CVAR_Int))
 							{
-								sc.ScriptMessage("CVar '%s' is not an int or bool", cvarName);
+								sc.ScriptMessage("CVar '%s' is not an int or bool", cvarName.GetChars());
 							}
 						}
 						else
 						{
-							sc.ScriptMessage("CVar '%s' does not exist", cvarName);
+							sc.ScriptMessage("CVar '%s' does not exist", cvarName.GetChars());
 						}
 						
 						if (parenthesized) sc.MustGetToken(')');
diff --git a/src/gl/models/gl_voxels.cpp b/src/gl/models/gl_voxels.cpp
index 288b1a9db..6e84bd7ab 100644
--- a/src/gl/models/gl_voxels.cpp
+++ b/src/gl/models/gl_voxels.cpp
@@ -314,12 +314,12 @@ void FVoxelModel::MakeSlabPolys(int x, int y, kvxslab_t *voxptr, FVoxelMap &chec
 		}
 		if (cull & 4)
 		{
-			AddFace(x, y, z, x+1, y, z, x, y, z+c, x+1, y, z+c, *col, check);
+			AddFace(x+1, y, z, x, y, z, x+1, y, z+c, x, y, z+c, *col, check);
 		}
 		if (cull & 8)
 		{
-			AddFace(x+1, y+1, z, x, y+1, z, x+1, y+1, z+c, x, y+1, z+c, *col, check);
-		}
+			AddFace(x, y+1, z, x+1, y+1, z, x, y+1, z+c, x+1, y+1, z+c, *col, check);
+		}	
 		z+=c;
 		col+=c;
 	}
diff --git a/src/gl/renderer/gl_renderbuffers.h b/src/gl/renderer/gl_renderbuffers.h
index 5fe05f5ac..4477718f4 100644
--- a/src/gl/renderer/gl_renderbuffers.h
+++ b/src/gl/renderer/gl_renderbuffers.h
@@ -2,7 +2,6 @@
 #define __GL_RENDERBUFFERS_H
 
 #include "gl/shaders/gl_shader.h"
-#include "gl/renderer/gl_renderer.h"
 
 class FGLBloomTextureLevel
 {
diff --git a/src/gl/renderer/gl_renderer.cpp b/src/gl/renderer/gl_renderer.cpp
index be1b96f15..0ffafef49 100644
--- a/src/gl/renderer/gl_renderer.cpp
+++ b/src/gl/renderer/gl_renderer.cpp
@@ -105,6 +105,8 @@ FGLRenderer::FGLRenderer(OpenGLFrameBuffer *fb)
 	mTonemapPalette = nullptr;
 	mBuffers = nullptr;
 	mPresentShader = nullptr;
+	mPresent3dCheckerShader = nullptr;
+	mPresent3dColumnShader = nullptr;
 	mPresent3dRowShader = nullptr;
 	mBloomExtractShader = nullptr;
 	mBloomCombineShader = nullptr;
@@ -139,6 +141,8 @@ void FGLRenderer::Initialize(int width, int height)
 	mFXAAShader = new FFXAAShader;
 	mFXAALumaShader = new FFXAALumaShader;
 	mPresentShader = new FPresentShader();
+	mPresent3dCheckerShader = new FPresent3DCheckerShader();
+	mPresent3dColumnShader = new FPresent3DColumnShader();
 	mPresent3dRowShader = new FPresent3DRowShader();
 	m2DDrawer = new F2DDrawer;
 
@@ -192,6 +196,8 @@ FGLRenderer::~FGLRenderer()
 	}
 	if (mBuffers) delete mBuffers;
 	if (mPresentShader) delete mPresentShader;
+	if (mPresent3dCheckerShader) delete mPresent3dCheckerShader;
+	if (mPresent3dColumnShader) delete mPresent3dColumnShader;
 	if (mPresent3dRowShader) delete mPresent3dRowShader;
 	if (mBloomExtractShader) delete mBloomExtractShader;
 	if (mBloomCombineShader) delete mBloomCombineShader;
diff --git a/src/gl/renderer/gl_renderer.h b/src/gl/renderer/gl_renderer.h
index 65e31c52f..19bc93af0 100644
--- a/src/gl/renderer/gl_renderer.h
+++ b/src/gl/renderer/gl_renderer.h
@@ -31,6 +31,8 @@ class FLensShader;
 class FFXAALumaShader;
 class FFXAAShader;
 class FPresentShader;
+class FPresent3DCheckerShader;
+class FPresent3DColumnShader; 
 class FPresent3DRowShader;
 class F2DDrawer;
 class FHardwareTexture;
@@ -109,6 +111,8 @@ public:
 	FFXAALumaShader *mFXAALumaShader;
 	FFXAAShader *mFXAAShader;
 	FPresentShader *mPresentShader;
+	FPresent3DCheckerShader *mPresent3dCheckerShader;
+	FPresent3DColumnShader *mPresent3dColumnShader;
 	FPresent3DRowShader *mPresent3dRowShader;
 
 	FTexture *gllight;
diff --git a/src/gl/scene/gl_flats.cpp b/src/gl/scene/gl_flats.cpp
index 394ca1626..c41e7664f 100644
--- a/src/gl/scene/gl_flats.cpp
+++ b/src/gl/scene/gl_flats.cpp
@@ -187,21 +187,18 @@ void GLFlat::DrawSubsector(subsector_t * sub)
 		unsigned int vi[4];
 
 		vi[0] = 0;
-		for (unsigned int i = 1; i < sub->numlines-1; i += 2)
+		for (unsigned int i = 0; i < sub->numlines - 2; i += 2)
 		{
-			if (i < sub->numlines - 3)
+			for (unsigned int j = 1; j < 4; j++)
 			{
-				for (unsigned int j = 1; j < 4; j++)
-				{
-					vi[j] = MIN(i + j, sub->numlines - 1);
-				}
-				for (unsigned int x = 0; x < 4; x++)
-				{
-					vertex_t *vt = sub->firstline[vi[x]].v1;
-					qd.Set(x, vt->fX(), plane.plane.ZatPoint(vt) + dz, vt->fY(), vt->fX() / 64.f, -vt->fY() / 64.f);
-				}
-				qd.Render(GL_TRIANGLE_FAN);
+				vi[j] = MIN(i + j, sub->numlines - 1);
 			}
+			for (unsigned int x = 0; x < 4; x++)
+			{
+				vertex_t *vt = sub->firstline[vi[x]].v1;
+				qd.Set(x, vt->fX(), plane.plane.ZatPoint(vt) + dz, vt->fY(), vt->fX() / 64.f, -vt->fY() / 64.f);
+			}
+			qd.Render(GL_TRIANGLE_FAN);
 		}
 	}
 
diff --git a/src/gl/scene/gl_sprite.cpp b/src/gl/scene/gl_sprite.cpp
index ab88f2f9c..e9748fba6 100644
--- a/src/gl/scene/gl_sprite.cpp
+++ b/src/gl/scene/gl_sprite.cpp
@@ -138,10 +138,10 @@ void GLSprite::CalculateVertices(FVector3 *v)
 			mat.Rotate(0, 1, 0, - actor->Angles.Roll.Degrees);
 			mat.Translate(-x, -z, -y);
 		}
-		v[0] = mat * FVector3(x1, z, y2);
-		v[1] = mat * FVector3(x2, z, y2);
-		v[2] = mat * FVector3(x1, z, y1);
-		v[3] = mat * FVector3(x2, z, y1);
+		v[0] = mat * FVector3(x2, z, y2);
+		v[1] = mat * FVector3(x1, z, y2);
+		v[2] = mat * FVector3(x2, z, y1);
+		v[3] = mat * FVector3(x1, z, y1);
 		return;
 	}
 	
diff --git a/src/gl/scene/gl_weapon.cpp b/src/gl/scene/gl_weapon.cpp
index cfa5c05d7..a2a12f947 100644
--- a/src/gl/scene/gl_weapon.cpp
+++ b/src/gl/scene/gl_weapon.cpp
@@ -129,19 +129,20 @@ void FGLRenderer::DrawPSprite (player_t * player,DPSprite *psp, float sx, float
 	y1 = viewwindowy + vh / 2 - (ftexturemid * scale);
 	y2 = y1 + (r.height * scale) + 1;
 
-	if (!mirror)
+	if (!(mirror) != !(psp->Flags & PSPF_FLIP))
 	{
-		fU1=tex->GetSpriteUL();
-		fV1=tex->GetSpriteVT();
-		fU2=tex->GetSpriteUR();
-		fV2=tex->GetSpriteVB();
+		fU2 = tex->GetSpriteUL();
+		fV1 = tex->GetSpriteVT();
+		fU1 = tex->GetSpriteUR();
+		fV2 = tex->GetSpriteVB();
 	}
 	else
 	{
-		fU2=tex->GetSpriteUL();
-		fV1=tex->GetSpriteVT();
-		fU1=tex->GetSpriteUR();
-		fV2=tex->GetSpriteVB();
+		fU1 = tex->GetSpriteUL();
+		fV1 = tex->GetSpriteVT();
+		fU2 = tex->GetSpriteUR();
+		fV2 = tex->GetSpriteVB();
+		
 	}
 
 	if (tex->GetTransparent() || OverrideShader != -1)
diff --git a/src/gl/shaders/gl_present3dRowshader.cpp b/src/gl/shaders/gl_present3dRowshader.cpp
index 607e4ede6..d8369a3b6 100644
--- a/src/gl/shaders/gl_present3dRowshader.cpp
+++ b/src/gl/shaders/gl_present3dRowshader.cpp
@@ -38,23 +38,37 @@
 #include "gl/system/gl_cvars.h"
 #include "gl/shaders/gl_present3dRowshader.h"
 
+void FPresentStereoShaderBase::Init(const char * vtx_shader_name, const char * program_name)
+{
+	FPresentShaderBase::Init(vtx_shader_name, program_name);
+	LeftEyeTexture.Init(mShader, "LeftEyeTexture");
+	RightEyeTexture.Init(mShader, "RightEyeTexture");
+	WindowPositionParity.Init(mShader, "WindowPositionParity");
+}
+
+void FPresent3DCheckerShader::Bind()
+{
+	if (!mShader)
+	{
+		Init("shaders/glsl/present_checker3d.fp", "shaders/glsl/presentChecker3d");
+	}
+	mShader.Bind();
+}
+
+void FPresent3DColumnShader::Bind()
+{
+	if (!mShader)
+	{
+		Init("shaders/glsl/present_column3d.fp", "shaders/glsl/presentColumn3d");
+	}
+	mShader.Bind();
+}
+
 void FPresent3DRowShader::Bind()
 {
 	if (!mShader)
 	{
-		mShader.Compile(FShaderProgram::Vertex, "shaders/glsl/screenquadscale.vp", "", 330);
-		mShader.Compile(FShaderProgram::Fragment, "shaders/glsl/present_row3d.fp", "", 330);
-		mShader.SetFragDataLocation(0, "FragColor");
-		mShader.Link("shaders/glsl/presentRow3d");
-		mShader.SetAttribLocation(0, "PositionInProjection");
-		mShader.SetAttribLocation(1, "UV");
-		LeftEyeTexture.Init(mShader, "LeftEyeTexture");
-		RightEyeTexture.Init(mShader, "RightEyeTexture");
-		InvGamma.Init(mShader, "InvGamma");
-		Contrast.Init(mShader, "Contrast");
-		Brightness.Init(mShader, "Brightness");
-		Scale.Init(mShader, "UVScale");
-		VerticalPixelOffset.Init(mShader, "VerticalPixelOffset");
+		Init("shaders/glsl/present_row3d.fp", "shaders/glsl/presentRow3d");
 	}
 	mShader.Bind();
 }
diff --git a/src/gl/shaders/gl_present3dRowshader.h b/src/gl/shaders/gl_present3dRowshader.h
index 8e3bd772d..10419d7f5 100644
--- a/src/gl/shaders/gl_present3dRowshader.h
+++ b/src/gl/shaders/gl_present3dRowshader.h
@@ -29,22 +29,35 @@
 #define GL_PRESENT3DROWSHADER_H_
 
 #include "gl_shaderprogram.h"
+#include "gl_presentshader.h"
 
-class FPresent3DRowShader
+class FPresentStereoShaderBase : public FPresentShaderBase
 {
 public:
-	void Bind();
-
 	FBufferedUniformSampler LeftEyeTexture;
 	FBufferedUniformSampler RightEyeTexture;
-	FBufferedUniform1f InvGamma;
-	FBufferedUniform1f Contrast;
-	FBufferedUniform1f Brightness;
-	FBufferedUniform2f Scale;
-	FBufferedUniform1i VerticalPixelOffset;
+	FBufferedUniform1i WindowPositionParity;
 
-private:
-	FShaderProgram mShader;
+protected:
+	void Init(const char * vtx_shader_name, const char * program_name) override;
+};
+
+class FPresent3DCheckerShader : public FPresentStereoShaderBase
+{
+public:
+	void Bind() override;
+};
+
+class FPresent3DColumnShader : public FPresentStereoShaderBase
+{
+public:
+	void Bind() override;
+};
+
+class FPresent3DRowShader : public FPresentStereoShaderBase
+{
+public:
+	void Bind() override;
 };
 
 // GL_PRESENT3DROWSHADER_H_
diff --git a/src/gl/shaders/gl_presentshader.cpp b/src/gl/shaders/gl_presentshader.cpp
index 39d20d0fc..15acc74fb 100644
--- a/src/gl/shaders/gl_presentshader.cpp
+++ b/src/gl/shaders/gl_presentshader.cpp
@@ -36,21 +36,26 @@
 #include "gl/system/gl_cvars.h"
 #include "gl/shaders/gl_presentshader.h"
 
+void FPresentShaderBase::Init(const char * vtx_shader_name, const char * program_name)
+{
+	mShader.Compile(FShaderProgram::Vertex, "shaders/glsl/screenquadscale.vp", "", 330);
+	mShader.Compile(FShaderProgram::Fragment, vtx_shader_name, "", 330);
+	mShader.SetFragDataLocation(0, "FragColor");
+	mShader.Link(program_name);
+	mShader.SetAttribLocation(0, "PositionInProjection");
+	mShader.SetAttribLocation(1, "UV");
+	InvGamma.Init(mShader, "InvGamma");
+	Contrast.Init(mShader, "Contrast");
+	Brightness.Init(mShader, "Brightness");
+	Scale.Init(mShader, "UVScale");
+}
+
 void FPresentShader::Bind()
 {
 	if (!mShader)
 	{
-		mShader.Compile(FShaderProgram::Vertex, "shaders/glsl/screenquadscale.vp", "", 330);
-		mShader.Compile(FShaderProgram::Fragment, "shaders/glsl/present.fp", "", 330);
-		mShader.SetFragDataLocation(0, "FragColor");
-		mShader.Link("shaders/glsl/present");
-		mShader.SetAttribLocation(0, "PositionInProjection");
-		mShader.SetAttribLocation(1, "UV");
+		Init("shaders/glsl/present.fp", "shaders/glsl/present");
 		InputTexture.Init(mShader, "InputTexture");
-		InvGamma.Init(mShader, "InvGamma");
-		Contrast.Init(mShader, "Contrast");
-		Brightness.Init(mShader, "Brightness");
-		Scale.Init(mShader, "UVScale");
 	}
 	mShader.Bind();
 }
diff --git a/src/gl/shaders/gl_presentshader.h b/src/gl/shaders/gl_presentshader.h
index 6f4e899bb..6f9e1bfd7 100644
--- a/src/gl/shaders/gl_presentshader.h
+++ b/src/gl/shaders/gl_presentshader.h
@@ -3,19 +3,27 @@
 
 #include "gl_shaderprogram.h"
 
-class FPresentShader
+class FPresentShaderBase
 {
 public:
-	void Bind();
+	virtual void Bind() = 0;
 
-	FBufferedUniformSampler InputTexture;
 	FBufferedUniform1f InvGamma;
 	FBufferedUniform1f Contrast;
 	FBufferedUniform1f Brightness;
 	FBufferedUniform2f Scale;
 
-private:
+protected:
+	virtual void Init(const char * vtx_shader_name, const char * program_name);
 	FShaderProgram mShader;
 };
 
+class FPresentShader : public FPresentShaderBase
+{
+public:
+	void Bind() override;
+
+	FBufferedUniformSampler InputTexture;
+};
+
 #endif
\ No newline at end of file
diff --git a/src/gl/stereo3d/gl_interleaved3d.cpp b/src/gl/stereo3d/gl_interleaved3d.cpp
index 329dec953..029d233e4 100644
--- a/src/gl/stereo3d/gl_interleaved3d.cpp
+++ b/src/gl/stereo3d/gl_interleaved3d.cpp
@@ -43,10 +43,25 @@
 EXTERN_CVAR(Float, vid_brightness)
 EXTERN_CVAR(Float, vid_contrast)
 EXTERN_CVAR(Bool, fullscreen)
-EXTERN_CVAR(Int, win_y) // pixel position of top of display window
+EXTERN_CVAR(Int, win_x) // screen pixel position of left of display window
+EXTERN_CVAR(Int, win_y) // screen pixel position of top of display window
 
 namespace s3d {
 
+/* static */
+const CheckerInterleaved3D& CheckerInterleaved3D::getInstance(float ipd)
+{
+	static CheckerInterleaved3D instance(ipd);
+	return instance;
+}
+
+/* static */
+const ColumnInterleaved3D& ColumnInterleaved3D::getInstance(float ipd)
+{
+	static ColumnInterleaved3D instance(ipd);
+	return instance;
+}
+
 /* static */
 const RowInterleaved3D& RowInterleaved3D::getInstance(float ipd)
 {
@@ -54,11 +69,7 @@ const RowInterleaved3D& RowInterleaved3D::getInstance(float ipd)
 	return instance;
 }
 
-RowInterleaved3D::RowInterleaved3D(double ipdMeters) 
-	: TopBottom3D(ipdMeters)
-{}
-
-void RowInterleaved3D::Present() const
+static void prepareInterleavedPresent(FPresentStereoShaderBase& shader)
 {
 	GLRenderer->mBuffers->BindOutputFB();
 	GLRenderer->ClearBorders();
@@ -79,39 +90,118 @@ void RowInterleaved3D::Present() const
 	const GL_IRECT& box = GLRenderer->mOutputLetterbox;
 	glViewport(box.left, box.top, box.width, box.height);
 
-	bool applyGamma = true;
+	shader.Bind();
+	shader.LeftEyeTexture.Set(0);
+	shader.RightEyeTexture.Set(1);
 
-	GLRenderer->mPresent3dRowShader->Bind();
-	GLRenderer->mPresent3dRowShader->LeftEyeTexture.Set(0);
-	GLRenderer->mPresent3dRowShader->RightEyeTexture.Set(1);
-
-	if (!applyGamma || GLRenderer->framebuffer->IsHWGammaActive())
+	if ( GLRenderer->framebuffer->IsHWGammaActive() )
 	{
-		GLRenderer->mPresent3dRowShader->InvGamma.Set(1.0f);
-		GLRenderer->mPresent3dRowShader->Contrast.Set(1.0f);
-		GLRenderer->mPresent3dRowShader->Brightness.Set(0.0f);
+		shader.InvGamma.Set(1.0f);
+		shader.Contrast.Set(1.0f);
+		shader.Brightness.Set(0.0f);
 	}
 	else
 	{
-		GLRenderer->mPresent3dRowShader->InvGamma.Set(1.0f / clamp<float>(Gamma, 0.1f, 4.f));
-		GLRenderer->mPresent3dRowShader->Contrast.Set(clamp<float>(vid_contrast, 0.1f, 3.f));
-		GLRenderer->mPresent3dRowShader->Brightness.Set(clamp<float>(vid_brightness, -0.8f, 0.8f));
+		shader.InvGamma.Set(1.0f / clamp<float>(Gamma, 0.1f, 4.f));
+		shader.Contrast.Set(clamp<float>(vid_contrast, 0.1f, 3.f));
+		shader.Brightness.Set(clamp<float>(vid_brightness, -0.8f, 0.8f));
 	}
-	GLRenderer->mPresent3dRowShader->Scale.Set(
+	shader.Scale.Set(
 		GLRenderer->mScreenViewport.width / (float)GLRenderer->mBuffers->GetWidth(),
 		GLRenderer->mScreenViewport.height / (float)GLRenderer->mBuffers->GetHeight());
+}
+
+// fixme: I don't know how to get absolute window position on Mac and Linux
+// fixme: I don't know how to get window border decoration size anywhere
+// So for now I'll hard code the border effect on my test machine.
+// Workaround for others is to fuss with vr_swap_eyes CVAR until it looks right.
+// Presumably the top/left window border on my test machine has an odd number of pixels
+//  in the horizontal direction, and an even number in the vertical direction.
+#define WINDOW_BORDER_HORIZONTAL_PARITY 1
+#define WINDOW_BORDER_VERTICAL_PARITY 0
+
+void CheckerInterleaved3D::Present() const
+{
+	prepareInterleavedPresent(*GLRenderer->mPresent3dCheckerShader);
 
 	// Compute absolute offset from top of screen to top of current display window
 	// because we need screen-relative, not window-relative, scan line parity
 	int windowVOffset = 0;
+	int windowHOffset = 0;
+
+#ifdef _WIN32
+	if (!fullscreen) {
+		I_SaveWindowedPos(); // update win_y CVAR
+		windowHOffset = (win_x + WINDOW_BORDER_HORIZONTAL_PARITY) % 2;
+		windowVOffset = (win_y + WINDOW_BORDER_VERTICAL_PARITY) % 2;
+	}
+#endif // _WIN32
+
+	GLRenderer->mPresent3dCheckerShader->WindowPositionParity.Set(
+		(windowVOffset
+			+ windowHOffset
+			+ GLRenderer->mOutputLetterbox.height + 1 // +1 because of origin at bottom
+		) % 2 // because we want the top pixel offset, but gl_FragCoord.y is the bottom pixel offset
+	);
+
+	GLRenderer->RenderScreenQuad();
+}
+
+void s3d::CheckerInterleaved3D::AdjustViewports() const
+{
+	// decrease the total pixel count by 2, but keep the same aspect ratio
+	const float sqrt2 = 1.41421356237f;
+	// Change size of renderbuffer, and align to screen
+	GLRenderer->mSceneViewport.height /= sqrt2;
+	GLRenderer->mSceneViewport.top /= sqrt2;
+	GLRenderer->mSceneViewport.width /= sqrt2;
+	GLRenderer->mSceneViewport.left /= sqrt2;
+
+	GLRenderer->mScreenViewport.height /= sqrt2;
+	GLRenderer->mScreenViewport.top /= sqrt2;
+	GLRenderer->mScreenViewport.width /= sqrt2;
+	GLRenderer->mScreenViewport.left /= sqrt2;
+}
+
+void ColumnInterleaved3D::Present() const
+{
+	prepareInterleavedPresent(*GLRenderer->mPresent3dColumnShader);
+
+	// Compute absolute offset from top of screen to top of current display window
+	// because we need screen-relative, not window-relative, scan line parity
+	int windowHOffset = 0;
+
+#ifdef _WIN32
+	if (!fullscreen) {
+		I_SaveWindowedPos(); // update win_y CVAR
+		windowHOffset = (win_x + WINDOW_BORDER_HORIZONTAL_PARITY) % 2;
+	}
+#endif // _WIN32
+
+	GLRenderer->mPresent3dColumnShader->WindowPositionParity.Set(windowHOffset);
+
+	GLRenderer->RenderScreenQuad();
+}
+
+void RowInterleaved3D::Present() const
+{
+	prepareInterleavedPresent(*GLRenderer->mPresent3dRowShader);
+
+	// Compute absolute offset from top of screen to top of current display window
+	// because we need screen-relative, not window-relative, scan line parity
+	int windowVOffset = 0;
+	
+#ifdef _WIN32
 	if (! fullscreen) {
 		I_SaveWindowedPos(); // update win_y CVAR
-		windowVOffset = win_y;
+		windowVOffset = (win_y + WINDOW_BORDER_VERTICAL_PARITY) % 2;
 	}
+#endif // _WIN32
 
-	GLRenderer->mPresent3dRowShader->VerticalPixelOffset.Set(
-		windowVOffset // fixme: vary with window location
-		+ box.height % 2 // because we want the top pixel offset, but gl_FragCoord.y is the bottom pixel offset
+	GLRenderer->mPresent3dRowShader->WindowPositionParity.Set(
+		(windowVOffset
+			+ GLRenderer->mOutputLetterbox.height + 1 // +1 because of origin at bottom
+		) % 2
 	);
 
 	GLRenderer->RenderScreenQuad();
diff --git a/src/gl/stereo3d/gl_interleaved3d.h b/src/gl/stereo3d/gl_interleaved3d.h
index 30aef7d5a..78bd19483 100644
--- a/src/gl/stereo3d/gl_interleaved3d.h
+++ b/src/gl/stereo3d/gl_interleaved3d.h
@@ -42,15 +42,30 @@
 #include "gl/system/gl_system.h"
 #include "gl/renderer/gl_renderstate.h"
 
-class FPresent3DRowShader;
-
 namespace s3d {
 
+class CheckerInterleaved3D : public SideBySideSquished
+{
+public:
+	static const CheckerInterleaved3D& getInstance(float ipd);
+	CheckerInterleaved3D(double ipdMeters) : SideBySideSquished(ipdMeters) {}
+	void Present() const override;
+	void AdjustViewports() const override;
+};
+
+class ColumnInterleaved3D : public SideBySideSquished
+{
+public:
+	static const ColumnInterleaved3D& getInstance(float ipd);
+	ColumnInterleaved3D(double ipdMeters) : SideBySideSquished(ipdMeters) {}
+	void Present() const override;
+};
+
 class RowInterleaved3D : public TopBottom3D
 {
 public:
 	static const RowInterleaved3D& getInstance(float ipd);
-	RowInterleaved3D(double ipdMeters);
+	RowInterleaved3D(double ipdMeters) : TopBottom3D(ipdMeters) {}
 	void Present() const override;
 };
 
diff --git a/src/gl/stereo3d/gl_stereo_cvars.cpp b/src/gl/stereo3d/gl_stereo_cvars.cpp
index 16296393d..fe5ae3261 100644
--- a/src/gl/stereo3d/gl_stereo_cvars.cpp
+++ b/src/gl/stereo3d/gl_stereo_cvars.cpp
@@ -107,6 +107,12 @@ const Stereo3DMode& Stereo3DMode::getCurrentMode()
 	case 12:
 		setCurrentMode(RowInterleaved3D::getInstance(vr_ipd));
 		break;
+	case 13:
+		setCurrentMode(ColumnInterleaved3D::getInstance(vr_ipd));
+		break;
+	case 14:
+		setCurrentMode(CheckerInterleaved3D::getInstance(vr_ipd));
+		break;
 	case 0:
 	default:
 		setCurrentMode(MonoView::getInstance());
diff --git a/src/p_acs.cpp b/src/p_acs.cpp
index 0a2ccddac..71ef65f22 100644
--- a/src/p_acs.cpp
+++ b/src/p_acs.cpp
@@ -5821,43 +5821,54 @@ doplaysound:			if (funcIndex == ACSF_PlayActorSound)
 		// [ZK] A_Warp in ACS
 		case ACSF_Warp:
 		{
-			int tid_dest = args[0];
-			int xofs = args[1];
-			int yofs = args[2];
-			int zofs = args[3];
-			int angle = args[4];
-			int flags = args[5];
-			const char *statename = argCount > 6 ? FBehavior::StaticLookupString(args[6]) : "";
-			bool exact = argCount > 7 ? !!args[7] : false;
-			int heightoffset = argCount > 8 ? args[8] : 0;
-			int radiusoffset = argCount > 9 ? args[9] : 0;
-			int pitch = argCount > 10 ? args[10] : 0;
-
-			FState *state = argCount > 6 ? activator->GetClass()->FindStateByString(statename, exact) : 0;
-
-			AActor *reference;
-			if((flags & WARPF_USEPTR) && tid_dest != AAPTR_DEFAULT)
+			if (nullptr == activator)
 			{
-				reference = COPY_AAPTR(activator, tid_dest);
-			}
-			else
-			{
-				reference = SingleActorFromTID(tid_dest, activator);
-			}
-
-			// If there is no activator or actor to warp to, fail.
-			if (activator == NULL || !reference)
 				return false;
-
-			if (P_Thing_Warp(activator, reference, ACSToDouble(xofs), ACSToDouble(yofs), ACSToDouble(zofs), ACSToAngle(angle), flags, ACSToDouble(heightoffset), ACSToDouble(radiusoffset), ACSToAngle(pitch)))
-			{
-				if (state && argCount > 6)
-				{
-					activator->SetState(state);
-				}
-				return true;
 			}
-			return false;
+			
+			const int dest = args[0];
+			const int flags = args[5];
+			
+			AActor* const reference = ((flags & WARPF_USEPTR) && (AAPTR_DEFAULT != dest))
+				? COPY_AAPTR(activator, dest)
+				: SingleActorFromTID(dest, activator);
+
+			if (nullptr == reference)
+			{
+				// there is no actor to warp to
+				return false;
+			}
+			
+			const double xofs = ACSToDouble(args[1]);
+			const double yofs = ACSToDouble(args[2]);
+			const double zofs = ACSToDouble(args[3]);
+			const DAngle angle = ACSToAngle(args[4]);
+			const double heightoffset = argCount > 8 ? ACSToDouble(args[8]) : 0.0;
+			const double radiusoffset = argCount > 9 ? ACSToDouble(args[9]) : 0.0;
+			const DAngle pitch = ACSToAngle(argCount > 10 ? args[10] : 0);
+
+			if (!P_Thing_Warp(activator, reference, xofs, yofs, zofs, angle, flags, heightoffset, radiusoffset, pitch))
+			{
+				return false;
+			}
+			
+			if (argCount > 6)
+			{
+				const char* const statename = FBehavior::StaticLookupString(args[6]);
+				
+				if (nullptr != statename)
+				{
+					const bool exact = argCount > 7 && !!args[7];
+					FState* const state = activator->GetClass()->FindStateByString(statename, exact);
+					
+					if (nullptr != state)
+					{
+						activator->SetState(state);
+					}
+				}
+			}
+			
+			return true;
 		}
 		case ACSF_GetMaxInventory:
 			actor = SingleActorFromTID(args[0], activator);
diff --git a/src/p_ceiling.cpp b/src/p_ceiling.cpp
index 63de657e0..182533013 100644
--- a/src/p_ceiling.cpp
+++ b/src/p_ceiling.cpp
@@ -215,6 +215,12 @@ DCeiling::DCeiling (sector_t *sec, double speed1, double speed2, int silent)
 	m_Speed = m_Speed1 = speed1;
 	m_Speed2 = speed2;
 	m_Silent = silent;
+	m_BottomHeight = 0;
+	m_TopHeight = 0;
+	m_Direction = 0;
+	m_Texture = FNullTextureID();
+	m_Tag = 0;
+	m_OldDirection = 0;
 }
 
 //============================================================================
diff --git a/src/p_mobj.cpp b/src/p_mobj.cpp
index 7e6b2839d..3a0eb16a5 100644
--- a/src/p_mobj.cpp
+++ b/src/p_mobj.cpp
@@ -635,6 +635,7 @@ void AActor::RemoveInventory(AInventory *item)
 
 bool AActor::TakeInventory(PClassActor *itemclass, int amount, bool fromdecorate, bool notakeinfinite)
 {
+	amount = abs(amount);
 	AInventory *item = FindInventory(itemclass);
 
 	if (item == NULL)
@@ -667,6 +668,7 @@ bool AActor::TakeInventory(PClassActor *itemclass, int amount, bool fromdecorate
 		item->IsKindOf(RUNTIME_CLASS(AAmmo)))
 	{
 		// Nothing to do here, except maybe res = false;? Would it make sense?
+		result = false;
 	}
 	else if (!amount || amount>=item->Amount)
 	{
@@ -686,11 +688,30 @@ bool AActor::TakeInventory(PClassActor *itemclass, int amount, bool fromdecorate
 
 void AActor::DestroyAllInventory ()
 {
-	while (Inventory != NULL)
+	AInventory *inv = Inventory;
+	if (inv != nullptr)
 	{
-		AInventory *item = Inventory;
-		item->Destroy ();
-		assert (item != Inventory);
+		TArray<AInventory *> toDelete;
+
+		// Delete the list in a two stage approach.
+		// This is necessary because an item may destroy another item (e.g. sister weapons)
+		// which would break the list and leave parts of it undestroyed, maybe doing bad things later.
+		while (inv != nullptr)
+		{
+			toDelete.Push(inv);
+			AInventory *item = inv->Inventory;
+			inv->Inventory = nullptr;
+			inv->Owner = nullptr;
+			inv = item;
+		}
+		for (auto p : toDelete)
+		{
+			// the item may already have been deleted by another one, so check this here to avoid problems.
+			if (!(p->ObjectFlags & OF_EuthanizeMe))
+			{
+				p->Destroy();
+			}
+		}
 	}
 }
 
diff --git a/src/p_pspr.cpp b/src/p_pspr.cpp
index fab7e38d8..e5b076acd 100644
--- a/src/p_pspr.cpp
+++ b/src/p_pspr.cpp
@@ -1006,7 +1006,7 @@ DEFINE_ACTION_FUNCTION(AActor, A_OverlayOffset)
 	PARAM_FLOAT_OPT(wx)		{ wx = 0.; }
 	PARAM_FLOAT_OPT(wy)		{ wy = 32.; }
 	PARAM_INT_OPT(flags)	{ flags = 0; }
-	A_OverlayOffset(self, layer, wx, wy, flags);
+	A_OverlayOffset(self, ((layer != 0) ? layer : stateinfo->mPSPIndex), wx, wy, flags);
 	return 0;
 }
 
@@ -1033,10 +1033,10 @@ DEFINE_ACTION_FUNCTION(AActor, A_OverlayFlags)
 	PARAM_INT(flags);
 	PARAM_BOOL(set);
 
-	if (self->player == nullptr)
+	if (!ACTION_CALL_FROM_PSPRITE())
 		return 0;
 
-	DPSprite *pspr = self->player->FindPSprite(layer);
+	DPSprite *pspr = self->player->FindPSprite(((layer != 0) ? layer : stateinfo->mPSPIndex));
 
 	if (pspr == nullptr)
 		return 0;
@@ -1049,6 +1049,52 @@ DEFINE_ACTION_FUNCTION(AActor, A_OverlayFlags)
 	return 0;
 }
 
+//---------------------------------------------------------------------------
+//
+// PROC OverlayX/Y
+// Action function to return the X/Y of an overlay.
+//---------------------------------------------------------------------------
+
+static double GetOverlayPosition(AActor *self, int layer, bool gety)
+{
+	if (layer)
+	{
+		DPSprite *pspr = self->player->FindPSprite(layer);
+
+		if (pspr != nullptr)
+		{
+			return gety ? (pspr->y) : (pspr->x);
+		}
+	}
+	return 0.;
+}
+
+DEFINE_ACTION_FUNCTION(AActor, OverlayX)
+{
+	PARAM_ACTION_PROLOGUE;
+	PARAM_INT_OPT(layer) { layer = 0; }
+
+	if (ACTION_CALL_FROM_PSPRITE())
+	{
+		double res = GetOverlayPosition(self, ((layer != 0) ? layer : stateinfo->mPSPIndex), false);
+		ACTION_RETURN_FLOAT(res);	
+	}
+	ACTION_RETURN_FLOAT(0.);
+}
+
+DEFINE_ACTION_FUNCTION(AActor, OverlayY)
+{
+	PARAM_ACTION_PROLOGUE;
+	PARAM_INT_OPT(layer) { layer = 0; }
+
+	if (ACTION_CALL_FROM_PSPRITE())
+	{
+		double res = GetOverlayPosition(self, ((layer != 0) ? layer : stateinfo->mPSPIndex), true);
+		ACTION_RETURN_FLOAT(res);
+	}
+	ACTION_RETURN_FLOAT(0.);
+}
+
 //---------------------------------------------------------------------------
 //
 // PROC OverlayID
diff --git a/src/p_pspr.h b/src/p_pspr.h
index 2735e0e01..8c2901050 100644
--- a/src/p_pspr.h
+++ b/src/p_pspr.h
@@ -58,6 +58,7 @@ enum PSPFlags
 	PSPF_ADDBOB		= 1 << 1,
 	PSPF_POWDOUBLE	= 1 << 2,
 	PSPF_CVARFAST	= 1 << 3,
+	PSPF_FLIP		= 1 << 6,
 };
 
 class DPSprite : public DObject
diff --git a/src/posix/cocoa/i_main.mm b/src/posix/cocoa/i_main.mm
index bbe344809..d8f0e74cf 100644
--- a/src/posix/cocoa/i_main.mm
+++ b/src/posix/cocoa/i_main.mm
@@ -130,6 +130,53 @@ void Mac_I_FatalError(const char* const message)
 }
 
 
+static void I_DetectOS()
+{
+	SInt32 majorVersion = 0;
+	Gestalt(gestaltSystemVersionMajor, &majorVersion);
+	
+	SInt32 minorVersion = 0;
+	Gestalt(gestaltSystemVersionMinor, &minorVersion);
+	
+	SInt32 bugFixVersion = 0;
+	Gestalt(gestaltSystemVersionBugFix, &bugFixVersion);
+	
+	const char* name = "Unknown version";
+	
+	if (10 == majorVersion) switch (minorVersion)
+	{
+		case  4: name = "Mac OS X Tiger";        break;
+		case  5: name = "Mac OS X Leopard";      break;
+		case  6: name = "Mac OS X Snow Leopard"; break;
+		case  7: name = "Mac OS X Lion";         break;
+		case  8: name = "OS X Mountain Lion";    break;
+		case  9: name = "OS X Mavericks";        break;
+		case 10: name = "OS X Yosemite";         break;
+		case 11: name = "OS X El Capitan";       break;
+		case 12: name = "macOS Sierra";          break;
+	}
+
+	char release[16] = {};
+	size_t size = sizeof release - 1;
+	sysctlbyname("kern.osversion", release, &size, nullptr, 0);
+	
+	const char* const architecture =
+#ifdef __i386__
+		"32-bit Intel";
+#elif defined __x86_64__
+		"64-bit Intel";
+#elif defined __ppc__
+		"32-bit PowerPC";
+#elif defined __ppc64__
+		"64-bit PowerPC";
+#else
+		"Unknown";
+#endif
+	
+	Printf("OS: %s %d.%d.%d (%s) %s\n", name, majorVersion, minorVersion, bugFixVersion, release, architecture);
+}
+
+
 DArgs* Args; // command line arguments
 
 
@@ -165,6 +212,8 @@ void OriginalMainTry(int argc, char** argv)
 	progdir += "/";
 
 	C_InitConsole(80 * 8, 25 * 8, false);
+	
+	I_DetectOS();
 	D_DoomMain();
 }
 
diff --git a/src/sound/music_hmi_midiout.cpp b/src/sound/music_hmi_midiout.cpp
index 97437a6c8..f2ae75d19 100644
--- a/src/sound/music_hmi_midiout.cpp
+++ b/src/sound/music_hmi_midiout.cpp
@@ -653,7 +653,7 @@ DWORD *HMISong::SendCommand (DWORD *events, TrackInfo *track, DWORD delay, ptrdi
 		if (event == MIDI_SYSEX || event == MIDI_SYSEXEND)
 		{
 			len = ReadVarLen(track);
-			if (len >= (MAX_EVENTS-1)*3*4)
+			if (len >= (MAX_EVENTS-1)*3*4 || DeviceType == MDEV_SNDSYS)
 			{ // This message will never fit. Throw it away.
 				track->TrackP += len;
 			}
diff --git a/src/sound/music_smf_midiout.cpp b/src/sound/music_smf_midiout.cpp
index 2557dc725..dfd2c5a65 100644
--- a/src/sound/music_smf_midiout.cpp
+++ b/src/sound/music_smf_midiout.cpp
@@ -598,7 +598,7 @@ DWORD *MIDISong2::SendCommand (DWORD *events, TrackInfo *track, DWORD delay, ptr
 		if (event == MIDI_SYSEX || event == MIDI_SYSEXEND)
 		{
 			len = track->ReadVarLen();
-			if (len >= (MAX_EVENTS-1)*3*4)
+			if (len >= (MAX_EVENTS-1)*3*4 || DeviceType == MDEV_SNDSYS)
 			{ // This message will never fit. Throw it away.
 				track->TrackP += len;
 			}
diff --git a/src/sound/music_xmi_midiout.cpp b/src/sound/music_xmi_midiout.cpp
index 47e60e505..25fa235fd 100644
--- a/src/sound/music_xmi_midiout.cpp
+++ b/src/sound/music_xmi_midiout.cpp
@@ -528,7 +528,7 @@ DWORD *XMISong::SendCommand (DWORD *events, EventSource due, DWORD delay, ptrdif
 		if (event == MIDI_SYSEX || event == MIDI_SYSEXEND)
 		{
 			len = track->ReadVarLen();
-			if (len >= (MAX_EVENTS-1)*3*4)
+			if (len >= (MAX_EVENTS-1)*3*4 || DeviceType == MDEV_SNDSYS)
 			{ // This message will never fit. Throw it away.
 				track->EventP += len;
 			}
diff --git a/src/thingdef/olddecorations.cpp b/src/thingdef/olddecorations.cpp
index 3ea0c4788..9afbfc965 100644
--- a/src/thingdef/olddecorations.cpp
+++ b/src/thingdef/olddecorations.cpp
@@ -140,6 +140,7 @@ DEFINE_CLASS_PROPERTY(respawns, 0, FakeInventory)
 // Reads an old style decoration object
 //
 //==========================================================================
+PClassActor *DecoDerivedClass(const FScriptPosition &sc, PClassActor *parent, FName typeName);
 
 void ParseOldDecoration(FScanner &sc, EDefinitionType def)
 {
@@ -154,7 +155,7 @@ void ParseOldDecoration(FScanner &sc, EDefinitionType def)
 
 	sc.MustGetString();
 	typeName = FName(sc.String);
-	type = static_cast<PClassActor *>(parent->CreateDerivedClass (typeName, parent->Size));
+	type = DecoDerivedClass(FScriptPosition(sc), parent, typeName);
 	ResetBaggage(&bag, parent);
 	bag.Info = type;
 #ifdef _DEBUG
diff --git a/src/thingdef/thingdef.cpp b/src/thingdef/thingdef.cpp
index 32d4f1ce0..9af031b04 100644
--- a/src/thingdef/thingdef.cpp
+++ b/src/thingdef/thingdef.cpp
@@ -68,13 +68,44 @@
 
 TDeletingArray<class FxExpression *> ActorDamageFuncs;
 
+
 // EXTERNAL FUNCTION PROTOTYPES --------------------------------------------
 void InitThingdef();
-void ParseDecorate (FScanner &sc);
+void ParseDecorate(FScanner &ctx);
 
 // STATIC FUNCTION PROTOTYPES --------------------------------------------
 PClassActor *QuestItemClasses[31];
 
+EXTERN_CVAR(Bool, strictdecorate);
+
+PClassActor *DecoDerivedClass(const FScriptPosition &sc, PClassActor *parent, FName typeName)
+{
+	PClassActor *type = static_cast<PClassActor *>(parent->CreateDerivedClass(typeName, parent->Size));
+	if (type == nullptr)
+	{
+		FString newname = typeName.GetChars();
+		FString sourcefile = sc.FileName;
+
+		sourcefile.Substitute(":", "@");
+		newname << '@' << sourcefile;
+		if (strictdecorate)
+		{
+			sc.Message(MSG_ERROR, "Tried to define class '%s' more than once.", typeName.GetChars());
+		}
+		else
+		{
+			// Due to backwards compatibility issues this cannot be an unconditional error.
+			sc.Message(MSG_WARNING, "Tried to define class '%s' more than once. Renaming class to '%s'", typeName.GetChars(), newname.GetChars());
+		}
+		type = static_cast<PClassActor *>(parent->CreateDerivedClass(newname, parent->Size));
+		if (type == nullptr)
+		{
+			// This we cannot handle cleanly anymore. Let's just abort and forget about the odd mod out that was this careless.
+			sc.Message(MSG_FATAL, "Tried to define class '%s' more than twice in the same file.", typeName.GetChars());
+		}
+	}
+	return type;
+}
 //==========================================================================
 //
 // Starts a new actor definition
@@ -141,7 +172,7 @@ PClassActor *CreateNewActor(const FScriptPosition &sc, FName typeName, FName par
 	else
 	{
 	create:
-		ti = static_cast<PClassActor *>(parent->CreateDerivedClass (typeName, parent->Size));
+		ti = DecoDerivedClass(sc, parent, typeName);
 	}
 
 	ti->Replacee = ti->Replacement = NULL;
@@ -428,6 +459,16 @@ void LoadActors ()
 	while ((lump = Wads.FindLump ("DECORATE", &lastlump)) != -1)
 	{
 		FScanner sc(lump);
+
+		if (Wads.GetLumpFile(lump) == 0)
+		{
+			// define namespace 'zdoom'
+		}
+		else
+		{
+			// use namespace 'zdoom'
+		}
+
 		ParseDecorate (sc);
 	}
 	FinishThingdef();
@@ -439,32 +480,3 @@ void LoadActors ()
 	if (!batchrun) Printf("DECORATE parsing took %.2f ms\n", timer.TimeMS());
 	// Base time: ~52 ms
 }
-
-
-//==========================================================================
-//
-// CreateDamageFunction
-//
-// Creates a damage function suitable for a constant, non-expressioned
-// value.
-//
-//==========================================================================
-
-VMScriptFunction *CreateDamageFunction(int dmg)
-{
-	if (dmg == 0)
-	{
-		// For zero damage, do not create a function so that the special collision detection case still works as before.
-		return NULL;
-	}
-	else
-	{
-		VMFunctionBuilder build;
-		build.Registers[REGT_POINTER].Get(1);		// The self pointer
-		build.EmitRetInt(0, false, dmg);
-		build.EmitRetInt(1, true, 0);
-		VMScriptFunction *sfunc = build.MakeFunction();
-		sfunc->NumArgs = 1;
-		return sfunc;
-	}
-}
diff --git a/src/thingdef/thingdef.h b/src/thingdef/thingdef.h
index 3a419b112..565da573a 100644
--- a/src/thingdef/thingdef.h
+++ b/src/thingdef/thingdef.h
@@ -164,14 +164,6 @@ inline void ResetBaggage (Baggage *bag, PClassActor *stateclass)
 	bag->statedef.MakeStateDefines(stateclass);
 }
 
-//==========================================================================
-//
-// Damage function creation
-//
-//==========================================================================
-
-VMScriptFunction *CreateDamageFunction(int dmg);
-
 //==========================================================================
 //
 // Action function lookup
diff --git a/src/thingdef/thingdef_codeptr.cpp b/src/thingdef/thingdef_codeptr.cpp
index 3b4e6203e..c6bc8215b 100644
--- a/src/thingdef/thingdef_codeptr.cpp
+++ b/src/thingdef/thingdef_codeptr.cpp
@@ -2622,6 +2622,92 @@ DEFINE_ACTION_FUNCTION_PARAMS(AActor, A_GiveToSiblings)
 	ACTION_RETURN_INT(count);
 }
 
+//===========================================================================
+//
+// A_SetInventory
+//
+//===========================================================================
+
+DEFINE_ACTION_FUNCTION_PARAMS(AActor, A_SetInventory)
+{
+	PARAM_ACTION_PROLOGUE;
+	PARAM_CLASS(itemtype, AInventory);
+	PARAM_INT(amount);
+	PARAM_INT_OPT(ptr)			{ ptr = AAPTR_DEFAULT; }
+	PARAM_BOOL_OPT(beyondMax)	{ beyondMax = false; }
+
+	bool res = false;
+
+	if (itemtype == nullptr)
+	{
+		ACTION_RETURN_BOOL(false);
+	}
+
+	AActor *mobj = COPY_AAPTR(self, ptr);
+
+	if (mobj == nullptr)
+	{
+		ACTION_RETURN_BOOL(false);
+	}
+
+	AInventory *item = mobj->FindInventory(itemtype);
+
+	if (item != nullptr)
+	{
+		// A_SetInventory sets the absolute amount. 
+		// Subtract or set the appropriate amount as necessary.
+
+		if (amount == item->Amount)
+		{
+			// Nothing was changed.
+			ACTION_RETURN_BOOL(false);
+		}
+		else if (amount <= 0)
+		{
+			//Remove it all.
+			res = (mobj->TakeInventory(itemtype, item->Amount, true, false));
+			ACTION_RETURN_BOOL(res);
+		}
+		else if (amount < item->Amount)
+		{
+			int amt = abs(item->Amount - amount);
+			res = (mobj->TakeInventory(itemtype, amt, true, false));
+			ACTION_RETURN_BOOL(res);
+		}
+		else
+		{
+			item->Amount = (beyondMax ? amount : clamp(amount, 0, item->MaxAmount));
+			ACTION_RETURN_BOOL(true);
+		}
+	}
+	else
+	{
+		if (amount <= 0)
+		{
+			ACTION_RETURN_BOOL(false);
+		}
+		item = static_cast<AInventory *>(Spawn(itemtype));
+		if (item == nullptr)
+		{
+			ACTION_RETURN_BOOL(false);
+		}
+		else
+		{
+			item->Amount = amount;
+			item->flags |= MF_DROPPED;
+			item->ItemFlags |= IF_IGNORESKILL;
+			item->ClearCounters();
+			if (!item->CallTryPickup(mobj))
+			{
+				item->Destroy();
+				ACTION_RETURN_BOOL(false);
+			}
+			ACTION_RETURN_BOOL(true);
+		}
+	}
+	ACTION_RETURN_BOOL(false);
+}
+
 //===========================================================================
 //
 // A_TakeInventory
diff --git a/src/thingdef/thingdef_expression.cpp b/src/thingdef/thingdef_expression.cpp
index 700ac94bb..c2b2d25e6 100644
--- a/src/thingdef/thingdef_expression.cpp
+++ b/src/thingdef/thingdef_expression.cpp
@@ -4956,6 +4956,25 @@ FxExpression *FxRuntimeStateIndex::Resolve(FCompileContext &ctx)
 	return this;
 }
 
+static bool VerifyJumpTarget(AActor *stateowner, FStateParamInfo *stateinfo, int index)
+{
+	PClassActor *cls = stateowner->GetClass();
+
+	while (cls != RUNTIME_CLASS(AActor))
+	{
+		// both calling and target state need to belong to the same class.
+		if (cls->OwnsState(stateinfo->mCallingState))
+		{
+			return cls->OwnsState(stateinfo->mCallingState + index);
+		}
+
+		// We can safely assume the ParentClass is of type PClassActor
+		// since we stop when we see the Actor base class.
+		cls = static_cast<PClassActor *>(cls->ParentClass);
+	}
+	return false;
+}
+
 static int DecoHandleRuntimeState(VMFrameStack *stack, VMValue *param, int numparam, VMReturn *ret, int numret)
 {
 	PARAM_PROLOGUE;
@@ -4963,7 +4982,7 @@ static int DecoHandleRuntimeState(VMFrameStack *stack, VMValue *param, int numpa
 	PARAM_POINTER(stateinfo, FStateParamInfo);
 	PARAM_INT(index);
 
-	if (index == 0 || !stateowner->GetClass()->OwnsState(stateinfo->mCallingState + index))
+	if (index == 0 || !VerifyJumpTarget(stateowner, stateinfo, index))
 	{
 		// Null is returned if the location was invalid which means that no jump will be performed
 		// if used as return value
diff --git a/src/thingdef/thingdef_properties.cpp b/src/thingdef/thingdef_properties.cpp
index d60cda823..80ee6c22f 100644
--- a/src/thingdef/thingdef_properties.cpp
+++ b/src/thingdef/thingdef_properties.cpp
@@ -1435,7 +1435,7 @@ DEFINE_PROPERTY(spriterotation, F, Actor)
 //==========================================================================
 //
 //==========================================================================
-DEFINE_PROPERTY(visibleangles, Ff, Actor)
+DEFINE_PROPERTY(visibleangles, FF, Actor)
 {
 	PROP_DOUBLE_PARM(visstart, 0);
 	PROP_DOUBLE_PARM(visend, 1);
@@ -1446,7 +1446,7 @@ DEFINE_PROPERTY(visibleangles, Ff, Actor)
 //==========================================================================
 //
 //==========================================================================
-DEFINE_PROPERTY(visiblepitch, Ff, Actor)
+DEFINE_PROPERTY(visiblepitch, FF, Actor)
 {
 	PROP_DOUBLE_PARM(visstart, 0);
 	PROP_DOUBLE_PARM(visend, 1);
diff --git a/src/v_blend.cpp b/src/v_blend.cpp
index e1552105f..c13ed547c 100644
--- a/src/v_blend.cpp
+++ b/src/v_blend.cpp
@@ -52,7 +52,8 @@
 #include "v_palette.h"
 #include "d_player.h"
 
-
+CVAR( Float, blood_fade_scalar, 1.0f, CVAR_ARCHIVE )	// [SP] Pulled from Skulltag - changed default from 0.5 to 1.0
+CVAR( Float, pickup_fade_scalar, 1.0f, CVAR_ARCHIVE )	// [SP] Uses same logic as blood_fade_scalar except for pickups
 
 // [RH] Amount of red flash for up to 114 damage points. Calculated by hand
 //		using a logarithmic scale and my trusty HP48G.
@@ -113,6 +114,9 @@ void V_AddPlayerBlend (player_t *CPlayer, float blend[4], float maxinvalpha, int
 	if (CPlayer->bonuscount)
 	{
 		cnt = CPlayer->bonuscount << 3;
+
+		// [SP] Allow player to tone down intensity of pickup flash.
+		cnt = (int)( cnt * pickup_fade_scalar );
 		
 		V_AddBlend (RPART(gameinfo.pickupcolor)/255.f, GPART(gameinfo.pickupcolor)/255.f, 
 					BPART(gameinfo.pickupcolor)/255.f, cnt > 128 ? 0.5f : cnt / 255.f, blend);
@@ -124,7 +128,10 @@ void V_AddPlayerBlend (player_t *CPlayer, float blend[4], float maxinvalpha, int
 	if (painFlash.a != 0)
 	{
 		cnt = DamageToAlpha[MIN (113, CPlayer->damagecount * painFlash.a / 255)];
-			
+
+		// [BC] Allow users to tone down the intensity of the blood on the screen.
+		cnt = (int)( cnt * blood_fade_scalar );
+
 		if (cnt)
 		{
 			if (cnt > maxpainblend)
diff --git a/src/zscript/zcc_expr.cpp b/src/zscript/zcc_expr.cpp
index 0cd399d64..e1a091b18 100644
--- a/src/zscript/zcc_expr.cpp
+++ b/src/zscript/zcc_expr.cpp
@@ -162,11 +162,11 @@ ZCC_OpProto *ZCC_OpInfoType::FindBestProto(
 		// [[float32 (op) int]] will choose the integer version instead of the floating point
 		// version, which we do not want.
 		int test_dist1 = dist1, test_dist2 = dist2;
-		if (routes[0][cur_route1][0]->ConvertConstant == FtoD)
+		if (test_dist1 > 0 && routes[0][cur_route1][0]->ConvertConstant == FtoD)
 		{
 			test_dist1--;
 		}
-		if (routes[1][cur_route2][0]->ConvertConstant == FtoD)
+		if (test_dist2 > 0 && routes[1][cur_route2][0]->ConvertConstant == FtoD)
 		{
 			test_dist2--;
 		}
diff --git a/wadsrc/static/actors/actor.txt b/wadsrc/static/actors/actor.txt
index 44b801f51..eecc843d3 100644
--- a/wadsrc/static/actors/actor.txt
+++ b/wadsrc/static/actors/actor.txt
@@ -58,6 +58,8 @@ ACTOR Actor native //: Thinker
 	native float GetSpriteRotation(int ptr = AAPTR_DEFAULT);
 	native int GetMissileDamage(int mask, int add, int ptr = AAPTR_DEFAULT);
 	action native int OverlayID();
+	action native float OverlayX(int layer = 0);
+	action native float OverlayY(int layer = 0);
 
 	// Action functions
 	// Meh, MBF redundant functions. Only for DeHackEd support.
@@ -209,6 +211,7 @@ ACTOR Actor native //: Thinker
 	native state A_JumpIfTargetInsideMeleeRange(state label);
 	native state A_JumpIfInventory(class<Inventory> itemtype, int itemamount, state label, int owner = AAPTR_DEFAULT);
 	native state A_JumpIfArmorType(name Type, state label, int amount = 1);
+	action native bool A_SetInventory(class<Inventory> itemtype, int amount, int ptr = AAPTR_DEFAULT, bool beyondMax = false);
 	native bool A_GiveInventory(class<Inventory> itemtype, int amount = 0, int giveto = AAPTR_DEFAULT);
 	native bool A_TakeInventory(class<Inventory> itemtype, int amount = 0, int flags = 0, int giveto = AAPTR_DEFAULT);
 	action native bool A_SpawnItem(class<Actor> itemtype = "Unknown", float distance = 0, float zheight = 0, bool useammo = true, bool transfer_translation = false);
diff --git a/wadsrc/static/actors/constants.txt b/wadsrc/static/actors/constants.txt
index dce2488c2..df3bb860b 100644
--- a/wadsrc/static/actors/constants.txt
+++ b/wadsrc/static/actors/constants.txt
@@ -598,6 +598,7 @@ enum
 	PSPF_ADDBOB		= 1 << 1,
 	PSPF_POWDOUBLE	= 1 << 2,
 	PSPF_CVARFAST	= 1 << 3,
+	PSPF_FLIP		= 1 << 6,
 };
 
 // Default psprite layers
diff --git a/wadsrc/static/actors/shared/sharedmisc.txt b/wadsrc/static/actors/shared/sharedmisc.txt
index bfe7d5519..fe5ac72d8 100644
--- a/wadsrc/static/actors/shared/sharedmisc.txt
+++ b/wadsrc/static/actors/shared/sharedmisc.txt
@@ -75,12 +75,14 @@ ACTOR PointPusher
 {
 	+NOBLOCKMAP
 	+INVISIBLE
+	+NOCLIP
 }
 
 ACTOR PointPuller
 {
 	+NOBLOCKMAP
 	+INVISIBLE
+	+NOCLIP
 }
 
 // Bloody gibs -------------------------------------------------------------
diff --git a/wadsrc/static/language.enu b/wadsrc/static/language.enu
index 224764a87..c560192e0 100644
--- a/wadsrc/static/language.enu
+++ b/wadsrc/static/language.enu
@@ -1782,6 +1782,8 @@ DSPLYMNU_CAPFPS					= "Rendering Interpolation";
 DSPLYMNU_COLUMNMETHOD			= "Column render mode";
 DSPLYMNU_WIPETYPE				= "Screen wipe style";
 DSPLYMNU_SHOWENDOOM				= "Show ENDOOM screen";
+DSPLYMNU_BLOODFADE				= "Blood Flash Intensity";
+DSPLYMNU_PICKUPFADE				= "Pickup Flash Intensity";
 DSPLYMNU_PALLETEHACK			= "DirectDraw palette hack"; // Not used
 DSPLYMNU_ATTACHEDSURFACES		= "Use attached surfaces"; // Not used
 DSPLYMNU_STRETCHSKY				= "Stretch short skies";
@@ -2706,6 +2708,8 @@ OPTVAL_SBSFULL		    	= "Side-by-side Full";
 OPTVAL_SBSNARROW			= "Side-by-side Narrow";
 OPTVAL_TOPBOTTOM			= "Top/Bottom";
 OPTVAL_ROWINTERLEAVED		= "Row Interleaved";
+OPTVAL_COLUMNINTERLEAVED	= "Column Interleaved";
+OPTVAL_CHECKERBOARD			= "Checkerboard";
 OPTVAL_QUADBUFFERED 		= "Quad-buffered";
 OPTVAL_UNCHARTED2			= "Uncharted 2";
 OPTVAL_HEJLDAWSON			= "Hejl Dawson";
diff --git a/wadsrc/static/menudef.txt b/wadsrc/static/menudef.txt
index 5153a372f..2a4a6f059 100644
--- a/wadsrc/static/menudef.txt
+++ b/wadsrc/static/menudef.txt
@@ -674,6 +674,9 @@ OptionMenu "VideoOptions"
 
 	Option "$DSPLYMNU_VSYNC",					"vid_vsync", "OnOff"
 	Option "$DSPLYMNU_CAPFPS",					"cl_capfps", "OffOn"
+	Slider "$DSPLYMNU_BLOODFADE",				"blood_fade_scalar", 0.0, 1.0, 0.05, 1
+	Slider "$DSPLYMNU_PICKUPFADE",				"pickup_fade_scalar", 0.0, 1.0, 0.05, 1
+	Option "$DSPLYMNU_COLUMNMETHOD",			"r_columnmethod", "ColumnMethods"
 
 	StaticText " "
 	Option "$DSPLYMNU_WIPETYPE",				"wipetype", "Wipes"
@@ -704,6 +707,7 @@ OptionMenu "VideoOptions"
 	Slider "$DSPLYMNU_MOVEBOB",			"movebob", 0, 1.0, 0.05, 2
 	Slider "$DSPLYMNU_STILLBOB",		"stillbob", 0, 1.0, 0.05, 2
 	Slider "$DSPLYMNU_BOBSPEED",				"wbobspeed", 0, 2.0, 0.1, 2
+
 }
 
 //-------------------------------------------------------------------------------------------
diff --git a/wadsrc/static/menudef.zz b/wadsrc/static/menudef.zz
index 7e798a039..253f984eb 100644
--- a/wadsrc/static/menudef.zz
+++ b/wadsrc/static/menudef.zz
@@ -180,6 +180,8 @@ OptionValue VRMode
 	4, "$OPTVAL_SBSNARROW"
 	11, "$OPTVAL_TOPBOTTOM"
 	12, "$OPTVAL_ROWINTERLEAVED"
+	13, "$OPTVAL_COLUMNINTERLEAVED"
+	14, "$OPTVAL_CHECKERBOARD"
 	5, "$OPTVAL_LEFTEYE"
 	6, "$OPTVAL_RIGHTEYE"
 	7, "$OPTVAL_QUADBUFFERED"
diff --git a/wadsrc/static/shaders/glsl/present_checker3d.fp b/wadsrc/static/shaders/glsl/present_checker3d.fp
new file mode 100644
index 000000000..5247261e0
--- /dev/null
+++ b/wadsrc/static/shaders/glsl/present_checker3d.fp
@@ -0,0 +1,37 @@
+
+in vec2 TexCoord;
+out vec4 FragColor;
+
+uniform sampler2D LeftEyeTexture;
+uniform sampler2D RightEyeTexture;
+uniform float InvGamma;
+uniform float Contrast;
+uniform float Brightness;
+uniform int WindowPositionParity; // top-of-window might not be top-of-screen
+
+vec4 ApplyGamma(vec4 c)
+{
+	vec3 val = c.rgb * Contrast - (Contrast - 1.0) * 0.5;
+	val += Brightness * 0.5;
+	val = pow(max(val, vec3(0.0)), vec3(InvGamma));
+	return vec4(val, c.a);
+}
+
+void main()
+{
+	int thisVerticalPixel = int(gl_FragCoord.y); // Bottom row is typically the right eye, when WindowHeight is even
+	int thisHorizontalPixel = int(gl_FragCoord.x); // column
+	bool isLeftEye = (thisVerticalPixel // because we want to alternate eye view on each row
+			+ thisHorizontalPixel // and each column
+			+ WindowPositionParity // because the window might not be aligned to the screen
+		) % 2 == 0;
+	vec4 inputColor;
+	if (isLeftEye) {
+		inputColor = texture(LeftEyeTexture, TexCoord);
+	}
+	else {
+		// inputColor = vec4(0, 1, 0, 1);
+		inputColor = texture(RightEyeTexture, TexCoord);
+	}
+	FragColor = ApplyGamma(inputColor);
+}
diff --git a/wadsrc/static/shaders/glsl/present_column3d.fp b/wadsrc/static/shaders/glsl/present_column3d.fp
new file mode 100644
index 000000000..b46246cf0
--- /dev/null
+++ b/wadsrc/static/shaders/glsl/present_column3d.fp
@@ -0,0 +1,35 @@
+
+in vec2 TexCoord;
+out vec4 FragColor;
+
+uniform sampler2D LeftEyeTexture;
+uniform sampler2D RightEyeTexture;
+uniform float InvGamma;
+uniform float Contrast;
+uniform float Brightness;
+uniform int WindowPositionParity; // top-of-window might not be top-of-screen
+
+vec4 ApplyGamma(vec4 c)
+{
+	vec3 val = c.rgb * Contrast - (Contrast - 1.0) * 0.5;
+	val += Brightness * 0.5;
+	val = pow(max(val, vec3(0.0)), vec3(InvGamma));
+	return vec4(val, c.a);
+}
+
+void main()
+{
+	int thisHorizontalPixel = int(gl_FragCoord.x); // zero-based column index from left
+	bool isLeftEye = (thisHorizontalPixel // because we want to alternate eye view on each column
+		 + WindowPositionParity // because the window might not be aligned to the screen
+		) % 2 == 0;
+	vec4 inputColor;
+	if (isLeftEye) {
+		inputColor = texture(LeftEyeTexture, TexCoord);
+	}
+	else {
+		// inputColor = vec4(0, 1, 0, 1);
+		inputColor = texture(RightEyeTexture, TexCoord);
+	}
+	FragColor = ApplyGamma(inputColor);
+}
diff --git a/wadsrc/static/shaders/glsl/present_row3d.fp b/wadsrc/static/shaders/glsl/present_row3d.fp
index 8ae72d1e0..50b27ac26 100644
--- a/wadsrc/static/shaders/glsl/present_row3d.fp
+++ b/wadsrc/static/shaders/glsl/present_row3d.fp
@@ -7,7 +7,7 @@ uniform sampler2D RightEyeTexture;
 uniform float InvGamma;
 uniform float Contrast;
 uniform float Brightness;
-uniform int VerticalPixelOffset; // top-of-window might not be top-of-screen
+uniform int WindowPositionParity; // top-of-window might not be top-of-screen
 
 vec4 ApplyGamma(vec4 c)
 {
@@ -19,9 +19,9 @@ vec4 ApplyGamma(vec4 c)
 
 void main()
 {
-	int thisVerticalPixel = int(gl_FragCoord.y + 1.0); // Bottom row is typically the right eye, when WindowHeight is even
+	int thisVerticalPixel = int(gl_FragCoord.y); // Bottom row is typically the right eye, when WindowHeight is even
 	bool isLeftEye = (thisVerticalPixel // because we want to alternate eye view on each row
-		 + VerticalPixelOffset // because the window might not be aligned to the screen
+		 + WindowPositionParity // because the window might not be aligned to the screen
 		) % 2 == 0;
 	vec4 inputColor;
 	if (isLeftEye) {