diff --git a/src/gl/data/gl_vertexbuffer.cpp b/src/gl/data/gl_vertexbuffer.cpp
index 01c7ee3b95..6fd7f3f98c 100644
--- a/src/gl/data/gl_vertexbuffer.cpp
+++ b/src/gl/data/gl_vertexbuffer.cpp
@@ -136,24 +136,29 @@ FFlatVertexBuffer::~FFlatVertexBuffer()
 //
 //==========================================================================
 
-CVAR(Bool, gl_testbuffer, false, 0)
+CUSTOM_CVAR(Int, gl_rendermethod, 0, CVAR_ARCHIVE | CVAR_GLOBALCONFIG)
+{
+}
 
 void FFlatVertexBuffer::ImmRenderBuffer(unsigned int primtype, unsigned int offset, unsigned int count)
 {
-#if 0
-	if (!gl_testbuffer)	// todo: remove the immediate mode calls once the uniform array method has been tested.
+	switch (gl_rendermethod)
 	{
-		glBegin(primtype);
-		for (unsigned int i = 0; i < count; i++)
+	case 0:
+#ifndef CORE_PROFILE
+		if (!(gl.flags & RFL_COREPROFILE))
 		{
-			glTexCoord2fv(&map[offset + i].u);
-			glVertex3fv(&map[offset + i].x);
+			glBegin(primtype);
+			for (unsigned int i = 0; i < count; i++)
+			{
+				glVertexAttrib2fv(VATTR_TEXCOORD, &map[offset + i].u);
+				glVertexAttrib3fv(VATTR_VERTEX, &map[offset + i].x);
+			}
+			glEnd();
+			break;
 		}
-		glEnd();
-	}
-	else
 #endif
-	{
+	case 1:
 		if (count > 20)
 		{
 			int start = offset;
@@ -186,6 +191,14 @@ void FFlatVertexBuffer::ImmRenderBuffer(unsigned int primtype, unsigned int offs
 			glUniform1fv(GLRenderer->mShaderManager->GetActiveShader()->fakevb_index, count * 5, &map[offset].x);
 			glDrawArrays(primtype, 0, count);
 		}
+		break;
+
+	case 2:
+		// glBufferSubData
+
+	case 3:
+		// glMapBufferRange
+		break;
 	}
 }
 
diff --git a/src/gl/renderer/gl_renderstate.cpp b/src/gl/renderer/gl_renderstate.cpp
index 6f5b9be14f..73d7d5ef3c 100644
--- a/src/gl/renderer/gl_renderstate.cpp
+++ b/src/gl/renderer/gl_renderstate.cpp
@@ -149,7 +149,29 @@ bool FRenderState::ApplyShader()
 	activeShader->muInterpolationFactor.Set(mInterpolationFactor);
 	activeShader->muClipHeightTop.Set(mClipHeightTop);
 	activeShader->muClipHeightBottom.Set(mClipHeightBottom);
-	activeShader->muAlphaThreshold.Set(mAlphaThreshold);
+
+#ifndef CORE_PROFILE
+	if (!(gl.flags & RFL_COREPROFILE))
+	{
+		if (mAlphaThreshold != glAlphaThreshold)
+		{
+			glAlphaThreshold = mAlphaThreshold;
+			if (mAlphaThreshold < 0.f)
+			{
+				glDisable(GL_ALPHA_TEST);
+			}
+			else
+			{
+				glEnable(GL_ALPHA_TEST);
+				glAlphaFunc(GL_GREATER, mAlphaThreshold);
+			}
+		}
+	}
+	else
+#endif
+	{
+		activeShader->muAlphaThreshold.Set(mAlphaThreshold);
+	}
 
 	if (mGlowEnabled)
 	{
diff --git a/src/gl/renderer/gl_renderstate.h b/src/gl/renderer/gl_renderstate.h
index d8168bca57..a07cd3401b 100644
--- a/src/gl/renderer/gl_renderstate.h
+++ b/src/gl/renderer/gl_renderstate.h
@@ -74,8 +74,10 @@ class FRenderState
 	int mEffectState;
 	int mColormapState;
 
-	int glSrcBlend, glDstBlend;
 	float glAlphaThreshold;
+	bool glClipOn;
+
+	int glSrcBlend, glDstBlend;
 	bool glAlphaTest;
 	int glBlendEquation;
 
diff --git a/src/gl/scene/gl_portal.cpp b/src/gl/scene/gl_portal.cpp
index afc5cf1f29..7c7efc91de 100644
--- a/src/gl/scene/gl_portal.cpp
+++ b/src/gl/scene/gl_portal.cpp
@@ -286,8 +286,10 @@ bool GLPortal::Start(bool usestencil, bool doquery)
 	}
 	planestack.Push(gl_RenderState.GetClipHeightTop());
 	planestack.Push(gl_RenderState.GetClipHeightBottom());
-	gl_RenderState.SetClipHeightTop(65536.f);
+	glDisable(GL_CLIP_DISTANCE0);
+	glDisable(GL_CLIP_DISTANCE1);
 	gl_RenderState.SetClipHeightBottom(-65536.f);
+	gl_RenderState.SetClipHeightTop(65536.f);
 
 	// save viewpoint
 	savedviewx=viewx;
@@ -351,8 +353,10 @@ void GLPortal::End(bool usestencil)
 	float f;
 	planestack.Pop(f);
 	gl_RenderState.SetClipHeightBottom(f);
+	if (f > -65535.f) glEnable(GL_CLIP_DISTANCE0);
 	planestack.Pop(f);
 	gl_RenderState.SetClipHeightTop(f);
+	if (f < 65535.f) glEnable(GL_CLIP_DISTANCE1);
 
 	if (usestencil)
 	{
@@ -790,15 +794,27 @@ void GLPlaneMirrorPortal::DrawContents()
 
 	validcount++;
 
-	float f = FIXED2FLOAT(planez);
-	if (PlaneMirrorMode < 0) gl_RenderState.SetClipHeightTop(f);	// ceiling mirror: clip everytihng with a z lower than the portal's ceiling
-	else gl_RenderState.SetClipHeightBottom(f);	// floor mirror: clip everything with a z higher than the portal's floor
-
 	PlaneMirrorFlag++;
 	GLRenderer->SetupView(viewx, viewy, viewz, viewangle, !!(MirrorFlag&1), !!(PlaneMirrorFlag&1));
 	ClearClipper();
 
+	float f = FIXED2FLOAT(planez);
+	if (PlaneMirrorMode < 0)
+	{
+		gl_RenderState.SetClipHeightTop(f);	// ceiling mirror: clip everytihng with a z lower than the portal's ceiling
+		glEnable(GL_CLIP_DISTANCE1);
+	}
+	else
+	{
+		gl_RenderState.SetClipHeightBottom(f);	// floor mirror: clip everything with a z higher than the portal's floor
+		glEnable(GL_CLIP_DISTANCE0);
+	}
+
 	GLRenderer->DrawScene();
+	glDisable(GL_CLIP_DISTANCE0);
+	glDisable(GL_CLIP_DISTANCE1);
+	gl_RenderState.SetClipHeightBottom(-65536.f);
+	gl_RenderState.SetClipHeightTop(65536.f);
 	PlaneMirrorFlag--;
 	PlaneMirrorMode=old_pm;
 }
diff --git a/src/gl/scene/gl_walls_draw.cpp b/src/gl/scene/gl_walls_draw.cpp
index 932451298e..b9965972ca 100644
--- a/src/gl/scene/gl_walls_draw.cpp
+++ b/src/gl/scene/gl_walls_draw.cpp
@@ -186,14 +186,7 @@ void GLWall::RenderWall(int textured, unsigned int *store)
 
 	if (!(textured & RWF_NORENDER))
 	{
-		// disable the clip plane if it isn't needed (which can be determined by a simple check.)
-		float ct = gl_RenderState.GetClipHeightTop();
-		float cb = gl_RenderState.GetClipHeightBottom();
-		if (ztop[0] <= ct && ztop[1] <= ct) gl_RenderState.SetClipHeightTop(65536.f);
-		if (zbottom[0] >= cb && zbottom[1] >= cb) gl_RenderState.SetClipHeightBottom(-65536.f);
 		gl_RenderState.Apply();
-		gl_RenderState.SetClipHeightTop(ct);
-		gl_RenderState.SetClipHeightBottom(cb);
 	}
 
 	// the rest of the code is identical for textured rendering and lights
diff --git a/src/gl/system/gl_interface.cpp b/src/gl/system/gl_interface.cpp
index f00202fd8e..0b1e885488 100644
--- a/src/gl/system/gl_interface.cpp
+++ b/src/gl/system/gl_interface.cpp
@@ -134,6 +134,7 @@ void gl_LoadExtensions()
 	if (CheckExtension("GL_EXT_texture_compression_s3tc")) gl.flags|=RFL_TEXTURE_COMPRESSION_S3TC;
 	if (CheckExtension("GL_ARB_buffer_storage")) gl.flags |= RFL_BUFFER_STORAGE;
 	if (CheckExtension("GL_ARB_separate_shader_objects")) gl.flags |= RFL_SEPARATE_SHADER_OBJECTS;
+	if (!CheckExtension("GL_ARB_compatibility")) gl.flags |= RFL_COREPROFILE;
 	
 
 	glGetIntegerv(GL_MAX_TEXTURE_SIZE,&gl.max_texturesize);
diff --git a/src/gl/system/gl_interface.h b/src/gl/system/gl_interface.h
index 0f569d2c42..2209f0bed5 100644
--- a/src/gl/system/gl_interface.h
+++ b/src/gl/system/gl_interface.h
@@ -13,6 +13,7 @@ enum RenderFlags
 	RFL_BUFFER_STORAGE = 8,				// allows persistently mapped buffers, which are the only efficient way to actually use a dynamic vertex buffer. If this isn't present, a workaround with uniform arrays is used.
 	RFL_SHADER_STORAGE_BUFFER = 16,		// to be used later for a parameter buffer
 	RFL_BASEINDEX = 32,					// currently unused
+	RFL_COREPROFILE = 64,
 };
 
 enum TexMode
diff --git a/src/gl/textures/gl_material.h b/src/gl/textures/gl_material.h
index 81f4dd10f5..259f881b45 100644
--- a/src/gl/textures/gl_material.h
+++ b/src/gl/textures/gl_material.h
@@ -63,7 +63,6 @@ private:
 
 	bool bHasColorkey;		// only for hires
 	bool bExpand;
-	float AlphaThreshold;
 
 	unsigned char * LoadHiresTexture(FTexture *hirescheck, int *width, int *height, bool alphatexture);
 
diff --git a/src/win32/win32gliface.cpp b/src/win32/win32gliface.cpp
index db414bae62..3b630177ad 100644
--- a/src/win32/win32gliface.cpp
+++ b/src/win32/win32gliface.cpp
@@ -17,6 +17,7 @@
 #include "i_system.h"
 #include "doomstat.h"
 #include "v_text.h"
+#include "m_argv.h"
 //#include "gl_defs.h"
 
 #include "gl/renderer/gl_renderer.h"
@@ -739,21 +740,42 @@ bool Win32GLVideo::InitHardware (HWND Window, int multisample)
 	m_hRC = NULL;
 	if (myWglCreateContextAttribsARB != NULL)
 	{
-		// let's try to get the best version possible.
-		static int versions[] = { 44, 43, 42, 41, 40, 33, 32, -1 };
+#ifndef CORE_PROFILE
+		bool core = !!Args->CheckParm("-core");
+#else
+		bool core = true;
+#endif
+		if (core)
+		{
+			// let's try to get the best version possible.
+			static int versions[] = { 44, 43, 42, 41, 40, 33, 32, -1 };
 
-		for (int i = 0; versions[i] > 0; i++)
+			for (int i = 0; versions[i] > 0; i++)
+			{
+				int ctxAttribs[] = {
+					WGL_CONTEXT_MAJOR_VERSION_ARB, versions[i] / 10,
+					WGL_CONTEXT_MINOR_VERSION_ARB, versions[i] % 10,
+					WGL_CONTEXT_FLAGS_ARB, gl_debug ? WGL_CONTEXT_DEBUG_BIT_ARB : 0,
+					WGL_CONTEXT_PROFILE_MASK_ARB, WGL_CONTEXT_CORE_PROFILE_BIT_ARB,
+					0
+				};
+
+				m_hRC = myWglCreateContextAttribsARB(m_hDC, 0, ctxAttribs);
+				if (m_hRC != NULL) break;
+			}
+		}
+		else
 		{
 			int ctxAttribs[] = {
-				WGL_CONTEXT_MAJOR_VERSION_ARB, versions[i] / 10,
-				WGL_CONTEXT_MINOR_VERSION_ARB, versions[i] % 10,
+				WGL_CONTEXT_MAJOR_VERSION_ARB, 3,
+				WGL_CONTEXT_MINOR_VERSION_ARB, 0,
 				WGL_CONTEXT_FLAGS_ARB, gl_debug ? WGL_CONTEXT_DEBUG_BIT_ARB : 0,
-				WGL_CONTEXT_PROFILE_MASK_ARB, WGL_CONTEXT_CORE_PROFILE_BIT_ARB,
+				WGL_CONTEXT_PROFILE_MASK_ARB, WGL_CONTEXT_COMPATIBILITY_PROFILE_BIT_ARB,
 				0
 			};
 
 			m_hRC = myWglCreateContextAttribsARB(m_hDC, 0, ctxAttribs);
-			if (m_hRC != NULL) break;
+
 		}
 	}
 	if (m_hRC == 0)
diff --git a/wadsrc/static/shaders/glsl/fogboundary.fp b/wadsrc/static/shaders/glsl/fogboundary.fp
index 1a2b0b0f9b..d8259c8452 100644
--- a/wadsrc/static/shaders/glsl/fogboundary.fp
+++ b/wadsrc/static/shaders/glsl/fogboundary.fp
@@ -10,12 +10,6 @@ out vec4 FragColor;
 
 void main()
 {
-#ifndef NO_DISCARD
-	// clip plane emulation for plane reflections. These are always perfectly horizontal so a simple check of the pixelpos's y coordinate is sufficient.
-	if (pixelpos.y > uClipHeightTop) discard;
-	if (pixelpos.y < uClipHeightBottom) discard;
-#endif
-
 	float fogdist;
 	float fogfactor;
 	
diff --git a/wadsrc/static/shaders/glsl/main.fp b/wadsrc/static/shaders/glsl/main.fp
index c560574019..a4e4d782b3 100644
--- a/wadsrc/static/shaders/glsl/main.fp
+++ b/wadsrc/static/shaders/glsl/main.fp
@@ -220,16 +220,10 @@ vec4 applyFog(vec4 frag, float fogfactor)
 
 void main()
 {
-#ifndef NO_DISCARD
-	// clip plane emulation for plane reflections. These are always perfectly horizontal so a simple check of the pixelpos's y coordinate is sufficient.
-	if (pixelpos.y > uClipHeightTop) discard;
-	if (pixelpos.y < uClipHeightBottom) discard;
-#endif
-
 	vec4 frag = ProcessTexel();
 	
-#ifndef NO_DISCARD
-	// alpha testing
+#ifdef CORE_PROFILE
+	// alpha testing - only for the core profile, in compatibility mode we use the alpha test.
 	if (frag.a <= uAlphaThreshold) discard;
 #endif
 
diff --git a/wadsrc/static/shaders/glsl/main.vp b/wadsrc/static/shaders/glsl/main.vp
index c404268293..e355141f0c 100644
--- a/wadsrc/static/shaders/glsl/main.vp
+++ b/wadsrc/static/shaders/glsl/main.vp
@@ -65,4 +65,6 @@ void main()
 	#endif
 	
 	gl_Position = ProjectionMatrix * eyeCoordPos;
+	gl_ClipDistance[0] = worldcoord.y - uClipHeightBottom;
+	gl_ClipDistance[1] = uClipHeightTop - worldcoord.y;
 }
diff --git a/wadsrc/static/shaders/glsl/shaderdefs.i b/wadsrc/static/shaders/glsl/shaderdefs.i
index 56e6965b21..2870cf975c 100644
--- a/wadsrc/static/shaders/glsl/shaderdefs.i
+++ b/wadsrc/static/shaders/glsl/shaderdefs.i
@@ -1,10 +1,13 @@
 // This file contains common data definitions for both vertex and fragment shader
 
 uniform vec4 uCameraPos;
+uniform int uTextureMode;
 uniform float uClipHeightTop, uClipHeightBottom;
 
-uniform int uTextureMode;
+#ifdef CORE_PROFILE
 uniform float uAlphaThreshold;
+#endif
+
 
 // colors
 uniform vec4 uObjectColor;