diff --git a/src/gl/compatibility/gl_20.cpp b/src/gl/compatibility/gl_20.cpp
index fb4064d3b..c1ba2d2db 100644
--- a/src/gl/compatibility/gl_20.cpp
+++ b/src/gl/compatibility/gl_20.cpp
@@ -772,11 +772,13 @@ void GLWall::RenderLightsCompat(int pass)
 		}
 		if (PrepareLight(light, pass))
 		{
+			vertcount = 0;
 			RenderWall(RWF_TEXTURED, NULL);
 		}
 		node = node->nextLight;
 	}
 	memcpy(tcs, save, sizeof(tcs));
+	vertcount = 0;
 }
 
 //==========================================================================
diff --git a/src/gl/data/gl_vertexbuffer.h b/src/gl/data/gl_vertexbuffer.h
index c9abae0da..5e6912a10 100644
--- a/src/gl/data/gl_vertexbuffer.h
+++ b/src/gl/data/gl_vertexbuffer.h
@@ -83,6 +83,13 @@ class FFlatVertexBuffer : public FVertexBuffer
 	static const unsigned int BUFFER_SIZE_TO_USE = 1999500;
 
 public:
+	enum
+	{
+		QUAD_INDEX = 0,
+		FULLSCREEN_INDEX = 4,
+		PRESENT_INDEX = 8
+	};
+
 	TArray<FFlatVertex> vbo_shadowdata;	// this is kept around for updating the actual (non-readable) buffer and as stand-in for pre GL 4.x
 
 	FFlatVertexBuffer(int width, int height);
diff --git a/src/gl/renderer/gl_postprocess.cpp b/src/gl/renderer/gl_postprocess.cpp
index a9aec57c6..c1b39e556 100644
--- a/src/gl/renderer/gl_postprocess.cpp
+++ b/src/gl/renderer/gl_postprocess.cpp
@@ -114,7 +114,7 @@ void FGLRenderer::RenderScreenQuad()
 {
 	mVBO->BindVBO();
 	gl_RenderState.ResetVertexBuffer();
-	glDrawArrays(GL_TRIANGLE_STRIP, 8, 4);
+	glDrawArrays(GL_TRIANGLE_STRIP, FFlatVertexBuffer::PRESENT_INDEX, 4);
 }
 
 //-----------------------------------------------------------------------------
diff --git a/src/gl/renderer/gl_quaddrawer.cpp b/src/gl/renderer/gl_quaddrawer.cpp
index 4dbd52a98..eedac12e5 100644
--- a/src/gl/renderer/gl_quaddrawer.cpp
+++ b/src/gl/renderer/gl_quaddrawer.cpp
@@ -81,6 +81,6 @@ void FQuadDrawer::DoRender(int type)
 	glUniformMatrix4fv(shader->vertexmatrix_index, 1, false, matV);
 	glUniformMatrix4fv(shader->texcoordmatrix_index, 1, false, matT);
 	glUniform1i(shader->quadmode_index, 1);
-	GLRenderer->mVBO->RenderArray(type, 0, 4);
+	GLRenderer->mVBO->RenderArray(type, FFlatVertexBuffer::QUAD_INDEX, 4);
 	glUniform1i(shader->quadmode_index, 0);
 }
diff --git a/src/gl/scene/gl_wall.h b/src/gl/scene/gl_wall.h
index 674d7fffd..59f21dc37 100644
--- a/src/gl/scene/gl_wall.h
+++ b/src/gl/scene/gl_wall.h
@@ -172,6 +172,9 @@ public:
 	float zceil[2];
 	float zfloor[2];
 
+	unsigned int vertindex;
+	unsigned int vertcount;
+
 public:
 	seg_t * seg;			// this gives the easiest access to all other structs involved
 	subsector_t * sub;		// For polyobjects
diff --git a/src/gl/scene/gl_walls.cpp b/src/gl/scene/gl_walls.cpp
index d8cef9771..54ba8c731 100644
--- a/src/gl/scene/gl_walls.cpp
+++ b/src/gl/scene/gl_walls.cpp
@@ -1467,6 +1467,8 @@ void GLWall::Process(seg_t *seg, sector_t * frontsector, sector_t * backsector)
 	// note: we always have a valid sidedef and linedef reference when getting here.
 
 	this->seg = seg;
+	vertindex = 0;
+	vertcount = 0;
 
 	if ((seg->sidedef->Flags & WALLF_POLYOBJ) && seg->backsector)
 	{
@@ -1760,6 +1762,9 @@ void GLWall::ProcessLowerMiniseg(seg_t *seg, sector_t * frontsector, sector_t *
 
 	float ffh = frontsector->GetPlaneTexZ(sector_t::floor);
 	float bfh = backsector->GetPlaneTexZ(sector_t::floor);
+
+	vertindex = 0;
+	vertcount = 0;
 	if (bfh > ffh)
 	{
 		this->seg = seg;