From 2125f8b9d1a12b14d9ff5159b14ffad34b68bab5 Mon Sep 17 00:00:00 2001
From: Christoph Oelckers <coelckers@users.noreply.github.com>
Date: Sat, 19 May 2018 14:42:25 +0200
Subject: [PATCH] - use triangles instead of triangle fans to render flats.

---
 src/gl/scene/gl_flats.cpp            |  7 +++----
 src/hwrenderer/data/flatvertices.cpp | 15 ++++++++++++---
 2 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/src/gl/scene/gl_flats.cpp b/src/gl/scene/gl_flats.cpp
index 41e4e0e691..ac6415b439 100644
--- a/src/gl/scene/gl_flats.cpp
+++ b/src/gl/scene/gl_flats.cpp
@@ -181,24 +181,23 @@ void FDrawInfo::DrawSubsectors(GLFlat *flat, int pass, bool processlights, bool
 		for (int i=0; i<flat->sector->subsectorcount; i++)
 		{
 			subsector_t * sub = flat->sector->subsectors[i];
+			if (sub->numlines <= 2) continue;
 				
 			if (gl_drawinfo->ss_renderflags[sub->Index()]& flat->renderflags || istrans)
 			{
 				if (processlights) SetupSubsectorLights(flat, GLPASS_ALL, sub, &dli);
 				drawcalls.Clock();
-				//glDrawArrays(GL_TRIANGLE_FAN, index, sub->numlines);
-				glDrawElements(GL_TRIANGLE_FAN, sub->numlines, GL_UNSIGNED_INT, GLRenderer->mVBO->GetIndexPointer() + index);
+				glDrawElements(GL_TRIANGLES, (sub->numlines - 2) * 3, GL_UNSIGNED_INT, GLRenderer->mVBO->GetIndexPointer() + index);
 				drawcalls.Unclock();
 				flatvertices += sub->numlines;
 				flatprimitives++;
 			}
-			index += sub->numlines;
+			index += (sub->numlines - 2) * 3;
 		}
 	}
 	else
 	{
 		// Draw the subsectors belonging to this sector
-		// (can this case even happen?)
 		for (int i=0; i<flat->sector->subsectorcount; i++)
 		{
 			subsector_t * sub = flat->sector->subsectors[i];
diff --git a/src/hwrenderer/data/flatvertices.cpp b/src/hwrenderer/data/flatvertices.cpp
index a5149a60e2..cfa929d31e 100644
--- a/src/hwrenderer/data/flatvertices.cpp
+++ b/src/hwrenderer/data/flatvertices.cpp
@@ -234,11 +234,20 @@ void FFlatVertexGenerator::CreateFlatVertices()
 
 int FFlatVertexGenerator::CreateIndexedSubsectorVertices(subsector_t *sub, const secplane_t &plane, int floor, int vi, FFlatVertexGenerator::FIndexGenerationInfo &gen)
 {
-	int idx = ibo_data.Reserve(sub->numlines);
-	for (unsigned int k = 0; k<sub->numlines; k++)
+	if (sub->numlines < 3) return -1;
+
+	int idx = ibo_data.Reserve((sub->numlines - 2) * 3);
+	int idxc = idx;
+	int firstndx = gen.GetIndex(sub->firstline[0].v1);
+	int secondndx = gen.GetIndex(sub->firstline[1].v1);
+	for (unsigned int k = 2; k<sub->numlines; k++)
 	{
 		auto ndx = gen.GetIndex(sub->firstline[k].v1);
-		ibo_data[idx + k] = vi + ndx;
+
+		ibo_data[idx++] = vi + firstndx;
+		ibo_data[idx++] = vi + secondndx;
+		ibo_data[idx++] = vi + ndx;
+		secondndx = ndx;
 	}
 	return idx;
 }