@ Added some code that I helps fixing some triangulation issues, but now it breaks other shapes. Also added test cases so I can figure this out later.

Source/Core/Geometry/EarClipVertex.cs

@@ -126,7 +126,9 @@ namespace CodeImp.DoomBuilder.Geometry
 		// This adds to reflexes list
 		public void AddReflex(LinkedList<EarClipVertex> reflexes)
 		{
+			#if DEBUG
 			if(vertslink == null) throw new Exception();
+			#endif
 			if(reflexlink == null) reflexlink = reflexes.AddLast(this);
 		}
 
@@ -140,7 +142,9 @@ namespace CodeImp.DoomBuilder.Geometry
 		// This adds to eartips list
 		internal void AddEarTip(LinkedList<EarClipVertex> eartips)
 		{
+			#if DEBUG
 			if(vertslink == null) throw new Exception();
+			#endif
 			if(eartiplink == null) eartiplink = eartips.AddLast(this);
 		}
 

Source/Core/Geometry/Triangulation.cs

@@ -613,7 +613,7 @@ namespace CodeImp.DoomBuilder.Geometry
 
 		// This clips a polygon and returns the triangles
 		// The polygon may not have any holes or islands
-		/// See: http://www.geometrictools.com/Documentation/TriangulationByEarClipping.pdf
+		// See: http://www.geometrictools.com/Documentation/TriangulationByEarClipping.pdf
 		private int DoEarClip(EarClipPolygon poly, List<Vector2D> verticeslist, List<Sidedef> sidedefslist)
 		{
 			LinkedList<EarClipVertex> verts = new LinkedList<EarClipVertex>();
@@ -720,7 +720,8 @@ namespace CodeImp.DoomBuilder.Geometry
 				
 				// Test first neighbour
 				t1 = GetTriangle(v1);
-				if(IsReflex(t1))
+				bool t1a = true;	//TriangleHasArea(t1);
+				if(t1a && IsReflex(t1))
 				{
 					// List as reflex if not listed yet
 					if(!v1.IsReflex) v1.AddReflex(reflexes);
@@ -734,7 +735,8 @@ namespace CodeImp.DoomBuilder.Geometry
 				
 				// Test second neighbour
 				t2 = GetTriangle(v2);
-				if(IsReflex(t2))
+				bool t2a = true;	//TriangleHasArea(t2);
+				if(t2a && IsReflex(t2))
 				{
 					// List as reflex if not listed yet
 					if(!v2.IsReflex) v2.AddReflex(reflexes);
@@ -747,8 +749,8 @@ namespace CodeImp.DoomBuilder.Geometry
 				}
 				
 				// Check if any neightbour have become a valid or invalid ear
-				if(!v1.IsReflex && CheckValidEar(t1, reflexes)) v1.AddEarTip(eartips); else v1.RemoveEarTip();
-				if(!v2.IsReflex && CheckValidEar(t2, reflexes)) v2.AddEarTip(eartips); else v2.RemoveEarTip();
+				if(!v1.IsReflex && (!t1a || CheckValidEar(t1, reflexes))) v1.AddEarTip(eartips); else v1.RemoveEarTip();
+				if(!v2.IsReflex && (!t2a || CheckValidEar(t2, reflexes))) v2.AddEarTip(eartips); else v2.RemoveEarTip();
 			}
 
 			#if DEBUG
@@ -810,8 +812,8 @@ namespace CodeImp.DoomBuilder.Geometry
 				float lineside12 = Line2D.GetSideOfLine(t[1].Position, t[2].Position, p.Value.Position);
 				float lineside20 = Line2D.GetSideOfLine(t[2].Position, t[0].Position, p.Value.Position);
 
-				// If any of the lineside results are 0 then that means the point lies on that edge and we
-				// need to test if the lines adjacent to the point are in the triangle or not.
+				// If any of the lineside results are 0 then that means the point p lies on that edge and we
+				// need to test if the lines adjacent to the point p are in the triangle or not.
 				// If the lines are intersecting the triangle, we also consider the point inside.
 				if((lineside01 == 0.0f) || (lineside12 == 0.0f) || (lineside20 == 0.0f))
 				{
@@ -834,18 +836,30 @@ namespace CodeImp.DoomBuilder.Geometry
 		}
 		
 		// This checks if a line is inside a triangle (touching the triangle is allowed)
-		// NOTE: Does NOT check if p1 is inside the triangle, because we only use the
-		// method when point-in-triangle is already tested for p1
+		// NOTE: We already know p1 is on an edge of the triangle.
 		private bool LineInsideTriangle(EarClipVertex[] t, Vector2D p1, Vector2D p2)
 		{
+			float s01 = Line2D.GetSideOfLine(t[0].Position, t[1].Position, p2);
+			float s12 = Line2D.GetSideOfLine(t[1].Position, t[2].Position, p2);
+			float s20 = Line2D.GetSideOfLine(t[2].Position, t[0].Position, p2);
+			
 			// Test if p2 is inside the triangle
-			if((Line2D.GetSideOfLine(t[0].Position, t[1].Position, p2) < 0.0f) &&
-			   (Line2D.GetSideOfLine(t[1].Position, t[2].Position, p2) < 0.0f) &&
-			   (Line2D.GetSideOfLine(t[2].Position, t[0].Position, p2) < 0.0f))
+			if((s01 < 0.0f) && (s12 < 0.0f) && (s20 < 0.0f))
 			{
 				// Line is inside triangle, because p2 is
 				return true;
 			}
+			// Test if p2 is on an edge of the triangle
+			else if((s01 == 0.0f) || (s12 == 0.0f) || (s20 == 0.0f))
+			{
+				// The line is inside the triangle if p2 is not on the same edge as p1,
+				// because only then the line has to go across the surface of the triangle.
+				bool sameedge = (p1 == p2);
+				sameedge |= (s01 == 0.0f) && (Line2D.GetSideOfLine(t[0].Position, t[1].Position, p1) == 0.0f);
+				sameedge |= (s12 == 0.0f) && (Line2D.GetSideOfLine(t[1].Position, t[2].Position, p1) == 0.0f);
+				sameedge |= (s20 == 0.0f) && (Line2D.GetSideOfLine(t[2].Position, t[0].Position, p1) == 0.0f);
+				return !sameedge;
+			}
 			else
 			{
 				// Test for line intersections
@@ -857,11 +871,11 @@ namespace CodeImp.DoomBuilder.Geometry
 				
 				// Test intersections
 				t01.GetIntersection(p, out pu, out pt);
-				if(!float.IsNaN(pu) && (pu > 0.0f) && (pu < 1.0f) && (pt > 0.0f) && (pt < 1.0f)) return true;
+				if(!float.IsNaN(pu) && (pu > 0.0f) && (pu < 1.0f) && (pt >= 0.0f) && (pt <= 1.0f)) return true;
 				t12.GetIntersection(p, out pu, out pt);
-				if(!float.IsNaN(pu) && (pu > 0.0f) && (pu < 1.0f) && (pt > 0.0f) && (pt < 1.0f)) return true;
+				if(!float.IsNaN(pu) && (pu > 0.0f) && (pu < 1.0f) && (pt >= 0.0f) && (pt <= 1.0f)) return true;
 				t20.GetIntersection(p, out pu, out pt);
-				return !float.IsNaN(pu) && (pu > 0.0f) && (pu < 1.0f) && (pt > 0.0f) && (pt < 1.0f);
+				return !float.IsNaN(pu) && (pu > 0.0f) && (pu < 1.0f) && (pt >= 0.0f) && (pt <= 1.0f);
 			}
 		}
 		

Source/Plugins/BuilderModes/BuilderModes.csproj

@@ -235,6 +235,7 @@
     <EmbeddedResource Include="Resources\HeightsMode.png" />
   </ItemGroup>
   <ItemGroup>
+    <Compile Include="ClassicModes\TriangulatorMode.cs" />
     <Compile Include="ErrorChecks\CheckMissingTextures.cs" />
     <Compile Include="ErrorChecks\CheckUnknownTextures.cs" />
     <Compile Include="ErrorChecks\ResultNoErrors.cs" />

Source/Plugins/BuilderModes/ClassicModes/TriangulatorMode.cs

@@ -380,7 +380,7 @@ namespace CodeImp.DoomBuilder.BuilderModes.Editing
 		// This shows a line
 		private void ShowLine(Vector2D v1, Vector2D v2, PixelColor c)
 		{
-			for(int a = 0; a < 6; a++)
+			for(int a = 0; a < 3; a++)
 			{
 				OnRedrawDisplay();
 				Thread.Sleep(20);
@@ -402,7 +402,7 @@ namespace CodeImp.DoomBuilder.BuilderModes.Editing
 				}
 
 				// Wait a bit
-				Thread.Sleep(60);
+				Thread.Sleep(50);
 				Application.DoEvents();
 			}
 		}
@@ -416,7 +416,7 @@ namespace CodeImp.DoomBuilder.BuilderModes.Editing
 			// Go for all vertices in the polygon
 			while(v != null)
 			{
-				for(int a = 0; a < 10; a++)
+				for(int a = 0; a < 5; a++)
 				{
 
 					// Start with a clear display
@@ -434,7 +434,7 @@ namespace CodeImp.DoomBuilder.BuilderModes.Editing
 						renderer.Present();
 					}
 					
-					Thread.Sleep(20);
+					Thread.Sleep(10);
 					
 					// Start with a clear display
 					if(renderer.StartPlotter(true))
@@ -452,7 +452,7 @@ namespace CodeImp.DoomBuilder.BuilderModes.Editing
 					}
 
 					// Wait a bit
-					Thread.Sleep(60);
+					Thread.Sleep(40);
 					Application.DoEvents();
 				}
 				
@@ -506,7 +506,7 @@ namespace CodeImp.DoomBuilder.BuilderModes.Editing
 		{
 			EarClipVertex prev, first;
 			
-			for(int a = 0; a < 5; a++)
+			for(int a = 0; a < 8; a++)
 			{
 				// Start with a clear display
 				if(renderer.StartPlotter(true))
@@ -543,7 +543,7 @@ namespace CodeImp.DoomBuilder.BuilderModes.Editing
 					renderer.Finish();
 					renderer.Present();
 				}
-				Thread.Sleep(50);
+				Thread.Sleep(60);
 				Application.DoEvents();
 
 				// Start with a clear display
@@ -573,7 +573,7 @@ namespace CodeImp.DoomBuilder.BuilderModes.Editing
 					renderer.Finish();
 					renderer.Present();
 				}
-				Thread.Sleep(20);
+				Thread.Sleep(40);
 			}
 		}
 		

Source/Plugins/BuilderModes/Resources/Actions.cfg

@@ -76,6 +76,16 @@ thingsmode
 	allowscroll = true;
 }
 
+triangulatormode
+{
+	title = "Triangulator Mode";
+	category = "modes";
+	description = "Debug stuff";
+	allowkeys = true;
+	allowmouse = true;
+	allowscroll = true;
+}
+
 drawlinesmode
 {
 	title = "Start Drawing";