diff --git a/src/swrenderer/drawers/r_draw.cpp b/src/swrenderer/drawers/r_draw.cpp
index 048d0807a..d7a6ddd41 100644
--- a/src/swrenderer/drawers/r_draw.cpp
+++ b/src/swrenderer/drawers/r_draw.cpp
@@ -289,6 +289,8 @@ namespace swrenderer
 		float idepth;
 	};
 
+	// #define DEPTH_DEBUG
+
 	class DepthSpanCommand : public DrawerCommand
 	{
 	public:
@@ -297,6 +299,9 @@ namespace swrenderer
 			y = args.DestY();
 			x1 = args.DestX1();
 			x2 = args.DestX2();
+			#ifdef DEPTH_DEBUG
+			dest = (uint32_t*)args.Viewport()->GetDest(0, args.DestY());
+			#endif
 		}
 
 		FString DebugInfo() override { return "DepthSpanCommand"; }
@@ -314,9 +319,16 @@ namespace swrenderer
 			if (idepth1 == idepth2)
 			{
 				float depth = idepth1;
+				#ifdef DEPTH_DEBUG
+				uint32_t gray = clamp<int32_t>((int32_t)(1.0f / depth / 4.0f), 0, 255);
+				uint32_t color = MAKEARGB(255, gray, gray, gray);
+				#endif
 				for (int x = x1; x <= end; x++)
 				{
 					values[x] = depth;
+					#ifdef DEPTH_DEBUG
+					dest[x] = color;
+					#endif
 				}
 			}
 			else
@@ -325,6 +337,12 @@ namespace swrenderer
 				float step = (idepth2 - idepth1) / (x2 - x1 + 1);
 				for (int x = x1; x <= end; x++)
 				{
+					#ifdef DEPTH_DEBUG
+					uint32_t gray = clamp<int32_t>((int32_t)(1.0f / depth / 4.0f), 0, 255);
+					uint32_t color = MAKEARGB(255, gray, gray, gray);
+					dest[x] = color;
+					#endif
+
 					values[x] = depth;
 					depth += step;
 				}
@@ -334,6 +352,9 @@ namespace swrenderer
 	private:
 		int y, x1, x2;
 		float idepth1, idepth2;
+		#ifdef DEPTH_DEBUG
+		uint32_t *dest;
+		#endif
 	};
 
 	void SWPixelFormatDrawers::DrawDepthSkyColumn(const SkyDrawerArgs &args, float idepth)
diff --git a/src/swrenderer/plane/r_slopeplane.cpp b/src/swrenderer/plane/r_slopeplane.cpp
index 6c2a5175b..df307e425 100644
--- a/src/swrenderer/plane/r_slopeplane.cpp
+++ b/src/swrenderer/plane/r_slopeplane.cpp
@@ -89,11 +89,10 @@ namespace swrenderer
 		auto viewport = Thread->Viewport.get();
 
 		DVector3 worldNormal = pl->height.Normal();
-		double worldD = pl->height.fD();
 		planeNormal.X = worldNormal.X * viewport->viewpoint.Sin - worldNormal.Y * viewport->viewpoint.Cos;
 		planeNormal.Y = worldNormal.X * viewport->viewpoint.Cos + worldNormal.Y * viewport->viewpoint.Sin;
 		planeNormal.Z = worldNormal.Z;
-		planeD = worldD + planeNormal.Z * viewport->viewpoint.Pos.Z;
+		planeD = -planeNormal.Z * (pl->height.ZatPoint(viewport->viewpoint.Pos.X, viewport->viewpoint.Pos.Y) - viewport->viewpoint.Pos.Z);
 
 
 		drawerargs.SetSolidColor(3);
@@ -213,24 +212,23 @@ namespace swrenderer
 
 	void RenderSlopePlane::RenderLine(int y, int x1, int x2)
 	{
+		drawerargs.DrawTiltedSpan(Thread, y, x1, x2, plane_sz, plane_su, plane_sv, plane_shade, planeshade, planelightfloat, pviewx, pviewy, basecolormap);
+
 		if (r_models)
 		{
-			// Calculate normalized device coordinates for the span
-			// 1.5 is not a typo. The range is [x1,x2]
-			double devY = -(y + 0.5 - Thread->Viewport->CenterY) / Thread->Viewport->InvZtoScale / Thread->Viewport->viewwindow.FocalTangent;
-			double devX1 = (x1 + 0.5 - Thread->Viewport->CenterX) / Thread->Viewport->CenterX;
-			double devX2 = (x2 + 1.5 - Thread->Viewport->CenterX) / Thread->Viewport->CenterX;
+			double viewZ = 1.0;
+			double viewX1 = Thread->Viewport->ScreenToViewX(x1, viewZ);
+			double viewX2 = Thread->Viewport->ScreenToViewX(x2 + 1, viewZ);
+			double viewY = Thread->Viewport->ScreenToViewY(y, viewZ);
 
 			// Find depth values for the span
-			float zbufferdepth1 = (float)-planeD / (planeNormal | DVector3(devX1, 1.0, devY));
-			float zbufferdepth2 = (float)-planeD / (planeNormal | DVector3(devX2, 1.0, devY));
+			float zbufferdepth1 = (float)(-planeD / (planeNormal | DVector3(viewX1, viewZ, viewY)));
+			float zbufferdepth2 = (float)(-planeD / (planeNormal | DVector3(viewX2, viewZ, viewY)));
 
 			drawerargs.SetDestX1(x1);
 			drawerargs.SetDestX2(x2);
 			drawerargs.SetDestY(Thread->Viewport.get(), y);
 			drawerargs.DrawDepthSpan(Thread, 1.0f / zbufferdepth1, 1.0f / zbufferdepth2);
 		}
-
-		drawerargs.DrawTiltedSpan(Thread, y, x1, x2, plane_sz, plane_su, plane_sv, plane_shade, planeshade, planelightfloat, pviewx, pviewy, basecolormap);
 	}
 }
diff --git a/src/swrenderer/viewport/r_viewport.h b/src/swrenderer/viewport/r_viewport.h
index 0eee6fa1c..1c6510cca 100644
--- a/src/swrenderer/viewport/r_viewport.h
+++ b/src/swrenderer/viewport/r_viewport.h
@@ -68,7 +68,17 @@ namespace swrenderer
 			else
 				return FocalLengthY / (screenY + 0.5 - CenterY) * planeHeight;
 		}
-		
+
+		double ScreenToViewX(int screenX, double viewZ) const
+		{
+			return (screenX + 0.5 - CenterX) / FocalLengthX * viewZ;
+		}
+
+		double ScreenToViewY(int screenY, double viewZ) const
+		{
+			return (CenterY - screenY - 0.5) / FocalLengthY * viewZ;
+		}
+
 	private:
 		void InitTextureMapping();
 		void SetupBuffer();