From b443ac8f719e02050382efd1b7c097d6d75d6113 Mon Sep 17 00:00:00 2001
From: Christoph Oelckers <c.oelckers@zdoom.fake>
Date: Sun, 1 May 2016 19:30:51 +0200
Subject: [PATCH] - let Trace() be a bit smarter about checking 3D slopes. If
 one matches right up with the floor or ceiling at the line where the trace
 enters a sector, check a second point to see whether we deal with something
 inside valid sector bounds or not.

---
 src/p_trace.cpp | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/src/p_trace.cpp b/src/p_trace.cpp
index 8a3664ac8..67a516f9c 100644
--- a/src/p_trace.cpp
+++ b/src/p_trace.cpp
@@ -494,6 +494,17 @@ bool FTraceInfo::LineCheck(intercept_t *in, double dist, DVector3 hit)
 					// clip to the part of the sector we are in
 					if (hit.Z > ff_top)
 					{
+						// 3D floor height is the same as the floor height. We need to test a second spot to see if it is above or below
+						if (fabs(bf - ff_top) < EQUAL_EPSILON)
+						{
+							double cf = entersector->floorplane.ZatPoint(entersector->centerspot);
+							double ffc = rover->top.plane->ZatPoint(entersector->centerspot);
+							if (ffc > cf)
+							{
+								bf = ff_top - EQUAL_EPSILON;
+							}
+						}
+						
 						// above
 						if (bf < ff_top)
 						{
@@ -505,6 +516,17 @@ bool FTraceInfo::LineCheck(intercept_t *in, double dist, DVector3 hit)
 					}
 					else if (hit.Z < ff_bottom)
 					{
+						// 3D floor height is the same as the ceiling height. We need to test a second spot to see if it is above or below
+						if (fabs(bc - ff_bottom) < EQUAL_EPSILON)
+						{
+							double cc = entersector->ceilingplane.ZatPoint(entersector->centerspot);
+							double fcc = rover->bottom.plane->ZatPoint(entersector->centerspot);
+							if (fcc < cc)
+							{
+								bc = ff_bottom + EQUAL_EPSILON;
+							}
+						}
+
 						//below
 						if (bc > ff_bottom)
 						{