From a6458d2716488d7f3b36ba22aee971fbacf348aa Mon Sep 17 00:00:00 2001
From: staphen <staphen@gmail.com>
Date: Sun, 19 Dec 2021 00:56:49 -0500
Subject: [PATCH] Prevent erroneous diagonal traversal in span filling
 algorithm

---
 Source/gendung.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Source/gendung.cpp b/Source/gendung.cpp
index 5ad9231bb..0b3a226c5 100644
--- a/Source/gendung.cpp
+++ b/Source/gendung.cpp
@@ -360,7 +360,8 @@ void FindTransparencyValues(Point floor, uint8_t floorID)
 			seedStack.push(std::make_tuple(scanLeft, scanRight - 1, y + dy, dy));
 			if (scanRight - 1 > scanEnd)
 				seedStack.push(std::make_tuple(scanEnd + 1, scanRight - 1, y - dy, -dy));
-			checkDiagonals({ scanRight - 1, y }, right);
+			if (scanLeft < scanRight)
+				checkDiagonals({ scanRight - 1, y }, right);
 
 			while (scanRight < scanEnd && !isInside(scanRight, y))
 				scanRight++;