- flip backwards oriented wall sprites before submitting them to the render list.

Backwards orientation will break the translucent object sorter so this needs to be sorted out beforehand.

source/core/gamefuncs.h

@@ -91,6 +91,12 @@ inline double PointOnLineSide(const DVector2 &pos, const walltype *line)
     return (pos.X - WallStartX(line)) * WallDelta(line).Y - (pos.Y - WallStartY(line)) * WallDelta(line).X;
 }
 
+template<class T>
+inline double PointOnLineSide(const TVector2<T>& pos, const TVector2<T>& linestart, const TVector2<T>& lineend)
+{
+    return (pos.X - linestart.X) * (lineend.Y - linestart.Y) - (pos.Y - linestart.Y) * (lineend.X - linestart.X);
+}
+
 inline int sectorofwall(int wallNum)
 {
     if ((unsigned)wallNum < (unsigned)numwalls) return wall[wallNum].sector;

source/core/rendering/scene/hw_walls.cpp

@@ -1089,7 +1089,6 @@ void HWWall::ProcessWallSprite(HWDrawInfo* di, spritetype* spr, sectortype* sect
 	zbottom[0] = zbottom[1] = (sprz) * (1 / -256.);
 	ztop[0] = ztop[1] =  (sprz - ((height * spr->yrepeat) << 2)) * (1 / -256.);
 
-
 	// Clip sprites to ceilings/floors
 	float origz = ztop[0];
 	float polyh = (zbottom[0] - origz);
@@ -1113,5 +1112,16 @@ void HWWall::ProcessWallSprite(HWDrawInfo* di, spritetype* spr, sectortype* sect
 			zbottom[0] = zbottom[1] = floorz;
 		}
 	}
+
+	// If the sprite is backward, flip it around so that we have guaranteed orientation when this is about to be sorted.
+	if (PointOnLineSide(di->Viewpoint.Pos.XY(), DVector2(glseg.x1, glseg.y1), DVector2(glseg.x2, glseg.y2)) < 0)
+	{
+		std::swap(glseg.x1, glseg.x2);
+		std::swap(glseg.y1, glseg.y2);
+		// z is always the same on both sides.
+		std::swap(tcs[LOLFT], tcs[LORGT]);
+		std::swap(tcs[UPLFT], tcs[UPRGT]);
+	}
+
 	PutWall(di, spriteHasTranslucency(sprite));
 }