From 872e2f82c154359ac5f8e33e5c9ffeeef5a84017 Mon Sep 17 00:00:00 2001
From: Monster Iestyn <iestynjealous@ntlworld.com>
Date: Fri, 28 Sep 2018 17:02:02 +0100
Subject: [PATCH] Redo OpenGL skewing support, so that it now itself supports
 the lower unpegged effect for FOFs in OpenGL!

---
 src/hardware/hw_main.c | 46 ++++++++++++++++++++++++++++++++++--------
 1 file changed, 38 insertions(+), 8 deletions(-)

diff --git a/src/hardware/hw_main.c b/src/hardware/hw_main.c
index d7c73415..92925c9b 100644
--- a/src/hardware/hw_main.c
+++ b/src/hardware/hw_main.c
@@ -2183,6 +2183,10 @@ static void HWR_StoreWallRange(double startfrac, double endfrac)
 				else if (drawtextured)
 				{
 					fixed_t texturevpeg;
+					boolean attachtobottom = false;
+#ifdef ESLOPE
+					boolean slopeskew = false; // skew FOF walls with slopes?
+#endif
 
 					// Wow, how was this missing from OpenGL for so long?
 					// ...Oh well, anyway, Lower Unpegged now changes pegging of FOFs like in software
@@ -2190,24 +2194,50 @@ static void HWR_StoreWallRange(double startfrac, double endfrac)
 					if (newline)
 					{
 						texturevpeg = sides[newline->sidenum[0]].rowoffset;
-						if (newline->flags & ML_DONTPEGBOTTOM)
-							texturevpeg -= *rover->topheight - *rover->bottomheight;
+						attachtobottom = !!(newline->flags & ML_DONTPEGBOTTOM);
+#ifdef ESLOPE
+						slopeskew = !!(newline->flags & ML_DONTPEGTOP);
+#endif
 					}
 					else
 					{
 						texturevpeg = sides[rover->master->sidenum[0]].rowoffset;
-						if (gr_linedef->flags & ML_DONTPEGBOTTOM)
-							texturevpeg -= *rover->topheight - *rover->bottomheight;
+						attachtobottom = !!(gr_linedef->flags & ML_DONTPEGBOTTOM);
+#ifdef ESLOPE
+						slopeskew = !!(rover->master->flags & ML_DONTPEGTOP);
+#endif
 					}
 
 					grTex = HWR_GetTexture(texnum);
 
 #ifdef ESLOPE
-					wallVerts[3].t = (*rover->topheight - h + texturevpeg) * grTex->scaleY;
-					wallVerts[2].t = (*rover->topheight - hS + texturevpeg) * grTex->scaleY;
-					wallVerts[0].t = (*rover->topheight - l + texturevpeg) * grTex->scaleY;
-					wallVerts[1].t = (*rover->topheight - lS + texturevpeg) * grTex->scaleY;
+					if (!slopeskew) // no skewing
+					{
+						if (attachtobottom)
+							texturevpeg -= *rover->topheight - *rover->bottomheight;
+						wallVerts[3].t = (*rover->topheight - h + texturevpeg) * grTex->scaleY;
+						wallVerts[2].t = (*rover->topheight - hS + texturevpeg) * grTex->scaleY;
+						wallVerts[0].t = (*rover->topheight - l + texturevpeg) * grTex->scaleY;
+						wallVerts[1].t = (*rover->topheight - lS + texturevpeg) * grTex->scaleY;
+					}
+					else
+					{
+						if (!attachtobottom) // skew by top
+						{
+							wallVerts[3].t = wallVerts[2].t = texturevpeg * grTex->scaleY;
+							wallVerts[0].t = (h - l + texturevpeg) * grTex->scaleY;
+							wallVerts[1].t = (hS - lS + texturevpeg) * grTex->scaleY;
+						}
+						else // skew by bottom
+						{
+							wallVerts[0].t = wallVerts[1].t = texturevpeg * grTex->scaleY;
+							wallVerts[3].t = wallVerts[0].t - (h - l) * grTex->scaleY;
+							wallVerts[2].t = wallVerts[1].t - (hS - lS) * grTex->scaleY;
+						}
+					}
 #else
+					if (attachtobottom)
+						texturevpeg -= *rover->topheight - *rover->bottomheight;
 					wallVerts[3].t = wallVerts[2].t = (*rover->topheight - h + texturevpeg) * grTex->scaleY;
 					wallVerts[0].t = wallVerts[1].t = (*rover->topheight - l + texturevpeg) * grTex->scaleY;
 #endif