From 3c71164406ec9cdcd1e2f7d11a686cd509b5d837 Mon Sep 17 00:00:00 2001
From: Jaycie Ewald <jaycie.ewald@outlook.com>
Date: Sun, 10 Dec 2023 15:32:35 -0600
Subject: [PATCH] rt_floor.c: fixed incorrect floor rendering

from rottexpr #48: https://github.com/LTCHIPS/rottexpr/pull/48/
---
 source/rt_floor.c | 30 ++++++++++++++----------------
 1 file changed, 14 insertions(+), 16 deletions(-)

diff --git a/source/rt_floor.c b/source/rt_floor.c
index 9203f4b..831d744 100644
--- a/source/rt_floor.c
+++ b/source/rt_floor.c
@@ -495,6 +495,9 @@ void DrawHLine(int xleft, int xright, int yp)
 		buf = floor;
 		hd = yp - centery;
 		height = (hd << 13) / (maxheight - pheight + 32);
+
+		mr_xstep = FixedScale(viewsin << 8, maxheight - pheight + 32, hd);
+		mr_ystep = FixedScale(viewcos << 8, maxheight - pheight + 32, hd);
 	}
 	else
 	{
@@ -508,16 +511,14 @@ void DrawHLine(int xleft, int xright, int yp)
 
 		hd = centery - yp;
 		height = (hd << 13) / pheight;
+
+		mr_xstep = FixedScale(viewsin << 8, pheight, hd);
+		mr_ystep = FixedScale(viewcos << 8, pheight, hd);
 	}
 	SetFCLightLevel(height >> (8 - HEIGHTFRACTION - 1));
-	mr_xstep = ((viewsin << 8) / (height));
-	mr_ystep = ((viewcos << 8) / (height));
 
-	startxfrac = ((viewx >> 1) + FixedMulShift(mr_ystep, scale, 2)) -
-				 FixedMulShift(mr_xstep, (centerx - xleft), 2);
-
-	startyfrac = ((viewy >> 1) - FixedMulShift(mr_xstep, scale, 2)) -
-				 FixedMulShift(mr_ystep, (centerx - xleft), 2);
+	startxfrac = ((viewx << 14) + mr_ystep * scale) - (mr_xstep * (centerx - xleft));
+	startyfrac = ((viewy << 14) - mr_xstep * scale) - (mr_ystep * (centerx - xleft));
 
 	dest = (byte *)bufferofs + ylookup[yp];
 
@@ -529,10 +530,6 @@ void DrawHLine(int xleft, int xright, int yp)
 			mr_xfrac = startxfrac;
 			mr_yfrac = startyfrac;
 
-			// back off the pixel increment (orig. is 4x)
-			mr_xstep >>= 2;
-			mr_ystep >>= 2;
-
 			mr_count = xright - xleft + 1;
 
 			if (mr_count)
@@ -596,18 +593,19 @@ void DrawPlanes(void)
 
 void DrawRow(int count, byte *dest, byte *src)
 {
-	unsigned frac, fracstep;
+	unsigned xfrac, yfrac;
 	int coord;
 
-	frac = (mr_yfrac << 16) + (mr_xfrac & 0xffff);
-	fracstep = (mr_ystep << 16) + (mr_xstep & 0xffff);
+	xfrac = mr_xfrac;
+	yfrac = mr_yfrac;
 
 	while (count--)
 	{
 		/* extract the x/y coordinates */
-		coord = ((frac >> (32 - 7)) | ((frac >> (32 - 23)) << 7)) & 16383;
+		coord = ((yfrac >> 24) & 0b00000001111111) | ((xfrac >> 17) & 0b11111110000000);
 
 		*dest++ = shadingtable[src[coord]];
-		frac += fracstep;
+		xfrac += mr_xstep;
+		yfrac += mr_ystep;
 	}
 }