From 3209e4e467bdee42f1e51f009a7aa3dbb70d10f4 Mon Sep 17 00:00:00 2001
From: Fabian Greffrath <fabian@greffrath.com>
Date: Tue, 8 Aug 2023 12:30:57 +0200
Subject: [PATCH] alternative approach to stable vissprite sorting in Vanilla
 order (#1169)

---
 src/r_things.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/src/r_things.c b/src/r_things.c
index e31e3006..21eae9be 100644
--- a/src/r_things.c
+++ b/src/r_things.c
@@ -901,7 +901,7 @@ static void msort(vissprite_t **s, vissprite_t **t, int n)
       msort(s1, t, n1);
       msort(s2, t, n2);
 
-      while ((*s1)->scale > (*s2)->scale ?
+      while ((*s1)->scale >= (*s2)->scale ?
              (*d++ = *s1++, --n1) : (*d++ = *s2++, --n2));
 
       if (n2)
@@ -917,12 +917,10 @@ static void msort(vissprite_t **s, vissprite_t **t, int n)
       for (i = 1; i < n; i++)
         {
           vissprite_t *temp = s[i];
-          // [FG] change '<' to '<=' here and below, so that vissprites with the same scale
-          // are reordered, so that the object with the higher map index appears in front
-          if (s[i-1]->scale <= temp->scale)
+          if (s[i-1]->scale < temp->scale)
             {
               int j = i;
-              while ((s[j] = s[j-1])->scale <= temp->scale && --j);
+              while ((s[j] = s[j-1])->scale < temp->scale && --j);
               s[j] = temp;
             }
         }
@@ -946,8 +944,12 @@ void R_SortVisSprites (void)
                                   * sizeof *vissprite_ptrs, PU_STATIC, 0);
         }
 
+      // Sprites of equal distance need to be sorted in inverse order.
+      // This is most easily achieved by filling the sort array
+      // backwards before the sort.
+
       while (--i>=0)
-        vissprite_ptrs[i] = vissprites+i;
+        vissprite_ptrs[num_vissprite-i-1] = vissprites+i;
 
       // killough 9/22/98: replace qsort with merge sort, since the keys
       // are roughly in order to begin with, due to BSP rendering.