- Instead of setting the default skybox in every sector without an explicit skybox set, set it

once in the level struct and then use that for sectors with a NULL skybox. This fixes zpack's
  E2M3 so that when it removes its sector stacks, you will get the default skybox in their
  place, since stacked sectors and skyboxes use the same pointers in a sector.

SVN r4224 (trunk)

src/dobjgc.cpp

@@ -307,6 +307,7 @@ static void MarkRoot()
 	DThinker::MarkRoots();
 	FCanvasTextureInfo::Mark();
 	Mark(DACSThinker::ActiveThinker);
+	Mark(level.DefaultSkybox);
 	// Mark dead bodies.
 	for (i = 0; i < BODYQUESIZE; ++i)
 	{

src/g_level.cpp

@@ -77,6 +77,7 @@
 #include "d_netinf.h"
 #include "v_palette.h"
 #include "menu/menu.h"
+#include "a_sharedglobal.h"
 #include "a_strifeglobal.h"
 #include "r_data/colormaps.h"
 #include "farchive.h"
@@ -1272,6 +1273,7 @@ void G_InitLevelLocals ()
 	NormalLight.ChangeFade (level.fadeto);
 
 	level.DefaultEnvironment = info->DefaultEnvironment;
+	level.DefaultSkybox = NULL;
 }
 
 //==========================================================================
@@ -1435,6 +1437,11 @@ void G_SerializeLevel (FArchive &arc, bool hubLoad)
 	P_SerializeSubsectors(arc);
 	StatusBar->Serialize (arc);
 
+	if (SaveVersion >= 4222)
+	{ // This must be done *after* thinkers are serialized.
+		arc << level.DefaultSkybox;
+	}
+
 	arc << level.total_monsters << level.total_items << level.total_secrets;
 
 	// Does this level have custom translations?

src/g_level.h

@@ -426,6 +426,8 @@ struct FLevelLocals
 	int			airsupply;
 	int			DefaultEnvironment;		// Default sound environment.
 
+	TObjPtr<class ASkyViewpoint> DefaultSkybox;
+
 	FSectorScrollValues	*Scrolls;		// NULL if no DScrollers in this level
 
 	SBYTE		WallVertLight;			// Light diffs for vert/horiz walls

src/g_shared/a_skies.cpp

@@ -49,21 +49,9 @@ void ASkyViewpoint::BeginPlay ()
 {
 	Super::BeginPlay ();
 
-	if (tid == 0)
+	if (tid == 0 && level.DefaultSkybox == NULL)
 	{
-		int i;
-
-		for (i = 0; i <numsectors; i++)
-		{
-			if (sectors[i].FloorSkyBox == NULL)
-			{
-				sectors[i].FloorSkyBox = this;
-			}
-			if (sectors[i].CeilingSkyBox == NULL)
-			{
-				sectors[i].CeilingSkyBox = this;
-			}
-		}
+		level.DefaultSkybox = this;
 	}
 }
 
@@ -87,6 +75,10 @@ void ASkyViewpoint::Destroy ()
 			sectors[i].CeilingSkyBox = NULL;
 		}
 	}
+	if (level.DefaultSkybox == this)
+	{
+		level.DefaultSkybox = NULL;
+	}
 	Super::Destroy();
 }
 

src/r_bsp.cpp

@@ -1015,6 +1015,7 @@ void R_Subsector (subsector_t *sub)
 	int          ceilinglightlevel;		// killough 4/11/98
 	bool		 outersubsector;
 	int	fll, cll, position;
+	ASkyViewpoint *skybox;
 
 	// kg3D - fake floor stuff
 	visplane_t *backupfp;
@@ -1082,9 +1083,10 @@ void R_Subsector (subsector_t *sub)
 		basecolormap = frontsector->ColorMap;
 	}
 
+	skybox = frontsector->CeilingSkyBox != NULL ? frontsector->CeilingSkyBox : level.DefaultSkybox;
 	ceilingplane = frontsector->ceilingplane.PointOnSide(viewx, viewy, viewz) > 0 ||
 		frontsector->GetTexture(sector_t::ceiling) == skyflatnum ||
-		(frontsector->CeilingSkyBox != NULL && frontsector->CeilingSkyBox->bAlways) ||
+		(skybox != NULL && skybox->bAlways) ||
 		(frontsector->heightsec && 
 		 !(frontsector->heightsec->MoreFlags & SECF_IGNOREHEIGHTSEC) &&
 		 frontsector->heightsec->GetTexture(sector_t::floor) == skyflatnum) ?
@@ -1099,7 +1101,7 @@ void R_Subsector (subsector_t *sub)
 					frontsector->GetYScale(sector_t::ceiling),
 					frontsector->GetAngle(sector_t::ceiling),
 					frontsector->sky,
-					frontsector->CeilingSkyBox
+					skybox
 					) : NULL;
 
 	if (fixedlightlev < 0 && frontsector->e && frontsector->e->XFloor.lightlist.Size())
@@ -1121,9 +1123,10 @@ void R_Subsector (subsector_t *sub)
 	// killough 3/7/98: Add (x,y) offsets to flats, add deep water check
 	// killough 3/16/98: add floorlightlevel
 	// killough 10/98: add support for skies transferred from sidedefs
+	skybox = frontsector->FloorSkyBox != NULL ? frontsector->FloorSkyBox : level.DefaultSkybox;
 	floorplane = frontsector->floorplane.PointOnSide(viewx, viewy, viewz) > 0 || // killough 3/7/98
 		frontsector->GetTexture(sector_t::floor) == skyflatnum ||
-		(frontsector->FloorSkyBox != NULL && frontsector->FloorSkyBox->bAlways) ||
+		(skybox != NULL && skybox->bAlways) ||
 		(frontsector->heightsec &&
 		 !(frontsector->heightsec->MoreFlags & SECF_IGNOREHEIGHTSEC) &&
 		 frontsector->heightsec->GetTexture(sector_t::ceiling) == skyflatnum) ?
@@ -1138,7 +1141,7 @@ void R_Subsector (subsector_t *sub)
 					frontsector->GetYScale(sector_t::floor),
 					frontsector->GetAngle(sector_t::floor),
 					frontsector->sky,
-					frontsector->FloorSkyBox
+					skybox
 					) : NULL;
 
 	// kg3D - fake planes rendering