Seriously improves the reliability of the linkdraw system (in Software, the only rendering engine which actually does anything with this information), reducing the number of visual errors it produces significantly.

Of particular note: Standing in shallow pools of water as Smiles will now reliably associate the bottom half of the tails following mobj with the underwater portion of Smiles' body, and the flicking ends with his un-submerged head.

Deliberately engineered to only affect objects with MF2_LINKDRAW applied.

src/r_things.c

@@ -1049,6 +1049,8 @@ static void R_SplitSprite(vissprite_t *sprite, mobj_t *thing)
 		// adjust the heights.
 		newsprite = M_Memcpy(R_NewVisSprite(), sprite, sizeof (vissprite_t));
 
+		newsprite->cut |= (sprite->cut & SC_LINKDRAW);
+
 		sprite->cut |= SC_BOTTOM;
 		sprite->gz = testheight;
 
@@ -1109,6 +1111,7 @@ static void R_SplitSprite(vissprite_t *sprite, mobj_t *thing)
 //
 static void R_ProjectSprite(mobj_t *thing)
 {
+	mobj_t *oldthing = thing;
 	fixed_t tr_x, tr_y;
 	fixed_t gxt, gyt;
 	fixed_t tx, tz;
@@ -1128,6 +1131,8 @@ static void R_ProjectSprite(mobj_t *thing)
 
 	vissprite_t *vis;
 
+	spritecut_e cut = SC_NONE;
+
 	angle_t ang = 0; // compiler complaints
 	fixed_t iscale;
 	fixed_t scalestep;
@@ -1326,24 +1331,10 @@ static void R_ProjectSprite(mobj_t *thing)
 	if ((thing->flags2 & MF2_LINKDRAW) && thing->tracer) // toast 16/09/16 (SYMMETRY)
 	{
 		fixed_t linkscale;
-#if 0 // support for chains of linkdraw - probably not network safe to modify mobjs during rendering
-		mobj_t *link, *link2;
 
-		for (link = thing->tracer; (link->tracer && (link->flags2 & MF2_LINKDRAW)); link = link->tracer)
+		thing = thing->tracer;
-			link->flags2 &= ~MF2_LINKDRAW; // to prevent infinite loops, otherwise would just be a ;
+		tr_x = thing->x - viewx;
-
+		tr_y = thing->y - viewy;
-		for (link2 = thing->tracer; (link2->tracer && (link2 != link)); link2 = link2->tracer)
-			link->flags2 |= MF2_LINKDRAW; // only needed for correction of the above
-
-		if (link->flags2 & MF2_LINKDRAW)
-			link->flags2 &= ~MF2_LINKDRAW; // let's try and make sure this doesn't happen again...
-
-		tr_x = link->x - viewx;
-		tr_y = link->y - viewy;
-#else
-		tr_x = thing->tracer->x - viewx;
-		tr_y = thing->tracer->y - viewy;
-#endif
 		gxt = FixedMul(tr_x, viewcos);
 		gyt = -FixedMul(tr_y, viewsin);
 		tz = gxt-gyt;
@@ -1356,6 +1347,7 @@ static void R_ProjectSprite(mobj_t *thing)
 			dispoffset *= -1; // if it's physically behind, make sure it's ordered behind (if dispoffset > 0)
 
 		sortscale = linkscale; // now make sure it's linked
+		cut = SC_LINKDRAW;
 	}
 
 	// PORTAL SPRITE CLIPPING
@@ -1374,18 +1366,18 @@ static void R_ProjectSprite(mobj_t *thing)
 		// When vertical flipped, draw sprites from the top down, at least as far as offsets are concerned.
 		// sprite height - sprite topoffset is the proper inverse of the vertical offset, of course.
 		// remember gz and gzt should be seperated by sprite height, not thing height - thing height can be shorter than the sprite itself sometimes!
-		gz = thing->z + thing->height - FixedMul(spritecachedinfo[lump].topoffset, this_scale);
+		gz = oldthing->z + oldthing->height - FixedMul(spritecachedinfo[lump].topoffset, this_scale);
 		gzt = gz + FixedMul(spritecachedinfo[lump].height, this_scale);
 	}
 	else
 	{
-		gzt = thing->z + FixedMul(spritecachedinfo[lump].topoffset, this_scale);
+		gzt = oldthing->z + FixedMul(spritecachedinfo[lump].topoffset, this_scale);
 		gz = gzt - FixedMul(spritecachedinfo[lump].height, this_scale);
 	}
 
 	if (thing->subsector->sector->cullheight)
 	{
-		if (R_DoCulling(thing->subsector->sector->cullheight, viewsector->cullheight, viewz, gz, gzt))
+		if (R_DoCulling(oldthing->subsector->sector->cullheight, viewsector->cullheight, viewz, gz, gzt))
 			return;
 	}
 
@@ -1469,7 +1461,7 @@ static void R_ProjectSprite(mobj_t *thing)
 	vis->sector = thing->subsector->sector;
 	vis->szt = (INT16)((centeryfrac - FixedMul(vis->gzt - viewz, sortscale))>>FRACBITS);
 	vis->sz = (INT16)((centeryfrac - FixedMul(vis->gz - viewz, sortscale))>>FRACBITS);
-	vis->cut = SC_NONE;
+	vis->cut = cut;
 	if (thing->subsector->sector->numlights)
 		vis->extra_colormap = thing->subsector->sector->lightlist[light].extra_colormap;
 	else
@@ -1506,12 +1498,12 @@ static void R_ProjectSprite(mobj_t *thing)
 	// specific translucency
 	if (!cv_translucency.value)
 		; // no translucency
-	else if (thing->flags2 & MF2_SHADOW) // actually only the player should use this (temporary invisibility)
+	else if (oldthing->flags2 & MF2_SHADOW) // actually only the player should use this (temporary invisibility)
 		vis->transmap = transtables + ((tr_trans80-1)<<FF_TRANSSHIFT); // because now the translucency is set through FF_TRANSMASK
-	else if (thing->frame & FF_TRANSMASK)
+	else if (oldthing->frame & FF_TRANSMASK)
-		vis->transmap = transtables + (thing->frame & FF_TRANSMASK) - 0x10000;
+		vis->transmap = transtables + (oldthing->frame & FF_TRANSMASK) - 0x10000;
 
-	if (((thing->frame & FF_FULLBRIGHT) || (thing->flags2 & MF2_SHADOW))
+	if (((oldthing->frame & FF_FULLBRIGHT) || (oldthing->flags2 & MF2_SHADOW))
 		&& (!vis->extra_colormap || !vis->extra_colormap->fog))
 	{
 		// full bright: goggles
@@ -1535,7 +1527,7 @@ static void R_ProjectSprite(mobj_t *thing)
 	vis->isScaled = false;
 
 	if (thing->subsector->sector->numlights)
-		R_SplitSprite(vis, thing);
+		R_SplitSprite(vis, oldthing);
 
 	// Debug
 	++objectsdrawn;
@@ -1559,7 +1551,7 @@ static void R_ProjectPrecipitationSprite(precipmobj_t *thing)
 	fixed_t iscale;
 
 	//SoM: 3/17/2000
-	fixed_t gz ,gzt;
+	fixed_t gz, gzt;
 
 	// transform the origin point
 	tr_x = thing->x - viewx;
@@ -1821,13 +1813,17 @@ void R_SortVisSprites(void)
 
 		ds->next = dsnext;
 		ds->prev = dsprev;
+		ds->linkdraw = NULL;
 	}
 
 	// Fix first and last. ds still points to the last one after the loop
 	dsfirst->prev = &unsorted;
 	unsorted.next = dsfirst;
 	if (ds)
+	{
 		ds->next = &unsorted;
+		ds->linkdraw = NULL;
+	}
 	unsorted.prev = ds;
 
 	// pull the vissprites out by scale
@@ -1857,6 +1853,47 @@ void R_SortVisSprites(void)
 		vsprsortedhead.prev->next = best;
 		vsprsortedhead.prev = best;
 	}
+
+	// bundle linkdraw
+	for (ds = vsprsortedhead.prev; ds != &vsprsortedhead; ds = ds->prev)
+	{
+		if (!(ds->cut & SC_LINKDRAW))
+			continue;
+
+		// reuse dsfirst...
+		for (dsfirst = vsprsortedhead.prev; dsfirst != &vsprsortedhead; dsfirst = dsfirst->prev)
+		{
+			// don't connect if it's also a link
+			if (dsfirst->cut & SC_LINKDRAW)
+				continue;
+
+			// don't connect if it's not the tracer
+			if (dsfirst->mobj != ds->mobj)
+				continue;
+
+			// don't connect if the tracer's top is cut off, but lower than the link's top
+			if ((dsfirst->cut & SC_TOP)
+			&& dsfirst->szt > ds->szt)
+				continue;
+
+			// don't connect if the tracer's bottom is cut off, but higher than the link's bottom
+			if ((dsfirst->cut & SC_BOTTOM)
+			&& dsfirst->sz < ds->sz)
+				continue;
+
+			break;
+		}
+
+		// remove from chain
+		ds->next->prev = ds->prev;
+		ds->prev->next = ds->next;
+
+		if (dsfirst != &vsprsortedhead)
+		{
+			ds->next = dsfirst->linkdraw;
+			dsfirst->linkdraw = ds;
+		}
+	}
 }
 
 //
@@ -2453,8 +2490,29 @@ void R_DrawMasked(void)
 			// Tails 08-18-2002
 			if (r2->sprite->precip == true)
 				R_DrawPrecipitationSprite(r2->sprite);
-			else
+			else if (!r2->sprite->linkdraw)
 				R_DrawSprite(r2->sprite);
+			else // unbundle linkdraw
+			{
+				vissprite_t *ds = ds = r2->sprite->linkdraw;
+
+				if (r2->sprite->dispoffset < ds->dispoffset)
+				{
+					r2->sprite->next = r2->sprite->linkdraw;
+					r2->sprite->linkdraw = r2->sprite;
+				}
+				else
+				{
+					for (; ds->next != NULL; ds = ds->next)
+						if (r2->sprite->dispoffset < ds->next->dispoffset)
+							break;
+					r2->sprite->next = ds->next;
+					ds->next = r2->sprite;
+				}
+
+				for (ds = r2->sprite->linkdraw; ds != NULL; ds = ds->next)
+					R_DrawSprite(ds);
+			}
 
 			R_DoneWithNode(r2);
 			r2 = next;

src/r_things.h

@@ -129,7 +129,8 @@ typedef enum
 {
 	SC_NONE = 0,
 	SC_TOP = 1,
-	SC_BOTTOM = 2
+	SC_BOTTOM = 2,
+	SC_LINKDRAW = 4
 } spritecut_e;
 
 // A vissprite_t is a thing that will be drawn during a refresh,
@@ -140,6 +141,9 @@ typedef struct vissprite_s
 	struct vissprite_s *prev;
 	struct vissprite_s *next;
 
+	// Bonus Linkdraw pointer.
+	struct vissprite_s *linkdraw;
+
 	mobj_t *mobj; // for easy access
 
 	INT32 x1, x2;