--- a/src/elrail.cpp	Tue Sep 18 20:41:59 2007 +0000
+++ b/src/elrail.cpp	Wed Sep 19 16:36:42 2007 +0000
@@ -168,6 +168,38 @@
 	return (z + 2) & ~3; // this means z = (z + TILE_HEIGHT / 4) / (TILE_HEIGHT / 2) * (TILE_HEIGHT / 2);
 }
 
+/**
+ * Draws wires on a tunnel tile
+ *
+ * DrawTile_TunnelBridge() calls this function to draw the wires as SpriteCombine with the tunnel roof.
+ *
+ * @param ti The Tileinfo to draw the tile for
+ */
+void DrawCatenaryOnTunnel(const TileInfo *ti)
+{
+	/* xmin, ymin, xmax + 1, ymax + 1 of BB */
+	static const int _tunnel_wire_BB[4][4] = {
+		{ 0, 1, 16, 15 }, // NE
+		{ 1, 0, 15, 16 }, // SE
+		{ 0, 1, 16, 15 }, // SW
+		{ 1, 0, 15, 16 }, // NW
+	};
+
+	if ((GetRailType(ti->tile) != RAILTYPE_ELECTRIC) || _patches.disable_elrails) return;
+
+	DiagDirection dir = GetTunnelDirection(ti->tile);
+
+	const SortableSpriteStruct *sss = &CatenarySpriteData_Tunnel[dir];
+	const int *BB_data = _tunnel_wire_BB[dir];
+	AddSortableSpriteToDraw(
+		sss->image, PAL_NONE, ti->x + sss->x_offset, ti->y + sss->y_offset,
+		BB_data[2] - sss->x_offset, BB_data[3] - sss->y_offset, BB_Z_SEPARATOR - sss->z_offset + 1,
+		GetTileZ(ti->tile) + sss->z_offset,
+		HASBIT(_transparent_opt, TO_BUILDINGS),
+		BB_data[0] - sss->x_offset, BB_data[1] - sss->y_offset, BB_Z_SEPARATOR - sss->z_offset
+	);
+}
+
 /** Draws wires and, if required, pylons on a given tile
  * @param ti The Tileinfo to draw the tile for
  */
@@ -300,9 +332,9 @@
 						continue; /* No neighbour, go looking for a better position */
 					}
 
-					AddSortableSpriteToDraw(pylon_sprites[temp], PAL_NONE, x, y, 1, 1, 10,
+					AddSortableSpriteToDraw(pylon_sprites[temp], PAL_NONE, x, y, 1, 1, BB_HEIGHT_UNDER_BRIDGE,
 							GetPCPElevation(ti->tile, i),
-							HASBIT(_transparent_opt, TO_BUILDINGS));
+							HASBIT(_transparent_opt, TO_BUILDINGS), -1, -1);
 					break; /* We already have drawn a pylon, bail out */
 				}
 			}
@@ -319,17 +351,7 @@
 	/* Drawing of pylons is finished, now draw the wires */
 	for (t = TRACK_BEGIN; t < TRACK_END; t++) {
 		if (HASBIT(trackconfig[TS_HOME], t)) {
-			if (IsTunnelTile(ti->tile)) {
-				const SortableSpriteStruct *sss = &CatenarySpriteData_Tunnel[GetTunnelDirection(ti->tile)];
-
-				AddSortableSpriteToDraw(
-					sss->image, PAL_NONE, ti->x + sss->x_offset, ti->y + sss->y_offset,
-					sss->x_size, sss->y_size, sss->z_size,
-					GetTileZ(ti->tile) + sss->z_offset,
-					HASBIT(_transparent_opt, TO_BUILDINGS)
-				);
-				break;
-			}
+			if (IsTunnelTile(ti->tile)) break; // drawn together with tunnel-roof (see DrawCatenaryOnTunnel())
 			byte PCPconfig = HASBIT(PCPstatus, PCPpositions[t][0]) +
 				(HASBIT(PCPstatus, PCPpositions[t][1]) << 1);
 
@@ -391,7 +413,7 @@
 		if (HASBIT(tlg, (axis == AXIS_X ? 0 : 1))) PPPpos = ReverseDir(PPPpos);
 		uint x = ti->x + x_pcp_offsets[PCPpos] + x_ppp_offsets[PPPpos];
 		uint y = ti->y + y_pcp_offsets[PCPpos] + y_ppp_offsets[PPPpos];
-		AddSortableSpriteToDraw(pylon_sprites[PPPpos], PAL_NONE, x, y, 1, 1, 10, height, HASBIT(_transparent_opt, TO_BUILDINGS));
+		AddSortableSpriteToDraw(pylon_sprites[PPPpos], PAL_NONE, x, y, 1, 1, BB_HEIGHT_UNDER_BRIDGE, height, HASBIT(_transparent_opt, TO_BUILDINGS), -1, -1);
 	}
 
 	/* need a pylon on the southern end of the bridge */
@@ -401,7 +423,7 @@
 		if (HASBIT(tlg, (axis == AXIS_X ? 0 : 1))) PPPpos = ReverseDir(PPPpos);
 		uint x = ti->x + x_pcp_offsets[PCPpos] + x_ppp_offsets[PPPpos];
 		uint y = ti->y + y_pcp_offsets[PCPpos] + y_ppp_offsets[PPPpos];
-		AddSortableSpriteToDraw(pylon_sprites[PPPpos], PAL_NONE, x, y, 1, 1, 10, height, HASBIT(_transparent_opt, TO_BUILDINGS));
+		AddSortableSpriteToDraw(pylon_sprites[PPPpos], PAL_NONE, x, y, 1, 1, BB_HEIGHT_UNDER_BRIDGE, height, HASBIT(_transparent_opt, TO_BUILDINGS), -1, -1);
 	}
 }
 

--- a/src/gfxinit.cpp	Tue Sep 18 20:41:59 2007 +0000
+++ b/src/gfxinit.cpp	Wed Sep 19 16:36:42 2007 +0000
@@ -384,6 +384,8 @@
 	assert(load_index == SPR_ONEWAY_BASE);
 	load_index += LoadGrfFile("oneway.grf", load_index, i++);
 
+	load_index++; // SPR_EMPTY_BOUNDING_BOX
+
 	assert(load_index == SPR_FLAGS_BASE);
 	load_index += LoadGrfFile("flags.grf", load_index, i++);
 

--- a/src/rail.h	Tue Sep 18 20:41:59 2007 +0000
+++ b/src/rail.h	Wed Sep 19 16:36:42 2007 +0000
@@ -788,6 +788,7 @@
  * @see DrawCatenaryRailway
  */
 void DrawCatenary(const TileInfo *ti);
+void DrawCatenaryOnTunnel(const TileInfo *ti);
 
 Foundation GetRailFoundation(Slope tileh, TrackBits bits);
 

--- a/src/rail_cmd.cpp	Tue Sep 18 20:41:59 2007 +0000
+++ b/src/rail_cmd.cpp	Wed Sep 19 16:36:42 2007 +0000
@@ -1262,7 +1262,7 @@
 		sprite = _signal_base + (GetSignalType(tile, track) - 1) * 16 + GetSignalVariant(tile, track) * 64 + image + condition;
 	}
 
-	AddSortableSpriteToDraw(sprite, PAL_NONE, x, y, 1, 1, 10, GetSlopeZ(x,y));
+	AddSortableSpriteToDraw(sprite, PAL_NONE, x, y, 1, 1, BB_HEIGHT_UNDER_BRIDGE, GetSlopeZ(x,y));
 }
 
 static uint32 _drawtile_track_palette;

--- a/src/road_cmd.cpp	Tue Sep 18 20:41:59 2007 +0000
+++ b/src/road_cmd.cpp	Wed Sep 19 16:36:42 2007 +0000
@@ -914,8 +914,8 @@
 		front = SPR_TRAMWAY_BASE + _road_frontwire_sprites_1[tram];
 	}
 
-	AddSortableSpriteToDraw(back,  PAL_NONE, ti->x, ti->y, 16, 16, 0x1F, ti->z, HASBIT(_transparent_opt, TO_BUILDINGS));
-	AddSortableSpriteToDraw(front, PAL_NONE, ti->x, ti->y, 16, 16, 0x1F, ti->z, HASBIT(_transparent_opt, TO_BUILDINGS));
+	AddSortableSpriteToDraw(back,  PAL_NONE, ti->x, ti->y, 16, 16, TILE_HEIGHT + BB_HEIGHT_UNDER_BRIDGE, ti->z, HASBIT(_transparent_opt, TO_BUILDINGS));
+	AddSortableSpriteToDraw(front, PAL_NONE, ti->x, ti->y, 16, 16, TILE_HEIGHT + BB_HEIGHT_UNDER_BRIDGE, ti->z, HASBIT(_transparent_opt, TO_BUILDINGS));
 }
 
 /**

--- a/src/table/sprites.h	Tue Sep 18 20:41:59 2007 +0000
+++ b/src/table/sprites.h	Wed Sep 19 16:36:42 2007 +0000
@@ -176,8 +176,11 @@
 	/* One way road sprites */
 	SPR_ONEWAY_BASE = SPR_TRAMWAY_BASE + 113,
 
+	/* Not really a sprite, but an empty bounding box. Used to construct bounding boxes, that help sorting the sprites, but do not have a sprite associated. */
+	SPR_EMPTY_BOUNDING_BOX = SPR_ONEWAY_BASE + 6,
+
 	/* Flags sprites (in same order as enum NetworkLanguage) */
-	SPR_FLAGS_BASE = SPR_ONEWAY_BASE + 6,
+	SPR_FLAGS_BASE = SPR_EMPTY_BOUNDING_BOX + 1,
 
 	/* Manager face sprites */
 	SPR_GRADIENT = 874, // background gradient behind manager face

--- a/src/tree_cmd.cpp	Tue Sep 18 20:41:59 2007 +0000
+++ b/src/tree_cmd.cpp	Wed Sep 19 16:36:42 2007 +0000
@@ -459,7 +459,7 @@
 
 			if (tep == NULL) break;
 
-			AddSortableSpriteToDraw(tep->image, tep->pal, ti->x + tep->x, ti->y + tep->y, 5, 5, 0x10, z, HASBIT(_transparent_opt, TO_TREES));
+			AddSortableSpriteToDraw(tep->image, tep->pal, ti->x + tep->x, ti->y + tep->y, 16 - tep->x, 16 - tep->y, 0x30, z, HASBIT(_transparent_opt, TO_TREES), -tep->x, -tep->y);
 			tep->image = 0;
 		}
 	}

--- a/src/tunnelbridge_cmd.cpp	Tue Sep 18 20:41:59 2007 +0000
+++ b/src/tunnelbridge_cmd.cpp	Wed Sep 19 16:36:42 2007 +0000
@@ -859,11 +859,11 @@
 			 * sprites is at the top
 			 */
 			if (z >= front_height) { // front facing pillar
-				AddSortableSpriteToDraw(image, psid->pal, x, y, p[4], p[5], 1, z, HASBIT(_transparent_opt, TO_BRIDGES));
+				AddSortableSpriteToDraw(image, psid->pal, x, y, p[4], p[5], BB_HEIGHT_UNDER_BRIDGE - 5, z, HASBIT(_transparent_opt, TO_BRIDGES), 0, 0, -5);
 			}
 
 			if (drawfarpillar && z >= back_height && z < i - TILE_HEIGHT) { // back facing pillar
-				AddSortableSpriteToDraw(image, psid->pal, x - p[6], y - p[7], p[4], p[5], 1, z, HASBIT(_transparent_opt, TO_BRIDGES));
+				AddSortableSpriteToDraw(image, psid->pal, x - p[6], y - p[7], p[4], p[5], BB_HEIGHT_UNDER_BRIDGE - 5, z, HASBIT(_transparent_opt, TO_BRIDGES), 0, 0, -5);
 			}
 		}
 	}
@@ -890,14 +890,23 @@
 	static const SpriteID back_offsets[6]    =   {  95,  96,  99, 102, 100, 101 };
 	static const SpriteID front_offsets[6]   =   {  97,  98, 103, 106, 104, 105 };
 
-	static const uint size_x[6] = { 11, 16, 16, 16, 16, 16 };
-	static const uint size_y[6] = { 16, 11, 16, 16, 16, 16 };
+	static const uint size_x[6] = {  1, 16, 16,  1, 16,  1 };
+	static const uint size_y[6] = { 16,  1,  1, 16,  1, 16 };
+	static const uint front_bb_offset_x[6] = { 15,  0,  0, 15,  0, 15 };
+	static const uint front_bb_offset_y[6] = {  0, 15, 15,  0, 15,  0 };
 
-	AddSortableSpriteToDraw(SPR_TRAMWAY_BASE + tram_offsets[overlay][offset], PAL_NONE, x, y, size_x[offset], size_y[offset], offset >= 2 ? 1 : 0, z, HASBIT(_transparent_opt, TO_BRIDGES));
+	/* The sprites under the vehicles are drawn as SpriteCombine. StartSpriteCombine() has already been called
+	 * The bounding boxes here are the same as for bridge front/roof */
+	AddSortableSpriteToDraw(SPR_TRAMWAY_BASE + tram_offsets[overlay][offset], PAL_NONE, x, y, size_x[offset], size_y[offset], 0x28, z, HASBIT(_transparent_opt, TO_BRIDGES));
 
-	AddSortableSpriteToDraw(SPR_TRAMWAY_BASE + back_offsets[offset],  PAL_NONE, x, y, size_x[offset], size_y[offset], 0, z, HASBIT(_transparent_opt, TO_BUILDINGS));
+	AddSortableSpriteToDraw(SPR_TRAMWAY_BASE + back_offsets[offset],  PAL_NONE, x, y, size_x[offset], size_y[offset], 0x28, z, HASBIT(_transparent_opt, TO_BUILDINGS));
+
+	/* Start a new SpriteCombine for the front part */
+	EndSpriteCombine();
+	StartSpriteCombine();
+
 	/* For sloped sprites the bounding box needs to be higher, as the pylons stop on a higher point */
-	AddSortableSpriteToDraw(SPR_TRAMWAY_BASE + front_offsets[offset], PAL_NONE, x, y, size_x[offset], size_y[offset], offset >= 2 ? 0x30 : 0x10, z, HASBIT(_transparent_opt, TO_BUILDINGS));
+	AddSortableSpriteToDraw(SPR_TRAMWAY_BASE + front_offsets[offset], PAL_NONE, x, y, size_x[offset] + front_bb_offset_x[offset], size_y[offset] + front_bb_offset_y[offset], 0x28, z, HASBIT(_transparent_opt, TO_BUILDINGS), front_bb_offset_x[offset], front_bb_offset_y[offset]);
 }
 
 /**
@@ -918,6 +927,27 @@
 	SpriteID image;
 
 	if (IsTunnel(ti->tile)) {
+		/* Front view of tunnel bounding boxes:
+		 *
+		 *   122223  <- BB_Z_SEPARATOR
+		 *   1    3
+		 *   1    3                1,3 = empty helper BB
+		 *   1    3                  2 = SpriteCombine of tunnel-roof and catenary (tram & elrail)
+		 *
+		 */
+
+		static const int _tunnel_BB[4][12] = {
+			/*  tunnnel-roof  |  Z-separator  | tram-catenary
+			 * w  h  bb_x bb_y| x   y   w   h |bb_x bb_y w h */
+			{  1,  0, -15, -14,  0, 15, 16,  1, 0, 1, 16, 15 }, // NE
+			{  0,  1, -14, -15, 15,  0,  1, 16, 1, 0, 15, 16 }, // SE
+			{  1,  0, -15, -14,  0, 15, 16,  1, 0, 1, 16, 15 }, // SW
+			{  0,  1, -14, -15, 15,  0,  1, 16, 1, 0, 15, 16 }, // NW
+		};
+		static const int *BB_data = _tunnel_BB[GetTunnelDirection(ti->tile)];
+
+		bool catenary = false;
+
 		if (GetTunnelTransportType(ti->tile) == TRANSPORT_RAIL) {
 			image = GetRailTypeInfo(GetRailType(ti->tile))->base_sprites.tunnel;
 		} else {
@@ -936,13 +966,27 @@
 				static const SpriteID tunnel_sprites[2][4] = { { 28, 78, 79, 27 }, {  5, 76, 77,  4 } };
 
 				DrawGroundSprite(SPR_TRAMWAY_BASE + tunnel_sprites[rts - ROADTYPES_TRAM][dir], PAL_NONE);
-				AddSortableSpriteToDraw(SPR_TRAMWAY_TUNNEL_WIRES + dir, PAL_NONE, ti->x, ti->y, 16, 16, 16, (byte)ti->z, HASBIT(_transparent_opt, TO_BUILDINGS));
+
+				catenary = true;
+				StartSpriteCombine();
+				AddSortableSpriteToDraw(SPR_TRAMWAY_TUNNEL_WIRES + dir, PAL_NONE, ti->x, ti->y, BB_data[10], BB_data[11], TILE_HEIGHT, ti->z, HASBIT(_transparent_opt, TO_BUILDINGS), BB_data[8], BB_data[9], BB_Z_SEPARATOR);
 			}
 		} else if (GetRailType(ti->tile) == RAILTYPE_ELECTRIC) {
 			DrawCatenary(ti);
+
+			catenary = true;
+			StartSpriteCombine();
+			DrawCatenaryOnTunnel(ti);
 		}
 
-		AddSortableSpriteToDraw(image + 1, PAL_NONE, ti->x + TILE_SIZE - 1, ti->y + TILE_SIZE - 1, 1, 1, 8, (byte)ti->z);
+		AddSortableSpriteToDraw(image + 1, PAL_NONE, ti->x + TILE_SIZE - 1, ti->y + TILE_SIZE - 1, BB_data[0], BB_data[1], TILE_HEIGHT, ti->z, false, BB_data[2], BB_data[3], BB_Z_SEPARATOR);
+
+		if (catenary) EndSpriteCombine();
+
+		/* Add helper BB for sprite sorting, that separate the tunnel from things beside of it */
+		AddSortableSpriteToDraw(SPR_EMPTY_BOUNDING_BOX, PAL_NONE, ti->x             , ti->y             , BB_data[6], BB_data[7], TILE_HEIGHT, ti->z);
+		AddSortableSpriteToDraw(SPR_EMPTY_BOUNDING_BOX, PAL_NONE, ti->x + BB_data[4], ti->y + BB_data[5], BB_data[6], BB_data[7], TILE_HEIGHT, ti->z);
+
 		DrawBridgeMiddle(ti);
 	} else if (IsBridge(ti->tile)) { // XXX is this necessary?
 		const PalSpriteID *psid;
@@ -977,6 +1021,9 @@
 
 		/* draw ramp */
 
+		/* Draw Trambits as SpriteCombine */
+		if (GetBridgeTransportType(ti->tile) == TRANSPORT_ROAD) StartSpriteCombine();
+
 		/* HACK set the height of the BB of a sloped ramp to 1 so a vehicle on
 		 * it doesn't disappear behind it
 		 */
@@ -996,8 +1043,10 @@
 				} else {
 					offset += 2;
 				}
+				/* DrawBridgeTramBits() calls EndSpriteCombine() and StartSpriteCombine() */
 				DrawBridgeTramBits(ti->x, ti->y, z, offset, HASBIT(rts, ROADTYPE_ROAD));
 			}
+			EndSpriteCombine();
 		} else if (GetRailType(ti->tile) == RAILTYPE_ELECTRIC) {
 			DrawCatenary(ti);
 		}
@@ -1043,6 +1092,25 @@
 
 void DrawBridgeMiddle(const TileInfo* ti)
 {
+	/* Sectional view of bridge bounding boxes:
+	 *
+	 *  1           2                                1,2 = SpriteCombine of Bridge front/(back&floor) and TramCatenary
+	 *  1           2                                  3 = empty helper BB
+	 *  1     7     2                                4,5 = pillars under higher bridges
+	 *  1 6 88888 6 2                                  6 = elrail-pylons
+	 *  1 6 88888 6 2                                  7 = elrail-wire
+	 *  1 6 88888 6 2  <- TILE_HEIGHT                  8 = rail-vehicle on bridge
+	 *  3333333333333  <- BB_Z_SEPARATOR
+	 *                 <- unused
+	 *    4       5    <- BB_HEIGHT_UNDER_BRIDGE
+	 *    4       5
+	 *    4       5
+	 *
+	 */
+
+	/* Z position of the bridge sprites relative to bridge height (downwards) */
+	static const int BRIDGE_Z_START = 3;
+
 	const PalSpriteID* psid;
 	uint base_offset;
 	TileIndex rampnorth;
@@ -1078,12 +1146,19 @@
 	x = ti->x;
 	y = ti->y;
 	uint bridge_z = GetBridgeHeight(rampsouth);
-	z = bridge_z - 3;
+	z = bridge_z - BRIDGE_Z_START;
 
+	/* Add a bounding box, that separates the bridge from things below it. */
+	AddSortableSpriteToDraw(SPR_EMPTY_BOUNDING_BOX, PAL_NONE, x, y, 16, 16, 1, bridge_z - TILE_HEIGHT + BB_Z_SEPARATOR);
+
+	/* Draw Trambits as SpriteCombine */
+	if (GetBridgeTransportType(rampsouth) == TRANSPORT_ROAD) StartSpriteCombine();
+
+	/* Draw floor and far part of bridge*/
 	if (axis == AXIS_X) {
-		AddSortableSpriteToDraw(psid->sprite, psid->pal, x, y, 16, 11, 1, z, HASBIT(_transparent_opt, TO_BRIDGES));
+		AddSortableSpriteToDraw(psid->sprite, psid->pal, x, y, 16, 1, 0x28, z, HASBIT(_transparent_opt, TO_BRIDGES), 0, 0, BRIDGE_Z_START);
 	} else {
-		AddSortableSpriteToDraw(psid->sprite, psid->pal, x, y, 11, 16, 1, z, HASBIT(_transparent_opt, TO_BRIDGES));
+		AddSortableSpriteToDraw(psid->sprite, psid->pal, x, y, 1, 16, 0x28, z, HASBIT(_transparent_opt, TO_BRIDGES), 0, 0, BRIDGE_Z_START);
 	}
 
 	psid++;
@@ -1092,7 +1167,11 @@
 		RoadTypes rts = GetRoadTypes(rampsouth);
 
 		if (HASBIT(rts, ROADTYPE_TRAM)) {
+			/* DrawBridgeTramBits() calls EndSpriteCombine() and StartSpriteCombine() */
 			DrawBridgeTramBits(x, y, bridge_z, axis ^ 1, HASBIT(rts, ROADTYPE_ROAD));
+		} else {
+			EndSpriteCombine();
+			StartSpriteCombine();
 		}
 	} else if (GetRailType(rampsouth) == RAILTYPE_ELECTRIC) {
 		DrawCatenary(ti);
@@ -1101,12 +1180,15 @@
 	/* draw roof, the component of the bridge which is logically between the vehicle and the camera */
 	if (axis == AXIS_X) {
 		y += 12;
-		if (psid->sprite & SPRITE_MASK) AddSortableSpriteToDraw(psid->sprite, psid->pal, x, y, 16, 1, 0x28, z, HASBIT(_transparent_opt, TO_BRIDGES));
+		if (psid->sprite & SPRITE_MASK) AddSortableSpriteToDraw(psid->sprite, psid->pal, x, y, 16, 4, 0x28, z, HASBIT(_transparent_opt, TO_BRIDGES), 0, 3, BRIDGE_Z_START);
 	} else {
 		x += 12;
-		if (psid->sprite & SPRITE_MASK) AddSortableSpriteToDraw(psid->sprite, psid->pal, x, y, 1, 16, 0x28, z, HASBIT(_transparent_opt, TO_BRIDGES));
+		if (psid->sprite & SPRITE_MASK) AddSortableSpriteToDraw(psid->sprite, psid->pal, x, y, 4, 16, 0x28, z, HASBIT(_transparent_opt, TO_BRIDGES), 3, 0, BRIDGE_Z_START);
 	}
 
+	/* Draw TramFront as SpriteCombine */
+	if (GetBridgeTransportType(rampsouth) == TRANSPORT_ROAD) EndSpriteCombine();
+
 	psid++;
 	if (ti->z + 5 == z) {
 		/* draw poles below for small bridges */

--- a/src/unmovable_cmd.cpp	Tue Sep 18 20:41:59 2007 +0000
+++ b/src/unmovable_cmd.cpp	Wed Sep 19 16:36:42 2007 +0000
@@ -148,7 +148,7 @@
 
 			AddSortableSpriteToDraw(
 				SPR_BOUGHT_LAND, PLAYER_SPRITE_COLOR(GetTileOwner(ti->tile)),
-				ti->x + TILE_SIZE / 2, ti->y + TILE_SIZE / 2, 1, 1, 10, GetSlopeZ(ti->x + TILE_SIZE / 2, ti->y + TILE_SIZE / 2)
+				ti->x + TILE_SIZE / 2, ti->y + TILE_SIZE / 2, 1, 1, BB_HEIGHT_UNDER_BRIDGE, GetSlopeZ(ti->x + TILE_SIZE / 2, ti->y + TILE_SIZE / 2)
 			);
 			DrawBridgeMiddle(ti);
 			break;

--- a/src/viewport.cpp	Tue Sep 18 20:41:59 2007 +0000
+++ b/src/viewport.cpp	Wed Sep 19 16:36:42 2007 +0000
@@ -513,8 +513,8 @@
 {
 	ViewportDrawer *vd = _cur_vd;
 	ParentSpriteToDraw *ps;
-	const Sprite *spr;
 	Point pt;
+	int32 right, bottom;
 
 	assert((image & SPRITE_MASK) < MAX_SPRITES);
 
@@ -551,11 +551,22 @@
 	pt = RemapCoords(x, y, z);
 	ps->x = pt.x;
 	ps->y = pt.y;
-	spr = GetSprite(image & SPRITE_MASK);
-	if ((ps->left   = (pt.x += spr->x_offs)) >= vd->dpi.left + vd->dpi.width ||
-			(             (pt.x +  spr->width )) <= vd->dpi.left ||
-			(ps->top    = (pt.y += spr->y_offs)) >= vd->dpi.top + vd->dpi.height ||
-			(             (pt.y +  spr->height)) <= vd->dpi.top) {
+	if (image == SPR_EMPTY_BOUNDING_BOX) {
+		ps->left = RemapCoords(x + w          , y + bb_offset_y, z + bb_offset_z).x;
+		right    = RemapCoords(x + bb_offset_x, y + h          , z + bb_offset_z).x;
+		ps->top  = RemapCoords(x + bb_offset_x, y + bb_offset_y, z + dz         ).y;
+		bottom   = RemapCoords(x + w          , y + h          , z + bb_offset_z).y;
+	} else {
+		const Sprite *spr = GetSprite(image & SPRITE_MASK);
+		ps->left = (pt.x += spr->x_offs);
+		right    = (pt.x +  spr->width );
+		ps->top  = (pt.y += spr->y_offs);
+		bottom   = (pt.y +  spr->height);
+	}
+	if (ps->left >= vd->dpi.left + vd->dpi.width ||
+	    right    <= vd->dpi.left ||
+	    ps->top  >= vd->dpi.top + vd->dpi.height ||
+	    bottom   <= vd->dpi.top) {
 		return;
 	}
 
@@ -1212,7 +1223,7 @@
 		const ParentSpriteToDraw* ps = *psd;
 		const ChildScreenSpriteToDraw* cs;
 
-		DrawSprite(ps->image, ps->pal, ps->x, ps->y);
+		if (ps->image != SPR_EMPTY_BOUNDING_BOX) DrawSprite(ps->image, ps->pal, ps->x, ps->y);
 
 		for (cs = ps->child; cs != NULL; cs = cs->next) {
 			DrawSprite(cs->image, cs->pal, ps->left + cs->x, ps->top + cs->y);

--- a/src/viewport.h	Tue Sep 18 20:41:59 2007 +0000
+++ b/src/viewport.h	Wed Sep 19 16:36:42 2007 +0000
@@ -44,6 +44,17 @@
 	while (DoZoomInOutWindow(how, w)) {};
 }
 
+/**
+ * Some values for constructing bounding boxes (BB). The Z positions under bridges are:
+ * z=0..5  Everything that can be built under low bridges.
+ * z=6     reserved, currently unused.
+ * z=7     Z separator between bridge/tunnel and the things under/above it.
+ */
+enum {
+	BB_HEIGHT_UNDER_BRIDGE = 6, ///< Everything that can be built under low bridges, must not exceed this Z height.
+	BB_Z_SEPARATOR  = 7,        ///< Separates the bridge/tunnel from the things under/above it.
+};
+
 void OffsetGroundSprite(int x, int y);
 
 void DrawGroundSprite(SpriteID image, SpriteID pal);