Simple decorations: Make 'place_offset_y' usable with simple decorations

Necessary for placing the base cube of 'plantlike_rooted' drawtype in the
seabed instead of on it.
Useful for placing decorations sunk into, or buried in, the ground.

doc/lua_api.txt

@@ -4814,20 +4814,19 @@ Definition tables
         y_min = -31000
         y_max = 31000
     --  ^ Lower and upper limits for decoration.
-    --  ^ This parameter refers to the `y` position of the decoration base, so
-    --    the actual maximum height would be `height_max + size.Y`.
+    --  ^ These parameters refer to the Y co-ordinate of the 'place_on' node.
         spawn_by = "default:water",
     --  ^ Node (or list of nodes) that the decoration only spawns next to.
     --  ^ Checks two horizontal planes of neighbouring nodes (including diagonal neighbours),
-    --  ^ one plane at Y = surface and one plane at Y = surface = + 1.
+    --  ^ one plane level with the 'place_on' node and a plane one node above that.
         num_spawn_by = 1,
     --  ^ Number of spawn_by nodes that must be surrounding the decoration position to occur.
     --  ^ If absent or -1, decorations occur next to any nodes.
         flags = "liquid_surface, force_placement",
     --  ^ Flags for all decoration types.
-    --  ^ "liquid_surface": Instead of placement on the highest solid surface
-    --  ^ in a mapchunk column, placement is on the highest liquid surface.
-    --  ^ Placement is disabled if solid nodes are found above the liquid surface.
+    --  ^ "liquid_surface": Instead of placement on the highest solid surface in
+    --  ^ a mapchunk column, placement is on the highest liquid surface. Placement
+    --  ^ is disabled if solid nodes are found above the liquid surface.
     --  ^ "force_placement": Nodes other than "air" and "ignore" are replaced by the decoration.
 
         ----- Simple-type parameters
@@ -4835,17 +4834,23 @@ Definition tables
     --  ^ The node name used as the decoration.
     --  ^ If instead a list of strings, a randomly selected node from the list is placed as the decoration.
         height = 1,
-    --  ^ Number of nodes high the decoration is made.
-    --  ^ If height_max is not 0, this is the lower bound of the randomly selected height.
+    --  ^ Decoration height in nodes.
+    --  ^ If height_max is not 0, this is the lower limit of a randomly selected height.
         height_max = 0,
-    --  ^ Number of nodes the decoration can be at maximum.
+    --  ^ Upper limit of the randomly selected height.
     --  ^ If absent, the parameter 'height' is used as a constant.
         param2 = 0,
-    --  ^ Param2 value of placed decoration node.
-    --  ^ If param2_max is not 0, this is the lower bound of the randomly selected param2.
+    --  ^ Param2 value of decoration nodes.
+    --  ^ If param2_max is not 0, this is the lower limit of a randomly selected param2.
         param2_max = 0,
-    --  ^ Upper bound of the randomly selected param2.
+    --  ^ Upper limit of the randomly selected param2.
     --  ^ If absent, the parameter 'param2' is used as a constant.
+        place_offset_y = 0,
+    --  ^ Y offset of the decoration base node relative to the standard
+    --  ^ base node position for simple decorations.
+    --  ^ Can be positive or negative. Default is 0.
+    --  ^ Ignored by 'y_min', 'y_max' and 'spawn_by' checks, which always refer
+    --  ^ to the 'place_on' node.
 
         ----- Schematic-type parameters
         schematic = "foobar.mts",

src/mg_decoration.cpp

@@ -219,6 +219,10 @@ size_t DecoSimple::generate(MMVManip *vm, PcgRandom *pr, v3s16 p)
 	if (c_decos.empty())
 		return 0;
 
+	// Check for a negative place_offset_y causing placement below the voxelmanip
+	if (p.Y + 1 + place_offset_y < vm->m_area.MinEdge.Y)
+		return 0;
+
 	if (!canPlaceDecoration(vm, p))
 		return 0;
 
@@ -234,9 +238,10 @@ size_t DecoSimple::generate(MMVManip *vm, PcgRandom *pr, v3s16 p)
 
 	const v3s16 &em = vm->m_area.getExtent();
 	u32 vi = vm->m_area.index(p);
+	vm->m_area.add_y(em, vi, place_offset_y);
+
 	for (int i = 0; i < height; i++) {
 		vm->m_area.add_y(em, vi, 1);
-
 		content_t c = vm->m_data[vi].getContent();
 		if (c != CONTENT_AIR && c != CONTENT_IGNORE &&
 				!force_placement)
@@ -251,7 +256,8 @@ size_t DecoSimple::generate(MMVManip *vm, PcgRandom *pr, v3s16 p)
 
 int DecoSimple::getHeight()
 {
-	return (deco_height_max > 0) ? deco_height_max : deco_height;
+	return ((deco_height_max > 0) ? deco_height_max : deco_height)
+		+ place_offset_y;
 }
 
 

src/mg_decoration.h

@@ -69,6 +69,7 @@ public:
 	NoiseParams np;
 	std::vector<content_t> c_spawnby;
 	s16 nspawnby;
+	s16 place_offset_y = 0;
 
 	std::unordered_set<u8> biomes;
 };
@@ -96,7 +97,6 @@ public:
 	virtual int getHeight();
 
 	Rotation rotation;
-	s16 place_offset_y = 0;
 	Schematic *schematic = nullptr;
 };
 

src/script/lua_api/l_mapgen.cpp

@@ -911,6 +911,7 @@ int ModApiMapgen::l_register_decoration(lua_State *L)
 	deco->y_min          = getintfield_default(L, index, "y_min", -31000);
 	deco->y_max          = getintfield_default(L, index, "y_max", 31000);
 	deco->nspawnby       = getintfield_default(L, index, "num_spawn_by", -1);
+	deco->place_offset_y = getintfield_default(L, index, "place_offset_y", 0);
 	deco->sidelen        = getintfield_default(L, index, "sidelen", 8);
 	if (deco->sidelen <= 0) {
 		errorstream << "register_decoration: sidelen must be "
@@ -1024,8 +1025,6 @@ bool read_deco_schematic(lua_State *L, SchematicManager *schemmgr, DecoSchematic
 	deco->rotation = (Rotation)getenumfield(L, index, "rotation",
 		ModApiMapgen::es_Rotation, ROTATE_0);
 
-	deco->place_offset_y = getintfield_default(L, index, "place_offset_y", 0);
-
 	StringMap replace_names;
 	lua_getfield(L, index, "replacements");
 	if (lua_istable(L, -1))