Decoration: Add height_min and height_max parameters

Also set default height_min/height_max to -31000 and 31000,
respectively, for ore and biomes

doc/lua_api.txt

@@ -2758,6 +2758,10 @@ Decoration definition (register_decoration)
     biomes = {"Oceanside", "Hills", "Plains"},
     ^ List of biomes in which this decoration occurs.  Occurs in all biomes if this is omitted,
     ^ and ignored if the Mapgen being used does not support biomes.
+    height_min = -31000
+    height_max = 31000
+    ^ Minimum and maximum y positions these decorations can be generated at.  This parameter refers to the
+    ^ y position of the decoration base, so the actual maximum height would be (height_max + size.Y).
 
     ----- Simple-type parameters
     decoration = "default:grass",

src/mg_decoration.cpp

@@ -140,7 +140,8 @@ size_t Decoration::placeDeco(Mapgen *mg, u32 blockseed, v3s16 nmin, v3s16 nmax)
 					mg->heightmap[mapindex] :
 					mg->findGroundLevel(v2s16(x, z), nmin.Y, nmax.Y);
 
-			if (y < nmin.Y || y > nmax.Y)
+			if (y < nmin.Y     || y > nmax.Y ||
+				y < height_min || y > height_max)
 				continue;
 
 			int height = getHeight();

src/mg_decoration.h

@@ -66,6 +66,8 @@ class Decoration : public GenElement, public NodeResolver {
 	int mapseed;
 	std::vector<content_t> c_place_on;
 	s16 sidelen;
+	s16 height_min;
+	s16 height_max;
 	float fill_ratio;
 	NoiseParams np;
 

src/script/lua_api/l_mapgen.cpp

@@ -423,14 +423,14 @@ int ModApiMapgen::l_register_biome(lua_State *L)
 	Biome *b = bmgr->create(biometype);
 
 	b->name            = getstringfield_default(L, index, "name", "");
-	b->depth_top       = getintfield_default(L, index, "depth_top",    1);
-	b->depth_filler    = getintfield_default(L, index, "depth_filler", 3);
-	b->height_shore    = getintfield_default(L, index, "height_shore", 3);
-	b->depth_water_top = getintfield_default(L, index, "depth_water_top", 0);
-	b->height_min      = getintfield_default(L, index, "height_min",   0);
-	b->height_max      = getintfield_default(L, index, "height_max",   0);
-	b->heat_point      = getfloatfield_default(L, index, "heat_point",     0.);
-	b->humidity_point  = getfloatfield_default(L, index, "humidity_point", 0.);
+	b->depth_top       = getintfield_default(L, index, "depth_top",          1);
+	b->depth_filler    = getintfield_default(L, index, "depth_filler",       3);
+	b->height_shore    = getintfield_default(L, index, "height_shore",       3);
+	b->depth_water_top = getintfield_default(L, index, "depth_water_top",    0);
+	b->height_min      = getintfield_default(L, index, "height_min",    -31000);
+	b->height_max      = getintfield_default(L, index, "height_max",     31000);
+	b->heat_point      = getfloatfield_default(L, index, "heat_point",     0.f);
+	b->humidity_point  = getfloatfield_default(L, index, "humidity_point", 0.f);
 	b->flags           = 0; //reserved
 
 	u32 id = bmgr->add(b);
@@ -501,6 +501,8 @@ int ModApiMapgen::l_register_decoration(lua_State *L)
 
 	deco->name       = getstringfield_default(L, index, "name", "");
 	deco->fill_ratio = getfloatfield_default(L, index, "fill_ratio", 0.02);
+	deco->height_min = getintfield_default(L, index, "height_min", 31000);
+	deco->height_max = getintfield_default(L, index, "height_max", -31000);
 	deco->sidelen    = getintfield_default(L, index, "sidelen", 8);
 	if (deco->sidelen <= 0) {
 		errorstream << "register_decoration: sidelen must be "
@@ -660,8 +662,8 @@ int ModApiMapgen::l_register_ore(lua_State *L)
 	ore->clust_scarcity = getintfield_default(L, index, "clust_scarcity", 1);
 	ore->clust_num_ores = getintfield_default(L, index, "clust_num_ores", 1);
 	ore->clust_size     = getintfield_default(L, index, "clust_size", 0);
-	ore->height_min     = getintfield_default(L, index, "height_min", 0);
-	ore->height_max     = getintfield_default(L, index, "height_max", 0);
+	ore->height_min     = getintfield_default(L, index, "height_min", -31000);
+	ore->height_max     = getintfield_default(L, index, "height_max", 31000);
 	ore->nthresh        = getfloatfield_default(L, index, "noise_threshhold", 0);
 	ore->noise          = NULL;
 	ore->flags          = 0;