Biomes: Add vertical biome blend (#6853)

Add 'vertical blend' parameter to biome registration that defines how
many nodes above the biome's 'y max' limit the blend will extend.

doc/lua_api.txt

@@ -4989,18 +4989,18 @@ Definition tables
         y_min = 1,
         y_max = 31000,
     --  ^ Lower and upper limits for biome.
+        vertical_blend = 8,
+    --  ^ Vertical distance in nodes above 'y_max' over which the biome will
+    --  ^ blend with the biome above.
+    --  ^ Set to 0 for no vertical blend. Defaults to 0.
         heat_point = 0,
         humidity_point = 50,
-    --  ^ Characteristic average temperature and humidity for the biome.
-    --  ^ These values create 'biome points' on a voronoi diagram that has heat
-    --  ^ and humidity as axes. The resulting voronoi cells determine which
-    --  ^ heat/humidity points belong to which biome, and therefore determine
-    --  ^ the area and location of each biome in the world.
-    --  ^ The biome points need to be carefully and evenly spaced on the voronoi
-    --  ^ diagram to result in roughly equal size biomes.
+    --  ^ Characteristic temperature and humidity for the biome.
+    --  ^ These values create 'biome points' on a voronoi diagram with heat and
+    --  ^ humidity as axes. The resulting voronoi cells determine the
+    --  ^ distribution of the biomes.
     --  ^ Heat and humidity have average values of 50, vary mostly between
-    --  ^ 0 and 100 but also often exceed these values.
-    --  ^ Heat is not in degrees Celsius, both values are abstract.
+    --  ^ 0 and 100 but can exceed these values.
     }
 
 ### Decoration definition (`register_decoration`)

src/mapgen/mg_biome.cpp

@@ -50,6 +50,7 @@ BiomeManager::BiomeManager(Server *server) :
 	b->y_max           = MAX_MAP_GENERATION_LIMIT;
 	b->heat_point      = 0.0;
 	b->humidity_point  = 0.0;
+	b->vertical_blend  = 0;
 
 	b->m_nodenames.emplace_back("mapgen_stone");
 	b->m_nodenames.emplace_back("mapgen_stone");
@@ -201,25 +202,41 @@ Biome *BiomeGenOriginal::getBiomeAtIndex(size_t index, s16 y) const
 
 Biome *BiomeGenOriginal::calcBiomeFromNoise(float heat, float humidity, s16 y) const
 {
-	Biome *b, *biome_closest = NULL;
+	Biome *biome_closest = nullptr;
+	Biome *biome_closest_blend = nullptr;
 	float dist_min = FLT_MAX;
+	float dist_min_blend = FLT_MAX;
 
 	for (size_t i = 1; i < m_bmgr->getNumObjects(); i++) {
-		b = (Biome *)m_bmgr->getRaw(i);
-		if (!b || y > b->y_max || y < b->y_min)
+		Biome *b = (Biome *)m_bmgr->getRaw(i);
+		if (!b || y > b->y_max + b->vertical_blend || y < b->y_min)
 			continue;
 
 		float d_heat = heat - b->heat_point;
 		float d_humidity = humidity - b->humidity_point;
-		float dist = (d_heat * d_heat) +
-					 (d_humidity * d_humidity);
+		float dist = (d_heat * d_heat) + (d_humidity * d_humidity);
+
+		if (y <= b->y_max) { // Within y limits of biome b
 			if (dist < dist_min) {
 				dist_min = dist;
 				biome_closest = b;
 			}
+		} else if (dist < dist_min_blend) { // Blend area above biome b
+			dist_min_blend = dist;
+			biome_closest_blend = b;
+		}
 	}
 
-	return biome_closest ? biome_closest : (Biome *)m_bmgr->getRaw(BIOME_NONE);
+	// Carefully tune pseudorandom seed variation to avoid single node dither
+	// and create larger scale blending patterns.
+	mysrand(y + (heat - humidity) * 2);
+
+	if (biome_closest_blend &&
+			myrand_range(0, biome_closest_blend->vertical_blend) >=
+			y - biome_closest_blend->y_max)
+		return biome_closest_blend;
+
+	return (biome_closest) ? biome_closest : (Biome *)m_bmgr->getRaw(BIOME_NONE);	
 }
 
 

src/mapgen/mg_biome.h

@@ -67,6 +67,7 @@ public:
 	s16 y_max;
 	float heat_point;
 	float humidity_point;
+	s16 vertical_blend;
 
 	virtual void resolveNodeNames();
 };

src/script/lua_api/l_mapgen.cpp

@@ -392,6 +392,7 @@ Biome *read_biome_def(lua_State *L, int index, INodeDefManager *ndef)
 	b->y_max           = getintfield_default(L,    index, "y_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->vertical_blend  = getintfield_default(L,    index, "vertical_blend",  0);
 	b->flags           = 0; //reserved
 
 	std::vector<std::string> &nn = b->m_nodenames;