Generate biomes: Recalculate biome at biome lower limit

Prevents biome nodes passing below the defined y_min of that biome.

doc/lua_api.txt

@@ -4630,9 +4630,6 @@ Definition tables
 
 ### Biome definition (`register_biome`)
 
-**Note**
-The Biome API is still in an experimental phase and subject to change.
-
     {
         name = "tundra",
         node_dust = "default:snow",
@@ -4659,13 +4656,6 @@ The Biome API is still in an experimental phase and subject to change.
         y_max = 31000,
     --  ^ Lower and upper limits for biome.
     --  ^ Limits are relative to y = water_level - 1.
-    --  ^ Because biome is not recalculated for every node in a node column
-    --  ^ some biome materials can exceed their limits, especially stone.
-    --  ^ For each node column in a mapchunk, biome is only recalculated at column
-    --  ^ top and at each of these surfaces:
-    --  ^ Ground below air, water below air, ground below water.
-    --  ^ The selected biome then stays in effect for all nodes below until
-    --  ^ column base or the next biome recalculation.
         heat_point = 0,
         humidity_point = 50,
     --  ^ Characteristic average temperature and humidity for the biome.

src/mapgen.cpp

@@ -633,6 +633,7 @@ void MapgenBasic::generateBiomes(MgStoneType *mgstone_type,
 		u16 base_filler = 0;
 		u16 depth_water_top = 0;
 		u16 depth_riverbed = 0;
+		s16 biome_y_min = -MAX_MAP_GENERATION_LIMIT;
 		u32 vi = vm->m_area.index(x, node_max.Y, z);
 
 		// Check node at base of mapchunk above, either a node of a previously
@@ -650,22 +651,20 @@ void MapgenBasic::generateBiomes(MgStoneType *mgstone_type,
 
 		for (s16 y = node_max.Y; y >= node_min.Y; y--) {
 			content_t c = vm->m_data[vi].getContent();
-
+			// Biome is (re)calculated:
-			// Biome is recalculated each time an upper surface is detected while
-			// working down a column. The selected biome then remains in effect for
-			// all nodes below until the next surface and biome recalculation.
-			// Biome is recalculated:
 			// 1. At the surface of stone below air or water.
 			// 2. At the surface of water below air.
 			// 3. When stone or water is detected but biome has not yet been calculated.
+			// 4. When stone or water is detected just below a biome's lower limit.
 			bool is_stone_surface = (c == c_stone) &&
-				(air_above || water_above || !biome);
+				(air_above || water_above || !biome || y < biome_y_min); // 1, 3, 4
 
 			bool is_water_surface =
 				(c == c_water_source || c == c_river_water_source) &&
-				(air_above || !biome);
+				(air_above || !biome || y < biome_y_min); // 2, 3, 4
 
 			if (is_stone_surface || is_water_surface) {
+				// (Re)calculate biome
 				// Limit to +-MAX MAP GENERATION LIMIT to work with biome y_min / y_max.
 				s32 relative_y = rangelim(y - biome_zero_level,
 					-MAX_MAP_GENERATION_LIMIT, MAX_MAP_GENERATION_LIMIT);
@@ -677,9 +676,11 @@ void MapgenBasic::generateBiomes(MgStoneType *mgstone_type,
 				depth_top = biome->depth_top;
 				base_filler = MYMAX(depth_top +
 					biome->depth_filler +
-					noise_filler_depth->result[index], 0.f);
+					noise_filler_depth->result[index], 0.0f);
 				depth_water_top = biome->depth_water_top;
 				depth_riverbed = biome->depth_riverbed;
+				biome_y_min = rangelim(biome->y_min + biome_zero_level,
+					-MAX_MAP_GENERATION_LIMIT, MAX_MAP_GENERATION_LIMIT);
 
 				// Detect stone type for dungeons during every biome calculation.
 				// If none detected the last selected biome stone is chosen.