Stratum ore: Allow use with no noise for simple horizontal strata

If either of the 2 noise parameters are omitted the ore will occur from y_min
to y_max in a simple horizontal stratum. As this does not compute noise
performance improves, and is ideal for placing many layers.

Clean up some nearby ore documentation.

doc/lua_api.txt

@@ -1098,12 +1098,17 @@ Creates a deformed sphere of ore according to 3d perlin noise described by
 ### `vein`
 Creates veins of ore varying in density by according to the intersection of two
 instances of 3d perlin noise with diffferent seeds, both described by
-`noise_params`.  `random_factor` varies the influence random chance has on
-placement of an ore inside the vein, which is `1` by default. Note that
-modifying this parameter may require adjusting `noise_threshold`.
+`noise_params`.
+
+`random_factor` varies the influence random chance has on placement of an ore
+inside the vein, which is `1` by default. Note that modifying this parameter may
+require adjusting `noise_threshold`.
+
 The parameters `clust_scarcity`, `clust_num_ores`, and `clust_size` are ignored
-by this ore type.  This ore type is difficult to control since it is sensitive
-to small changes.  The following is a decent set of parameters to work from:
+by this ore type.
+
+This ore type is difficult to control since it is sensitive to small changes.
+The following is a decent set of parameters to work from:
 
 	noise_params = {
 	    offset  = 0,
@@ -1122,16 +1127,24 @@ computationally expensive than any other ore.
 ### `stratum`
 Creates a single undulating ore stratum that is continuous across mapchunk
 borders and horizontally spans the world.
+
 The 2D perlin noise described by `noise_params` varies the Y co-ordinate of the
 stratum midpoint. The 2D perlin noise described by `np_stratum_thickness`
 varies the stratum's vertical thickness (in units of nodes). Due to being
 continuous across mapchunk borders the stratum's vertical thickness is
 unlimited.
+
 `y_min` and `y_max` define the limits of the ore generation and for performance
 reasons should be set as close together as possible but without clipping the
 stratum's Y variation.
+
+If either of the 2 noise parameters are omitted the ore will occur from y_min
+to y_max in a simple horizontal stratum. As this does not compute noise
+performance improves, and is ideal for placing many layers.
+
 Each node in the stratum has a 1-in-`clust_scarcity` chance of being ore, so a
 solid-ore stratum would require a `clust_scarcity` of 1.
+
 The parameters `clust_num_ores`, `clust_size`, `noise_threshold` and
 `random_factor` are ignored by this ore type.
 
@@ -4713,24 +4726,36 @@ Definition tables
     --  ^ Number of ores in a cluster
         clust_size = 3,
     --  ^ Size of the bounding box of the cluster
-    --  ^ In this example, there is a 3x3x3 cluster where 8 out of the 27 nodes are coal ore
+    --  ^ In this example, there is a 3x3x3 cluster where 8 out of the 27 nodes
+    --  ^ are coal ore.
         y_min = -31000,
         y_max = 64,
     --  ^ Lower and upper limits for ore.
         flags = "",
     --  ^ Attributes for this ore generation
         noise_threshold = 0.5,
-    --  ^ If noise is above this threshold, ore is placed.  Not needed for a uniform distribution
-        noise_params = {offset=0, scale=1, spread={x=100, y=100, z=100}, seed=23, octaves=3, persist=0.70}
-    --  ^ NoiseParams structure describing the perlin noise used for ore distribution.
-    --  ^ Needed for sheet ore_type.  Omit from scatter ore_type for a uniform ore distribution
+    --  ^ If noise is above this threshold, ore is placed. Not needed for a
+    --  ^ uniform distribution.
+        noise_params = {
+            offset = 0,
+            scale = 1,
+            spread = {x = 100, y = 100, z = 100},
+            seed = 23,
+            octaves = 3,
+            persist = 0.7
+        },
+    --  ^ NoiseParams structure describing one of the perlin noises used for ore
+    --  ^ distribution.
+    --  ^ Omit from "scatter" ore for a uniform ore distribution.
+    --  ^ Omit from "stratum ore for a simple horizontal strata from y_min to y_max.
         random_factor = 1.0,
     --  ^ Multiplier of the randomness contribution to the noise value at any
     --  ^ given point to decide if ore should be placed.  Set to 0 for solid veins.
     --  ^ This parameter is only valid for ore_type == "vein".
         biomes = {"desert", "rainforest"}
-    --  ^ 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.
+    --  ^ 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.
     --  ^ Can be a list of (or a single) biome names, IDs, or definitions.
     }
 

src/mg_ore.cpp

@@ -433,7 +433,8 @@ void OreStratum::generate(MMVManip *vm, int mapseed, u32 blockseed,
 	PcgRandom pr(blockseed + 4234);
 	MapNode n_ore(c_ore, 0, ore_param2);
 
-	if (!noise) {
+	if (flags & OREFLAG_USE_NOISE) {
+		if (!(noise && noise_stratum_thickness)) {
 			int sx = nmax.X - nmin.X + 1;
 			int sz = nmax.Z - nmin.Z + 1;
 			noise = new Noise(&np, 0, sx, sz);
@@ -441,6 +442,7 @@ void OreStratum::generate(MMVManip *vm, int mapseed, u32 blockseed,
 		}
 		noise->perlinMap2D(nmin.X, nmin.Z);
 		noise_stratum_thickness->perlinMap2D(nmin.X, nmin.Z);
+	}
 
 	size_t index = 0;
 
@@ -452,10 +454,18 @@ void OreStratum::generate(MMVManip *vm, int mapseed, u32 blockseed,
 				continue;
 		}
 
+		int y0;
+		int y1;
+
+		if (flags & OREFLAG_USE_NOISE) {
 			float nmid = noise->result[index];
 			float nhalfthick = noise_stratum_thickness->result[index] / 2.0f;
-		int y0 = MYMAX(nmin.Y, nmid - nhalfthick);
-		int y1 = MYMIN(nmax.Y, nmid + nhalfthick);
+			y0 = MYMAX(nmin.Y, nmid - nhalfthick);
+			y1 = MYMIN(nmax.Y, nmid + nhalfthick);
+		} else {
+			y0 = nmin.Y;
+			y1 = nmax.Y;
+		}
 
 		for (int y = y0; y <= y1; y++) {
 			if (pr.range(1, clust_scarcity) != 1)

src/mg_ore.h

@@ -135,7 +135,7 @@ public:
 
 class OreStratum : public Ore {
 public:
-	static const bool NEEDS_NOISE = true;
+	static const bool NEEDS_NOISE = false;
 
 	NoiseParams np_stratum_thickness;
 	Noise *noise_stratum_thickness = nullptr;

src/script/lua_api/l_mapgen.cpp

@@ -1156,7 +1156,9 @@ int ModApiMapgen::l_register_ore(lua_State *L)
 			OreStratum *orestratum = (OreStratum *)ore;
 
 			lua_getfield(L, index, "np_stratum_thickness");
-			read_noiseparams(L, -1, &orestratum->np_stratum_thickness);
+			// If thickness noise missing unset 'use noise' flag
+			if (!read_noiseparams(L, -1, &orestratum->np_stratum_thickness))
+				ore->flags &= ~OREFLAG_USE_NOISE;
 			lua_pop(L, 1);
 
 			break;