Ore: Add ore sheet column height range selection

Modders are now able to select the range of ore column height,
and the midpoint at which they 'grow' starting from.
This commit adds three new parameters for the 'sheet' ore type:
column_height_min, column_height_max, and column_midpoint_factor.
clust_size is now deprecated for this ore type.

doc/lua_api.txt

@@ -714,23 +714,28 @@ a non-equal distribution of ore.
 
 ### `sheet`
 Creates a sheet of ore in a blob shape according to the 2D perlin noise
-described by `noise_params`. The relative height of the sheet can be
-controlled by the same perlin noise as well, by specifying a non-zero
-`scale` parameter in `noise_params`.
+described by `noise_params`. This is essentially an improved version of
+the so-called "stratus" ore seen in some unofficial mods.
 
-**IMPORTANT**: The noise is not transformed by `offset` or `scale` when comparing
-against the noise threshold, but scale is used to determine relative height.
-The height of the blob is randomly scattered, with a maximum height of `clust_size`.
+This sheet consists of vertical columns of uniform randomly distributed height,
+varying between the inclusive range `column_height_min` and `column_height_max`.
+If `column_height_min` is not specified, this parameter defaults to 1.
+If `column_height_max` is not specified, this parameter defaults to `clust_size`
+for reverse compatibility.  New code should prefer `column_height_max`.
 
-`clust_scarcity` and `clust_num_ores` are ignored.
+The `column_midpoint_factor` parameter controls the position of the column at which
+ore eminates from.  If 1, columns grow upward.  If 0, columns grow downward.  If 0.5,
+columns grow equally starting from each direction.  `column_midpoint_factor` is a
+decimal number ranging in value from 0 to 1.  If this parameter is not specified,
+the default is 0.5.
 
-This is essentially an improved version of the so-called "stratus" ore seen in
-some unofficial mods.
+The ore parameters `clust_scarcity` and `clust_num_ores` are ignored for this ore type.
 
 ### `blob`
 Creates a deformed sphere of ore according to 3d perlin noise described by
 `noise_params`.  The maximum size of the blob is `clust_size`, and
 `clust_scarcity` has the same meaning as with the `scatter` type.
+
 ### `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

src/mg_ore.cpp

@@ -176,8 +176,8 @@ void OreSheet::generate(MMVManip *vm, int mapseed, u32 blockseed,
 	PseudoRandom pr(blockseed + 4234);
 	MapNode n_ore(c_ore, 0, ore_param2);
 
-	int max_height = clust_size;
-	int y_start = pr.range(nmin.Y, nmax.Y - max_height);
+	u16 max_height = column_height_max;
+	int y_start = pr.range(nmin.Y + max_height, nmax.Y - max_height);
 
 	if (!noise) {
 		int sx = nmax.X - nmin.X + 1;
@@ -200,10 +200,12 @@ void OreSheet::generate(MMVManip *vm, int mapseed, u32 blockseed,
 				continue;
 		}
 
-		int height = max_height * (1. / pr.range(1, 3));
-		int y0 = y_start + np.scale * noiseval; //pr.range(1, 3) - 1;
+		u16 height = pr.range(column_height_min, column_height_max);
+		int ymidpoint = y_start + noiseval;
+		int y0 = ymidpoint - height * (1 - column_midpoint_factor);
 		int y1 = y0 + height;
-		for (int y = y0; y != y1; y++) {
+
+		for (int y = y0; y < y1; y++) {
 			u32 i = vm->m_area.index(x, y, z);
 			if (!vm->m_area.contains(i))
 				continue;

src/mg_ore.h

@@ -87,6 +87,10 @@ class OreSheet : public Ore {
 public:
 	static const bool NEEDS_NOISE = true;
 
+	u16 column_height_min;
+	u16 column_height_max;
+	float column_midpoint_factor;
+
 	virtual void generate(MMVManip *vm, int mapseed, u32 blockseed,
 		v3s16 nmin, v3s16 nmax, u8 *biomemap);
 };

src/script/lua_api/l_mapgen.cpp

@@ -937,8 +937,19 @@ int ModApiMapgen::l_register_ore(lua_State *L)
 	}
 	lua_pop(L, 1);
 
-	if (oretype == ORE_VEIN) {
+	//// Get type-specific parameters
+	if (oretype == ORE_SHEET) {
+		OreSheet *oresheet = (OreSheet *)ore;
+
+		oresheet->column_height_min = getintfield_default(L, index,
+			"column_height_min", 1);
+		oresheet->column_height_max = getintfield_default(L, index,
+			"column_height_max", ore->clust_size);
+		oresheet->column_midpoint_factor = getfloatfield_default(L, index,
+			"column_midpoint_factor", 0.5f);
+	} else if (oretype == ORE_VEIN) {
 		OreVein *orevein = (OreVein *)ore;
+
 		orevein->random_factor = getfloatfield_default(L, index,
 			"random_factor", 1.f);
 	}