World Generation
World generation is the game building the blocks of a chunk from the world seed and the chunk's coordinates.
Chunks
A chunk is 16 blocks in x, 16 in z and 128 in y. Sea level is y=64.
A chunk is built in two passes. The first runs when the chunk is first needed and does five things in order:
- look up the biome and temperature of each of the chunk's 256 columns,
- place stone, water and ice from the terrain noise,
- replace the top blocks of each column with the biome's surface, and add bedrock,
- carve caves,
- build the skylight map.
The second pass, population, adds ores, features and plants. It runs on a chunk only once the chunks at +1 x, +1 z and both have also been built. Population places its features in a 16×16 area offset 8 blocks in x and z from the chunk origin, which overlaps four chunks — hence the wait.
Population seeds its random numbers from the chunk coordinates and the world seed together, so a chunk's features do not depend on when it was generated.
Biomes
Two noise fields cover the world: temperature and rainfall, each a value from 0 to 1. Neither varies with height or with the terrain below.
The biome is read from a 64×64 table indexed by the two values. Taking r as
rainfall multiplied by temperature, the first matching row wins:
| Condition | Biome |
|---|---|
| temperature < 0.1 | Tundra |
r < 0.2 and temperature < 0.5 |
Tundra |
r < 0.2 and temperature < 0.95 |
Savanna |
r < 0.2 |
Desert |
r > 0.5 and temperature < 0.7 |
Swampland |
| temperature < 0.5 | Taiga |
temperature < 0.97 and r < 0.35 |
Shrubland |
| temperature < 0.97 | Forest |
r < 0.45 |
Plains |
r < 0.9 |
Seasonal Forest |
| otherwise | Rainforest |
Temperature and rainfall are rounded down to a step of 1/63 before the table is read, so each boundary above falls at the next step. Tundra's temperature below 0.1 takes in everything below 7/63, about 0.111, and Taiga and Tundra reach up to 32/63, about 0.508. The ice and snow of generation read the unrounded temperature, so a thin band at the warm edge of Taiga and Tundra gets no sea ice, and no snow on its lowest ground.
Ice Desert is never returned by that table and does not generate. Hell is the Nether's only biome. Sky belongs to the Sky dimension, which the game never creates a world for.
Each biome carries the block it puts on the surface and the block underneath it. Every biome uses grass over dirt except Desert and Ice Desert, which use sand over sand.
Each biome also stores a colour, given on its page, which nothing in the game reads.
Terrain
Terrain is a density field. Stone is placed wherever the density is above zero. The field is sampled every 4 blocks in x and z and every 8 in y, then interpolated between the samples, which is what gives Beta terrain its smooth, rounded slopes.
The noise fields
Every field the generator reads is Perlin gradient noise. A lattice of random gradient vectors is laid over space, and a point's value is a smoothly faded blend of the gradients at the corners of the cell it falls in. Values drift rather than jump, so a field looks like a spread of soft rounded lumps and never like static.
Each field is a stack of octaves, and each octave is a separate lattice seeded from the world seed. Every octave has half the frequency and twice the amplitude of the one before it. The last octave alone therefore carries half the field's range and sets its shape, and the earlier octaves only roughen it.
Five fields build the density:
| Field | Octaves | Read in | Broadest lump | Role |
|---|---|---|---|---|
| Landscape A | 16 | 3D | about 190 blocks | a complete landscape |
| Landscape B | 16 | 3D | about 190 blocks | a second complete landscape |
| Selector | 8 | 3D | about 60 blocks | chooses between A and B |
| Depth | 16 | 2D | about 650 blocks | the column's centre height |
| Stretch | 10 | 2D | about 1800 blocks | how far the column may stray from its centre |
The two landscape fields are read on the 4×8×4 sample grid at the same numeric scale in every direction, so their lumps come out twice as tall as they are wide. That stretch is what produces overhangs, arches and floating islands.
The selector is mapped so that a value below 0 takes landscape A whole and a value above 1 takes landscape B whole, with a crossfade between. Most sample points land outside that range, so most of the world is one landscape or the other and the blend shows only in narrow bands.
The depth field separates sea from land. Below its threshold the column's centre height is set somewhere between y=56 and y=68 and the stretch is forced to its minimum, which gives the flat low ground of an ocean basin. Above the threshold the centre height rises as far as y=72 and the stretch is left alone.
The stretch field is the broadest of the five, so whether a region is mountainous
or gentle holds across more than a thousand blocks. It is then scaled by
temperature multiplied by rainfall. Cold or dry columns are pulled flat towards
their centre height, and hot wet ones keep their full relief. A column keeps
nearly all of it once r passes 0.7, and less than 60% below 0.2. Every
Rainforest column keeps its full relief. Tundra, Desert and
Savanna, where r is below 0.2, keep the least.
Above y=112 the finished density is blended towards a large negative value, reaching it at y=128. Terrain does not reach the top of the world.
Water, ice and bedrock
Any space below y=64 that is not stone becomes water. At y=63 exactly it becomes ice instead, where the column's temperature is below 0.5.
Bedrock is placed by rolling a number from 0 to 4 separately for every block from y=0 to y=4, and placing bedrock where the roll is at or above that block's y:
| y | Chance of bedrock |
|---|---|
| 0 | always |
| 1 | 4 in 5 |
| 2 | 3 in 5 |
| 3 | 2 in 5 |
| 4 | 1 in 5 |
Every block is rolled on its own, so a column can have bedrock at y=3 and stone at y=2. Y=0 is bedrock everywhere in the world.
Surface
The surface pass walks each column from y=127 downwards and rewrites the first few blocks below every air gap, so it shapes cave roofs and overhangs as well as the open surface.
A noise field sets how deep the filler runs beneath the top block. Where it comes out at zero or less the column gets no top block at all, and bare stone reaches the surface.
Between y=60 and y=65 two more noise fields override the biome's choice:
- about half of columns take sand as both top and filler, which is what makes beaches;
- a much rarer roll takes gravel as filler and leaves the top block empty, so gravel shores sit under water.
A top block left empty below y=64 becomes water. Sand filler is followed by 0 to 3 blocks of sandstone once the sand runs out.
Caves
Caves are carved into a chunk before it is populated. Every chunk within 8 chunks of the one being built is asked to seed caves into it, so one cave system can run 128 blocks across chunk borders.
One chunk in fifteen seeds any caves. Such a chunk starts up to 39 systems, strongly biased towards one or two. Each starts at a random point in the chunk, at a y biased towards the bottom of the world.
One system in four opens with a wide room and then sends out 1 to 4 tunnels. The rest are a single tunnel. A tunnel is a chain of overlapping ellipsoids that turns as it advances, and one tunnel in six turns more sharply than the others. A tunnel may fork in two at its midpoint, each fork narrower than the parent.
Carving replaces stone, dirt and grass and nothing else. Below y=10 the carved block becomes flowing lava rather than air. Dirt left exposed by a removed grass block becomes grass.
Carving runs before population, so the sand and gravel the surface pass laid down are already in place and a tunnel that reaches a beach leaves it standing. The gravel and dirt veins that population adds are not: they replace stone only, so a vein that meets an existing tunnel is cut off at the tunnel wall.
A segment whose bounding box contains water is skipped entirely, so caves do not breach the bottom of an ocean or a lake.
Ores
Every ore is placed by the same generator. It draws a line through the chunk at a random horizontal angle, walks along it, and fills an ellipsoid at each step. Only stone is replaced, so a vein running into a cave or into dirt loses those blocks. Size is the generator's block count, not the number of blocks a player finds.
| Ore | Veins per chunk | Size | Y |
|---|---|---|---|
 Dirt |
20 | 32 | 0–127 |
 Gravel |
10 | 32 | 0–127 |
 Coal Ore |
20 | 16 | 0–127 |
 Iron Ore |
20 | 8 | 0–63 |
 Gold Ore |
2 | 8 | 0–31 |
 Redstone Ore |
8 | 7 | 0–15 |
 Diamond Ore |
1 | 7 | 0–15 |
 Lapis Lazuli Ore |
1 | 6 | 0–30, peaking at 15 |
The y in the table is where the line is drawn. The generator then lifts the vein 2 to 4 blocks above it, so a diamond vein reaches a few blocks higher than 15.
Clay uses the same shape at size 32, 10 times per chunk, but replaces sand instead of stone and gives up unless the point it starts from is water.
Population
Every feature below is attempted at a random point in the populated area, and most of them fail most of the time. Attempts are per chunk.
| Feature | Attempts | Y |
|---|---|---|
| Water lake | 1 chunk in 4 | 0–127 |
| Lava lake | 1 chunk in 8 | biased low; above y=63 only 1 time in 10 |
| Dungeon | 8 | 0–127 |
| Trees | by biome | surface |
| Flowers, grass, mushrooms | by biome | 0–127 |
| Sugar cane | 10 runs of 20 | 0–127 |
| Pumpkin | 1 chunk in 32 | 0–127 |
| Cactus | 10 patches in Desert, else none | 0–127 |
| Water spring | 50 | biased low |
| Lava spring | 20 | strongly biased low |
The biome that sets the counts is read once per chunk, at the centre of the populated area, and holds for the whole of it. Near a border, one biome's trees and plants can appear in another's columns, even in Plains, Desert or Tundra, which grow no trees of their own.
Sand and gravel that a feature undercuts are left hanging unless something updates them. One updated while a chunk is being populated, as beside a spring, drops at once to where it would land, with no falling sand entity.
Lakes
A lake is 4 to 7 overlapping ellipsoids inside a box 16 across, 16 deep and 8 tall, placed from four blocks below the first solid block under the chosen point. The bottom four layers are filled with the liquid and the top four are cleared to air.
The whole shape is abandoned if any block bordering the upper half is liquid, or if any block bordering the lower half is neither solid nor the lake's own liquid. A lake cannot form against open air, and cannot merge into an existing body of water.
Dirt under the lake's cleared upper layers, its banks, becomes grass where sky light reaches it. Dirt under the liquid stays dirt. A lava lake seals itself in a shell of stone: every solid neighbour of the lower half is replaced, and half of those around the upper half.
Dungeons
A dungeon is attempted 8 times per chunk, at a y from 0 to 127, and is built only where the rock around it passes the dungeon's tests.
Trees
Every chunk starts with a tree count of zero, and one time in ten that count is raised to one. The biome is then applied on top of it.
| Biome | Trees |
|---|---|
| Forest, Rainforest, Taiga | n + 5, plus the bonus |
| Seasonal Forest | n + 2, plus the bonus |
| Shrubland, Savanna, Swampland | the bonus alone |
| Desert, Plains, Tundra | none |
n is a noise value read from the same field the game uses for mob spawning, and
averages 2 before it is truncated to a whole number. It drifts over about sixteen
chunks, so tree density shifts across a forest rather than jumping from one chunk
to the next.
Desert, Plains and Tundra subtract 20 and add nothing, so their count stays negative and the bonus can never lift it above zero. Shrubland, Savanna and Swampland appear in none of the biome terms, so the bonus is the only tree they ever get.
Which tree is built depends on the biome as well:
| Biome | Trees built |
|---|---|
| Forest | birch 1 in 5; of the rest, big 1 in 3 |
| Rainforest | big 1 in 3 |
| Taiga | pine 1 in 3, spruce otherwise |
| Everything else | big 1 in 10 |
Every tree needs grass or dirt beneath it and a clear column. Every tree but the big tree turns the block under its trunk to dirt. Trunk heights are 4 to 6 for the plain tree, 5 to 7 for birch, 6 to 9 for spruce, 7 to 11 for pine and 5 to 16 for the big tree.
Plants
A plant generator is given one point and makes many attempts around it, so each count below is a patch rather than a plant. Attempts land up to 7 blocks from the point in x and z, and up to 3 in y.
Tall grass and dead bush patches first drop from their point, through air and leaves, to the ground. Flower, rose, mushroom and cactus patches do not, so they place anything only when their point lands within 3 blocks of ground they can grow on.
| Plant | Patches per chunk | Attempts per patch |
|---|---|---|
 Flower |
4 in Seasonal Forest, 3 in Plains, 2 in Forest and Taiga, else none | 64 |
 Rose |
1 chunk in 2 | 64 |
 Tall Grass |
10 in Rainforest and Plains, 2 in Forest and Seasonal Forest, 1 in Taiga, else none | 128 |
 Brown Mushroom |
1 chunk in 4 brown, 1 chunk in 8 red | 64 |
 Dead Bush |
2 in Desert, else none | 4 |
Two patches in three in Rainforest place ferns instead of tall grass.
Sugar cane is placed by ten separate runs per chunk. Each run picks one point at a random y from 0 to 127, then makes 20 attempts to plant a stalk within 3 blocks of that point in x and z, all at the run's single y. An attempt needs air at that y and water beside the block below it, so almost every one fails. A stalk that does take hold is 2 to 4 blocks tall.
A cactus patch makes 10 attempts, and each cactus is 1 to 3 blocks tall.
Springs
A spring is a single source block set into a wall of stone. It is placed only where the blocks above and below are stone, exactly three of the four horizontal neighbours are stone, and exactly one is air.
Water springs are attempted 50 times per chunk and lava springs 20, both at a y biased towards the bottom of the world. The source flows immediately rather than waiting for a tick.
Snow
The last step of population lays snow over the populated area. A column
is covered when the top solid block is not ice, and the column's temperature
reduced by (y − 64) ÷ 64 × 0.3 comes out below 0.5.
The height term is what puts snow on mountain tops in biomes that are otherwise bare. It reads the temperature directly, not the biome's snow flag.
The Nether
The Nether uses a generator of its own. It shares the density-field approach but replaces stone with netherrack and water with lava, and its lava level is y=32.
A fixed vertical profile is added to the density, pushing the outermost four sample levels at each end of the world towards solid and doing it far harder at the last two. The result is netherrack floor and ceiling under every column.
Bedrock is then rolled the same way as in the Overworld, once at each end:
| y | Chance of bedrock |
|---|---|
| 0 and 127 | always |
| 1 and 126 | 4 in 5 |
| 2 and 125 | 3 in 5 |
| 3 and 124 | 2 in 5 |
| 4 and 123 | 1 in 5 |
The surface pass runs the same way as the Overworld's, with netherrack as both top and filler. Between y=60 and y=65 about half of columns take gravel and about half take soul sand, with soul sand winning where both apply.
Caves are seeded in one chunk in five rather than one in fifteen, and their tunnels are twice as wide and half as tall. They carve netherrack only, and a segment is skipped where its box contains lava.
Population is short:
| Feature | Attempts per chunk |
|---|---|
| Lava spring | 8 |
| Fire patch | 1 to 10 |
| Glowstone cluster | 0 to 9, plus 10 more, all hanging from ceilings |
| Mushroom patch | 1 brown and 1 red |
A glowstone cluster starts from one block against a netherrack ceiling and makes 1500 attempts to grow downwards, adding a block only where exactly one of the six neighbours is already glowstone.
There are no ores, lakes, dungeons or trees in the Nether. Its population step does not reseed its random numbers per chunk, so which features a chunk receives depends on the order chunks were generated in.
The world spawn point
On a new world the game walks outwards from x=0, z=0 in random steps of up to 63 blocks until it finds a column whose topmost uncovered block is sand. Every world's spawn point is on sand.
The spawn y is recorded as 64 whatever the terrain does. A player joining is placed on the first non-air column found by stepping up to 7 blocks at a time from the recorded point, at the height of the first block with air above it.