Terrain generation
In Banished, the map comes from a seed. The same seed and the same settings always give the same map. Un-Banished makes the same maps, bit for bit. This page explains the steps. The code is in banished-terrain.
The inputs
The New Game dialog chooses three profiles and a seed:
| Setting | Resource | Class |
|---|---|---|
| Terrain type (Valleys, Mountains) | Game/Terrain/*.rsc | TerrainGenerator |
| Climate (Mild, Fair, Harsh) | Game/Climate/*.rsc | ClimateGenerator |
| Starting conditions (Easy, Medium, Hard) | Game/StartCondition/*.rsc | MapGenerator |
A terrain profile holds one size record for each map size. Its first value, n, is 256, 384 or 512. The terrain is 2n cells wide, so it has 2n + 1 height samples on each side. The playable map is the central n × n cells; the outer ring is scenery.
The game's random number generator is seeded with the map seed. Every step below draws from it, in a fixed order.
The five stages
The generator runs five stages on a float height grid:
| Stage | What it does |
|---|---|
| 1 | Fills the grid from two Perlin noise fields, each with its own random seeds. |
| 2 | Makes the river paths (random end points, with midpoints displaced again and again) and the lake points. A path that crosses itself or another path is rejected. |
| 3 | Adds random local changes, then draws the paths and points into extra grids and combines them with the heights. (Inferred: this shapes the river beds and the lakes.) |
| 4 | Walks from random cells downhill and moves part of the height difference (inferred: a simple erosion). |
| 5 | Applies the paths and points once more, then smooths the grid twice. |
Then the float heights are multiplied by 256 and stored as 16-bit integers. These are the heights that a save file stores.
After the heights
The surface image: one 32-bit value per cell, which packs how much dirt, rock and light the terrain shader uses there. It draws two more random numbers for its own Perlin noise.
The mesh: the terrain is cut into tiles of 32 × 32 cells. Each tile has 33 × 33 vertices, and the diagonal of each cell alternates by position, like a chequerboard.
The water: the river paths become a triangle strip drawn with the River material and the lake points become the lake surface.
The cell types: the game casts a ray down at the centre of each map cell and reads the ground height and slope. Deep water is below -1.4, shallow water below -0.25, buildable ground is between -0.25 and 0.75 on flat ground, rough ground is below 2.0, and high ground is the rest. Water and high ground block walking.
The start point: the game searches in rings outward from the centre for the first place where all cells within the start radius are buildable ground.
How it was proved
The terrain was the first large part ported, and it set the method for the rest:
- The original's generator was run in the emulator with many seeds and sizes.
tools/emu/terrain_stages.pywrote the grid after each of the five stages. - The Rust port was compared stage by stage. When a stage differed, the first differing cell showed where to look in the disassembly.
- Then real saves were used: the port regenerates the terrain from a save's seed and settings, and compares it with the heights that the original stored in the save.
The tests terrain_emu and saves in crates/game/tests/ do this. The cell types match 99.95 % of the free cells of a large save; the cells that differ are where buildings changed the ground.
Read more
- WORLD-GENERATION.md: from the New Game dialog to the generator.
- TERRAIN-GRID-TRANSFORMS.md: the five stages in detail.
- TERRAIN-PROFILES.md: the values in each profile and size.
- TERRAIN-DERIVED-GEOMETRY.md and TERRAIN-WATER-GEOMETRY.md: the mesh, the surface image and the water.
- MAP-CELLS.md: the gameplay map, the ground query and the cell types.
- START-CONDITION-PROFILES.md: what a start condition places.