procedural-rock

Rocks made from a seed, with no model file. rockGeometry returns the positions, normals and indices of one rock. The same seed always gives the same rock, so a scattered field is reproducible and needs nothing from the network.

b3d-decorator uses it for every boulder and stone: a rule names a model rock:boulder:3 and the decorator builds it instead of looking it up in a library. It shades them with the terrain's own biome colours, so a rock on Mars is Mars-coloured and a rock above the snow line carries snow.

import { rockGeometry } from 'tosijs-3d'

const rock = rockGeometry({ seed: 7, cuts: 9, squash: 0.75 })
// rock.positions, rock.normals, rock.indices

Winding is counter-clockwise (the glTF convention), and Babylon's front face is the other way round. Handed to Babylon as they are, the triangles draw inside out: you see the far wall of the rock through the near one. Swap two indices of each triangle first:

const indices = new Uint16Array(rock.indices.length)
for (let i = 0; i < indices.length; i += 3) {
  indices[i] = rock.indices[i]
  indices[i + 1] = rock.indices[i + 2]
  indices[i + 2] = rock.indices[i + 1]
}
const data = new BABYLON.VertexData()
data.positions = rock.positions
data.normals = rock.normals
data.indices = indices
data.applyToMesh(mesh)

The decorator does this for you.

Names a scatter rule can use

rock:<kind>:<n>, where the kind is one of boulder, tall, stone or slab (ROCK_KINDS) and n picks the seed. rockNames('boulder', 6) lists six of them.

Under the decorator, a rock's own sink does nothing: the decorator seats every model by its lowest point and then buries it by the RULE's sink (ScatterRule.sink), so that is the one to change.

How a rock is made

1. Start from a sphere.
2. Cut it with a handful of random planes. This is what makes it read as rock: flat faces meeting at blunt edges, where noise alone gives a potato.
3. Push the surface in and out with a little low-frequency noise.
4. Stretch it unevenly and squash it, so no two share a silhouette.
5. Sink the base a little below y = 0, so it sits IN the ground.

The result is about one unit across, with its origin on the ground under it: the same convention as a library model, so scatter rules scale it the same way.

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