Terrain you can change while the world runs, and terrain described by a distance field. Everything here is deterministic, goes into snapshots and hashes, is recorded by replays and is carried by forks. The normative descriptions are in collision2d and collision3d.
Carve a 2D bitmask terrain
var world = new PhysicsWorld2D(); var solid = new bool[128 * 64]; // x-fastest; true = ground for (int y = 0; y < 32; y++) for (int x = 0; x < 128; x++) solid[y * 128 + x] = true; Body ground = world.AddBitmaskTerrain(128, 64, solid, cellSize: 0.25f); ground.CarveCircle(new Vector2D(10, 8), 2.5); // a crater; bodies resting on it wake and fall ground.FillPolygon([new(4, 6), new(8, 6), new(8, 8)]); Body[] pieces = ground.DetachIslands(); // chunks cut loose become dynamic bodies
Each nonempty cell is one sample at its centre and the boundary is the marching-squares contour of the samples, so craters are smoother than cell staircases. An edit rebuilds the boundary only near the changed cells: segments elsewhere keep their shape handles and cached contacts, and bodies far from the edit are not woken. DetachIslands keeps pieces that touch the bottom row (TerrainAnchors), turns the rest into bodies covered by boxes and returns them in piece order.
Dig a 3D voxel grid
var world = new PhysicsWorld3D(); Body ground = world.AddVoxelGrid(64, 8, 64, occupancy, cellSize: 0.5f); Shape voxels = ground.Voxels; voxels.DigSphere(new Vector3D(10, 2, 10), 1.8); // cells whose centres are inside voxels.SetVoxels(14, 0, 14, 4, 4, 4, solid: true); // a block of cells, in cell coordinates
Triangle ids are 12 * cell + 2 * face + half, so faces nobody touched keep their ids; characters and bodies elsewhere keep their state bit for bit. Bodies near the edit wake and re-find the changed surface.
Signed distance fields
SdfGrid3D hill = SdfGrid3D.FromFunction(new(-8, -3, -8), new(8, 2, 8), 0.25f, SdfGrid3D.Union(SdfGrid3D.HalfSpace(default, new(0, 1, 0)), SdfGrid3D.Sphere(new(0, 0, 0), 2f))); world.AddSdf(hill); // static terrain SdfGrid3D ball = SdfGrid3D.FromMesh(vertices, indices, cellSize: 0.1f); // a closed, consistently oriented mesh Body body = world.AddSdf(ball, at: new Vector3D(0, 5, 0), type: BodyType.Dynamic); // mass from the inside volume
The field is trilinear between samples; contacts use its gradient. A dynamic field collides with convex shapes and other fields; characters, particles and rods see the field's iso-surface as triangles; rays are sphere-traced. A field is at most 2^22 samples and is copied into every snapshot, so keep the grid as coarse as the contact quality allows. FromMesh rejects open, inconsistently wound or degenerate meshes with an SdfBuildException naming the problem. In 2D, SdfGrid2D and PhysicsWorld2D.AddSdf make static terrain from the field's zero contour, or (with type: BodyType.Dynamic) a moving body covered by convex polygons that tile the contour, with its mass from the inside area; 2D contacts use those polygons rather than field samples.