ludic/packages/ludic.nav/README.md

ludic.nav

A walkable mesh of the world and the ways across it, over Recast & Detour (zlib) built here from a pinned tag (phase 17). Uses ludic.base and nothing else.

import "ludic.nav"

The rules it keeps

• One mesh per kind of walker. NAV_PERSON (0.35 m wide, the hiker's 0.55 m step), NAV_LARGE (an elk, a bear, a horse) and NAV_SMALL (a hare, a marmot) each have their own, because what a hare slips between a bear walks round. A NavConfig says what a mesh is built for: its voxels and its walker's height, radius, step and steepest slope.
• Built from ground, or loaded from a bake. A NavGround is triangles - each with an area byte, 0 not walkable and 1..62 a kind of ground - and what nothing stands in: cylinders (a trunk, a post) and convex footprints (a boulder). Ground steeper than the slope is cleared whatever its area. nav_build makes one mesh of it; a map is square tiles (nav_tiled, then nav_tile_build for each tile from the ground under it and a few metres round), and a tile must be a whole number of cells across or its seams never join - the shim refuses one that is not. A game bakes its maps once and loads the bytes (nav_save_file / nav_load_file, one format for one tile or many): a map is never built at start-up. Polygon refs are 64-bit, so a map may have 16384 tiles.
• A path is corners. nav_path answers the way from one point to another as the corners where it turns, the first the start and the last the end. When the end cannot be reached the path stops as near it as the mesh allows and nav_partial says so - a walker goes there and gives up, never through a river. Either end off the mesh is -1.
• A wander's goal is somewhere it can go. nav_random_near answers a point about r away that a walker standing at the start can reach - never across a river without a ford - from a seed the caller draws from its own Rng, so the same seed is the same point on every machine.
• A kind of ground has a cost, and a walker has a taste. A mesh carries NAV_FILTERS (8) sets of costs per area (nav_area_cost(kind, filter, area, cost)), and every path, straight line and random point names the filter it walks by: a deer's dislikes scree, a marmot's likes it, a person's prefers the town. The nearest point uses filter 0.
• The same question gets the same answer. Detour is deterministic for the same mesh and the same points, so co-op's order of dice is untouched by asking it.

API

NAV_PERSON, NAV_LARGE, NAV_SMALL, NAV_FILTERS, NAV_CORNERS	the kinds of walker, the filters a mesh has (8), and a path's most corners (256)
NavConfig { cell, cell_h, height, radius, climb, slope }	what a mesh is built for (a person by default, 0.25 m cells)
NavGround { v, nv, t, nt, area, cyl, nc, foot, nf }	what it is built from: triangles, an area byte each, cylinders (x, y, z, r, h) and footprints (n, n points x z, ymin, ymax)
nav_build(kind, ground, config) -> bool, nav_polygons(kind)	one mesh over all of it
nav_tiled(kind, ox, oz, tile, max_tiles, max_polys) -> bool, nav_tile_build(kind, tx, tz, ground, config) -> int	a map of square tiles, a tile at a time (its polygons, 0 none, -1 failed)
nav_save_file(kind, path), nav_load_file(kind, path), nav_reset()	a baked mesh written and read; every mesh let go
nav_nearest(kind, x, y, z) -> bool, nav_near_x/y/z()	the nearest walkable point within a couple of metres
nav_path(kind, filter, sx, sy, sz, ex, ey, ez) -> int, nav_corners(), nav_corner_x/y/z(i), nav_partial(kind)	a way as corners
nav_next_corner(kind, filter, sx, sy, sz, ex, ey, ez) -> bool	the next corner of that way, read back as the nearest point is (a walker's one question a frame)
nav_straight(kind, filter, sx, sy, sz, ex, ez) -> bool	does the straight line stay walkable
nav_random_near(kind, filter, x, y, z, r, seed) -> bool, nav_near_x/y/z()	a reachable point about r away, from the caller's seed
nav_asked(), nav_found(), nav_short(), nav_us()	paths asked for, found and cut short since the meshes loaded, and the microseconds they took (timed in the shim by the OS clock: a game's measure of how often its walkers had a way, and what it cost)
nav_area_cost(kind, filter, area, cost)	how dear a kind of ground is to cross by that filter

Tests

ludic test packages/ludic.nav

A 40 m meadow built by hand: a path round a post and a boulder's footprint, round a band of thicket by a filter that dislikes it and straight through by the plain one, over a river's ford, stopping at the bank of a river with none, the nearest point, random points that never cross the river, and a saved mesh answering as the built one did. And a 128 m meadow as four tiles: a path across the seams, a tile under water with no polygons, and the set saved and loaded.

The native library

native/build.sh fetches Recast & Detour v1.6.0, checks its SHA-256, and builds Recast, Detour and the shim (native/shim/nav_shim.cpp) into lib/<target>/ - the same script on the Mac and on the PC (Git Bash, the LLVM installer's clang). native/LICENSE-recastnavigation ships with it.

Crowds (phase 18)

nav_crowd_start(kind, max, max_radius) puts a DetourCrowd on a kind's mesh, with the mesh's tastes as its filters. nav_crowd_add(kind, x, y, z, radius, height, speed, filter, separation) adds a walker (snapped to the mesh, -1 off it), nav_crowd_target and nav_crowd_speed say where and how fast, and nav_crowd_step(dt) steps every crowd. nav_crowd_read puts a walker into nav_agent_x/y/z/vx/vz, and nav_crowd_us() is what the stepping has cost. nav_crowd_add_fixed adds a walker the crowd avoids but never steers (a player), which nav_crowd_place puts where it is, and how fast it goes, each frame. The mechanic decides and the crowd moves, and dropping a mesh drops its crowd first. On the test meadow, twenty walkers crossing head-on never come closer than their two radii, and a step costs about 12 us.