Walking through a scan

Walking through a scan

extensions/scan/walk.mojo puts a person in a scanned building, as a game does. The person is a capsule in a physics world that holds the scan as a static collider. A tick of the world is a tick of the game loop. A camera at the walker's eye gives the first-person view.

The picture comes from examples/scan_walk.mojo. The scan is "Studio 11" by milanoski, under CC BY 4.0. The walker crosses the living room, with a rug, an armchair and a television, and turns round at a wall.

The loop

var scan = open_scan("studio-11")
var filled = fill_grid(scan)
var walker = Walker(
    filled.grid,
    filled.floor,
    Vec3d(filled.region.low.x, filled.region.low.y, 0),
    Vec3d(filled.region.high.x, filled.region.high.y, 0),
    default_walk_options(),
    PhysicsMaterial.default(),
)
for _ in range(frames):
    walker.step(Duration(Float32(1.0 / 60), SECOND))
    var eye = walker.eye()          # meters east, north and up
    var ahead = walker.forward()    # a unit vector in the plan
    # Draw the scene from the eye.

open_scan and fill_grid are in tools/_scan_demo.mojo, which the examples share. They read a scan, clean it, and fill its occupancy grid with the voxel grown until it fits. A caller with its own grid passes it to Walker.

A tick does three things.

1. The walker turns toward its goal, at most turn radians a second. It stops and turns where its way is more than 0.6 radians from the goal.
2. It sets its horizontal velocity to speed along the way it faces.
3. The world steps. Gravity, the floor, the walls and the furniture of the scan decide the rest.

The capsule cannot turn over: its inverse inertia is locked, as locked_inverse_inertia gives it. The walker turns round when it comes within 0.3 m of an end.

The ends of the walk

The goal is one of two points of the floor. walk_ends finds them from the occupancy grid.

1. A plan point is clear when the floor is under it. No solid voxel is within the walker's radius, from step to clearance over the floor.
2. The clear points are tried on a lattice, spacing apart.
3. The first end is the clear point nearest the middle of the clear points.
4. The second is the clear point farthest from the first with a clear line between them. is_walkable checks the line every 0.1 m.
5. The first end then moves to the point farthest from the second in the same way.

A scan with fewer than two clear points has no walk, and walk_ends refuses it.

is_clear tests one point, and is_walkable tests one line. Each checks its options first. They refuse options that WalkOptions.check refuses.

Options

WalkOptions holds sizes and paces as typed quantities. default_walk_options() gives an adult.

Option	Default	Meaning
radius	0.3 m	The capsule's radius
height	1.8 m	Feet to crown
eye	0.75 m	The eye over the capsule's center, inside the capsule
speed	1.4 m/s	The walking pace
turn	2.5 rad/s	The most the walker turns in a second
step	0.25 m	The lowest solid voxel that stops the walker. A lower one is a step
clearance	1.7 m	The height over the floor that must be clear
spacing	0.5 m	The distance between the points tried for the ends
mass	80 kg	The walker's mass

The speed

The numbers come from a Linux machine with 24 threads and an NVIDIA TITAN RTX. The scan holds 4,759,156 splats after the cleaning. Its tree for levels of detail holds 6,301,303.

Draw	Size	Physics	Draw	The loop
scan_walk.mojo, on every core	480 by 270	0.7 ms	24.3 ms	40 frames a second
scan_walk_gpu.mojo, on the GPU	1280 by 720	0.7 ms	13.2 ms	72 frames a second

The physics of the whole scan, as one static mesh of merged faces, costs about a millisecond a tick. The draw is the rest. See GPU splats for where its time goes.

Limits

• The walker follows two points and no more. It is a test of the scan's collider and of the frame rate, not a path finder.
• A scan holds surfaces, and a collider made of its voxels is a shell one voxel thick. A walker that walks at 1.4 m/s does not pass through it. A fast body can: see Scanned buildings.
• A stair is a sequence of steps. A step higher than step stops the walker.
• A scan has holes where the camera did not see. A hole in a wall is a door to the walker.