VastuSLAM

A high-performance 2D SLAM library for robot vacuum cleaners, featuring multi-sensor occupancy grid mapping, scan matching, and loop closure.

Features

• Multi-Sensor Occupancy Grid: Semantic cell types (Floor, Wall, Cliff, Bump) with Bayesian probability updates
• Scan Matching: Correlative and branch-and-bound scan-to-map alignment with Gauss-Newton refinement
• Submap Architecture: Cartographer-inspired reversible map updates for loop closure correction
• Loop Closure: LiDAR-IRIS descriptors with pose graph optimization
• SIMD Optimized: Explicit std::simd operations with f32x4 for portable SIMD (ARM NEON, x86 SSE/AVX)
• SoA Data Layout: Structure-of-Arrays for cache-friendly, SIMD-efficient memory access

Quick Start

use vastu_slam::{OccupancyGridMap, MapConfig, Pose2D, LidarScan};

// Create map with default configuration (800×800 cells, 2.5cm resolution = 20m×20m)
let config = MapConfig::default();
let mut map = OccupancyGridMap::new(config);

// Process lidar scan
let pose = Pose2D::new(0.0, 0.0, 0.0);
let scan = LidarScan::new(ranges, angles, 0.15, 8.0);
let result = map.observe_lidar(&scan, pose);

println!("Updated {} cells", result.cells_updated);

System Architecture

┌─────────────────────────────────────────────────────────────────────────────────┐
│                              VastuSLAM Pipeline                                 │
├─────────────────────────────────────────────────────────────────────────────────┤
│                                                                                 │
│  Sensors                          SLAM Core                         Output      │
│  ───────                          ─────────                         ──────      │
│                                                                                 │
│  ┌─────────┐     ┌─────────────────────────────────────────────┐   ┌─────────┐  │
│  │ LiDAR   │────▶│           Scan Matching                     │──▶│ Pose    │  │
│  │ 360°    │     │  ┌──────────────────────────────────────┐   │   │ Output  │  │
│  └─────────┘     │  │ Coarse: Correlative / Branch-Bound   │   │   └─────────┘  │
│                  │  │ Fine: Gauss-Newton + Distance Field  │   │                │
│  ┌─────────┐     │  └──────────────────────────────────────┘   │   ┌─────────┐  │
│  │ Cliff   │────▶│                    │                        │──▶│ Map     │  │
│  │ Sensors │     │                    ▼                        │   │ .vastu  │  │
│  └─────────┘     │  ┌──────────────────────────────────────┐   │   │ .pgm    │  │
│                  │  │         Occupancy Grid               │   │   └─────────┘  │
│  ┌─────────┐     │  │  • Log-odds Bayesian updates         │   │                │
│  │ Bumper  │────▶│  │  • Priority: Bump > Cliff > Wall     │   │   ┌─────────┐  │
│  │ Sensors │     │  │  • Bresenham ray casting             │   │──▶│ Paths   │  │
│  └─────────┘     │  └──────────────────────────────────────┘   │   │ A*      │  │
│                  │                    │                        │   └─────────┘  │
│  ┌─────────┐     │                    ▼                        │                │
│  │ Encoder │────▶│  ┌──────────────────────────────────────┐   │                │
│  │ Odom    │     │  │         Submap Manager               │   │                │
│  └─────────┘     │  │  • Local grid per submap             │   │                │
│                  │  │  • Stored scans for regeneration     │   │                │
│                  │  │  • Origin-only optimization          │   │                │
│                  │  └──────────────────────────────────────┘   │                │
│                  │                    │                        │                │
│                  │                    ▼                        │                │
│                  │  ┌──────────────────────────────────────┐   │                │
│                  │  │         Loop Closure                 │   │                │
│                  │  │  • LiDAR-IRIS descriptors            │   │                │
│                  │  │  • Pose graph optimization           │   │                │
│                  │  │  • Submap origin correction          │   │                │
│                  │  └──────────────────────────────────────┘   │                │
│                  └─────────────────────────────────────────────┘                │
│                                                                                 │
└─────────────────────────────────────────────────────────────────────────────────┘

Core Algorithms

1. Occupancy Grid with Log-Odds

VastuSLAM uses a Bayesian occupancy grid with log-odds representation for numerically stable probability updates:

Log-Odds Update:
L(cell | observation) = L(cell) + L(sensor_model)

Where:
• L_hit = +20 (observation confirms obstacle)
• L_miss = -4 (observation confirms free space)
• L_occupied_threshold = +50 → classify as Wall
• L_free_threshold = -50 → classify as Floor

The asymmetric weights (20 vs -4) mean a single hit strongly indicates an obstacle, while multiple passes are needed to clear a false positive.

2. Multi-Resolution Scan Matching

Scan matching corrects encoder drift by aligning LiDAR scans to the existing map:

Pipeline:
┌───────────────────────────────────────────────────────────────────┐
│ Step 1: COARSE SEARCH                                             │
│   • Window: ±30cm × ±30cm × ±8.6°                                 │
│   • Resolution: 4cm linear, 2.3° angular                          │
│   • Method: Brute-force or branch-and-bound                       │
│   • Scoring: Gaussian distance field (smooth gradients)           │
├───────────────────────────────────────────────────────────────────┤
│ Step 2: FINE SEARCH                                               │
│   • Window: ±6cm × ±6cm × ±3.4°                                   │
│   • Resolution: 2cm linear, 1.1° angular                          │
│   • Prior-penalized scoring                                       │
├───────────────────────────────────────────────────────────────────┤
│ Step 3: GAUSS-NEWTON REFINEMENT                                   │
│   • Adaptive Levenberg-Marquardt                                  │
│   • Sub-millimeter accuracy                                       │
│   • Convergence: cost change < 1e-6                               │
└───────────────────────────────────────────────────────────────────┘

Gaussian Scoring provides smooth gradients for optimization:

Score(point) = exp(-d² / 2σ²)

Where d = distance to nearest wall, σ = 0.10m (typical)

3. Submap Architecture

Inspired by Google Cartographer, VastuSLAM uses submaps for reversible map updates:

┌───────────────────────────────────────────────────────────────┐
│                        World Frame                            │
│                                                               │
│    Submap 0              Submap 1              Submap 2       │
│   ┌─────────┐           ┌─────────┐           ┌─────────┐     │
│   │ Local   │  ──T01──▶ │ Local   │  ──T12──▶ │ Local   │     │
│   │ Grid    │           │ Grid    │           │ Grid    │     │
│   │ [scans] │           │ [scans] │           │ [scans] │     │
│   └────┬────┘           └────┬────┘           └────┬────┘     │
│        ▼                     ▼                     ▼          │
│   origin_0              origin_1              origin_2        │
│   (fixed)               (adjustable)          (adjustable)    │
└───────────────────────────────────────────────────────────────┘

Key Insight: Only submap origins change during optimization.
             Local grids regenerate from stored scans.

Submap Lifecycle:

1. Active: Receives scans, builds local grid
2. Filling: Overlap period (scans go to both old and new submap)
3. Finalized: No more updates, eligible for loop closure

4. Loop Closure Detection

Uses LiDAR-IRIS descriptors for efficient place recognition:

LiDAR-IRIS Pipeline:
1. Project scan → radial signature (range histogram by angle)
2. Extract features via Fourier transform
3. Create binary descriptor (rotation-invariant)
4. Compare using Hamming distance (fast bit operations)
5. Geometric validation with scan matching
6. Pose graph optimization if validated

5. Cell Type Priority

Multiple sensors update the same cells. Priority ensures correct classification:

Priority (highest to lowest):
┌─────────────────────────────────────────────────────┐
│ 4. Bump (invisible obstacle - glass, chair legs)    │ ← Bumper sensor
├─────────────────────────────────────────────────────┤
│ 3. Cliff (floor drop-off - stairs, ledges)          │ ← Cliff sensor
├─────────────────────────────────────────────────────┤
│ 2. Wall (visible obstacle)                          │ ← LiDAR endpoint
├─────────────────────────────────────────────────────┤
│ 1. Floor (traversable)                              │ ← LiDAR ray
├─────────────────────────────────────────────────────┤
│ 0. Unknown (not observed)                           │ ← Initial state
└─────────────────────────────────────────────────────┘

Module Documentation

Module	Description	Documentation
core	Fundamental types: Pose2D, WorldPoint, GridCoord, LidarScan, Cell	README
grid	Occupancy grid storage with SoA layout, ray casting, sensor updates	README
matching	Scan matching (correlative, branch-bound), loop closure, pose graph	README
submap	Submap manager, pose graph, multi-submap matching	README
config	YAML configuration loading with sensible defaults	README
io	Persistence: native .vastu format, ROS .pgm export, SVG visualization	README
modes	Operation modes: localization-only (no map updates)	README
evaluation	Cartographer-style evaluation: ATE, RPE, pose relations metrics	README

Coordinate System

All coordinates follow the ROS REP-103 convention:

                    +X (Forward)
                        ▲
                        │
                        │
        +Y (Left) ◄─────┼───── -Y (Right)
                        │
                        │
                        ▼
                    -X (Backward)

Rotation: Counter-clockwise positive (θ = 0 points to +X)

Configuration

YAML Configuration

# config.yaml
grid:
  resolution: 0.025       # 2.5cm cells
  initial_width: 800      # 20m × 20m
  initial_height: 800

sensor:
  robot:
    radius: 0.17          # Robot radius for collision
  lidar:
    offset_x: -0.110      # LiDAR position relative to robot center
    max_range: 8.0

slam:
  correlative:
    enabled: true
    search_x: 0.30        # ±30cm search window
    search_y: 0.30
    search_theta: 0.15    # ±8.6° search window
    multi_resolution: true
    use_gaussian_scoring: true

  loop_closure:
    enabled: true
    min_score_threshold: 0.6

Programmatic Configuration

use vastu_slam::config::VastuConfig;
use std::path::Path;

// Load from YAML file
let config = VastuConfig::load(Path::new("config.yaml"))?;

// Or use defaults
let config = VastuConfig::default();

// Convert to runtime configs
let map_config = config.to_map_config();
let matcher_config = config.correlative_matcher_config();

Memory Layout

VastuSLAM uses Structure-of-Arrays (SoA) for SIMD efficiency:

Traditional AoS (Array of Structs):
┌──────────────────────────────────────────────────┐
│ Cell0{type,conf,swept} │ Cell1{...} │ Cell2{...} │ ...
└──────────────────────────────────────────────────┘
  ↑ Cache line may span multiple cells, mixed data types

VastuSLAM SoA (Struct of Arrays):
┌───────────────────────────────────────┐
│ types:  [t0, t1, t2, t3, t4, t5, ...] │ ← f32x4 SIMD loads
├───────────────────────────────────────┤
│ confs:  [c0, c1, c2, c3, c4, c5, ...] │ ← Contiguous memory
├───────────────────────────────────────┤
│ swept:  [s0, s1, s2, s3, s4, s5, ...] │ ← Cache-friendly
└───────────────────────────────────────┘

Performance Characteristics

Metric	Typical Value	Notes
Binary size	~100KB	Stripped, statically linked
Memory (800×800)	~4MB	SoA grid storage
Scan matching	5-15ms	ARM Cortex-A7 @ 1GHz
Map save/load	<10ms	Native .vastu format
Resolution	2.5cm	Default, configurable

Building

Development

# Requires nightly Rust for portable_simd
rustup override set nightly

# Build and test
cargo build --release
cargo test

Cross-Compilation (ARM)

rustup target add armv7-unknown-linux-musleabihf
cargo build --release --target armv7-unknown-linux-musleabihf

Documentation

cargo +nightly doc --no-deps --open

Usage Examples

Basic Mapping

use vastu_slam::{OccupancyGridMap, MapConfig, Pose2D, LidarScan, SensorObservation};

let mut map = OccupancyGridMap::new(MapConfig::default());

// Process complete sensor observation
let observation = SensorObservation {
    pose: Pose2D::new(0.0, 0.0, 0.0),
    lidar: Some(scan),
    cliffs: cliff_sensors,
    bumpers: bumper_sensors,
    timestamp_us: 0,
};

let result = map.observe(&observation);
println!("Floor: {}, Wall: {}, Cliff: {}, Bump: {}",
    result.cells_floor, result.cells_wall,
    result.cells_cliff, result.cells_bump);

Scan Matching with Drift Correction

use vastu_slam::{OccupancyGridMap, CorrelativeMatcherConfig};

let mut map = OccupancyGridMap::new(MapConfig::default());
let matcher_config = CorrelativeMatcherConfig::default();

// Match scan and integrate at corrected pose
let (result, corrected_pose, match_result) =
    map.observe_lidar_with_matching(&scan, encoder_pose, &matcher_config);

if match_result.converged {
    println!("Corrected pose: ({:.3}, {:.3})",
        corrected_pose.x, corrected_pose.y);
}

Localization on Known Map

use vastu_slam::modes::{Localizer, LocalizerConfig};
use vastu_slam::io::load_vastu;

// Load pre-built map
let map = load_vastu(Path::new("map.vastu"))?;

// Create localizer
let mut localizer = Localizer::new(map, LocalizerConfig::default());
localizer.set_initial_pose(dock_pose);

// Localization loop
let result = localizer.localize(&scan, Some(odom_delta));
if result.converged {
    println!("Robot at: ({:.2}, {:.2})", result.pose.x, result.pose.y);
}

Submap-Based SLAM

use vastu_slam::submap::{SubmapManager, SubmapConfig};

let mut manager = SubmapManager::new(SubmapConfig::default(), &map_config);

// Insert scans (manager handles submap transitions)
let result = manager.insert_scan(&scan, pose, timestamp_us);
if result.new_submap_created {
    println!("Started new submap");
}

// Get composed global grid
let global_grid = manager.global_grid();

File Formats

Format	Extension	Purpose	Preserves
Native	.vastu	Full persistence	All cell data (type, confidence, swept)
ROS Map	.pgm + .yaml	ROS integration	Occupancy only (Free/Occupied/Unknown)
SVG	.svg	Visualization	Visual map + trajectory overlay

Thread Safety

Type	Thread Safety	Typical Usage
OccupancyGridMap	Send	Single-threaded map owner
GridStorage	Send	Single-threaded access
CorrelativeMatcher	Send + Sync	Shared across threads
SubmapManager	Send	Single-threaded manager
Localizer	Send	Single-threaded localizer

License

MIT