# cartographer/sensor/internal/voxel_filter.h

## Overview

`VoxelFilter` downsamples point clouds by dividing space into 3D voxels and keeping one point per voxel (typically the first or centroid).

## Class: `cartographer::sensor::VoxelFilter`

### Construction

```cpp
explicit VoxelFilter(float voxel_size);
```

- `voxel_size` — Voxel edge length in meters

### Filtering

```cpp
PointCloud Filter(const PointCloud& point_cloud) const;
```

Returns downsampled point cloud.

### Algorithm

1. Compute voxel index for each point: `index = floor(point / voxel_size)`
2. Use hash map to track first point per voxel
3. Return collected points

### Complexity

- Time: O(N) for N points
- Space: O(M) for M occupied voxels

## Usage

```cpp
// Create filter
sensor::VoxelFilter filter(0.025f);  // 2.5cm voxels

// Filter point cloud
sensor::PointCloud raw = lidar->GetScan();
sensor::PointCloud filtered = filter.Filter(raw);

// Typical reduction: 100k points -> 10-20k points
```

## Configuration

Used in trajectory builder configs:

```lua
TRAJECTORY_BUILDER_2D = {
  voxel_filter_size = 0.025,  -- 2.5cm
}

TRAJECTORY_BUILDER_3D = {
  voxel_filter_size = 0.15,   -- 15cm (3D lidars denser)
}
```

## See Also

- [PointCloud](../point_cloud.md) — Input/output format
- [RangeData](../range_data.md) — Uses filtered point clouds