AI Colmap Camera Tracking

Automated pipeline for camera tracking and scene reconstruction using COLMAP and NeRF-compatible formats. Processes video inputs, performs sparse 3D reconstruction, undistorts footage, and exports camera data for use in Houdini or NeRF training.

Features

• Automated Workflow: Batch processes multiple video files.
• Frame Extraction: Uses FFmpeg to extract frames from input videos.
• Feature Extraction & Matching: Utilises COLMAP for feature extraction and sequential matching.
• Sparse Reconstruction: Uses COLMAP Global Mapper (requires COLMAP 4.0+).
• NeRF Conversion: Converts COLMAP data to transforms.json (NeRF format).
• Undistortion: Expands the undistorted canvas to preserve all valid pixels while keeping the focal length unchanged. Optionally crops to the original canvas size (--crop).
• Houdini Integration: Automatically generates a Houdini .hip scene with the reconstructed point cloud and animated camera. Correctly handles sensor size, canvas expansion, and principal-point offset.

Prerequisites

1. uv — Python package and environment manager.
2. FFmpeg — video processing.
3. COLMAP 4.0+ — feature extraction, matching, and Global Mapper reconstruction.
4. Houdini (hython) — required only for Houdini scene generation.

Python Dependencies

Dependencies are declared in pyproject.toml and managed by uv. To create the virtual environment and install all dependencies:

uv sync

Optional: For automatic object masking in colmap2nerf.py, PyTorch and Detectron2 are required.

Usage

The main entry point is run_autotracker.py.

uv run python run_autotracker.py <input_videos_dir> <output_dir> [options]

Arguments

Argument	Default	Description
input_videos_dir	—	Directory containing source video files (.mp4, .mov, …)
output_dir	—	Directory for all output data
--scale	0.5	Image scaling factor applied before processing
--overlap	12	Sequential matching overlap (number of frames)
--skip-houdini	off	Skip Houdini .hip generation
--hfs	—	Path to Houdini installation directory (e.g. C:\Program Files\Side Effects Software\Houdini 20.0.625). If omitted, hython must be in PATH.
--multi-cams	off	Treat each video as a separate camera (useful for multi-device shoots)
--acescg	off	Convert input from ACEScg to sRGB before processing
--lut	—	Path to a .cube LUT file for colour-space conversion
--mask	—	Path to a directory containing per-frame masks
--focal_length_mm	—	Lens focal length in mm (e.g. 24). Locks COLMAP to this value instead of estimating it.
--sensor_width_mm	36.0	Physical sensor width in mm. Used together with --focal_length_mm. Common values: full-frame=36.0, ARRI LF=36.7, Super35=24.89, MFT=17.3
--crop	off	Keep original canvas size during undistortion instead of expanding it. Houdini focal length and aperture remain at exact physical values (e.g. 20 mm / 36 mm).
--camera_model	auto	COLMAP camera model (e.g. OPENCV, PINHOLE, SIMPLE_RADIAL)
--loop	off	Enable loop detection in sequential matching
--loop_period	5	Loop detection period
--loop_num_images	50	Number of images considered per loop detection pass
--vocab_tree_path	vocab_tree_faiss_flickr100K_words32K.bin	Path to vocabulary tree for loop detection
--extra_fe	—	Extra COLMAP feature-extraction arguments. Accepts a JSON string or .json file path.
--extra_sm	—	Extra COLMAP sequential-matching arguments.
--extra_ma	—	Extra COLMAP global-mapper arguments.

Specifying Focal Length

When the shooting focal length is known, providing it improves reconstruction accuracy by preventing COLMAP from freely estimating it:

python run_autotracker.py ./videos ./output --focal_length_mm 24 --sensor_width_mm 36

Extra Arguments Example

Create a params.json:

{
    "SiftExtraction.peak_threshold": 0.01,
    "SiftExtraction.max_num_features": 8192
}

Then pass it:

uv run python run_autotracker.py ./in ./out --extra_fe params.json

Masking

The pipeline supports per-frame masks to exclude moving objects or unwanted regions from reconstruction.

Rules:

1. Auto-detection: For a video shot01.mp4, the script looks for a sibling directory named shot01_mask.
2. Custom root: --mask <path> looks for <video_name>_mask inside the specified path.
3. Filename format: PNG files named frame_000001.jpg.png. If frame_000001.png is found it is automatically renamed to match COLMAP requirements.

Example

uv run python run_autotracker.py ./videos ./output \
    --scale 0.5 \
    --focal_length_mm 20 \
    --sensor_width_mm 36 \
    --hfs "C:/Program Files/Side Effects Software/Houdini 20.0.625"

Batch Processing

batch_run.py processes multiple folders within a target directory. Per-folder settings can be defined in a batch_config.ini placed in the target directory.

Usage

uv run python batch_run.py <target_directory>

Configuration Format (batch_config.ini)

If a section name matches a folder name, its settings override the defaults for that folder.

[global]
scale = 0.5
hfs = C:/Program Files/Side Effects Software/Houdini 20.0.625

[shot_01]
scale = 0.8
camera_model = OPENCV
focal_length_mm = 24
sensor_width_mm = 36

[shot_02]
focal_length_mm = 20
sensor_width_mm = 24.89
acescg = true

Supported INI Keys:

Key	Type	Description
scale	float	Image scaling factor
overlap	int	Sequential matching overlap
camera_model	string	e.g. OPENCV, PINHOLE
focal_length_mm	float	Lens focal length in mm
sensor_width_mm	float	Physical sensor width in mm
mask	string	Path to mask directory
lut	string	Path to .cube LUT file
hfs	string	Path to Houdini installation
crop	bool	true / false
multi_cams	bool	true / false
acescg	bool	true / false
skip_houdini	bool	true / false
loop	bool	true / false
loop_period	int	Loop detection period
loop_num_images	int	Images per loop detection pass
vocab_tree_path	string	Path to vocabulary tree

Advanced Parameter Injection (INI Only)

Pass any COLMAP internal parameter using these prefixes:

• fe.<Parameter> — injected into feature_extractor
• sm.<Parameter> — injected into sequential_matcher
• ma.<Parameter> — injected into global_mapper

Example:

[global]
fe.SiftExtraction.peak_threshold = 0.01
sm.SequentialMatching.min_num_matches = 20

Quick Start / Demo

run_demo_test.bat

Processes ./demo-test/walking-forest and outputs to ./demo-test/walking-forest-output. Edit the .bat file to point to your Houdini installation if hython is not in PATH.

Pipeline Steps

1. Frame extraction — FFmpeg extracts frames from each input video.
2. Feature extraction & matching — COLMAP feature_extractor + sequential_matcher.
3. Sparse reconstruction — COLMAP global_mapper (requires COLMAP 4.0+).
4. Model export — Sparse model converted to TXT and PLY formats.
5. NeRF conversion — colmap2nerf.py generates transforms.json.
6. Undistortion — undistortionNerfstudioColmap.py removes lens distortion.
   • Default: canvas is expanded to include all valid pixels; sensor_w/sensor_h are recorded so downstream tools can recover the physical focal length.
   • --crop: keeps the original canvas size; Houdini focal length and aperture remain at their exact physical values (e.g. 20 mm / 36 mm).
7. Houdini scene — build_houdini_scene.py imports the point cloud and creates an animated camera with correct focal length, aperture, and principal-point offset.

Scripts Overview

Script	Description
run_autotracker.py	Master script — orchestrates the full pipeline
autotracker.py	Core photogrammetry: FFmpeg, COLMAP feature extraction, matching, and Global Mapper
colmap2nerf.py	Converts COLMAP sparse model to transforms.json
undistortionNerfstudioColmap.py	Undistorts images; expands canvas or crops to original size
restore_distortion.py	Utility to apply or remove lens distortion from rendered images. Supports EXR via --exr
build_houdini_scene.py	Generates a .hip file with point cloud and animated camera
batch_run.py	Batch runner with per-folder INI configuration

Output Structure

For each processed video:

<output_dir>/<video_name>/
├── images/                  # Extracted frames
├── sparse/                  # COLMAP sparse reconstruction
├── database.db              # COLMAP feature database
├── transforms.json          # Camera poses (NeRF format)
├── points3D.ply             # Point cloud
├── undistort/
│   ├── images_undistorted/  # Undistorted frames
│   └── transforms_undistorted.json
└── <video_name>.hip         # Houdini project file

References

• Inspired by: Video Link
• Demo test video: Pexels — Tranquil Autumn Forest Walkway Path