Python bindings for COLMAP

This repository exposes to Python most COLMAP reconstruction objects, such as Cameras and Points3D, as well as estimators for absolute and relative poses.

Getting started

Wheels for Python 3.7/8/9/10 on Linux and macOS 11 can be install using pip:

pip install pycolmap

The wheels are automatically built and pushed to pypi at each release.

Building from source

Alternatively, we explain below how to compile PyCOLMAP from source.

COLMAP should first be installed as a library following the official guide. We recommend always building PyCOLMAP against the latest commit of the COLMAP dev branch - using a previous COLMAP build might not work. Currently, the earliest supported COLMAP commit is colmap/colmap@33e2692.

Then clone the repository and its submodules:

git clone --recursive [email protected]:colmap/pycolmap.git

And finally build PyCOLMAP:

cd pycolmap
pip install .

Windows

Building on Windows is currently not supported - please do not open any issues regarding this. Any contributions in this direction are welcome - please refer to issue #34. As a temporary workaround, we suggest using WSL.

Reconstruction pipeline

PyCOLMAP provides bindings for multiple steps of the standard reconstruction pipeline. They are defined in pipeline/ and include:

extracting and matching SIFT features
importing an image folder into a COLMAP database
inferring the camera parameters from the EXIF metadata of an image file
running two-view geometric verification of matches on a COLMAP database
triangulating points into an existing COLMAP model
running incremental reconstruction from a COLMAP database
dense reconstruction with multi-view stereo

Sparse & Dense reconstruction from a folder of images can be performed with:

output_path: pathlib.Path
image_dir: pathlib.Path

output_path.mkdir()
mvs_path = output_path / "mvs"
database_path = output_path / "database.db"

pycolmap.extract_features(database_path, image_dir)
pycolmap.match_exhaustive(database_path)
maps = pycolmap.incremental_mapping(database_path, image_dir, output_path)
maps[0].write(output_path)
# dense reconstruction
pycolmap.undistort_images(mvs_path, output_path, image_dir)
pycolmap.patch_match_stereo(mvs_path)  # requires compilation with CUDA
pycolmap.stereo_fusion(mvs_path / "dense.ply", mvs_path)

PyCOLMAP can leverage the GPU for feature extraction, matching, and multi-view stereo if COLMAP was compiled with CUDA support.

Similarly, PyCOLMAP can run Delauney Triangulation if COLMAP was compiled with CGAL support. This requires to build the package from source and is not available with the PyPI wheels.

All of the above steps are easily configurable with python dicts which are recursively merged into their respective defaults, e.g.

pycolmap.extract_features(database_path, image_dir, sift_options={"max_num_features": 512})
# equivalent to
ops = pycolmap.SiftExtractionOptions()
ops.max_num_features = 512
pycolmap.extract_features(database_path, image_dir, sift_options=ops)

To list available options, use

help(pycolmap.SiftExtractionOptions)

The default parameters can be looked up with

print(pycolmap.SiftExtractionOptions().summary())
# or
print(pycolmap.SiftExtractionOptions().todict())

For another example of usage, see hloc/reconstruction.py.

Reconstruction object

We can load and manipulate an existing COLMAP 3D reconstruction:

import pycolmap
reconstruction = pycolmap.Reconstruction("path/to/my/reconstruction/")
print(reconstruction.summary())

for image_id, image in reconstruction.images.items():
    print(image_id, image)

for point3D_id, point3D in reconstruction.points3D.items():
    print(point3D_id, point3D)

for camera_id, camera in reconstruction.cameras.items():
    print(camera_id, camera)

reconstruction.write("path/to/new/reconstruction/")

The object API mirrors the COLMAP C++ library. The bindings support many other operations, for example:

projecting a 3D point into an image with arbitrary camera model:

uv = camera.world_to_image(image.project(point3D.xyz))

aligning two 3D reconstructions by their camera poses:

tfm = reconstruction1.align_poses(reconstruction2)  # transforms reconstruction1 in-place
print(tfm.rotation, tfm.translation)

pycolmap also allows exporting models to TXT/NVM//CAM/Bundler/VRML/PLY (also supports pathlib.Path inputs). skip_distortion == True enables exporting more camera models, with the caveat of averaging the focal length parameters.

# Exports reconstruction to COLMAP text format.
reconstruction.write_text("path/to/new/reconstruction/")

# Exports in NVM format http://ccwu.me/vsfm/doc.html#nvm.
reconstruction.export_NVM("rec.nvm", skip_distortion=False)

# Creates a <img_name>.cam file for each image with pose/intrinsics information.
reconstruction.export_CAM("image_dir/", skip_distortion=False)

# Exports in Bundler format https://www.cs.cornell.edu/~snavely/bundler/.
reconstruction.export_bundler("rec.bundler.out", "rec.list.txt", skip_distortion=False)

# exports 3D points to PLY format.
reconstruction.export_PLY("rec.ply")

# Exports in VRML format https://en.wikipedia.org/wiki/VRML.
reconstruction.export_VRML("rec.images.wrl", "rec.points3D.wrl",
                           image_scale=1.0, image_rgb=[1.0, 0.0, 0.0])

Estimators

We provide robust RANSAC-based estimators for absolute camera pose (single-camera and multi-camera-rig), essential matrix, fundamental matrix, homography, and two-view relative pose for calibrated cameras.

All RANSAC and estimation parameters are exposed as objects that behave similarly as Python dataclasses. The RANSAC options are described in colmap/src/optim/ransac.h and their default values are:

ransac_options = pycolmap.RANSACOptions(
    max_error=4.0,  # reprojection error in pixels
    min_inlier_ratio=0.01,
    confidence=0.9999,
    min_num_trials=1000,
    max_num_trials=100000,
)

Absolute pose estimation

For instance, to estimate the absolute pose of a query camera given 2D-3D correspondences:

# Parameters:
# - points2D: Nx2 array; pixel coordinates
# - points3D: Nx3 array; world coordinates
# - camera: pycolmap.Camera
# Optional parameters:
# - max_error_px: float; RANSAC inlier threshold in pixels (default=12.0)
# - estimation_options: dict or pycolmap.AbsolutePoseEstimationOptions
# - refinement_options: dict or pycolmap.AbsolutePoseRefinementOptions
answer = pycolmap.absolute_pose_estimation(points2D, points3D, camera, max_error_px=12.0)
# Returns: dictionary of estimation outputs

2D and 3D points are passed as Numpy arrays or lists. The options are defined in estimators/absolute_pose.cc and can be passed as regular (nested) Python dictionaries:

pycolmap.absolute_pose_estimation(
    points2D, points3D, camera,
    estimation_options={'ransac': {'max_error': 12.0}},
    refinement_options={'refine_focal_length': True},
)

Absolute Pose Refinement

# Parameters:
# - tvec: List of 3 floats, translation component of the pose (world to camera)
# - qvec: List of 4 floats, quaternion component of the pose (world to camera)
# - points2D: Nx2 array; pixel coordinates
# - points3D: Nx3 array; world coordinates
# - inlier_mask: array of N bool; inlier_mask[i] is true if correpondence i is an inlier
# - camera: pycolmap.Camera
# Optional parameters:
# - refinement_options: dict or pycolmap.AbsolutePoseRefinementOptions
answer = pycolmap.pose_refinement(tvec, qvec, points2D, points3D, inlier_mask, camera)
# Returns: dictionary of refinement outputs

Essential matrix estimation

# Parameters:
# - points2D1: Nx2 array; pixel coordinates in image 1
# - points2D2: Nx2 array; pixel coordinates in image 2
# - camera1: pycolmap.Camera of image 1
# - camera2: pycolmap.Camera of image 2
# Optional parameters:
# - max_error_px: float; RANSAC inlier threshold in pixels (default=4.0)
# - options: dict or pycolmap.RANSACOptions
answer = pycolmap.essential_matrix_estimation(points2D1, points2D2, camera1, camera2)
# Returns: dictionary of estimation outputs

Fundamental matrix estimation

answer = pycolmap.fundamental_matrix_estimation(
    points2D1,
    points2D2,
    [max_error_px],  # optional RANSAC inlier threshold in pixels
    [options],       # optional dict or pycolmap.RANSACOptions
)

Homography estimation

answer = pycolmap.homography_matrix_estimation(
    points2D1,
    points2D2,
    [max_error_px],  # optional RANSAC inlier threshold in pixels
    [options],       # optional dict or pycolmap.RANSACOptions
)

Two-view geometry estimation

COLMAP can also estimate a relative pose between two calibrated cameras by estimating both E and H and accounting for the degeneracies of each model.

# Parameters:
# - points2D1: Nx2 array; pixel coordinates in image 1
# - points2D2: Nx2 array; pixel coordinates in image 2
# - camera1: pycolmap.Camera of image 1
# - camera2: pycolmap.Camera of image 2
# Optional parameters:
# - max_error_px: float; RANSAC inlier threshold in pixels (default=4.0)
# - options: dict or pycolmap.TwoViewGeometryOptions
answer = pycolmap.homography_matrix_estimation(points2D1, points2D2)
# Returns: dictionary of estimation outputs

The options are defined in estimators/two_view_geometry.cc and control how each model is selected. The return dictionary contains the relative pose, inlier mask, as well as the type of camera configuration, such as degenerate or planar. This type is an instance of the enum pycolmap.TwoViewGeometry whose values are explained in colmap/src/estimators/two_view_geometry.h.

Camera argument

All estimators expect a COLMAP camera object, which can be created as follow:

camera = pycolmap.Camera(
    COLMAP_CAMERA_MODEL_NAME,
    IMAGE_WIDTH, 
    IMAGE_HEIGHT,
    EXTRA_CAMERA_PARAMETERS,
)

The different camera models and their extra parameters are defined in colmap/src/base/camera_models.h. For example for a pinhole camera:

camera = pycolmap.Camera(
    model='SIMPLE_PINHOLE',
    width=width,
    height=height,
    params=[focal_length, cx, cy],
)

Alternatively, we can also pass a camera dictionary:

camera_dict = {
    'model': COLMAP_CAMERA_MODEL_NAME,
    'width': IMAGE_WIDTH,
    'height': IMAGE_HEIGHT,
    'params': EXTRA_CAMERA_PARAMETERS_LIST
}

SIFT feature extraction

import numpy as np
import pycolmap
from PIL import Image, ImageOps

# Input should be grayscale image with range [0, 1].
img = Image.open('image.jpg').convert('RGB')
img = ImageOps.grayscale(img)
img = np.array(img).astype(np.float) / 255.

# Optional parameters:
# - options: dict or pycolmap.SiftExtractionOptions
# - device: default pycolmap.Device.auto uses the GPU if available
sift = pycolmap.Sift()

# Parameters:
# - image: HxW float array
keypoints, scores, descriptors = sift.extract(img)
# Returns:
# - keypoints: Nx4 array; format: x (j), y (i), sigma, angle
# - scores: N array; DoG scores
# - descriptors: Nx128 array; L2-normalized descriptors

TODO

Add documentation
Add more detailed examples
Add unit tests for reconstruction bindings

Created and maintained by Mihai Dusmanu, Philipp Lindenberger, John Lambert, Paul-Edouard Sarlin, and other contributors.

colmap/pycolmap

colmap

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