nanofractal 0.2.0

High-performance fiducial marker detection (ArUco Nano v6 + Fractal)

nanofractal

High-performance fiducial-marker detection for Python. nanofractal wraps two compact, header-only C++ detectors with nanobind:

• ArUco Nano v6 — square markers: all standard OpenCV ArUco dictionaries (4×4, 5×5, 6×6, 7×7) plus ARUCO_MIP_36h12 and AprilTag 36h11.
• Fractal markers — nested markers that stay detectable under heavy occlusion and expose many inner corner correspondences for accurate, long-range pose.

It is built for speed: zero-copy NumPy ↔ cv::Mat, the GIL is released during detection, and a parallel batch API scales across cores.

single-frame detect():   ~0.43 ms @ 640×480   ~1.0 ms @ 1280×720   ~3.1 ms @ 1920×1080
detection_scale=0.5:     ~4× faster on the threshold/contour stage (corners refined at full res)
batch detect_batch():    ~3.2× throughput on 4 threads

Measured on a desktop CPU with max_attempts=1; your numbers will vary.

---

Installation

pip install nanofractal

Wheels bundle a minimal OpenCV, so no system OpenCV is required at runtime.

Build from source

You need a C++17 compiler, CMake ≥ 3.18 and a development OpenCV (core, imgproc, calib3d, features2d):

# Debian/Ubuntu
sudo apt-get install -y build-essential cmake libopencv-dev

pip install .

Local dev build with CPU tuning

# Enable -march=native + -ffast-math for maximum local performance:
NF_NATIVE=1 pip install -e . --no-build-isolation

---

Quick start

Inputs are plain NumPy uint8 arrays — either (H, W) grayscale or (H, W, 3) BGR, and C-contiguous (use np.ascontiguousarray if unsure). Any image loader works; the examples use OpenCV.

Detect ArUco markers

import cv2
import nanofractal as nf

image = cv2.imread("scene.png")                  # (H, W, 3) uint8 BGR

# Standard 4×4 dictionary (50 unique markers)
det = nf.ArucoDetector(nf.Dict.DICT_4X4_50)

# Or the legacy / AprilTag dictionaries:
# det = nf.ArucoDetector(nf.Dict.ARUCO_MIP_36h12)
# det = nf.ArucoDetector(nf.Dict.APRILTAG_36h11)

res = det.detect(image)
print(res.ids)        # int32   (N,)       e.g. [ 7 42]
print(res.corners)    # float32 (N, 4, 2)  clockwise corners, subpixel

Tune detection parameters

params = nf.DetectorParams()
params.min_contour_size    = 30    # detect smaller markers (default: 50)
params.adaptive_block_size = 11    # adaptive threshold window (must be odd, ≥3)
params.adaptive_c          = 7.0   # threshold constant (default: 7)
params.approx_poly_rate    = 0.05  # polygon approx rate (default: 0.05)

det = nf.ArucoDetector(nf.Dict.DICT_5X5_100, params=params)

# Or change params after creation:
det.params.min_contour_size = 80

For high-resolution input with reasonably large markers, detection_scale is the single biggest speed lever — the dominant cost (adaptiveThreshold + findContours) is already SIMD-optimized inside OpenCV, so the win comes from feeding it fewer pixels. Corners are still refined at full resolution, and it works for both ArucoDetector and FractalDetector:

params = nf.DetectorParams()
params.detection_scale = 0.5   # ~4x faster threshold/contour stage @1080p

det  = nf.ArucoDetector(nf.Dict.DICT_4X4_50, params=params)
fdet = nf.FractalDetector("FRACTAL_5L_6", params=params)

FractalDetector also supports all the same parameters plus two extras:

fparams = nf.DetectorParams()
fparams.subpix_win_size  = 4    # corner sub-pixel half-window (0 = off)
fparams.kfilter_min_dist = 10.0 # min pixel distance between FAST keypoints

Estimate pose

estimate_pose runs solvePnP (IPPE) for every detected marker at once.

import numpy as np

camera_matrix = np.array([[600, 0, 320],
                          [0, 600, 240],
                          [0,   0,   1]], dtype=np.float64)
dist_coeffs = np.zeros(5, dtype=np.float64)

rvecs, tvecs = det.estimate_pose(res.corners, camera_matrix, dist_coeffs,
                                 marker_size=0.05)   # marker side in metres
# rvecs, tvecs: float64 (N, 3) — rotation (Rodrigues) and translation per marker

Detect fractal markers

fdet = nf.FractalDetector("FRACTAL_5L_6", marker_size=0.85)  # size in metres (optional)

res = fdet.detect(image)
print(res.ids, res.corners.shape)   # outer 4 corners of each fractal marker

Fractal pose + visualization (occlusion-robust)

FractalDetector.estimate_pose returns one marker pose (rvec, tvec, reproj_err) or None. It uses every visible inner and outer corner correspondence when available (accurate, robust to occlusion) and otherwise falls back to the four outer corners — so you never call solvePnP yourself or worry about the empty-inner-points case. reproj_err (RMS pixels) lets you gate noisy poses.

fdet = nf.FractalDetector("FRACTAL_5L_6", marker_size=0.85)  # size in metres

res = fdet.detect(image, with_inner_points=True)
pose = fdet.estimate_pose(res, camera_matrix, dist_coeffs)
if pose is not None:
    rvec, tvec, reproj_err = pose      # rvec, tvec: float64 (3,); reproj_err: px
    fdet.draw(image, res, camera_matrix, dist_coeffs, rvec, tvec)  # corners + axes

draw(image, result, ...) overlays marker outlines, ids and (given a pose) the frame axes in place — no cv2.polylines/drawFrameAxes boilerplate. Without a pose, fdet.draw(image, res) just draws the outlines.

The raw correspondences are still exposed if you prefer to run PnP yourself:

res.points_2d   # float32 (M, 2) image points  (None unless with_inner_points=True)
res.points_3d   # float32 (M, 3) object points (planar, z = 0)

Parallel batch (offline throughput)

Process many frames across a thread pool. The GIL is released, so it scales with cores. num_threads=0 uses all cores.

frames = [cv2.imread(p) for p in paths]            # list of uint8 arrays
results = det.detect_batch(frames, num_threads=0)  # list[DetectionResult]
for r in results:
    print(r.ids)

---

API

Dict — marker dictionaries

Name	Markers	Inner bits	Notes
DICT_4X4_50 … DICT_4X4_1000	50–1000	4×4	fewest bits, fastest matching
DICT_5X5_50 … DICT_5X5_1000	50–1000	5×5
DICT_6X6_50 … DICT_6X6_1000	50–1000	6×6
DICT_7X7_50 … DICT_7X7_1000	50–1000	7×7	most bits, best error detection
ARUCO_MIP_36h12	250	6×6	legacy ArUco MIP dictionary
APRILTAG_36h11	587	6×6	AprilTag 36h11

All dictionaries are identical to their OpenCV counterparts — markers printed with cv2.aruco.generateImageMarker are detected directly.

ArucoDetector(dictionary=Dict.ARUCO_MIP_36h12, max_attempts=1, params=None)

• dictionary: Dict — any Dict enum value.
• max_attempts: int — retries per candidate with small corner jitter. 1 is fastest (real-time default); raise (up to ~10) for harder images.
• params: DetectorParams | None — tuning parameters (see below). None uses defaults.
• .params — read/write access to the DetectorParams after creation.
• detect(image) -> DetectionResult
• detect_batch(images, num_threads=0) -> list[DetectionResult]
• estimate_pose(corners, camera_matrix, dist_coeffs, marker_size) -> (rvecs, tvecs) — corners is (N, 4, 2) float32; outputs are (N, 3) float64.

FractalDetector(config, marker_size=-1.0, params=None)

• config: str — one of FRACTAL_2L_6, FRACTAL_3L_6, FRACTAL_4L_6, FRACTAL_5L_6.
• marker_size: float — outer marker side in metres; if set, points_3d is returned in metres (otherwise normalized).
• params: DetectorParams | None — tuning parameters. None uses defaults.
• .params — read/write access to the DetectorParams after creation.
• detect(image, with_inner_points=False) -> DetectionResult
• detect_batch(images, num_threads=0) -> list[DetectionResult]
• estimate_pose(result, camera_matrix, dist_coeffs) -> (rvec, tvec, reproj_err) | None — single-marker pose; uses inner+outer points when ≥ 4, else the 4 outer corners; rvec/tvec are float64 (3,), reproj_err is RMS pixels.
• draw(image, result, camera_matrix=None, dist_coeffs=None, rvec=None, tvec=None, axis_length=None) -> image — draw outlines + ids (and frame axes when a pose is given) in place; image must be a writable BGR uint8 array.

DetectorParams

Shared by both detectors. All fields are optional — defaults reproduce the original hard-coded behaviour so existing code needs no changes.

Field	Default	Description
min_contour_size	-1 (auto)	Minimum contour perimeter in pixels. ArUco default: 50, Fractal: 120.
adaptive_block_size	-1 (auto)	Adaptive threshold block size (odd, ≥ 3). ArUco default: 13; Fractal: scales with image width.
adaptive_c	7.0	Constant subtracted from the local threshold mean.
approx_poly_rate	0.05	Polygon approximation: epsilon = perimeter × rate.
subpix_win_size	-1 (auto=4)	Corner sub-pixel half-window (Fractal only); 0 to disable.
kfilter_min_dist	10.0	Minimum distance (px) between FAST keypoints (Fractal only).
detection_scale	1.0	Downscale factor for the detection stage (both detectors). 0.5 runs threshold/contour/decode on ¼ the pixels (≈ 4× faster); corners are mapped back and sub-pixel refined at full resolution. min_contour_size stays in original-image pixels.

DetectionResult

field	dtype / shape	meaning
ids	int32 (N,)	marker ids
corners	float32 (N, 4, 2)	outer corners (subpixel, clockwise)
points_2d	float32 (M, 2) or None	inner+outer image points (fractal, with_inner_points=True)
points_3d	float32 (M, 3) or None	matching object points

Empty results are returned as correctly-shaped empty arrays ((0,), (0, 4, 2)), never None.

Errors

• Wrong dtype / non-contiguous input → TypeError.
• Unsupported shape, empty frame, invalid dictionary or fractal config → ValueError.

---

Performance notes

• Zero-copy input. A contiguous uint8 array is wrapped as a cv::Mat over the same buffer — no copy. Non-contiguous or wrong-dtype inputs raise instead of silently copying.
• GIL released during the native detection, so other Python threads keep running and detect_batch scales.
• Thread safety. The ArUco detector is stateless and shared across batch workers. The fractal detector is not thread-safe, so detect_batch uses a pool of independent detectors (one per worker). A single detector object is fine to call from one thread at a time.

---

Changelog

0.2.0

• detection_scale — opt-in downscale of the threshold/contour/decode stage for both detectors (~4× faster at 1080p; corners refined at full resolution).
• Lower-overhead decode — ArUco dictionary tables are cached per detector instead of rebuilt every frame; marker-id matching and rotation now run on stack buffers with no per-candidate heap allocations (both detectors).
• Pinned SIMD — CI wheels build OpenCV with an explicit SSE4_2 baseline and AVX/AVX2/AVX512 runtime dispatch.

0.1.x

• Initial release: ArUco Nano v6 (all standard OpenCV dictionaries plus ARUCO_MIP_36h12 / AprilTag 36h11), Fractal markers, pose, parallel batch, and DetectorParams tuning.

---

Citation

If you use this in research, please cite the original work:

• F. J. Romero-Ramirez, R. Muñoz-Salinas, R. Medina-Carnicer, "Speeded up detection of squared fiducial markers", Image and Vision Computing, 76, 2018.
• S. Garrido-Jurado, R. Muñoz-Salinas, F. J. Madrid-Cuevas, R. Medina-Carnicer, "Generation of fiducial marker dictionaries using mixed integer linear programming", Pattern Recognition, 51, 2016.
• F. J. Romero-Ramirez, R. Muñoz-Salinas, R. Medina-Carnicer, "Fractal Markers: A New Approach for Long-Range Marker Pose Estimation Under Occlusion", IEEE Access, 7, 2019.

License

Apache-2.0. The vendored detectors (ArUco Nano, Fractal markers) are © their authors and used under their terms; see third_party/ and PATCHES.md.