Multiview Ball Calibration

Here is a python + opencv toolbox that can help you calibrate your multi-camera extrinsic parameters. All 2D and 3D position data are numpy array based.

Python package install

I've assume that you've installed basic libs, like opencv, numpy, scipy. Here is the multive calib package, just install it. The multiview_calib is modified from https://github.com/cvlab-epfl/multiview_calib, take care use my version.

pip install -U git+https://github.com/chenxinfeng4/multiview_calib

Ball define and detect

You can use single-ball or triple-ball.

!!! The ball detector is not contain in this code, so you should manually give me the position of ball in each view.

The single-ball or triple-ball are most the some pipeline. Except the triple-ball can restore the scale information. It will be helpful when you don't care about the world origin point and axes direction. Other wise I recommand you to use single-ball plus checkboard, which can onstep register world axes.

Tutorial

The test data is contained in the tutorial.

Step1: Intrinsic Calibration

See the 1_intrinsic_calib/intrinsic_calib.ipynb. You will get the intrinsic_calib.json.

Step2: Ball Extrinsic Calibration

See the 2A_ball_extrinsic_calib/ball_extrinsic_calib.ipynb and 2B_triball_extrinsic_calib/tripleball_extrinsic_calib.ipynb. You will get the *calibpkl.

The calibpkl contains the all intrinsic & extrinsic parameters.

K (3, 3);
dist [K1, K2, T1, T2, K3];
t (3,);
R (3, 3);

Step3: World Axes Register

See the 3_worldaxes_register/worldaxes_register.ipynb. You will get the refined calibration model *.recalibpkl.

Step4: Mutural Transform 2D ⇌ 3D

See the 4_transform_between2D_and_3D/mutual_transform_2D_3D.ipynb

Performance and Precision

All 2D and 3D position data are numpy array based, so the calibration and reconstruction would be fast. The calibration error is 0.6 mm in 3D construction, and 1.6 pixel in 2D image projection. Only 50-100 time slice ball-allview samples are enough for calibration.

Adapt to your own data

1. Your multiview videos, sure. All cameras should be synchronized. I recommend your to use OBS Studio grid arange the cameras and record into one file.
2. One checkboard. It's for intrinsic calibration. Also, it's for world axes registration.
3. One single-ball or one triple-ball. And also a ball detection software. In my case, I use open-mmlab/mmpose, it offer 1000 images/sec ball detection. The config of mmpose is not release in this code yet.
4. No ball detection in you case? Manually label 50 time slice ball-all-view samples. And extract the positions into a numpy array as (NVIEW, NSAMPLE, 1, XYP).

Aknowledgement

• Great work from https://github.com/cvlab-epfl/multiview_calib .

Extent readings

• [Single-ball calibration] Perez-Cortes, Juan-Carlos, et al. "A system for in-line 3D inspection without hidden surfaces." Sensors 18.9 (2018): 2993.
• [Multi-ball calibration] Perez, Alberto J., Juan-Carlos Perez-Cortes, and Jose-Luis Guardiola. "Simple and precise multi-view camera calibration for 3D reconstruction." Computers in Industry 123 (2020): 103256.
• [Triple-ball calibration] 吴福朝，王亮，胡占义. “一种对多相机系统的标定方法及装置” . 发明专利 (2008): CN 101226638A