A cross-target-based accurate calibration method of binocular stereo

A cross-target-based accurate calibration method of binocular stereo systems with large-scale field-of-view


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a b s t r a c t
This paper presents an accurate calibration method of binocular 3D measurement systems
for industrial on-site inspection, which uses a cross target with ring coded points. The cross
target can be used to calibrate large-scale field-of-view stereo measurement systems and
obtain higher measurement precision conveniently. The world coordinates of these ring
coded points are not required. All we need is the distance of two ring coded points as a
scale. A new calibration model with 10 distortion parameters for each camera is proposed.
The calibration initial values are computed using the relative orientation method and the
Direct Linear Transform (DLT) method of photogrammetry. The bundle adjustment algorithm
is used to optimize the calibration parameters as well as the 3D coordinates of the
ring coded points. Experiment results show that the RMS error of the reprojection in our
method is less than 0.05 pixels and the measurement error is 0.011 mm compared with
the Coordinate Measuring Machine (CMM).



 



 



1. Introduction
With the rapid development of computer technology
and photogrammetry, the binocular 3D measurement has
been widely used in many fields such as quality inspection
and reverse engineering. Depending on the application,
there are different requirements for 3D measurement systems.
The dimension measurement for industrial products
inspection requires high accuracy and flexibility. Camera
calibration is a key issue that affects the accuracy of binocular
3D measurement systems.
Camera calibration is the process of determining the
internal and external parameters of camera via capturing
external reference objects. The single camera calibration
has been studied extensively [1–3,13,14]. A popular and
practical algorithm is the method developed by Tsai using
radial alignment constraint (RAC) [1], but in this method,
initial camera parameters are required and only lens radial
distortion is considered. Weng et al. proposed a camera
calibration with distortion models [2]. Zhang proposed a
flexible technique for camera calibration by viewing a
plane from different unknown orientations [3]. This algorithm
assumes that the calibration pattern is an ideal
plane, and ignores the actual errors in manufacturing.
However, all these approaches assume that the world coordinates
of the feature points of the calibration pattern are
accurately known. Therefore they did not consider the
inevitable manufacturing errors and measurement errors
of these points. These errors could affect the accuracy of
the calibration, especially in large-scale field-of-view stereo
measurement systems.
The calibration of camera systems without relying on
external 3D measurements was also investigated by a
number of researchers [4–9,15–17]. Self-calibration only
requires the corresponding points of images, and thus is
more flexible in practical applications. In [4], Heyden and
Astrom proposed a self-calibration algorithm that uses explicit
constraints from the assumption of the intrinsic
parameters of the camera. They proved that self-calibration
is possible under different kind of cameras with the
assumptions that the aspect ratio is known and there is


no skew. They solved the problem using the bundle adjustment
method [6] that requires simultaneous minimization
on all reconstructed points and cameras. However, the initialization
problem was not properly presented in it. Bougnoux
[5] proposed a practical self-calibration algorithm
that used the constraints derived from [4]. A linear initialization
step is used in the nonlinear minimization, and the
bundle adjustment is used in the projective reconstruction
step. But none of them is suitable for practical on-site calibration
in industrial inspection because of the limit of precision
and field of view.
In this paper, an accurate calibration method of binocular
3Dmeasurement systems for industrial on-site inspection is
proposed. Our method uses a cross target with ring coded
points as calibration pattern. The world coordinates of these
ring coded points are not required. The proposed procedure
does not require any expensive equipment. Moreover, it is
very fast and completely automatic, as the user is only requested
to capture a few images of the calibration cross target.
The calibration initial value is computed by using the
relative orientation method and the Direct Linear Transform
(DLT) method of photogrammetry. The bundle adjustment
algorithm is used to optimize the calibration parameters,
including the 3D coordinates of the ring coded points. It also
considers the errors of feature point identification, the radial
distortion, the tangential distortion and thin prism distor-


4. Conclusions
We have developed an accurate calibration method for
binocular stereo systems. A calibration model with 10 distortion
parameters for each camera is proposed. A calibration
procedure has been designed for industrial on-site
inspection by using a cross-shaped calibration target. A
set of experiments has been designed to demonstrate the
effectiveness of the proposed approach. The experiment
result shows that the RMS error of the reprojection in
our method is <0.05 pixels and the measurement error is
0.011 mm compared with the Coordinate Measuring Machine
(CMM). In order to make the calibration results valid
for longer time, and to adapt to temperature changes, temperature
compensation will be added in the future.


 

a quick question

Hey,

I happen to run a similar blog. I just wanted to
ask you if you can tell me what kind of theme
are you using for this blog?

I would love to use it on my own,
Gabriel
http://fblayoutsfree.org/

Pretty impressive works

I read your posts serveral times and also tried to look for your papers. One thing suprises me. Althought you claimed that your software yeilds very high accuracy, nobody cites your works! What happened? Probably they do not trust your technique? In regarding to that, I am completely agree with our Admin. Why don't you give the opticsit.org user a trial of your software? I believe it would be more convincing than just posting stuffs trying to draw attention.

Please provide a program copy

It will be great if you can provide an executable program of your camera calibration function to opticist.org users.

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