Journal of Sensor Science and Technology
Vol. 22, No. 6 (2013) pp. 387-392
http://dx.doi.org/10.5369/JSST.2013.22.6.387
pISSN 1225-5475/eISSN 2093-7563
Capturing Distance Parameters Using a Laser Sensor in a
Stereoscopic 3D Camera Rig System
Wan-Young Chung, Julian Ilham, and Jong-Jin Kim+
Abstract
Camera rigs for shooting 3D video are classified as manual, motorized, or fully automatic. Even in an automatic camera rig, the process
of Stereoscopic 3D (S3D) video capture is very complex and time-consuming. One of the key time-consuming operations is capturing the
distance parameters, which are near distance, far distance, and convergence distance. Traditionally these distances are measured by tape
measure or triangular indirect measurement methods. These two methods consume a long time for every scene in shot. In our study, a
compact laser distance sensing system with long range distance sensitivity is developed. The system is small enough to be installed on top
of a camera and the measuring accuracy is within 2% even at a range of 50 m. The shooting time of an automatic camera rig equipped
with the laser distance sensing system can be reduced significantly to less than a minute.
Keywords : Automatic camera rig, Stereoscopic 3D (S3D), Distance parameters, Laser sensor
1. INTRODUCTION
Three dimensional (3D) video is becoming popular
again. Between 1952 and 1954 more than 65 3D movies
were produced by Hollywood before other cinema formats
took over [1]. Recently the number of 3D video
productions has started increasing, however the impractical
process of 3D production remains a challenge.
To make 3D video using the stereoscopic method, two
cameras are used. They should be attached to one holder,
widely known as a 3D camera rig to get the correct pictures
which have the same height and size to reduce picture
distortion.
There are three types of 3D camera rigs. The first type is
a manual rig. This needs full human control to adjust the
cameras positions. The second is a motorized camera rig.
This still needs human control to operate through a human
interface panel. This type has a higher cost since it contains
a microprocessor for calculation and driving the
positioning motors. The last is an automatic camera rig.
This type can reduce human dependence because it can
Department of Electronic Engineering, Pukyong National University,
Nam-gu, Busan 608-737, Korea
+Corresponding author: kimjj@pknu.ac.kr
(Received : Aug. 7, 2013, Revised : Oct. 1, 2013, Accepted : Oct. 7, 2013)
This is an Open Access article distributed under the terms of the Creative Commons
Attribution Non-Commercial License(http://creativecommons.org/licenses/bync/3.0)which permits unrestricted non-commercial use, distribution, and
reproduction in any medium, provided the original work is properly cited.
automatically move the cameras to the desired positions.
The system structure of this type of camera rig is obviously
more complex and costly. Several parameters which are
needed for determining the camera position are usually
acquired from some kind of information devices or sensors
attached to the camera rig.
Efforts are needed to simplify the 3D shooting operation
in camera rigs. Several researchers have proposed their
own methods. Liu et al proposed motorized zoom lenses
for reconstructing Stereoscopic 3D (S3D) shooting with a
two digital image sensors [2]. Heinzle et al proposed a
computational camera system for their automatic camera
rig by embedding image processing capability inside [3].
In this study, an automatic camera rig is developed using
a low-cost laser sensor for capturing several distance
parameters to reduce the operation time required.
2. AUTOMATIC 3D CAMERA RIG SYSTEM
As seen in Fig. 1 and Fig. 2, the camera rig system can
be divided mainly into two movement parts. The horizontal
movement part is for adjusting the distance between the
two cameras on the rig. The distance between the two
cameras is known as the interaxial distance (T). The
cameras are moved simultaneously to the desired interaxial
distance. The convergence movement part is for adjusting
the convergence angle ( 3) of the two cameras. They are
Wan-Young Chung Julian Ilham Jong-Jin Kim
both converged around 1 simultaneously to create a
symmetric figure of a stereoscopic image. Proper interaxial
distance and convergence angle controls are crucial to
produce better stereoscopic image [4].
Making a 3D video takes much longer time than
traditional 2D video even when an automatic method is
used. This long operation time is due to the high
complexity level of the 3D camera rig system. Several
parameters are needed to determine the correct interaxial
distance and convergence angle for proper 3D video
shooting [4]. This will prevent any cyber sickness or visual
fatigue for viewers.
Based on equation (1), the interaxial distance (T) is
determined by the focal length of the cameras (Lens), the
desired disparity of two images (Parallax), nearest (Near)
and farthest (Far) object distances from the cameras.
Fig. 1. Relation between interaxial distance and convergence angle [4].
Fig. 2. The two main movement parts of a horizontal parallel S3D
camera rig.
(1)
This paper presents low-cost laser sensor for S3D
camera systems to simplify the shooting operation and
reduce the operation time.
3. THEORETICAL DESCRIPTIONS
3.1 Laser distance sensing system
The three main distance parameters to be measured are
the near distance, far distance, convergence distance. In our
previous study [5], the three distance parameters were
captured using indirect methods such as triangulation
calculation along with a tape measure on the camera rig, or
only using the tape measure. However these kinds of
methods are inefficient because of the large amount of time
required and the number of people required to perform the
measurements; for example, one person is needed to hold
the pole of the tape measure.
A laser sensor range finder provides a direct method. There
are several ways range finder can operate, such as rotational
scanning, triangulation, and single point method. A rotational
scanning laser sensor is generally used for 2D mapping [6].
Most triangulation laser sensors are used for obtaining more
precise distance measurement. Konolige et al proposed a
low-cost laser sensor using the triangulation method, but the
maximum range was 6 meters [7]. It therefore cannot be used
for S3D systems, which are sometimes used to capture video
from distances greater than 10 m. The stereoscopic 3D
camera rig system, which only needs to get the objects’
distance one by one, is more suitable for the single point
method. However, the common barrier for using this kind of
sensor is high price. Several low-cost single point sensors
such as Leica and Bosch products [8, 9], do not provide any
interface for integrating the sensor to the main system and
cannot easily be attached to the camera.
A laser sensor suitable for our system would be at least
smaller than the camera itself, with a distance measurement
greater than 30 meters, low cost with an embedded circuit
interface, easy to install, and with light weight. These
specifications force us to design a custom laser sensor.
A red laser diode with a wavelength of approximately
Capturing Distance Parameters Using a Laser Sensor in a Stereoscopic 3D Camera Rig System
650 nm is used as a transmitter. A Hamamatsu S4282-51
photodiode is used as a light receiver as it is quite sensitive
to the 650 nm wavelength, as shown in Fig 3 (a). When the
photodiode is receiving a low intensity reflected laser beam
or no laser beam at all, the logic level output is high
because the transistor is open circuit. If there is a sufficient
intensity of reflected laser beam detected by photodiode the
transistor is short circuit to the ground [10].
A single point scanning sensor system, which uses a
time- of- flight method, was designed for the low-cost
device and long-distance measurement. The sensor
measures the time taken from transmitting the light until it
is detected by sensor. The sequential process can be seen in
Fig. 4.
As shown in Fig. 6, the laser transmitter is triggered by
microcontroller periodically. A plano-convex lens collects
the light reflected by an object and focuses it onto the
photodiode.
The microcontroller takes the time taken and determines
the distance. The equation is given as:
(2)
(a)
Where c is the velocity of light in the atmosphere and t is
the measured time.
The red laser used in this report has a wavelength of
approximately 650 nm which means it is classified as a
class 2 laser. To make it safe for general use and to follow
international standards [11], the average maximum output
power must be limited at 1 mW.
(3)
(4)
(b)
The laser energy, E, typically refers to the output of the
pulsed
laser and it is related to the power output. The
Fig. 3. Hamamatsu S4282-51 photo sensor [10]; (a) Characteristic of
energy E is the multiplication of the laser peak power
photo sensor and (b) principle of photo sensor.
output PPEAK and the laser pulses duration t. Since the
average power output PAVG is the multiplication of the
energy E and repetition rate, f, the output power can be
reduced by minimizing the repetition rate or by lowering
the laser pulse length.
3.2 System structure and tests
Fig. 4. Sequential process of a laser sensor measurement.
A laser sensor using the time-of-flight technique deals
with nanosecond timing. Therefore, to achieve required
accuracy, one dedicated high speed microcontroller is used
to fully handle the laser sensor, particularly when using
polling technique, as shown in Fig. 5.
Serial Peripheral Interface (SPI) communication which
Wan-Young Chung Julian Ilham Jong-Jin Kim
Fig. 5. Diagram of the laser sensor process system.
is faster than Inter-Integrated Circuit (I2C) or Universal
Asynchronous Receiver Transmitter (UART), is used to
send data from the laser sensor’s microcontroller to the
general purpose microcontroller. The laser sensor’s
microcontroller sends the data out continuously after the
time-of-flight to distance conversion is finished. Finally,
the distance data can be sent to Programmable Logic
Control (PLC) board from the general purpose
microcontroller through UART.
Fig. 7 shows the use of the laser sensor module on the
3D camera rig. In order to check the measurement
resolution of sensor module, the distance between the
camera rig and the object is also determined using a tape
measure. They are aimed towards the same point and the
wall is used as an object for reflecting the laser beam. This
experiment was repeated at several distances, and the
results are summarized in Table 1.
Fig. 8 shows the process of capturing the distance
parameters in a simulated situation. For comparison the
direct and indirect are shown.
Fig. 6. The principle of a single point scanning laser sensor system.
Fig. 7. An accuracy test of the fabricated laser sensor module on the
3D camera rig; (a) Laser beam and (b) tape measure.
Table 1. Distance comparison between tape measure and
laser sensor
No
Tape measure
(m)
Laser sensor
(m)
1
2
3
4
5
6
7
5
7.5
10
20
30
40
50
5.006
7.509
10.013
20.03
30.051
40.073
50.103
Capturing Distance Parameters Using a Laser Sensor in a Stereoscopic 3D Camera Rig System
Table 2. Shortest
distance comparison based on theoretical
processes
of the microcontroller.
and experiment
Theoretical
(m)
Experiment
(m)
0.15
0.17
(a)
Fig. 9. Laser sensor error rate.
For an accuracy test from non-flat surfaces which are
harder to measure, a mannequin was used. The commercial
product DistoTM D2 (Leica Geosystem Ltd) was also
tested together with the tape measure and our developed
laser sensor module. The commercial laser sensor
guarantees the typical accuracy 1.5 mm [8].
Several colors can absorb the source light and affect the
resolution.
However, in this study there are no colors
(b)
Fig. 8. Simulated situation for capturing distance on the 3D camera which can introduce such an error.
rig; (a) Indirect method and (b) direct method.
Table 3. Distance measurement comparison during
practice
4. RESULT AND DISCUSSION
Measurement
The maximum range the laser sensor module reached
was approximately 50 m. At greater distances, the intensity
of reflected light was too low to trigger the photodiode.
The distances measured using tape measure and laser
sensor are compared in Table 1. The results indicate that
the laser sensor has less than 1% error as seen in Fig. 9.
The shortest distance which can be measured
experimentally using laser sensor is 0.17 m which is
equivalent to time travel 1.13 ns as shown in Table 2.
Based on equation (2), when a 1 GHz clock source is used,
the shortest distance that can theoretically be measured is
approximately 0.15 m which is equivalent to time travel 1
ns. The additional time delay is attributed to the internal
Near
distance
Convergence
distance
Far distance
Tape
measure
(m)
Laser
sensor Laser sensor
Disto D2(m)
(m)
TM
2.96
3.036
3.035
4.67
4.699
4.700
6.80
6.825
6.826
Indirect measurement method by using the tape measure
needs considerable efforts and time, usually minutes. On
the other hand, the laser sensor takes only seconds to
measure it even with one operator. The accuracy of the
distance measurement by the developed laser sensor
module is compared with that from the tape measure and
Wan-Young Chung Julian Ilham Jong-Jin Kim
commercial laser sensor module (DistoTM D2 laser sensor).
The errors in the range of 3 m to 7 m by both the designed
laser sensor module and commercial laser sensor module
for the non-flat object are 2.5% in the 3m range and 0.4%
in the 7 m range.
5. CONCLUSION
A laser sensor module was designed and fabricated using
single point scanning sensor system with time-of-flight
measurement technique to be applied to the fast 3D video
shooting. The maximum distance range obtainable from
this laser sensor module is approximately 50 m. Thus, this
laser sensor can be very useful for simplifying the shooting
process of an S3D camera rig system in the range of 50 m.
The error rate of the designed laser distance module for
vertical concrete surface is within 0.21% in the distance
range between 5 and 50 m. The measuring error is within
2.6% in the range between 3 to 7 m for the mannequin with
clothes as a reflecting object.
In addition, besides the impressive accuracy, using this
low-cost laser sensor module, the shooting operation time
to measure the three main distance parameters in a scene
becomes a few seconds. This processing time is much
shorter compared to the previous triangulation method. The
number of individuals involved is also reduce.
ACKNOWLEDGMENT
This work was supported by a Research Grant of
Pukyong National University (2013).
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