3D PTV
3D Particle Tracking Velocimetry software on Github ( http://3dptv.github.com )
3D-PTV benchmark program
Introduction
Recent WG1 meeting in Zurich (August 2009) concluded that the three-dimensional particle tracking velocimetry (3D-PTV) is one of the established experimental approaches in the field of Particles in Turbulence. As a consequence, many algorithms, software packages have been developed over years by various research groups worldwide. This action is an attempt to learn best practices from each and to develop the open source particle tracking velocimetry that will combine the know-how of all the involved parties.
The first step, proposed during that meeting, was to develop few test cases that different research groups can use to test their own algorithms and software. The report of the groups will help to identify those performing best at different stages of the analysis and to expose the best practices to the other members of the action. There is a single important condition - open science. The codes, algorithms, data should be open to all the members of the action. The main goal is to make particle tracking better and accessible.
Test cases
At the moment there are few test cases available. These are of the two major types:
- Synthetic images - for the demonstration purposes, we use the Standard PIV Images project. Specifically, we use the 3D standard images: http://www.piv.jp/image3d/image-e.html.
- Real experimental data
Synthetic images test cases
There are four cases on the Standard 3D Images website [1] that seem to fit our purpose of testing the tracking software packages:
Transient 3D flow field from 3 angles with wall refrection
Case 351 (Jet Shear flow)(x,y,z) = (-0.8:0.0:0.8, -0.8:0.0:0.8, -0.8:0.0:0.8) 27 points
- Number of particles = 2000
- Camera calibration Images are imX999.raw
Case 352 (Jet Shear flow)
- Three-cameras are on the holizontal plane
(x,y,z) = (-0.8:0.0:0.8, -0.8:0.0:0.8, -0.8:0.0:0.8) 27 points
- Number of particles = 300
- Camera calibration Images are imX999.raw
Transient 3D flow field from 3 angles with unknown wall refrection
- Three-cameras are on the holizontal plane
Case 371 (Jet Shear flow)(x,y,z) = (-0.4:0.0:0.4, -0.4:0.0:0.4, -0.4:0.0:0.4) 27 points
- Number of particles = 500
- Camera calibration Images are imX999.raw
Case 377 (Jet Impinging flow)
- camera parameters are not known
(x,y,z) = (-0.2:0.0:0.2, -0.2:0.0:0.2, -0.2:0.0:0.2) 125 points
- Number of particles = 500
- Camera calibration Images are imX999.raw
- cameras are set at the bottom of impinging plates
As we see, we can process the various cases and compare our algorithms versus the
- given particle positions (aspects of stereo-matching, center-of-gravity, calibration and camera models, etc.)
- given tracking (aspects of tracking, length of trajectory, filters along the trajectory, post-tracking "gluing" algorithms, etc.)
For example, comparing the case 352 and 371 will show if the effect of "known" vs "unknown" wall reflections on your camera model, comparing case 351 wit 352 will show the traceability parameter best, where the number of particles density is much higher in the "same" flow in case 351.
How to prepare and post the test case
For the demonstration purposes, we use the test case no 352: "Transient 3D flow field from 3 angles with wall refrection". We find that the way Standard PIV Images project defined the test cases is simple and comprehensible. We suggest to adopt this way of formatting your experimental data for the test case. Below we report the way 3D-PTV software (ETH Zurich) is used to analyze the case #352 and to report the results for further analysis.
General description
1. Demo images - to provide a quick overview
Source: 
2. Textual information of the authors:
PIV Three-dimensional Standard Images #352
Generator: K. Okamoto (Univ. Tokyo)
Program: ddr.c
Date: March 27, 1998
3. Parameters:
Target Flow Field: Jet impinging on the Wall Reynolds Number: 3000
2cm
------------| |------------
| 15cm/s
V
XXX
XXX
-----------------------------
Number of Particles inserted in the field 1000 Particles Number of Particles visualized in one Image 320 Particles (average)
Diameter of Particles 5 pixel (average)
2 (standard deviation)
1 (minimum)
Maximum Velocity 12 cm/s Target Flow field 2cm x 2cm x 2cm Time Interval between images 5 msec
Laser Illumination Cylindrical from bottom (radius = 5cm)
[almost whole illumination]
Water refractive index 1.33
Wall distance 3 cm from center
orientation parallel to #1 camera image
4. Camera positions
#0 Distance from center 20 cm
angle to X axis -30 degree
#1 Distance from center 20 cm
angle to X axis 0 degree
#2 Distance from center 20 cm
angle to X axis 30 degree
No orientation variations were considered.
5. Sketch of the coordinate system
^ y
|
|/
------------+---------------> x
/|
/ |
/
/z
L
TOP VIEW
|
O| Particles
--------------+-----------------> x-axis
/|
/ |
/ | n=1.33(water)
==============|===================== WALL
/ | n=1.00(air)
/theta|
/ (-30)|(30)
/ |
#0 | #2 camera
camera |
#1 camera
|
| z-axis
V
6. Data description
Image Files (imX???.raw)
X: camera# (0-2) ???: serial# (000-144) Total 720msec
Image size 256x256 pixel (8bit:256)
Calibration File (imX999.raw)
Particles (Total 27 particles)
x = -0.8, 0, 0.8 cm
y = -0.8, 0, 0.8 cm
z = -0.8, 0, 0.8 cm
Vector files (vec???.dat)
x y z u v w -0.80 -0.80 -0.80 0.17542 -4.51977 0.01204 -0.80 -0.80 -0.60 -0.18245 -4.11870 -0.26516 -0.80 -0.80 -0.40 -0.78761 -3.38484 -0.41633
unit: x,y,z [cm]
u,v,w [cm/s]
Particle Files (ptc???.dat)
#1 camera #2 camera #3 camera intensity ID x y z X Y X Y X Y 4 -0.301 0.584 0.893 147.61 64.91 95.55 65.10 53.07 65.92 208.18 6 -0.898 1.266 -0.258 0.00 0.00 0.00 0.00 46.22 0.15 208.69 8 1.178 0.886 0.149 249.88 38.07 251.10 35.42 219.99 32.46 191.52 9 0.720 -1.242 -0.530 183.76 252.88 201.24 254.40 0.00 0.00 211.21
unit: x,y,z [cm]
X,Y [pixel]
The particle ID is unique number, therefore, the same particle could be easily tracked with checking the ID for serial particle files.
Thanks to the authors of the Standard 3D PIV Images: Dr. Okamoto (okamoto@tokai.t.u-tokyo.ac.jp)