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3 EXPERIMENTAL SETUP
Measurements have been performed at the large circular wind-water tunnel at Heidelberg University.
Tracer particles ( Wierzimok 1990) have been spherical polysturol particles with diameters ranging from 50 to
150 um. Image sequences of an observation volume have been taken with two CCD-cameras (NTSC, 30Hz, in-
terlaced). Both camera signals (camera 1: red, camera 2: green) have been superimposed and recorded simul-
tanously using an analog Sony Laserdisc Component Videorecorder (YUV -Signal), which allows access on sin-
gle images. ;
7s red and green signal have been splitted during digitization. The digitized images have been stored on WORM
disk.
Measurements have been made in four camera positions:
- cameras positioned horizontal and vertical
- in each position with parallel and rotated optical axes
4 2-DPARTICLE TRACKING
Essentially the 2-D particle tracking consists of two steps: particle segmentation and particle tracking. The
first step is to separate the particles from the background and to determine the water surface. The particle
tracking automatically traces a particle 's path from image to image througout the complete sequence. por this
purpose two algorithms have been implemented. The first one (Netzsch 1992) uses global thresholds reducing
the computational time while the second one (Hering 1992, Wierzimok 1992) is based on a region growing algo-
rithm andis suitable for high particle densities.
Figure 1 shows the result of the 2-D particle tracking for a sequence of 70 images (2.3 sec) processed with al-
gorithm 1. The cameras have been positioned horizontally with parallel optical axes. Only trajectories of partic-
les that have been tracked mare than 5 images are presented. As in all following figures, the upper half repre-
sents the right camera, while the lower half represents the left camera.
As a result a time sorted linked list of particle trajectories is obtained. The time sorting occurs automatically,
because of the sequential processing of the image sequences. The trajectories itself consist of another linked
list containg the center of mass of the particle in camera coordinates and the image number as a time stamp.
The time resolution is given by the NTSC standard. :
Figure 1 Result of the 2D-particle tracking. The upper half shows the left camera, the lower half the right ca-
mera. :
APRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop "From Pixels to Sequences", Zurich, March 22-24 1995
