Download Francis - Thesis 2008

routine used to generate the correlation fringes shown in figure 4.10 was achieved
using a DaVis macro. All DaVis functions are defined within the command
language. Additional functions can be programmed in C++ and called from the main
CL program as a DLL (dynamic link library). CL contains functions that can be used
to build a graphical user interface (GUI) for a macro. An example of a macro GUI is
shown under label (9) in figure 4.11.
The ‘compute’ menu (10) contains a number of image processing functions such as
image filtering and Fourier transform calculations. Some of the features that can be
accessed from the ‘compute’ menu are the functions that are used to correct images
for perspective distortion. This is important within the context of this work because
each of the camera lenses views the target at an angle, introducing perspective
distortion into the recorded images. The effect of perspective distortion can be seen
in figure 4.6, for example. Errors will be present in the results of calculations made
from images obtained with the system if this distortion is not corrected for. The
process of correcting for perspective distortion, known as image ‘dewarping’,
requires the calibration of a test grid. The calibration process is carried out using the
functions in the ‘compute’ menu. The dewarping process is described in more detail
in section 6.1.
Functions within the ‘acquisition set up’ menu (11) can be used to select which
camera to use (if more than one camera is connected to the PC) and to set the
exposure time of the camera. The background subtraction function can be toggled on
or off under this menu. The background subtraction function works by averaging a
series of 10 images recorded sequentially with the laser beam switched off. The
resulting image is saved and subtracted from all subsequent recorded images.
Background subtraction is used to remove sources of background noise from the
recorded images. Sources of background noise include ambient light, such as light
from the PC monitor, and CCD dark current.
4.8 Image processing
The image processing scheme used to calculate the orthogonal displacement gradient
components is illustrated in figure 4.12. First, images are captured before (reference
frame) and after (signal frame) deformation of the test object. The signal and
reference frames are used to calculate the wrapped phase distribution across the field
of view. The phase analysis techniques used with this system are discussed in
chapter 5. The wrapped phase maps are filtered to remove high frequency noise. The
images are then separated into their individual image quadrants using the DaVis
rectangle functions described above. The phase maps are unwrapped before being
dewarped to remove the effects of perspective distortion. The sensitivity vectors are
then calculated across the field of view for each image quadrant. Finally, the
sensitivity vector maps are combined with the unwrapped phase maps to calculate
the orthogonal displacement gradient components. Only the results from three of the
four measurement channels are required for the displacement gradient calculation.
84