[1] presented a two-step approach for integer-pixel displacements searching using the digital-image-correlation DIC method.. First, the presented approach in the Letter [1], which compri
Trang 1High-speed digital-image correlation method: comment Zhaoyang Wang,* Thang M Hoang, Dung A Nguyen, Andrew C Urcinas, and John R Magro
Department of Mechanical Engineering, The Catholic University of America, Washington, DC 20064, USA
*Corresponding author: wangz@cua.edu
Received July 30, 2009; accepted October 5, 2009;
posted August 6, 2010 (Doc ID 114561); published August 20, 2010
We comment on the recent letter by Wang et al [Opt Lett 34, 1955 (2009)], in which the authors presented a
high-speed digital image correlation (DIC) method We consider that the so-called high-high-speed DIC method has
consider-able deficiencies and that the Letter is misleading in terms of applicability and measurement accuracy as well as
processing speed © 2010 Optical Society of America
OCIS codes: 100.2000, 120.3940, 120.6150, 120.6650.
In a recent Letter, Wang et al [1] presented a two-step
approach for integer-pixel displacements searching using
the digital-image-correlation (DIC) method The method
first obtains the correlation index at each pixel with a
small-size subset and identifies a group of pixels as
potential matching points Then a large-size subset is
em-ployed to reanalyze the potential points to find the
best-matching one After that, a peak-finding algorithm (e.g.,
curved-surface approximation or Lagrange interpolation)
is used to get the subpixel displacements This approach
is claimed to be a high-speed DIC method We feel that
the method has considerable deficiencies and that the
Letter seems misleading
First, the presented approach in the Letter [1], which
comprises two-step integer-pixel displacement searching
and subpixel displacement peak-finding, can handle only
a deformation field with relatively small rotation or
de-formation; otherwise, large errors will be expected With
respect to applicability and measurement accuracy, the
method is inferior to other well-known DIC techniques
[2,3] For instance, the iterative cross-correlation
algo-rithm (e.g., the Levenberg–Marquardt method and the
Newton–Raphson method [4,5]), which can easily handle
large deformation and rotation and provide very high
registration accuracy, has been proved to be the most
robust DIC algorithm [2,3,6]
Second, the Letter [1] seems misleading in regard to
the purpose of integer-pixel displacement searching and
the processing speed of DIC In practice, DIC normally
does not require searching of integer-pixel displacements
for all of the pixels defined in the region of interest;
in-stead, the initial estimation needs to be performed only
on a starting point After the initial estimation of the
start-ing point, the correspondstart-ing subpixel displacements can
be subsequently determined by using a popular DIC
algo-rithm, such as the Newton–Raphson method Then, the
determined displacements as well as their gradients of
the point can be used as the initial estimate of subset
parameters for the next point of investigation according
to the continuous deformation assumption [3,7] The
above handling scheme indicates that even though a
“high-speed” integer-pixel displacement-searching
ap-proach is useful in some cases, it is helpful only for
the analysis of the starting point Accordingly, the total
computation time of DIC employing the integer-pixel
dis-placement-searching scheme will not be evidently re-duced In reality, most computation time is consumed
by the subpixel registration process, as the image recon-struction at subpixel locations is required
Third, the approach proposed in the Letter [1] is very similar to a coarse–fine searching scheme However, a comparison of the two-step searching scheme with the coarse–fine searching algorithms [2,3] was not per-formed in the Letter [1] On the basis of the testing that
we conducted, we did not see a notable advantage of the proposed technique over the existing coarse–fine methods
Finally, it may be helpful to point out that an easy and fast, yet very effective, way to perform the initial estima-tion in DIC is to manually pick one corresponding point (if the rotation and deformation are small) or three cor-responding noncollinear points in both the reference and target images [2,3] It has been shown that this human– computer-interaction scheme can provide a reliable initial estimate for very complex deformation fields in practice It is also noteworthy that, using this initial es-timation method and the Newton–Raphson algorithm,
it usually takes less than 1 s after picking the point pairs
to obtain the final subpixel-accuracy displacements for every 1000 points This is considerably faster (also more reliable and practical) than the method presented in the Letter [1], where it takes around 2 s to get the integer-pixel displacements and additional time to obtain the subpixel ones at a single point
Z Wang acknowledges partial support from the National Science Foundation (NSF) under grant No 0825806
References
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3 M Sutton, in Handbook of Experimental Solid Mechanics,
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September 1, 2010 / Vol 35, No 17 / OPTICS LETTERS 2891
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