Assessment of Gradientbased Digital Speckle Correlation Measurement Errors
 Author: Jian Zhao, Dong Zhao, Zhe Zhang
 Organization: Jian Zhao; Dong Zhao; Zhe Zhang
 Publish: Journal of the Optical Society of Korea Volume 16, Issue4, p372~380, 25 Dec 2012

ABSTRACT
The optical method Digital Speckle Correlation Measurement (DSCM) has been extensively applied due its capability to measure the entire displacement field over a body surface. A formula of displacement measurement errors by the gradientbased DSCM method was derived. The errors were found to explicitly relate to the image grayscale errors consisting of subpixel interpolation algorithm errors, image noise, and subset deformation mismatch at each point of the subset. A powerlaw dependence of the standard deviation of displacement measurement errors on the subset size was established when the subset deformation was rigid body translation and random image noise was dominant and it was confirmed by both the numerical and experimental results. In a gradientbased algorithm the basic assumption is rigid body translation of the interrogated subsets, however, this is in contradiction to the real circumstances where strains exist. Numerical and experimental results also indicated that, subset shape function mismatch was dominant when the order of the assumed subset shape function was lower than that of the actual subset deformation field and the powerlaw dependence clearly broke down. The powerlaw relationship further leads to a simple criterion for choosing a suitable subset size, image quality, subpixel algorithm, and subset shape function for DSCM.

KEYWORD
Gradientbased digital speckle correlation method , Measurement errors , Subset deformation mismatch

I. INTRODUCTION
Digital Speckle Correlation Measurement (DSCM) is an optical technique developed for full field and noncontact measuring of surface displacement and deformation [1]. This method utilizes a natural or artificial surface speckle pattern as an information carrier and sum of squared differences or cross correlates two slightly different images captured before and after a deformation, to obtain the whole field displacement of a planar specimen surface.
An important issue about the DCSM method is accuracy and precision. There are several factors influencing the accuracy of the DSCM method. The measurement errors may be classified in two types: errors associated with the experimental setup (acquisition system, illumination conditions) and errors associated with the correlation algorithm. The experimental errors are related basically to the variations of illumination and the quality of the acquisition system, i.e., the noise during the acquisition and digitalization [2], the camera lens distortion [3], position of the camera respect to the specimen, etc. The errors related to the algorithm are due to different choices of the implementation of the algorithm, such as the choice of the subset size [4,5], correlation function [6], subpixel interpolation algorithm [7,8], subset shape function [9,10] and speckle pattern quality [11]. Recently, analytical results have been derived about the effect of random image noise on the precision of displacement measurements by the DSCM method for rigid body motion estimation [12]. In [7], the performance of three most used subpixel algorithms were compared for simulated images. These algorithms were known as curve fitting, gradientbased and NewtonRaphson algorithms. The results showed that the NewtonRaphson approach is the more accurate and stable. But its computation time is much longer than that of gradientbased algorithms. Taking into the requirements both in calculation accuracy and in calculation speed, the gradientbased method is a better replacement of the NewtonRaphson method. Other interesting approaches like the use of genetic algorithms, finite elements and Bsplines are reported in [13], but it appears to have a lower performance than gradientbased algorithms in terms of accuracy. A study of the systematic errors due to use of shape functions of lower order than the actual displacement field was presented in [9]. Some requirements were presented about properties of the pattern, subset size and order of the shape functions. In [4], a theoretical model of the displacement measurement accuracy of DSCM was accurately predicted based on the variance of image noise and Sum of Square of Subset Intensity Gradients (SSSIG). The model further led to a simple criterion for choosing a proper subset size for the DSCM analysis. Further, in [11], it proposed the Mean Intensity Gradient to obtain the optimal subset size, where the Newton Raphson algorithm was used, the simplest correlation coefficient and the zeroorder shape function for pure inplane translation tests. In [14], a method was presented to estimate an appropriate pattern density for different subset sizes determining the characteristic sampling length of the speckle granules. From the above descriptions, it can be seen that assessment of DSCM measurement errors in practice is important but confusing. The measurement accuracy of DSCM depends on a number of factors. The effects of these factors (e.g. speckle pattern, out of plane displacement, lens distortion, noise, subset size, subpixel registration algorithm, shape function, interpolation scheme) have been investigated separately in previous studies. Few reported quantitative works have been performed to systematically evaluate the gradientbased DSCM measurement errors. Therefore, understanding the major constituent of gradientbased digital speckle correlation measurement errors and describe them in mathematical derivation is necessary and helpful.
In this study, an analysis has been carried out on the standard deviation of displacement measurement errors by gradientbased DSCM. A new formulation will be used to explicitly account for the image grayscale error due to subpixel interpolation errors, image noise, and subset deformation mismatch at each point of the subset. Numerical and experiment results of error estimation will be presented to validate the analytical formulas on the standard deviation of displacement measurement errors, and to assess individual and combined effects of subpixel interpolation errors, image noise, and subset deformation mismatch on the precision of displacement measurements. The final formulation can not only be used to predict the displacement measurement error, but also to give us some hints on how to improve the displacement measurement precision of DSCM.
II. FUNDAMENTAL PRINCIPLE OF GRADIENTBASED DSCM
The DSCM involves recording, digitizing and processing a pair of speckle patterns of an object in different deformation states, one before deformation and the other after deformation. The DSCM process is generally divided into two different stages. The first stage takes discrete, one pixel steps and correlates each one. The highest correlation is then taken to be the starting point for the next stage, which uses interpolation for subpixel accuracy. In routine practice of the DSCM method, a crosscorrelation criterion or sum squareddifference correlation criterion is predefined to evaluate the similarity between the reference image subset and the current image subset. The subpixel registration algorithm is considered as a key technique to improve displacement measurement accuracy in DSCM [7]. There are multiple ways to achieve subpixel accuracy that have been used successfully.
The gradientbased method as one of the DSCM methods is introduced to measure subpixel displacement because of its high efficiency and precision [15,16]. This method was originally developed by Davis and Freeman for use in video compression and is based on firstorder spatiotemporal gradients. It starts with an assumption that the image misalignment is purely translational. This assumption is deemed to be reasonable when the subsets are small. For simplicity, both grayscale intensity value of pixels in the deformation direction of reference image
G (X ) before deformation and its corresponding current imageg (x ) after deformation will be given as a single row matrix [17]. The corresponding subsets in the current imageg (x ) is related to the subsetS in the reference imageG (X ) by a socalled subset shape functionu (X ) viax =X +u (X ), which is utilized to approximate the underlying deformation field. With gray level conservation assumption it can be assumed that the relationship in Equation (1) holds true.A zeroorder shape function is employed due to the pure inplane rigid body translation of the subset, it has
where
p _{1} is the wholepixel translational displacement, andp _{2} is the corresponding subpixel displacement in the deformation direction. The pixel points in the reference image subsetS = [X _{1}, ···,X _{2}] all have integral pixel coordinates, soX_{i} is always integralvalued. However, the corresponding pixel points in the current image subsets = [x _{1}, ···,x _{2}] are generally nonintegral pixel coordinates, sox_{i} is usually realvalued.Let us assume that the integer pixel portion of the displacement (
P ) is known. The unknown displacement field is sought in the formu (X ) =P +δu (X ), where the displacement correctionδu (X ) is assumed to be small enough to allow for a firstorder Taylor expansion. So, the grayscale intensity value of the current imageg (x ) at nonintegral pixel locations may be approximated by a Taylor expansion around the wholepixel locations of the current imagewhere the term
P =int (p _{1} +p _{2}) is the integer pixel portion of the displacement, [Δg_{i} ]_{T} are higher order terms of the Taylor expansion,δu (X ) =u (X ) P is nonintegral pixel portion of the displacement. Φ_{x} is the firstorder derivatives of grayscale intensities at pointX +P of the current image. It can be calculated by the convolution of the grey with the mask of [1/12, 8/12, 0, 8/12, 1/12](The mask is deduced through cubic spline interpolation of five neighboring pixels). Using this method, an approximation of the partial derivatives value at pointX +P can be calculated as follow [16]The following sumsquareddifference (SSD) correlation coefficient is introduced for deriving a theoretical model of the displacement measurement accuracy of DSCM. The SSD correlation function is defined as
Minimization of the SSD correlation coefficient would provide the best estimate of the desired displacements. Thus we have
The iterative solution methods such as the Newton Raphson method or LevenbergMarquardt method can be used to solve the above nonlinear equation.
III. ERROR EVALUATION OF GRADIENTBASED DSCM
It is noted that the displacement measurement accuracy
of DSCM relies heavily on the perfection of the imaging system and the selection of a particular correlation algorithm. The errors discussed in this study are given in Table 1.
The image noise defined here will consist of both random white noise and quantization error in each image. If noise [Δ
G_{i} ]_{N} in the reference image and noise [ΔG_{i} ]_{N} in the current image can be fully accounted for, there would be a perfect match between the corrected reference imageG (X_{i} )[ΔG_{i} ]_{N} , and the corrected current imageg (x_{i} )[ΔG_{i} ]_{N} , namely,The grayscale level of the current image
g (x_{i} ) at non integral pixel locations may be approximated by a Taylor expansion around the wholepixel locationX_{i} +P_{i} of the current imagewhere
δu (X_{i} ) =u (X_{i} )P_{i} . Equation (8) is the same as Equation (3). If the subset deformation is approximated by the zeroorder shape function as given in Equation (2), one haswhere [Δ
g_{i} ]_{S} is the error due to the approximate subset shape function used. If the gradient Φ_{x} is to be approximated by cubic spline interpolation in Equation (4), one has,where
δ Φ_{x} is the cubic spline interpolation error, Equation (10) can be rewritten aswhere [Δ
g_{i} ]_{I} =δ Φ_{x} [p _{1} +p _{2}P_{i} ] + [Δg_{i} ]_{T} is the interpolation error for approximating the current imageg (x_{i} ) at nonintegral pixel locations. By combining Equation (7) and (11), one hasIn practical applications, some ideal situation such as (1) both reference image and current image are noisefree; (2) the subset deformation can be accurately described by the a zeroorder shape function; may not be met. So under nonideal situations, the deformation parameters
p _{1} andp _{2} will be displaced byp _{1} + Δp _{1} andp _{2} + Δp _{2} respectively, where Δp _{1} and Δp _{2} are the error in the estimated wholepixel displacement and subpixel displacement due to the existence of image noise and shape function approximation. The zeroorder shape function can be written asThe minimization of the SSD correlation coefficient
C in Equation (5) can then be expressed asTerm Δ
a_{i} = [Δg_{i} ]_{I} + [Δg_{i} ]_{S} + [ΔG_{i} ]_{N}  [Δg_{i} ]_{N} represents a sum grayscale errors at each pixel point and its existence usually results in the nonzero errors Δ
p _{1} and Δp _{2}. That means, when the image grayscale error at each pixel point Δa_{i} = 0, one would have Δp _{1} = 0 and Δp _{2} = 0, namely, Δp = 0.If both
p _{1} andp _{2} are known, the image grayscale error Δa_{i} at each point of the subset can actually be computed fromSo displacement error Δ
p (contains both the error of wholepixel and subpixel displacements) can be computed fromAs shown in the analyses above, the deformation errors in DSCM are unavoidably. It is assumed that the image grayscale error Δ
a_{i} at each point maintains a statistically random character and has a Gaussian distribution with a zero mean. The variance of displacement errors are given asIf
var (Δa_{i} ) is the same for every point,var (Δa_{i} ) =var (Δa ), Equation (17) can be further simplify as the followingFurthermore, the standard deviation errors of the displacement measurement can be expressed as
It is evident from Equation (18) and (19) that the displacement accuracy of DSCM is determined by the image grayscale error Δ
a (consisting of subpixel interpolation error ([Δg_{i} ]_{I} ), image noise ([ΔG_{i} ]_{N} , [Δg_{i} ]_{N} ), and the mismatch between the assumed subset shape function and the actual deformation field of the subset ([Δg_{i} ]_{S} )) and firstorder derivatives of grayscale intensities Φ_{x} .In addition, if Φ
_{x} is a nearly constant contrast at every point in the subset, then the standard deviation errors from Equation (19) can be rewrite asIV. RESULTS OF NUMERICAL SIMULATIONS
4.1. Effects of Subpixel Interpolation When the reference and current image are
When the reference and current image are free of random white noise ([Δ
G_{i} ]_{N} = 0, [Δg_{i} ]_{N} = 0) and the zeroorder subset shape function matches the actual deformation field ([Δg_{i} ]_{S} = 0), the subpixel interpolation error [Δg_{i} ]_{I} will be the main factor affecting the accuracy and precision of deformation estimation. Fig. 1 shows a summary of the standard deviation in displacement estimation errorstd (Δp )over a range of subset sizes for 20 image pairs with 20 different rigid body translations (
p _{1} +p _{2} = 0.005, 0.007, 0.016, 0.024, 0.038, 0.041, 0.053, 0.069, 0.077, 0.082, 0.15, 0.78, 1.01, 1.40, 1.62, 1.81, 1.96, 2.39, 2.74, 3.18 pixels).While
std (Δp ) is found to decrease with increasing subset size, The standard variation of the average values ofstd (Δp ) of all 20 image pairs with the subset size (totaln pixels ) are also shown in Fig. 1 as a dashed line with discrete symbols for each data point. The dashed line is a powerlaw curve fitting of the discrete data points in the form ofstd (Δp ) =an^{b} witha = 0.04046 andb = 0.5683 (SSE: 2.1e8, Rsquare: 0.9935). The exponentb is close to 0.5 (see Equation 20), indicating that the image grayscale error does following the assumption made previously in deriving Equation (20). In addition, the actual value of average standard deviation due to subpixel interpolation error is rather small:std (Δp ) = 0.000511 pixels at subset sizen = 43.4.2. Effects of Random White Noise
Add Gaussian noise of 0 mean and variance (σ^{2}) of 0.2 pixels to the reference and current image. Fig. 2 shows the effect of random white noise on the standard deviation in displacement estimation error
std (Δp ) over a range of subset sizes for 20 image pairs with 20 different rigid body translations (p _{1} +p _{2} = 0.005, 0.007, 0.016, 0.024, 0.038, 0.041, 0.053, 0.069, 0.077, 0.082, 0.15, 0.78, 1.01, 1.40, 1.62, 1.81, 1.96, 2.39, 2.74, 3.18 pixels). The dashed line is a powerlaw curve fitting of the discrete data point in the form ofstd (Δp ) =an^{b} witha = 0.07484 andb = 0.6617 respectively (SSE: 2.96e8, Rsquare: 0.9937). Again thestd (Δp ) decrease in general with increasing subset sizes for all image pairs. But the exponentb is deviate slightly from 0.5. On average, the actual value of average standard deviation due to subpixel interpolation error and random white noise is still small:std (Δp ) = 0.0005823 pixels at subset sizen = 43, somewhat larger than the former case in above section.Figure 3 shows the effect of different random white noise levels (σ^{2} = 0.2, 0.4 and 0.6) on the standard deviation
in displacement estimation error
std (Δp ) over a range of subset sizes for an image pair with a rigid body translation only (p _{1} +p _{2} = 0.041). The standard deviation in displacement estimation error decreases with the increasing random white noise variance. Fig. 4 usingn ^{(0.5)} as the horizontalaxis coordinate instead ofn . A nearly linear dependence ofstd (Δp ) onn ^{(0.5)} seems to exist (std (Δp ) =an ^{(0.5)} +b ); dashdotted line, dashed line and solid line represent the linear fitting on the displacement estimation errorstd (Δp ) ofn ^{(0.5)} under different image noise levels. Specifically, the actual values ofa andb are 0.03102 and 3.6780e4 (with SSE:1.595e7, Rsquare: 0.9544), 0.03379 and 1.246e4 (with SSE: 1.995e7, Rsquare: 0.952), 0.04676 and 2.181e4 (with SSE: 6.128e7, Rsquare: 0.9252) respectively for three different image noise levels.
4.3. Effects of Subset Shape Function Mismatch
The analysis above does not consider the mismatch of shape function. In practice, the deformations may contain the displacement gradient
ε , which means that the deformations of subset are no longer pure rigid body translation. Fig. 5 shows the effect of displacement gradient (ε = 0.002,p _{1} +p _{2} = 0.041) on the standard deviations of displacement estimation errorstd (Δp ) under different image noise levels. Four image pairs with different random image noise levels (σ^{2} = 0, 0.2, 0.4 and 0.6) are shown in the figure. Clearly, the powerlaw relationship observed previously forstd (Δp ) no longer holds and the values ofstd (Δp ) are significantly increased. The diversity of variance in different noise levels is no longer a key factor affecting the displacement estimation error. In fact,std (Δp ) is found not to decrease monotonically with increasing subset sizes. The obvious fluctuation of these curves reflects the detrimental effect of major subset shape function mismatch [Δg_{i} ]_{S} when the order of the assumed subset shape function was lower than that of the actual subset shape field.V. RESULTS OF EXPERIMENT
5.1. The Selfcorrelation Experiment
The selfcorrelation experiment is proposed by Z.Y. Wang et al [12]. It acquires two images by CMOS without moving the object. The first image is denoted by
α , and the second image is denoted byβ ,α andβ are not identical due to noise. Then they calculate the displacement fields betweenα andβ by DSCM. The difference betweenα andβ is denoted byγ ,γ =α β . The elements ofγ are noise values of different pixels. By using this method, we can analyze the effect of noiseγ on the displacement estimation errors. As stated before, the acquired imageα is shown in Fig. 6. The imageβ is not presented because itis very similar to
α . The images are obtained by a CMOSbased digital image acquiring system, which permits recording of images with a size of 1280×1024 pixels with 256 gray levels each. The random speckle pattern in the images is created by lightly spraying some white and black paint on the specimen. The specimen is illuminated by ordinary white light.Figure 7 is the results concerning both the vertical and horizontal displacement estimation errors, it shows that the displacement estimation error decreases when subset size increases. For different directions, the powerlaw relationship observed roughly for
std (Δp ) in the form ofstd (Δp ) =an^{b} , Specifically, the actual values of a and b are 0.07034 and 0.5673 (with SSE: 5.765e7, Rsquare: 0.9441), 0.1646 and 0.7099 (with SSE: 8.429e7, Rsquare: 0.932) respectively for the vertical and horizontal directions.5.2. Rigid Body Translation
In this section, the specimen is the same as the one used in the selfcorrelation experiment. The sample is attached to the universal material experimental machine with a translation accuracy of 2 μm. Three images of the speckle pattern with a given vertical direction rigid body translation (0.214 mm, 0.332 mm and 0.455 mm respectively) are captured by the same CMOSbased digital image acquiring system.
The mean displacement of the rigid body translation calculated by DSCM is 16.2653 pixels, 25.3415 pixels and 34.9853 pixels, respectively. Clearly, there is a linear relationship between actual displacements (
U_{act} ) in millimetersand calculated displacements (
U_{cal} ) in pixels, it can be expressed in the form ofU_{cal} =aU_{act} +b witha = 0.01287 andb = 0.005018 respectively (SSE: 8.87e7, Rsquare: 1), the coefficienta is the conversion factor between actual displacements and calculated displacements, which means each pixel represents 12.87 μm.Figure 8 shows the standard deviations of displacement estimation error
std (Δp ) of different actual displacements. In fact, contrary to Equation (20),std (Δp ) is found to increase rather than decrease with increasing subset sizes. Furthermore,std (Δp ) increase with increasing actual displacements for these three image pairs. The most probable reason for the increasing trend instd (Δp ) with increasing subset sizes is that the displacement (in pixels) violates one of gradient based DSCM assumptions, the displacements of subsets are small. The underlying deformation field of a small subset can be readily and accurately approximated by a zeroorder subset shape function, whereas a larger subset size normally leads to larger errors in the approximation of the underlying deformations [4]. For this reason, to guarantee a reliable displacement measurement, a small subset size is preferable in this section. This problem can be avoided by increasing the distance between the lens and the specimen. It means that each pixel of current image represents a large displacement (in millimeters).Rigid body translation experiment was rerun. The sample is attached to the universal material experimental machine. Five images of the speckle pattern with a given vertical direction rigid body translation (0.008 mm, 0.015 mm, 0.023 mm, 0.030 mm and 0.035 mm respectively) are captured by the CMOSbased digital image acquiring system. Fig. 9 shows a summary of the standard deviation in displacement estimation error
std (Δp ) over a range of subset sizes for 5 image pairs with 5 different rigid body translations mentioned above.The powerlaw relationship observed roughly for the average values of
std (Δp ) (dashed line with discrete symbols) in the form ofstd (Δp ) =an^{b} , Specifically, the actual values ofa andb are 0.02846 and 0.3499 (with SSE: 1.573e7, Rsquare: 0.9842).5.3. Threepoint Bending
In this section, a threepoint bending test of a steel specimen was carried out. The dimensions of the threepoint bending specimen are shown in Fig. 10. The experimental device was a universal material experimental machine. First, the specimen was recorded before loaded as the reference image. Then a load
P was applied to the specimen. With the load increased, images of the Region of Interests (ROI) was captured at P = 4333.73N, 8659.89N and 12851.17N. These images were analyzed using the DSCM. Fig. 11 shows a summary of the standard deviation in displacement estimation errorstd (Δp ) of the center point of ROI over a range of subset sizes for 3 image pairs with 3 different load conditions.The powerlaw relationship observed previously for std (Δ
p ) no longer holds. In fact,std (Δp ) is found not to decrease monotonically with increasing subset sizes for all image pairs and the subset size that has a minimum value ofstd (Δp ) is usually between 31×31 to 37×37 pixels. The increasing trend instd (Δp ) with increasing subset sizes at large subset sizes (sayn > 43 pixels) reflects the detrimental effect of major subset shape function mismatch [Δg_{i} ]_{S} when the order of the assumed subset shape function (zeroorder) was lower than that of the actual subset deformation field.VI. DISCUSSION
Deformation estimation errors by DSCM have been reformulated with an explicit representation of the image grayscale error Δ
a_{i} at each point of a subset. Subpixel interpolation error ([Δg_{i} ]_{I} ), image noise ([ΔG_{i} ]_{N} , [Δg_{i} ]_{N} ) and the mismatch between the assumed subset shape function and the actual deformation field of the subset ([Δg_{i} ]_{S} ) have been identified as major sources of grayscale errors that affect the accuracy and precision of deformation measurement.The subpixel interpolation error and image noise error would always be present and they can be affected by the actual image intensity profile characteristics and deformation field characteristics [12]. Nevertheless, the image grayscale error due to subpixel interpolation and image noise in the simulated image pairs analyzed in this study were relatively small (the average values of
std (Δp ) even at a small subset size of 43 × 43 pixels were no more than 0.0005 pixels and 0.0006 pixels respectively). Similarly, the effect of subpixel interpolation and image noise in the self correlation experiment and rigid body translation experiment were still small (the average values ofstd (Δp ) even at a small subset size of 43 × 43 pixels were no more than 0.0012 pixels and 0.0022 pixels respectively; they were about 3 to 4 times larger than the simulated image pairs). Values ofstd (Δp ) at subset size 43 × 43 pixels were about 0.00076 pixels when the random white noise variance was σ^{2} = 0.6 and the displacement was 0.041 pixels. It was about 1.4 times of in images with zero random white noise at the same subset size. Variance of displacement errorsstd (Δp ) increased significantly due to the combined effect of subpixel interpolation, image random noise and their dependence on the subset size followed a powerlaw relationship with their exponents around 0.3499 and 0.7077. Finally, there was an even more significant effect due to the third type of errors [Δg_{i} ]_{S} ) when the zeroorder subset shape function was used for an image pair deformed with a nonzero displacement gradient. Whether numerical simulation, or threepoint bending test, not only the power law dependence on the subset size broke down, but the resulting errors were also much higher (see Fig. 5 and Fig. 11).Several assumptions were made to obtain the powerlaw relationships on the variances of displacement errors. Those assumptions might not all strictly apply to any real experimental conditions, even numerical simulation. Pan et al. extended the error analysis of twodimensional motion estimation [4] to include detailed analyses on bias in displacement estimation when random white noise existed in both reference and current images. Their results showed the bias in displacement measurements depends on both the variance of image noise and Sum of Square of Subset Intensity Gradients (SSSIG). But the effect of possible subset shape mismatch was not considered by them. If the image noise ([Δ
G_{i} ]_{N} , [Δg_{i} ]_{N} ) consisting of random white noise with a zero mean and a variance of σ^{2} dominates the image grayscale error Δa_{i} (Δa_{i} = [ΔG_{i} ]_{N}  [Δg_{i} ]_{N} ), Equation (19) can be rewritten as followEquation (21) becomes identical to Equation (18) and (19) given by Pan et al. in [4]. Similarly, Equation (21) provides a criterion for subset size selection in DSCM. For certain CMOSbased digital image acquiring system, the image noise can be accurately quantified by selfcorrelation experiment mentioned above. The displacement measurement accuracy can be controlled by adjusting the value of SSSIG [4], which can be increased or decreased by adjusting the subset size. Furthermore, Equation (20) leads to a simple criterion for choosing a suitable subset size, image quality, subpixel algorithm, and subset shape function for DSCM for the desired measurement precision.
VII. SUMMARY
Deformation measurement errors of gradientbased DSCM analysis have been presented in terms of subpixel interpolation error, image noise, and subset deformation mismatch. A new formulation is used to explicitly account for the image grayscale error due to above three error sources at each point of the subset. Numerical and experiment results of error estimation are presented to validate the analytical formulas on the standard deviation of displacement measurement errors. It has been shown that 1) Deformation measurement errors are directly linked to the image grayscale error at each point of the subset; 2) The standard deviation of displacement measurement errors
std (Δp ) on the subset size is actually a powerlaw relationship with an exponent of 0.3499 and 0.7077 when the subpixel interpolation error and image noise are dominant of the image grayscale error. The powerlaw relationship further leads to a simple criterion for choosing a suitable subset size, image quality, subpixel algorithm, and subset shape function for DSCM; 3) When the powerlaw dependence clearly breaks down, the subset deformation mismatch may become dominant and there is an upper limit of the subset size for minimizing errors.

[TABLE 1.] Error sources of gradientbased DSCM and their expression

[FIG. 1.] Summary of the standard deviation in displacement estimation error std (Δp) over a range of subset sizes for 20 image pairs with 20 different rigid body translations.

[FIG. 2.] Effect of random white noise σ2 = 0.2 on the standard deviation in displacement estimation error std (Δp) over a range of subset sizes for 20 image pairs with 20 different rigid body translations.

[FIG. 3.] Effect of different random white noise levels (σ2 = 0.2, 0.4 and 0.6) on the standard deviation in displacement estimation error std (Δp) over a range of subset sizes for an image pair with a rigid body translation only (p1 + p2 = 0.041).

[FIG. 4.] Powerlaw fittings of the same results in FIG. 3.

[FIG. 5.] Effect of displacement gradient (ε = 0.002, p1 + p2 = 0.041) on the standard deviations of displacement estimation error std (Δp) under different image noise levels.

[FIG. 6.] Image α used selfcorrelation experiment.

[FIG. 7.] Both the vertical and horizontal displacement estimation errors.

[FIG. 8.] Standard deviations of displacement estimation error std (Δp) of different actual displacements.

[FIG. 9.] Summary of the standard deviation in displacement estimation error std (Δp) over a range of subset sizes for 5 image pairs with 5 different rigid body translations.

[FIG. 10.] (a) Dimensions of the specimen (where L = 350 mm, s = 120 mm, h = 30 mm, and b = 15 mm), (b) Region of Interests on the threepoint bending steel specimen.

[FIG. 11.] Summary of the standard deviation in displacement estimation error std (Δp) of the center point of ROI over a range of subset sizes for 3 image pairs with 3 different load conditions.