## 4 Results and Discussion
### DIC-PDDO Method Performance Evaluation  
Despite the good agreement between DIC and DIC-PDDO strain fields noticed in Figure 9, a better metric should be used to evaluate the proposed DIC-PDDO strain computation method. Moreover, the agreement of maximum strain values is related to the spatial resolution not including the effect of measurement resolution. Recently created metrological efficiency indicator (MEI) [35,36] combines both spatial and measurements resolutions and for the case of strains is expressed as:  
\[\text{MEI}=n^{2}l_{10\%}, \tag{8}\]  
where \(n\) is the standard deviation of the noise level in analyzed strains, \(l_{10\%}\)-cutoff period to the 10% fractional strain bias. Lower MEI values indicate better method performance.  
The DIC Challenge 2.0 [11] Star 6 synthetic images are used to evaluate the DIC-PDDO method against conventional DIC. Star 6 contains 3 speckle images: an undeformed reference; one deformed image (sinusoidal vertical strain amplitude of 5% \"star\" pattern is used to calculate the strain spatial resolution); and a noise floor image to calculate the measurement resolution. According to the MEI calculation procedure and Star 6 images definition found in [35], the cutoff period \(l_{10\%}\) for the deformed Star 6 image is expressed as \(l_{10\%}=10+290/3999\cdot(X-1)\), where \(X\) is the minimum \(X\) coordinate in px of the deformed image when the 90% signal value (in this case 4.5% strain) is achieved at the image horizontal center line-cut as it is shown in Figure 10.  
Comparison of the DIC and DIC-PDDO strain on the Star 6 synthetic image is presented in Figure 10. The VSG size of 81 px for DIC and absolute horizon diameter of \(2\delta=140\) px for DIC-PDDO are selected to achieve the same spatial resolution at cutoff period of \(l_{10\%}=207\) px (\(X=2721\) px).  
For the same spatial resolution expressed by the 10% cutoff period of 207 px, standard deviations of computed strain noise levels are \(1.13\times 10^{-4}\) for DIC and \(0.957\cdot 10^{-4}\) for DIC-PDDO methods resulting in 1.4 lower MEI value for the DIC-PDDO strain computation method (\(2.65\mu\epsilon^{2}\cdot\text{px}\) for DIC and \(1.98\mu\epsilon^{2}\cdot\text{px}\) for DIC-PDDO respectively). Figures 10 and 11 present the DIC-PDDO method comparison against conventional DIC strain calculation algorithms. MEI-based DIC algorithms review was graphically presented in [35] where  
Figure 9: Comparison of \(\varepsilon_{y}\), \(\varepsilon_{x}\) Lagrangian strains computed on tensile experiment specimen with DIC (VSG = 57 px), DIC-PDDO (\(\delta=3.1\Delta x,\ \Delta x=0.167\) mm) and FE simulation.  
each DIC algorithm spatial resolution (\(x\)-axis) vs measurement resolution (\(y\)-axis) is plotted. Plot in Figure 11 replicates the plot from [35] by extending it with the same DIC-PDDO metrics while DIC algorithms data points from [35] are generalized and shown as grey area in the plot.  
Figure 11: Comparison of the DIC strain calculation algorithms (the data points replicated from [35] are marked as grey area) and DIC-PDDO method.  
Figure 10: DIC and DIC-PDDO strain computation methods comparison using MEI: (**a**) strain fields showing the same spatial resolution of both methods for the DIC VSG = 81 px and PD 2\(\delta\) = 140 px, (**b**) measurement resolution showing 1.2 times lower noise standard deviation in DIC-PDDO strains.  
The proposed PDDO strain calculation method for DIC has higher measurement resolution than the DIC VIC-2D software algorithm for each selected spatial resolution value as it is seen in Figure 11. Moreover, increasing DIC-PDDO performance with the strain spatial resolution (differences between the \"DIC-PDDO\" and \"DIC (VIC-2D)\" curves) shows the proposed method potential to accurately compute localized strains.