## 1 Introduction  
Digital image correlation (DIC) is a modern non-contact full-field method for displacement and strain measurements. It is based on the correlation of the digital images acquired by the cameras with the CCD (charge-coupled device) or CMOS (complementary metal oxide semiconductor) sensor. As the correlation is performed on facets (subsets) during the motion or deformation of the analysed object, there is a requirement to create a random high-contrast black-and-white pattern on the analysed object surface to ensure the uniqueness of those small picture element groups mostly of squared shape. Several factors can influence the accuracy of the obtained results, e.g., the quality of the pattern created, facet size defined for the correlation process, captured image noise, and errors related to the illumination variation or calibration process of the camera(s). When analysing the influence of the patterns in the early phases, the research has primarily been carried out so that multiple patterns were compared to identify the best suited [1; 2; 3]. [PERSON] and [PERSON] [4] identified that finer randomly created patterns with more speckles lead to more accurate results of rigid-body-motion tests when compared to larger dotted patterns. It was proved that the type of speckle patterns does not influence strain measurements for strains lower than 10%. However, it strongly influences the resultsby large strain measurements [5]. [PERSON] et al. [6] compared the two most common pattern application methods, i.e., spray paint and airbrush, dedicating that a more even distribution of speckle sizes in the airbrush patterns results in lower strain measurement errors. The development of patterning techniques such as lithography [7] or microstamping [8] and, consequently, pattern optimization methodologies [9; 10] have led to the minimization of the error introduced into DIC measurement from the pattern. Modern approaches to laser interference-based speckles, fringe and speckle pattern projection provide an alternative to the conventional invasive painted speckle patterns. The advantage of the laser speckle pattern is that such a speckle is not directly affected by temperature. Therefore, it is suitable for strain measurement in an ultra-high-temperature environment [11]. Fringe patterns combined with speckle patterns were used in displacement and strain [12] or vibrational analyses [13] using single-camera DIC. [PERSON] et al. [14] analysed an unconventional approach to creating a speckle pattern by photocopying or printing passages from several publications. They concluded that the uncertainty of the measurements using text patterns is comparable to those using random speckle patterns. The benefit of text pattern is evident when it already exists on a specimen, and applying a traditional speckle pattern is difficult. A recent study by [PERSON] et al. [15] presents that to evaluate the accuracy and precision of DIC measurements using different speckle patterns is convenient to combine a global assessment parameter based on the first- and second-order gradient with the mean intensity gradient. As each facet of the images captured during DIC measurement should be unique, the quality of the speckle pattern created also influences the correct definition of the facet size. Research carried out in the proper facet size selection [16; 17; 18] led to the conclusion that larger facets containing more information are more robust to random error. [PERSON] et al. [19] and [PERSON] et al. [20] developed methods to reduce the strong influence of the facet size on displacement analysis accuracy using the weighting factor and Gaussian window. A modified digital image correlation with the weight assigned to each pixel and computed by Butterworth function was provided [21]. A new approach, i.e., temporal subset digital image correlation technique, improving the measurement resolution in the case of small-sized facets was proposed by [PERSON] et al. [22]. Their research was further developed by [PERSON] et al.