# Panoramic Digital Image Correlation for 360-Deg Full-Field Displacement Measurement
## 1 Introduction  
As a flexible non-contact optical-numerical technology, digital image correlation (DIC) has the characteristics of relatively straightforward data processing, easy implementation, high accuracy, and wide applicability, that have become widespread in the mechanical properties testing of materials and biological tissues [1; 2; 3; 4; 5], health monitoring of engineering structures [6; 7; 8], and other fields. Stereo-DIC technology combines the principles of DIC and stereophotogrammetry, which is a powerful and practical full-field optical deformation measurement technology [8; 9; 10]. It obtains full-field complex geometric shapes, motions, and deformations by tracking and recovering the three-dimensional shape of the deformed object in the stereo image sequence [11]. The realization of the Stereo-DIC technology requires the use of a dual-camera synchronous imaging system or a single-camera pseudo-stereo imaging system, which can simultaneously record a pair of image sequences of angled views on the specimen surface under different states. Then, the intrinsic parameters and extrinsic parameters of the dual-camera setup are calibrated according to the specific stereo camera calibration procedures [12; 13]. Next, a correlation algorithm is used to match the correspondence of corresponding point pairs in the two stereo images by comparing the gray distribution of a subset of square pixels and tracking these points in a sequence of stereo images representing different states of deformation. Finally, the matched set ofimage points is used to reconstruct and track the 3D position of the points on the specimen surface over time by combining the calibrated parameters with stereo triangulation, and, then, determining the 3D displacement field on the specimen surface. Despite the wide use of Stereo-DIC, its field of view is limited to the front surface of the specimen due to the occlusion of the camera's line of sight, and it cannot measure the whole-body profile of the panoramic deformation of the sample with a complex geometric morphology. In order to extend the field of view of the deformation measurement to a panoramic view, the effective countermeasures are to adopt a multi-view imaging system composed of multiple synchronous cameras, a pseudo-multi-view imaging system composed of single camera combined with slewing ring, or a binocular imaging system assisted by mirrors. The whole-body shape and displacement measurement technique can usually be carried out according to one of the following system configurations: (i) Encircling configuration. The object to be measured is surrounded by a group of synchronous cameras deployed as an open or closed ring, with each pair of adjacent cameras having a common field of view to facilitate the integration of all cameras into a common world coordinate system [14; 15]. The encircling configuration requires two or more synchronous cameras, making the measurement technique both complicated and expensive, but, even so, this configuration is still the mainstream of full-field 360-deg displacement measurements, especially for the shape and displacement measurement of large-size specimens or specimens located in industrial sites. (ii) Pseudo-encircling configuration. The configuration is also measured the object by several adjacent and intersecting views, but the difference is that the different views are captured by rotating the camera while keeping the object still or rotating the object while fixing the camera [16; 17]. One serious limitation of this configuration is that it can only be used under static or quasi-static loading conditions. (iii) Discrete stereoscopic configuration. Consists of two synchronous Stereo-DIC systems with no common field of view that measure each surface of the plate or sheet specimen independently [18]. The discrete stereoscopic configuration requires four cameras to form two stereo camera pairs. On the one hand, it is similar to the encircling configuration and requires complicated calibration.