I was going through one publication, Atomic-Level Processes of Shear Band Nucleation in Metallic Glasses, PRL119,195503 (2017), Dr. Stukowski is also one of the coauthor. In Fig. 2 a) Representative vortex described by the displacement vectors and (b) displacement vectors and corresponding angle of rotation is shown. The displacements vectors are inbuilt and possible to get, but how did the calculation of "corresponding angle of rotation" was done (From the figures, I am assumed all of it was done in OVITO).