Flowchart of how the correlation coefficient (CC) is calculated. I_t1, I_t2, and I_t3 are three frames of the intensity observations. Two Γ₂ maps (Γ_t1 and Γ_t2) are generated from these three intensity observations employing ASDA (Liu et al. 2019a). They are further ternarised to contain only values of −1, 0, and 1. These ternarised Γ₂ maps are then used to calculate CC. For more information, see the text provided in Sect. 2.