Example showing how to identify collapsing regions from the curvature of velocity gradients. We use three different collapsing stages of subregion A1-2, i.e., 0.4 Myr (first column), 0.6 Myr (second column), and 0.8 Myr (third column). First row: orientation of velocity gradients in the range of [0, π) (i.e., red: ≃π, blue: ≃0, and green: ≃π/2). Second row: curvature of velocity gradients calculated via the RK2 method. Third row: gravitationally collapsing regions (red regions) identified from the curvature of velocity gradients. Fourth row: gravitationally collapsing regions (blue regions) identified from the N-PDFs.