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We propose a novel vortex core line extraction method based on the λ2 vortex region criterion in order to improve the detection of vortex features for 3D flow visualization. The core line is defined as a curve that connects λ2 minima restricted to planes that are perpendicular to the core line. The basic algorithm consists of the following stages: (1) λ2 field construction and isosurface extraction; (2) computation of the curve skeleton of the λ2 isosurface to build an initial prediction for the core line; (3) correction of the locations of the prediction by searching for λ2 minima on planes perpendicular to the core line. In particular, we consider the topology of the vortex core lines, guaranteeing the same topology as the initial curve skeleton. Furthermore, we propose a geometry-guided definition of vortex bifurcation, which represents the split of one core line into two parts. Finally, we introduce a user-guided approach in order to narrow down vortical regions taking into account the graph of λ2 along the computed vortex core line. We demonstrate the effectiveness of our method by comparing our results to previous core line detection methods with both simulated and experimental data; in particular, we show robustness of our method for noise-affected data. [ABSTRACT FROM AUTHOR]