Comparison between LIC imaging, vorticity, and vorticity strength of a solid rotation (upper panels) and that of a shear flow (lower panels) indicated by white arrows. Upper panels: a) the horizontal velocity amplitude of a swirling feature is illustrated by the LIC imaging, b) the tangential discontinuity at the edge of a swirling feature is enhanced by the vorticity, and c) the constant angular rotation speed of a swirling feature is depicted by the vorticity strength. Lower panels: d) same as a) but for a shear flow, e) a gap between the rightward and leftward velocity at y = 0 is illustrated by the vorticity, and f) there are no features shown by the vorticity strength.