Vorticity computed with streamlines


I was computing some flow streamlines and I saw this option to compute vorticity (see pic attached). I was wondering if there is any theory guide that can explain how this vorticity is computed in order to see the difference between this vorticity and other indices such as Q-criterion or Lambda 2.

Thank you in advance,

Screenshot 2020-11-17 at 19.07.05|344x187