The Hilbert transform along a vector field integrates along the straight line whose direction
is frozen at the output point. Its short hard truncation
is
Lacey, M. and Li, X. On a Conjecture of E. M. Stein on the Hilbert Transform on Vector Fields. Providence, RI: Amer. Math. Soc., 2010.
https://doi.org/10.1090/S0065-9266-10-00572-7.