You assume it maintains its current path after impact.Yeah though as per my edit, I believe the umpire is suppose to assume the ball goes straight after impact. Someone to confirm.
Like when a spinner hits the batsman's pad on the full.
Maintaining current path =/= straight, as some player-umpires seemingly don't understand. If a guy bowls a massive in-swinger and hits the guy on leg stump you don't simply assume the ball won't swing post-impact, and suddenly return running parallel to the pitch. Obviously, the swing will give a different trajectory.
Angle on the crease does the same thing - often ignored by many umpires, in my experience.