Puzzle caused by DistFromInside

I am trying to calculate distance from inside using the method DistFromInside of TGeoShape. I have an example code to illustrate my puzzle:

  • build a box
  • put a point in the box
  • define a direction and its opposite direction
  • calculate the distance based on the point via both the directions

I realized that one direction returns the correct distance, another one gives zero, which I am so puzzled by. Why the distance along one direction is zeor?!

I understand some experts would not suggest to use the methods like DistFromInside or Outside to trace tracks. But in the example, it is a very simple case, it is not working.

Any hints?

test_DistFromInside.C (1.66 KB)

Please disregard the puzzle above. I know what is my problem.