invisible to people

[Etude in B Minor]

Sometimes (quite often, actually) it seems like I am invisible. Walking on a sidewalk, not too crowded either, people will often walk right close to me, almost colliding. Often they would collide if I didn't jump out of the way. Its not that they are purposely walking into me as a display of dominance or something - these are just normal people who don't seem to notice me. Maybe they are just distracted, but they seem to avoid other obstacles just fine. This seems to happen in waves, i.e. some days it happens often and other days I don't have such problems.