Like most animals, pigs are more sensitive to movement rather than actually seeing & determining a human threat.
Like deer and dogs, they are color blind. If you have clothes that blend in reasonably into the background and don't move, hogs won't pick you out until they get very close. HOWEVER, they have noses that are better than a bloodhound so any wift of scent that gets blown their way will bust you. I was watching a big boar from over 200 yds away one day. All of a sudden, there was a slight shift in the wind and within seconds he lifted his head towards us and high-tailed it out of the field.