system
Given the following post, respond as if you're commenting on Reddit:
human
How effective are the black bars football players put under their eyes?

I understand it helps with the light, the glare and the reflection from the lightspots, I was just wondering if someone had answers concerning the usefulness of the black lines football players put under their eyes.
gpt
Eye black helps to reduce veiling glare (lens flare) which, in turn, boosts your ability to differentiate between light and dark. Being able to see that contrast helps players judge an object's speed as it travels towards them. So a receiver in football being able to judge when and where the ball will land while it's in the air. Or a batter in baseball judging the speed and type of throw.

*But* some sort of visor is basically necessary for any noticable difference. Just the eye black on it's own is negligible. But a hat and eye black helps to alleviate more glare than wearing just a hat.

Edit: it's fairly effective as long as you also wear a visor of some sort. When you reduce glare to your eye, you also reduce the amount of light your eye takes in. This means your eye will take in more light from things you're directly looking at and allow better focus and contrast. Eye black maybe also helps keep sweat from clinging to your cheeks and creating more glare.