I was putting up some wall decorations earlier today and was painstakingly realigning everything until it looked level to my eyes. It might be just a hair off, but if I don’t correct it, I’ll see the misalignment almost instantly and get bothered for the rest of time until I fix it. Has anyone investigated, or is there literature on the minimum perceptible angle from level to the naked eye?
Perpendicular to the normal vector to the center of the local gravity well
But, oddly, it’s not. How square it is with the walls is more important. Door and window frames, too, and those are more likely to be a touch off.
I was defining true level. Squaring with the room is inherently different, and absolutely more important for decor such as wall art. In contrast, hanging drapes to be level to a slanted ceiling can result in a loss of functionality.
You’re right. Especially with the drapes: if the ceiling isn’t level, it’s a no-win situation. It’s either going to look distracting, or function poorly.