Perhaps the best representation of blurred lines and how they apply to optical illusions is the Hering illusion. Its radial spokes are blurred lines, all emanating from a single point. Those lines tell us where we are heading: forwards, towards the center.
The reason the two vertical lines appear to bow in the middle is because the radial lines suck our field of vision towards the center, as if we were in motion. In fact, those vertical lines are parallel, despite what our brain tells us. Our perception is actually showing us what those parallel lines look like in the next tenth of a second, the moment our gaze “passes through” the vertical lines, towards the vanishing point of the radial lines.
To simplify things, Chingazi suggests we imagine walking through a very tall doorway of a cathedral. When we’re really far away, the doorway sides seem parallel to one another. The angular distance between the top, middle and bottom of the door are all roughly the same.
“Once you’re really close or going through the cathedral doorway, the parts at eye-level are going to be wider apart,” he said. “When you look up, they actually converge like railroad tracks in the sky.”
Essentially, this is the same phenomenon that happens in the Hering illusion.