when a plane is receding from us, it is more likely to contain color variation, which or eyes sees as a line. it is more likely to contain color variation simpl

when a plane is receding from us, it is more likely to contain color variation, which or eyes sees as a line. it is more likely to contain color variation simply because a plane receding from us has more surface area relative to a unit of space in our visual cortex, versus a plane we are viewing at the position of a normal vector. the edges of a 3d object tend to be receding planes, hence we evolved to detect this