[the_af]

Oh, you mean the physics of "seeing"? I thought the question was about why the vertical line is not 1-pixel wide.

As for "seeing": the real world is 3D. The physics of seeing work in 3D. I don't think there are true 1D or 2D objects in the universe, so we must use our imaginations when we say "Mario is seeing" in 1D or 2D.

We must, like others argued, imagine Mario possesses a way of sensing/seeing that works in 1D, something that doesn't exist in the real world.

PS: alternatively, the line is 2D in the sense that it's made of physical 3D pixels, and Mario can see those. But the line is also 1D in the sense its width offers no additional information because it's a single pixel wide; so it's physically 2D but "informationally" 1D.