To expand upon what the problem is: imagine rays of light reflecting from a point going in all directions (essentially a sphere). A cone of those rays make it i

To expand upon what the problem is: imagine rays of light reflecting from a point going in all directions (essentially a sphere). A cone of those rays make it into your eye through the pupil. Putting a 2D layer just near to the eye means light from that one point is actually spread across a circle on that 2D layer, with diameter close to the pupil size (getting on for 4mm in indoor lighting). There is therefore significant overlap between the circles from distinct 3D points - so to block out just one of the points you need to selectively block the rays over that entire circle, but only those in the direction of the point we wish to block.