To help you understand the problem, we begin by showing that human vision is based on imaging optics.

To the right, we see a 3d cactus. Rays of light travel away from it, and pass through the lens we see in the center of the diagram. The lens creates an image of the cactus on the screen to the left. Note that the image is inverted, and that the screen is a flat 2d surface. The image of the cactus is 2d, even though the cactus itself is 3d.