The retina receives info in 2-dimensions, length and width. Monocular depth cues only require the use of one eye to change 2d into 3d, these include: Light and shadow: use of light and shadow to trick the brain into 3d perception, e. g. a drawing of something coming out at you" on a 2d piece of paper. Interposition: objects closer to us may cut off part of our view of more distant objects. Clarity: we can see nearby objects more clearly than further away objects. Relative size: if we see 2 objects that we know to be of similar size then the one that looks smaller will be judged as further away. Motion parallax: if we are moving, nearby objects appear to move faster in the opposite direction than ones further away do. Binocular depth cues require both eyes, these include: Requires the brain to integrate info from many senses. Concludes" that you were moving so the pen was not.