How Far Away Is It? Depth Perception

Depth Perception The ability to see objects in three dimensions although the images that strike the retina are two-dimensional, allows us to judge distance How do we see a 3-D world using only the 2-D retinal images? We are able to see in 3-D because the visual system can utilize depth cues that appear in the retinal images

Gibson & Walk’s Visual Cliff A laboratory device for testing depth perception in infants and young animals Infants that have been crawling for about a month are reluctant to crawl past the “edge” of the visual cliff Other animals had similar results. Suggests that depth perception, to some extent, is inborn but needs experience of crawling

Visual Cliff The Visual Cliff Experiment Click on link above to view the visual cliff experiment

Depth Perception: Monocular Depth Cues Module 19

Monocular Cues Depth cues that require the use of only one eye Monocular depth cues include: relative size relative motion interposition relative height texture gradient relative clarity linear perspective.

Monocular Depth Cues – Relative Size Using the perceived size of a familiar object to determine depth The larger the object appears, the closer the object is to the viewer The smaller the object appears, the farther away it is to the viewer.

Monocular Depth Cues Relative Motion (Motional Parallax) A person who is moving can determine depth by focusing on a distant object. As we move, objects that are actually stable may appear to move Objects further away than the object of focus will appear to move slowly in the same direction as the subject is moving. Objects closer than the object of focus will appear to move quickly in the opposite direction.

Relative Motion In this example, the passenger is moving past a stable world. If she fixes her gaze on the bridge, objects behind it will appear to move forward. The farther away the object is, the more slowly it will appear to move. Objects in front of the fixation point appear to move backward.

Monocular Depth Cues Interposition/Overlap Method of determining depth by noting that closer objects partially obstruct/block the more distant objects Also called “overlap”

Monocular Depth Cues – Relative Height Method of determining depth by noting that distant objects appear higher in your field of vision than do closer objects This reversed above the horizon where higher objects seem closer. You know that the trees and houses are farther away than the lake because they are higher up in the drawing than the lake is.

Monocular Depth Cues – Texture Gradient Method of determining depth by noting that distant objects have a blended or smoother texture than nearby objects Individual flowers are visible in the foreground, but in the distance they look like a smooth carpet.

Monocular Depth Cues – Relative Clarity (Aerial Perspective) Method of determining depth by noting that distant objects are less clear than nearby objects Tends to work outdoors because light from distant objects passes through more atmosphere, we perceive hazy objects as farther away than sharp, clear objects The distant mountains look blue & hazy because of dust & moisture in the atmosphere.

Clarity as a Cue to Distance

Monocular Depth Cues– Linear Perspective Method of determining depth by noting that parallel lines appear to converge in the distance The lines appears to eventually merge on the horizon. The sides of the road or track seem to converge in the distance.

Monocular Depth Cues– Light and Shadow Nearby objects reflect more light into our eyes. Given two identical objects, the dimmer one seems farther away. Brain assumes that light comes from above. Watch as the picture flips upside down.

Relative Height

Can you apply these principles to a painting? Explain how the following terms are being used in the picture on the next slide: Relative Size Interposition Relative Height Texture Gradient Relative Clarity/Aerial Perspective Linear Perspective Light & Shadow

Accomodation of the Lens Accommodation is also used by the brain to estimate distance. Brain takes into account how the muscles in your eye affect your lens - thickens (for nearby objects) or flattens (for far away objects). However, we rely on pictorial cues more than this.

Depth Perception: Binocular Depth Cues

Binocular Cues Depth cues that require the use of both eyes There are two types: Retinal Disparity & Convergence

Retinal/Binocular Disparity Because our eyes are about 2 ½ inches apart, the retina receives slightly different images of the world. The greater the difference between the two images, the closer the object is to the viewer. The more alike the two images, the further away the object is perceived. Is most effective when the item is quite close to the person Example: hold a pencil just in front of your nose and close one eye, then the other. The pencil should move showing the disparity. Now focus on something far away and do the same. It should not move very much showing less disparity.

Some People’s Eyes are Further Apart than 2 ½ inches

Binocular Depth Cues: Finger Sausage Hold your two index fingers about 5 inches in front of your eyes, with their tips half an inch apart. Now look beyond them and note the weird result. Move your fingers out farther and the retinal disparity—and the finger sausage—will shrink.

Retinal Disparity – 3D Images

Stereogram Another way to create the illusion of depth through binocular stereopsis is with an Autostereogram An autostereogram is formed by superimposing two repeating patterns The two patterns are slightly offset, and when viewed properly, this offset is seen as a binocular disparity

Stereogram Do you see the Tiger? Try More HERE? To see the 3-D image, first view the picture closely. Focus your eyes as if you are looking at some far off distant object (look through the picture). Try relaxing your eyes so they swing outward. Without changing your focus, slowly move away from the picture. The image will begin to to come into focus. Do you see a Tiger? If you have trouble, it just means your eyes are difficult to trick, they want to focus at the proper depth! Do you see the Tiger? Try More HERE? Hockenbury powerpoint (Schulman) This image should be viewed by crossing the eyes until the two target circles at the top are seen as three. What you will see is a checkerboard pattern.

3D Fun Look at the image and cross your eyes to bring the two images together into one 3D image

Convergence A binocular depth cue related to the tension in the eye muscles when the eyes track inward to focus on objects close to the viewer The brain notes the angle of the convergence and then computes the distance of what you are focusing at. The more tension in the eye muscle, the closer the object is Works best at close distances

Julian Beever’s Sidewalk Chalk Art Depth is all a matter of perspective! Want to see more? Goto http://www.etntalk.com/sidewalk/

Julian Beever’s Sidewalk Chalk Art Watch How He Does It Watch a video montage of his work Want to see more? Goto http://users.skynet.be/J.Beever/pave.htm