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Visual Cues - seeing in 3DLight and Color for Nonscientists
PHYS 1230

Vision Cues
Ambiguous and unambiguous depth cues
Recall ambiguous silhouette illusion - we interpret a scene in away that makes sense to our brain
http://www.michaelbach.de/ot/sze_silhouette/index.html

Vision Cues
Ambiguous and unambiguous depth cues
Accomodation
Binocular disparity
Convergence
Parallax
Size
Perspective

Vision Cues
Which cues are ambiguous and which are unambiguous depth -lets see.
Accomodation
Binocular disparity
Convergence
Parallax
Size
Perspective

Human visual pathway
http://www.hhmi.org/senses

Binocular Vision
One obvious advantage of the second eye is that it provides an increased field ofview. You can check this by blocking one eye!
The second advantage is that the two images that differ slightly from both eyes allowus to gauge depth and allow objects to appear in 3-D.
Predators (such as humans or cats) have both eyes in front, to allow us to gauge thedistance from our prey. Rabbits or deer, on the other hand, who are likely to becomesomeone elses dinner, have eyes on each side of the head to quickly detectpredators!

The Peaceable Kingdom, by Edward Hicks
http://www.tfaoi.com/am/3am/3am219.jpg

Advantages of Binocular VisionHumans use numerous visual cues to perceive distance. One of the most accurate of thosecues is binocular disparity, which takes advantage of the fact that most people have twoeyes.
Unlike horses and some other animals, humans have two eyes located side-by-side in thefront of their heads. Thanks to the close side-by-side positioning, each eye takes a view ofthe same area from a slightly different angle. The two eye views have plenty in common,but each eye picks up visual information the other doesn't.
Each eye captures its own view and the two separate images are sent on to the brain forprocessing. Binocular vision results when the brain combines these disparate images todetermine true stereoscopic depth. Basically, the farther away an object is from you, themore similar the view in your two eyes, and the closer the object is to you, the moredisparate the two images. The two images are united into one 3-d picture.
The word "stereo" comes from the Greek word "stereos" which means firm or solid. Withstereo vision you see an object as solid in three spatial dimensions - width, height anddepth - or x, y and z. It is the added perception of the depth dimension that make stereovision so rich and special.
With stereo vision, we can see WHERE objects are in relation to us with much greaterprecision - especially when those objects are moving toward or away from us.

DEPTH PERCEPTIONHow can we tell if we are looking at a painting or photograph, or looking at a reallandscape?
Claude Lorrain, Landscape with Merchants, c. 1635, National Museum of Art, Washington, D.C.

DEPTH PERCEPTIONHow can we tell if we are looking at a painting, or looking at a real landscape?A. Nothing changesB. Far away objects are always out of focus since they were painted that way!Adjusting your eyes does not bring them into focus.
Claude Lorrain, Landscape with Merchants, c. 1635, National Museum of Art, Washington, D.C.

Visual Cues - AccomodationWe can roughly measure distance by noting the change in accommodation needed tobring an object into focus.
This works well for close objects, but not so well for far objects. Why?

Visual Cues - AccomodationWe can roughly measure distance by noting the change in accommodation needed tobring an object into focus.
This works well for close objects, but not so well for far objects. Why?
A. There is a big change inaccomodation for close objects asthey move, but not much change forfar objects.
B. Your eye muscles are relaxed for faraway objects
C. Far away objects are not in focus

Visual Cues - Convergence
The angle between your two eyeschanges depending on if the object isclose or far.
At the near point, this angle is at amaximum of 15 degrees.
It is only 1 degree for an object only4m from your eyes!
Like accommodation, convergencechanges most and is thus most usefulfor gauging depth in nearby objects.

Visual Cues - Parallax
The closer the object is to you, the more it appears to move in the direction oppositeto your motion.

Visual Cues - Parallax
To gauge depth, you rely much moreon the fact that the view is differentfrom different positions of your head- parallax.
This effect cannot be mimicked on aflat painting - as you move your head,objects that are behind other objectsdo not move by different amounts.e.g. the finger in the poster alwayspoints toward you, no matter whereyou head is. An actual finger wouldnot.
Alfred Leetes 1914 recruiting poster of Lord Kitchener

Visual Cues -Binocular Disparity
Because the two eyes are separated by 6.5cm, we do not have to move ourheads to get a perception of depth.
The difference between the views of thetwo eyes is called binocular disparity.
However, we see a 3-dimensionalworld, not two 2-dimensional images!
As before, binocular disparity also worksbest for close objects, because the eyessee almost the same for far-away objects.
http://www.yorku.ca/eye/toc-sub.htm

Binocular Disparity
http://ligwww.epfl.ch/~fua/vision/3/misc/exam/human/2/
In the diagram, we can see that the analysesof objects in three dimension is based on theretinal disparity between the images formedin the left and the right eye. Nevertheless, inthe reality, things are not so easy. Our brainmust perform a great deal of computationsto give us an interpretation which seems soevident to us at first sight. In humans, it isimportant for binocular stereoscopic depthperception that each of the possibly tworetinal images of a visual field be mappedonto the same region of the brain.

Binocular Vision
http://www1.uni-bremen.de/~i18m/research/model.html

Binocular Vision the Frankfurter Illusion
http://www.michaelbach.de/ot/sze_Frankfurter/index.html

ILLUSIONS of DEPTH PERCEPTION
http://www.hhmi.org/senses

Binocular Vision
http://lifesci.rutgers.edu/~auerbach/bmlec10/sld024.htm

Binocular Vision - Stereograms
The next two pictures are single-image random dot stereograms or SIRDS which willillustrate how depth is generated using binocular disparity. They were created by PeterChang and Gareth Richards in England. There are actually two sets of random dotsoverlayed on these images, one for each eye. What often helps is to place your nose on thescreen and slowly move backwards. Most people report seeing a three-dimensional face.Some people cannot relax their eyes enough.

http://psych.hanover.edu/Krantz/art/cues.html

http://eyetricks.com/stereograms/dancing-stars.jpg

Camouflage

Camouflage
This experiment shows that past experience can affect your perception ofsuch properties as form or depth. Consider what happens when you view this illustration. Atfirst most people cannot tell what this picture depicts, but with continued inspection or a hint,the fragments suddenly are perceptually reorganized and recognized, in this case, as aDalmatian dog. A recognizable image emerges that had no perceptual reality before. Hence,there is some sort of perceptual change among the neurons in your brain. This also leads to achange in the way in which you perceive the shape and depth of the scene. Perhaps mostimportantly, the figure now looks like the shape and depth relations of a Dalmatian dog.
Sometimes being told that a Dalmatian hides in this scene can provide the visual systemwith enough of a hint to recognize the dog. This is a case in which a high-level brain areaunderlying language comprehension tells a lower-level area, in this case the cortical areasdedicated to visual scene analysis, what might be going on.
If this dog was animated then it would be immediately apparent. Common motion of agroup of otherwise unrecognizable blobs is a very powerful cue for your visual system. Itenables your visual system to realize that it's dealing with a single object. This effect istermed grouping. It is important that an animal can do this well, otherwise it might not easilyspot a predator, prey or other food, such as apples. Animals must be able to separate thefigure from the ground. What we call camouflage is an attempt to deceive these processes.

More challenging camouflage - Bird still or in motion
The camouflage is broken by correlated motion!
http://www.michaelbach.de/ot/cog_hiddenBird/index.html

Impossible Paintings
Dr. Brook Taylor's Method of PerspectiveWilliam Hogarth, Frontispiece to Dr. BrookTaylor's Method of Perspective (1754)

Impossible Paintings
Waterfall by M.C. Escher

Depth Cues - SIZE
Distant objects appear smaller, and subtend asmaller angle on your retina. There image istherefore smaller also.
EXAMPLE: Objects A and B are the same size,but since A is farther away, it subtends a smallerangle and has a smaller imag

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