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The Apparatus. Seeing in Stereo. Seeing in Stereo. Seeing in Stereo. It’s very hard to read words if there are multiple images on your retina. It’s very hard to read words if there are multiple images on your retina. Seeing in Stereo.
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Seeing in Stereo Seeing in Stereo
Seeing in Stereo It’s very hard to read words if there are multiple images on your retina It’s very hard to read words if there are multiple images on your retina
Seeing in Stereo It’s very hard to read words if there are multiple images on your retina It’s very hard to read words if there are multiple images on your retina But how many images are there on your retinae?
Binocular Disparity • Your eyes have a different image on each retina • hold pen at arms length and fixate the spot • how many pens do you see? • which pen matches which eye?
Binocular Disparity • Your eyes have a different image on each retina • now fixate the pen • how many spots do you see? • which spot matches which eye?
Binocular Disparity • Binocular disparity is the difference between the two images
Binocular Disparity • Binocular disparity is the difference between the two images • Disparity depends on where the object is relative to the fixation point: • objects closer than fixation project images that “cross” • objects farther than fixation project images that do not “cross”
Binocular Disparity • Corresponding retinal points
Binocular Disparity • Points in space that have corresponding retinal points define a plane called the horopter The Horopter
Binocular Disparity • Points not on the horopter will be disparate on the retina (they project images onto non-corresponding points)
Binocular Disparity • Points not on the horopter will be disparate on the retina (they project images onto non-corresponding points) • The nature of the disparity depends on where they are relative to the horopter
Binocular Disparity • points nearer than horopter have crossed disparity • points farther than horopter have uncrossed disparity The Horopter
Binocular Disparity • Why don’t we see double vision?
Binocular Disparity • Why don’t we see double vision? • Images with a small enough disparity are fused into a single image • The region over which these are fused is called Panum’s Area
Stereoscopes • seeing depth requires “only” two different images on the retina • this could be accomplished by an optical device that projects separate images into the two eyes
Stereoscopes • seeing depth requires “only” two different images on the retina • this could be accomplished by an optical device that projects separate images into the two eyes • Note that some of you can’t see in stereo because you have amblyopia • Brain failed to learn to use one eye’s image during development • You might have amblyopia if: • you had a strabismus or “lazy eye” • you had poor vision in one eye as a child • you had to wear a patch over one eye when you were a child
Stereoscopes • 3D movies are just tricky stereoscopes in that the lenses allow one image to enter one eye and the other image to enter the other eye • Difference is that images are superposed on the screen
Stereoscopes Divider Left Eye Right Eye • Right eye sees face to the right; left eye sees face to the left therefore: • uncrossed disparity • Face appears behind the square
Stereoscopes Divider Left Eye Right Eye What would you see?
Stereoscopes Divider Left Eye Right Eye • Right eye sees face to the left; left eye sees face to the right therefore: • crossed disparity • Face appears in front of square
Autostereograms • Optically separate images aren’t needed
Autostereograms • Optically separate images aren’t needed • It’s possible to create disparity by misadjusting the vergence of your eyes
Autostereograms • WARNING! Tricky stuff coming in the next slides • Keep definitions of crossed/uncrossed disparity and crossed/uncrossed convergence separate in your mind!
Autostereograms What would happen if you remove the divider of a stereoscope? • Convergence is on a point at the same distance as the images • Boxes and faces are on the horopter • How many boxes would you see? boxes and faces are on the horopter
Autostereograms Now cross your eyes: • right-eye line of sight crosses left-eye line of sight in front of image (crossed convergence) • each retina is now pointed at the opposite box • How many boxes would you see? • What would happen to the face? crossed convergence
Autostereograms Now cross your eyes: • right-eye line of sight crosses left-eye line of sight in front of image (crossed convergence) • each retina is now pointed at the opposite box • How many boxes would you see? • What would happen to the face? crossed convergence Note the difference between convergence and disparity
Autostereograms This is what your brain would get from your eyes:
Autostereograms • There would be three boxes • middle box: right eye sees face shifted to right; left eye sees face shifted to left therefore: • uncrossed disparity • Face in the middle box appears behind square crossed convergence
Autostereograms What would happen if the convergence was uncrossed?
Autostereograms What would happen if the faces were switched relative to the boxes?
Autostereograms • Switching your convergence will switch the direction of depth (in/out) • Switching the elements of the image left/right will switch the direction of depth (in/out) • Notice the disconnection between convergence and disparity!
Autostereograms • one doesn’t even need two different images!
Autostereograms RIGHT EYE LEFT EYE Faces fuse Convergence tells your brain that the plane of the image is farther away than it really is
Autostereograms • Uneven spacing between identical objects in a single picture can appear as disparity if the angle of convergence is inappropriate
Autostereograms • Uneven spacing between identical objects in a single picture can appear as disparity if the angle of convergence is inappropriate • TRICK: Seeing depth in autostereograms requires you to suppress the reflexive coordination between convergence and accommodation
Autostereograms Any repeating objects that have a spacing different from the background will have either crossed or uncrossed disparity RIGHT EYE LEFT EYE If you uncross convergence, your right eye gets these faces shifted slightly to left, left eye gets them shifted to right = CROSSED DISPARITY What would you see?
Autostereograms Any repeating objects that have a spacing difference from the background will have either crossed or uncrossed disparity RIGHT EYE LEFT EYE If you uncross convergence, right eye gets these faces shifted slightly to right, left eye gets them shifted to left = UNCROSSED DISPARITY What would you see?
Autostereograms • by adjusting the disparity at different parts of the image (with a computer usually) one can make shapes that emerge or recede in depth
“Magic Eye” Stereograms • Usually viewed with uncrossed convergence • Imagine gazing farther than the surface (let your eyes “relax”) • Now try to notice objects or forms in the blurriness • As you become aware of shapes, try to focus (accommodate) the plane of the image without converging your eyes
