The Apparatus

Seeing in StereoSeeing in Stereo

Seeing in Stereo

It’s very hard to read words if there are multiple images on your retinaIt’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 retinaIt’s very hard to read words if there are multiple images on your retina

But how many images are there on yourretinae?

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

Now on to Magic Eye Stereograms

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

Left Eye Right EyeDivider

• Right eye sees face to the right; left eye sees face to the left therefore:

uncrossed disparity

• Face appears behind the square

Stereoscopes

Left Eye Right EyeDivider

What would you see?

Stereoscopes

Left Eye Right EyeDivider

• 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

• 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

What would happen if you remove the divider of a stereoscope?

Autostereograms

• 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

Now cross your eyes:

Autostereograms

• 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

Now cross your eyes:

Note the difference between convergence and disparity

AutostereogramsThis 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!

RIGHT EYE LEFT EYE

Convergence tells your brain that the plane of the image is farther away than it really is

Autostereograms

Faces fuse

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

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

AutostereogramsAny repeating objects that have a spacing different from the background will have either crossed or uncrossed disparity

What would you see?

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

AutostereogramsAny repeating objects that have a spacing difference from the background will have either crossed or 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

Autostereograms

Autostereograms

Autostereograms