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Today: Finish Gregory Discussion, finish stereograms
Tuesday: Start colour vision
Thursday: Finish colour vision - Read LAND for Thursday
Muller-Lyer Illusion
• What theory does Gregory take up?– Thiery / Woodworth : Muller-Lyer figure
“suggests perspective”
Muller-Lyer Illusion
• What theory does Gregory take up?– Thiery / Woodworth : Muller-Lyer figure
“suggests perspective”
– Is there something about the presence of pictorial depth cues in an image that distorts size?
Depth Cues and Size Distortion
• The illusion of depth in an image seems to distort the perception of size in the image
• This is due to misapplication of Size - Constancy
The Apparatus
• What does he do to remove the paradox so that the Muller-Lyer figure no longer looks flat?
Removing the Background
• With background removed, Muller-Lyer figure actually looks 3D
• What prediction can you make about how the perceived size and the perceived distance of the center line are related?
• How did Gregory test that?
Stereograms
Left Eye Right EyeDivider
•Right eye sees face to the left; left eye sees face to the right therefore:faces present crossed disparity to the brain
•Face appears in front of square
Autostereograms
• Optically separate images aren’t needed
• WARNING! Confusing stuff coming in the next slides
Autostereograms
• Optically separate images aren’t needed
• WARNING! Confusing stuff coming in the next slides
• Keep the definitions of convergence and disparity separate.
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
•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
Left Eye’s ImageRight Eye’s Image
Crossed convergence shifts right image to the right of the left image and vice versa:
Autostereograms
Left EyeRight Eye
Uncrossed convergence shifts right-eye’s image to the left of the left-eye image and vice versa:
RIGHT EYE LEFT EYE
Convergence tells your brain that the plane of the image is farther away than it really is
Autostereograms
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?
AutostereogramsAny repeating objects that have a spacing difference from the background will have either crossed or uncrossed disparity
How could you reproduce the same effects using crossed convergence?
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