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?

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 Ponzo Illusion:

Depth Cues and Size Distortion The Ponzo Illusion:

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

Pictures are Weird What does Gregory find paradoxical about pictures?

The Apparatus What does he do to remove the paradox so that the Muller-Lyer figure no longer looks flat?

The Apparatus

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?

Removing the Background Perceived size and perceived distance are tightly related

Now on to Magic Eye Stereograms

Stereograms Left Eye Right Eye Divider 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

Pinker: Autostereograms Optically separate images aren’t needed

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 Image Right Eye’s Image Crossed convergence shifts right image to the right of the left image and vice versa:

Autostereograms What would happen if the convergence was uncrossed?

Autostereograms Left Eye Right Eye Uncrossed convergence shifts right-eye’s image to the left of the left-eye image and vice versa:

Autostereograms What would happen if the faces were switched relative to the boxes?

Autostereograms one doesn’t even need two different images!

RIGHT EYELEFT EYE 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

RIGHT EYELEFT EYE If you uncross convergence, your right eye gets these faces shifted slightly to left, left eye gets them shifted to right = CROSSED DISPARITY Autostereograms Any repeating objects that have a spacing different from the background will have either crossed or uncrossed disparity What would you see?

RIGHT EYELEFT EYE If you uncross convergence, right eye gets these faces shifted slightly to right, left eye gets them shifted to left = UNCROSSED DISPARITY Autostereograms Any repeating objects that have a spacing difference from the background will have either crossed or uncrossed disparity What would you see?

Autostereograms Any 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

Autostereograms

Next Time: Colour Vision Read Land Article for Thursday