L8: Depth Perception

Why do our eyes face the same direction?

• Problem with images, don’t tell you how far they are

• Flat image so must be something in the context

  • By having essentially two views of the same world can you ‘maths’ to understand depth in our world

Stereopsis - Retinal Disparity

• Our eyes have two different views of the world 

• The use of these two views to get depth is termed “stereopsis”

• Any object not on the horopter (the locus of points in space that have the same disparity as fixation) has a retinal disparity

• If we can calculate this retinal disparity we know how far the object is away from our ‘horopter’

What should you see if you put different images in the left and right eye

Should see depth

What is the usual object distance for getting two different images in each eye and what is the rationale behind it

Eyes are about 6cm apart, so object distance usually 6cm apart

Random dot steregrams: Bela Julesz (1961)

• Two patterns of random dots presented to each eye

• Cut square of random dots out and stick it at different depth

• People see the square

• Conclusion

  • Depth must precede form

    • Without solving the depth problem, there is no square

Autosteregrams - Magic Eye

• A way of getting a stereogram from one image

  • Normally one image in each eye

• Trick to get one eye looking at one stripe, the other eye looking at the other stripe 

• Autosterogram is a 2D image that tricks the brain that it is a 3D image

What is the first place information from two eyes comes together

V1

What are many cells in the V1 area

Disparity sensitive

Around what percentage of people have problems with their stereovision

10%

What can cause stereoblindness

Problems in early life (e.g. squint) then the brain doesn’t learn to put the information from the two eyes together and this cue to depth is lost

Can still perceive depth with one eye, so what are some other depth cues we use?

• Motion parallax

• Pictorial cues to depth

• Size

• Texture gradients

• Shape and Shading

• Cast shadows

Motion parallax

Objects that are closer appear to be moving faster than objects that are further away

• Why cats bob up and down when close to prey, trying to use motion parallax to gauge depth

Pictorial cues to depth

• Despite a lack of stereo cues and lack of motion we can still see depth in pictures

• We use a lot of cues such as:

  • Interposition

  • Height

  • Size

  • Perspective

  • Shadows

Size

For a given object size it will cast a smaller and smaller retinal image as it gets further away

Texture gradients

Smaller cobblestone looks further away

Shape and Shading

Ramachandran (1988)

• Tend to see the middle element as sticking out

• Brain makes the assumption that light comes from the top… but top of what? The world of your head

• If light coming from the top, expect circles that are light at the top to be ‘sticking out’ and those with light at the bottom to be sticking in

  

Cast shadows

Kerten et al (1997)

• The shadow of an object, ball takes same trajectory only shadow cast differs

  

Emmert’s law (Distortions due to misplaced depth

Images of same retinal size will appear different sizes when located at different distances (linear relationship)

Example of an illusion from size contancy

Ames room

What point in our vision DOESN’T gave retinal disparity

The point we are looking at