1. CS790 Presentation Andrew Eggebeen 5/6/2015
Depth Perception
CS790 Presentation
Andrew Eggebeen
5/6/2015

2. Depth perception
“Cognitive processes that lead to the inference of the third dimension from 2D retinal images” [1]
Multiple cues are perceived
Depth can be perceived incorrectly
Relevance to Computer Science
More realistic 3-D graphics
More efficient vision systems
Visual prosthesis (bionic eye)

3. Depth cues Monocular cues Binocular cues Oculomotor cues
Gathered from one eye (one image)
Used to perceive depth from a ‘flat’ image (photo, computer screen, etc.)
Makes vision system robust
Not entirely impaired if one eye is closed, covered, or damaged
Binocular cues
Gathered from both eyes (two images)
Used in Virtual Reality headsets
Gives more information about depth (different info from both eyes)
Oculomotor cues
Physical (non-visual) feedback

4. Motion parallax
During lateral motion, closer objects move across field of view more rapidly than distant objects
Used in 2-D side-scrolling computer games to give feeling of depth
Still used in 3-D graphics
Pigeons bob their heads perceive depth from parallax

5. Size Relative size Familiar size Useful effect for films
For objects of the same size, distant objects will appear smaller and closer objects will appear larger
Familiar size
‘Known’ size of an object suggests how distant it is
Useful effect for films
Can create illusion of object size
Can create illusion of object distance

6. Perspective Linear perspective
Parallel lines converge at vanishing points
Significant depth cue used in 3-D graphics
Aerial perspective (atmospheric perspective)
Contrast of distant features decreases
Moisture in atmosphere scatters light and veils distant features
Popular effect in paintings/artwork

7. Muller Lyer illusion Are the center lines the same length? Explanation
Yes
Explanation
Angles on left suggest a ‘protruding’ (closer) feature, angles on right suggest a ‘receding’ (more distant) feature
A ‘close’ line and a ‘far’ line with the same length are perceived as shorter and longer, respectively
People from different cultures perceive this illusion to varying degrees [2]

8. Ames room illusion Are these people the same size? Explanation Yes
Room is not actually square
People appear to be the same distance from viewer, but one is closer
Size suggests closer person is much larger than distant person

9. Lighting and shading
Surfaces facing light sources are more well lit, surfaces facing away from light sources are less well lit
Visual system assumes that “light is shining from above” [3]
Visual system assumes that “there is only one light source illuminating the entire visual image” [3]
Another significant depth cue in 3-D graphics

10. Image from [3]

11. Defocus blur Vision system has limited depth of focus
Objects at focused distance are clear, others are blurry
Popular effect in photography
Jumping spiders use defocus blur for depth perception
But only with certain wavelengths of light
“When using just green light to illuminate prey, the spiders were always accurate in their strikes, just like in full spectrum light.  However, when green light was excluded and red light exclusively was used to illuminate prey, the spiders attacks on prey were typically short.” [5]

12. “Estimating Spatially Varying Defocus Blur From A Single Image” [5]
Xiang Zhu, Member, IEEE, Scott Cohen, Member, IEEE, Stephen Schiller, Member, IEEE, and Peyman Milanfar, Fellow, IEEE
December 2013
Algorithm analyzes defocus blur in an image
Can be used to segment foreground image from background
Image from [5]

13. Binocular disparity Difference in images received by each eye
Disparity is more significant for closer objects
Less effective at long distances
Less disparity of distant objects
Correspondence problem
How to merge the two images into a single image?
Virtual reality headsets (e.g. Oculus Rift) project offset images onto each eye

14. Autostereogram Converging eyes behind image overlaps patterns
Patterns repeating across smaller distances appear closer
Patterns repeating across larger distances appear farther
Same pattern appear flat
Sailboat

15. Stereoscopic images Displays two images on the same surface
Viewer wears glasses to separate images
Anaglyph
Red/blue images are superimposed
Red/blue lenses separate images
Polarized
Images are projected with polarized light
Polarizes lenses separate images
Eclipse
Images are displayed alternately
Active lenses shutter correspondingly to separate images
Stereoblindness - inability to perceive 3D from stereo vision
5% - 10% of the population is stereoblind
In 2012, a man unexpectedly overcame stereoblindness while watching a 3D film. “It was just literally like a whole new dimension of sight.” [6]

16. Accommodation Ciliary body
Part of eye which changes lens shape to focus image
Feedback from eye muscles suggests how near or far image in focus is
Lens shape correlates with distance
Works with convergence
Accommodation vs Defocus blur
Defocus blur is a visual cue
Accommodation is an oculomotor cue

17. Convergence Eyes move inward (converge) to focus on closer objects
Amount of convergence suggest how close object is
Only useful at very close distances
Less convergence change at further distances
Convergence vs Binocular disparity
Binocular disparity is a visual cue
Convergence is an oculomotor cue

18. Predators and prey Predators Prey Forward facing eyes
More stereo vision, less monocular vision
Narrower field of view
More focus on target
Prey
Side facing eyes
Less stereo vision, more monocular vision
Larger field of view
Wider range of threat detection

19. Prey Predator Peregrine falcons [7]
Use monocular vision to see sideways
Fly lateral to prey to surveil with side view
Use binocular vision to see forward
Dive at prey using front view

20. End

21. References
[1] L. Renier et al, “Cross-modal activation of visual cortex during depth perception using auditory substitution of vision”, March 2005, NeuroImage 26 [2] Cultural Differences in the Perception of Geometric Illusions, M. Segall et al, February 1963, Science, New Series, Vol. 139, No [3] On the Perception of Shape from Shading, D. Kleffner and V.S. Ramachandran, 1992, Perception & Psychophysics, Vol. 52 (1) [4] Referenced: 5/3/2015 [5] Estimating Spatially Varying Defocus Blur From A Single Image, X. Zhu et al, December 2013, IEEE Transactions on Image Processing, Vol. 22, No. 12 [6] Referenced: 5/4/2015 [7] Peregrine Falcons: Stories of the Blue Meanie, J. H. Enderson, March 2005, University of Texas Press