Stereopsis, Stereograms, Random Dot

• What is Stereopsis?

  • (3D vision) the perception of depth from binocular disparity

  *an object further away than the plane we are focussing on, will fall on the retina of each eye at different locations (nasal from the fovea)

  *retinal disparity gives rise to depth perception - how we know that the object is further away

  
• The Concept of Retinal Correspondence:

  • Corresponding  retinal  points  are points  on  each  retina  that  receive  input from the same point in space.​

  *For every each point in one retina, there is a corresponding point on the OPPOSITE side of the other retina (relative to fovea)

  
• What is the Horopter?

  • The  horopter  is  the  plane  wherein all locations are at the same depth as the fixation point

  *when an object can be seen as a single image and not double

  How can it be Empirically Derived?

  • the empirical horopter = everyone's horopter follows the same circular shape but distance away from them is unique
  • Haplopic Method = a pin is moved and noted when it is seen as single and double
  • Measured = as the midpoint​ b/w the inner and outer limits of singular (fused) vision.​

  
• What is Panum's Fusional Area?

  • INSIDE PANUM'S AREA = If  the  difference  in  visual directions b/w the two eyes disparity is very  small (stimuli  are  still seen  as  single images)
  • OUTSIDE PANUM'S AREA = If  the  difference in visual directions  b/w the two eyes disparity is very large (stimuli are seen as diplopic images)
  • The Haplopic method is based on the idea of that objects with very similar visual directions are fused into binocular percepts as they lie within Panum's area

  
• Crossed and Uncrossed Retinal Disparity:

  • Crossed = disparate receptive fields in front of the horopter

  *Stimulus closer to subject than Panum's area

  • Uncrossed = disparate receptive fields behind the horopter

  *Stimulus further from subject than Panum's area

  Horizontal Retinal Disparity:

  • Retinal disparity is encoded in V1 via ocular dominance columns

  *These form the basis for these disparate receptive fields
• Describe Mirror and Lens-Based Stereoscopes:

  • Mirror = allow 2 images to be viewed simultaneously such that   small differences  in  horizontal  disparity b/w objects in the 2 images give rise to vivid impressions of depth
  • Lens = taking 2 photographs of a visual scene with a camera separated by an appropriate amount (can be done using colour, polarisation, LCD)

  How are they made?

  • Stereograms - enable us to view a 2D image as 3D

  *have 2 copies of the object that is further away, at the plane of focus - these shifted views relative to each other.​

  • a DIFFERENT image is displayed to each eye
• 1)The Red-Green Anaglyph Approach to Depth Perception:

  • 2 coloured images are used and overlaid
  • Filters in the glasses ensure that each coloured image is only viewed by one eye
  • This will convey disparate binocular information

  *used in the 80s - 3D glasses for cinema viewing

  • Red wavelength image = red lens
  • Green wavelength image = green lens

  
• 2)The Polaroid Approach to Depth Perception:

  • Each image is polarised into orthogonal (perpendicular direction) components and projected onto a silver screen - 2 projectors were used for each image
  • Polaroid filters in the glasses ensure that each eye only receives one of the projected images (horizontal or vertical)

  *used in early 3D glasses for cinema viewing

  DISADVANTAGES:

  • Difficult to keep projectors in sync if 2 projectors

  MODERN POLAROID MOVIES:

  • use a single projector that presents images at a higher firing rate
  • a switchable filter is used to achieve dual projection

  
• 3)The LCD Shutters Approach to Depth Perception:

  • Liquid-Crystal device = One eye is blocked at a time and synchronised to the image displayed

  *one lens becomes opaque whilst the other transparent

  ADVANTAGES:

  • The 2 sets of images are presented sequentially but at fast enough frame rates that the brain integrates the images together to form VERY REALISTIC 3D percepts

  DISADVANTAGES:

  • Very expensive - used in 3D gaming industry

  
• The Purpose of Random Dot Stereograms:

  • The removal of monocular cues so that binocular disparity depth cues can be investigated - monocular images contain cues which gives it depth

  *random dot stereograms remove cues - ensures 3D effect is due to horizontal binocular disparity and not just common sense

  How are they made?

  • Random dot pattern identical in both eyes
  • A section of the dots are shifted laterally to create disparity
  • When the two images are fused, the shifted section of dots appears to float in front of/behind the background – EYES MUST DIVERGE BEYOND THE PICTURE ​

  
• Free-Fusing Random Dot Stereograms:

  • To see the 3D effect , you would use a stereogram, but can be seen without
  • Method 1:

  *To fuse a random dot stereogram, fixate on u
• Free-Fusing Random Dot Stereograms:

  • To see the 3D effect , you would use a stereogram, but can be seen without
  • Method 1:

  *To fuse a random dot stereogram, fixate on your finger in front of it (NOT the stereogram)

  *Move your finger backwards and forwards until the images fuse

  • Method 2:

  *Use the clear front of a CD case to focus on, in front of the stereogram
• Random Dot Autostereograms:

  • uses the dependence of depth on spacing to create 3D images from 2D
  • If distance of repeats is shorter over some area = area will appear CLOSER
  • If the distance of repeats is longer over some area = area will appear more DISTANT

  *examples usually have 2 dots above/below autostereogram to aid fusion of the eyes