Chapter 5 Human Stereopsis, Fusion, and Stereoscopic Virtual Environments

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• Chapter 5Human Stereopsis, Fusion, and
  Stereoscopic Virtual Environments

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What is Stereopsis?

• Binocular disparity - lateral difference between
  the two retinal images.
• Stereopsis - depth sense based solely on
  stimulation of disparate locations on retina.

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Stereoscopic Displays

• Stereoscopic displays are really 2D. Present two
  images, one to left eye and one to the right eye,
  to simulate disparity, and create 3D. Can not
  change viewing angle.
• Time multiplexed systems present the images in an
  alternating fashion beyond the critical flicker
  frequency of the eye. First to left eye than to
  right eye.

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When are 3D Stereoscopic Displays Useful?

• when info is presented in perspective
• when monocular cues are ambiguous
• when static displays are used
• for complex scenes and ambiguous objects
• for complex 3D manipulation tasks
• when inexperienced users are performing 3D
  manipulation tasks
• Stereopsis can help provide info about spatial
  layout

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Stereopsis and Fusion

• Fusion - the ability to fuse the two images into
  one image.
• Stereoacuity - smallest depth that can be
  detected.
• 5 of arc under optimal conditions

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Convergence Retinal Disparity

• Convergence - the amount of rotation of the eyes
  when they are fixed on a target.
• Disparity is the difference between two
  convergence angles of two points in space.
• Any part of the visual filed that does not
  project disparate retinal images is perceived as
  occupying a single point in space.

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Convergence Retinal Disparity

• Crossed disparity - The object is in front of the
  fixation point.
• Uncrossed disparity - The object is behind the
  fixation point.
• See handouts

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Horopter

• Locus of points in space that fall on
  corresponding points on the retinae.
• (see handouts)
• The observer interprets all intersected stimulus
  points as being at a common distance in 3D space.

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Horopter

• Corresponding retinal locations in each eye
  result in perception of a single image at that
  point.
• i.e. fusion. However, binocular fusion is not an
  exclusive property of precise stimulus locations
  crossed by the horopter. Image points in small
  volume surrounding the horopter (Panums area) is
  also fused).
• Stereopsis is the ability of the visual system to
  extract from disparate retinal images the depth
  location of objects relative to the locus defined
  by the horopter.

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Diplopia

• Large disparities can result in diplopia (double
  images).
• Larger amount of horizontal disparity can be
  fused (10 to 20 arc min) than vertical disparity
  (2.5 to 3.5 arc min).
• Do not induce vertical disparities in
  stereoscopic displays!

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Spatial Factors

• Spatial frequency
• Stereoacuity and fusion threshold depend on
  spatial frequency. Best for visual patterns that
  contain sf gt 2.5 cycles per degree of visual
  angle.
• Low sf results in poor stereoacuity
• a complex stimulus contain broad range of sf can
  result in simultaneous precept of fusion and
  diplopia.

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Spatial Factors

• Relative Spacing of stimuli
• length of the compared features and distance
  between them affect stereoacuity
• lines 10 to 15 arc min
• dots pairs separated by 10 to 15 arc min
• gaps of 10 to 30 arc between stimuli (larger
  distances or closer together results in lower
  stereoacuity
• Be careful when designing spatial enhancers to
  improve distance or elevation judgements.

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Spatial Factors

• Disparity scaling and disparity gradient limit
• The maximum disparity fused is proportional to
  the distance between objects.
• Highly dense visual scenes may result in
  difficulties fusing images.
• Orientation
• Stereoacuity worse for oblique or horizontal
  visual patterns than for vertical patterns

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Spatial Factors

• Visual field location
• Stereoacuity is best for stimuli that fall on or
  near the fixation point.
• Provide finer spatial resolution near the center
  of gaze then in peripheral areas of vision when
  using stereo.

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Stereo Displays for VE

• Stereo displays do not provide the same visual
  cues that are available in the real environment.
  Some problems include
• a) large amounts of disparity result in double
  images
• b) ghosting of stereo time multiplexed displays
• c) only horizontal disparity will result in depth
  perception. Vertical disparity results in
  suppression of one eyes image or in the
  perception of double images.
• d) stereo pairs can't be fused by 10 of the
  population
• e) most effective for near space or objects,
  objects that are very distance produce the same
  disparities. But you can induce depth by
  artificially creating large disparities.

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Some References

• Wickens, C.D., Todd, S., and Seidler, K.
  Three-dimensional displays perception,
  implementation, applications. CSERIAC
  SOAR-89-01. (1989).
• McAllister, D.F. (Ed.) (1993). Stereo Computer
  Graphics and Other True 3D Technologies. New
  JerseyPrinceton University Press.
• Yeh, Y. Silverstein L.D. (1990). Limits of
  fusion and depth judgement in stereoscopic color
  displays. Human Factors, 32, 45-60.

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• What are some general recommendations?
• Avoid stereoscopic displays with low spatial
  resolution for tasks with fine detail.
• Lower resolutions could be used for