Depth and Distance Perception - 5

Cues for Perceiving Depth and Distance

Introduction

  • Vision, though seemingly straightforward, involves interpreting two-dimensional images on the retina into three-dimensional perceptions.
  • Multiple 3D interpretations can arise from a single 2D image.
  • Understanding the cues available for depth and distance perception is crucial.
  • These cues can be monocular (available to one eye) or binocular (requiring both eyes).

Monocular Cues

Height in the Visual Field
  • Objects higher in the visual field appear further away.
  • Example: In a painting, castles are higher up and perceived as more distant than figures in the foreground.
  • Optical Explanation:
    • Objects at position A project to A' on the retina.
    • Objects at position D (further away) project below A'.
    • Since the retinal image is inverted, distant objects project higher in the visual field.
Geometrical Perspective
  • Parallel lines converge with distance, meeting at infinity.
  • Example: Parallel lines on a motorway or the entrance to the Taj Mahal appear to converge in the distance.
  • Renaissance period revolutionized depth cues using the camera obscura (pinhole camera).
    • The camera obscura projects an image of the outside world onto a surface, providing a correct perspective view.
    • Early Renaissance paintings did not always accurately represent converging parallel lines.
    • Later Renaissance paintings fully utilized geometrical cues.
Texture Gradient
  • Finer, denser textures appear further away.
  • Assumption: Texture elements are the same size at different distances.
  • Example: Cobblestones in the foreground appear larger than those in the distance.
  • Changing the texture density gradient can suggest different rates of recession.
Size of Familiar Objects
  • Image size decreases with increasing distance.
  • If you know the actual size of an object, you can gauge its distance.
  • The retinal image size gets smaller as people move further away, yet we recognize their actual size.
  • Object Constancy/Size Constancy: We compensate for the size on the retina by increasing distance.
Occlusion (Overlap)
  • If one object covers another, the overlapping object is perceived as closer.
  • Example: Chimney pots covering roof tiles suggest the chimney is in front of the roof.
Atmospheric Perspective
  • Distant objects appear more blurred and hazy due to dust particles scattering light.
    Blurring<br/>ewlineIncreasing<br/>ewlineblueBlurring <br /> ewline Increasing <br /> ewline bluewith distance.
  • Color Change: Distant objects appear bluer because shorter wavelengths (blue light) scatter more than longer wavelengths (red light).
Shadows
  • Interpretation of shadows depends on the assumed light direction.
  • Our interpretation of shadows is stable and can influence if we see objects protruding or dipping away from us.
  • Assumption: Light typically comes from above.
  • Inverted images can flip our interpretation of depth based on shadow cues.
  • Faces: We tend to interpret faces as protruding towards us, even if presented with conflicting shadow information.
Ambiguity of Pictorial Cues
  • Many 3D interpretations can arise from the same 2D image.
  • Example: A quadrilateral on the retina could be a square viewed directly, a closer square, or a tilted quadrilateral.
Hogarth's "False Perspective"
  • Hogarth's painting violates multiple perspective cues to highlight ambiguity.
  • Examples:
    • Overlapping cues contradict size and distance cues.
    • Reversed texture gradients.
    • Size of familiar objects is distorted.

Unambiguous Depth Cues

Motion Parallax
  • Relative speed of movement of different elements in the visual array.
  • When moving, stationary objects move across the retina in the opposite direction.
  • Close objects appear to move faster; far objects move slower.
  • Completely stationary objects appear infinitely distant.

Binocular Cues

Binocular Convergence
  • The angle between the two eyes when fixating on an object provides a cue to depth.
  • Muscles give a sense of the angle of convergence
  • Eyes turn inward more for closer objects.
Stereopsis (Binocular Disparity)
  • Arises from having two different views of the world.
  • The left and right eyes have different views, creating a horizontal shift that cues depth.
  • Red-Green Goggles: Used to project different images to each eye.
  • Lateral Shift: The size of the lateral shift in the views is a cue to how far it is away.
  • Stereograms: Generate a depth impression from 2D images by presenting slightly different images to each eye.
Victorian Era Views
  • Early theory: Depth perception was established after working out the form of elements in each eye separately and then combining the information.
  • Stereograms with random dots challenged this view.
  • Modern view: Depth is worked out first, and then the shape of the area at that depth is resolved.

Dynamic Cues

  • Motion parallax

Accommodation

  • The focus of the eye also gives a cue to depth.
  • To focus on close objects the lens shape changes and you can feel that.
  • Accommodation, then, is a further cue.

Stereopsis

  • A powerful cue that arises because of the left/right difference in the position of the two eyes.
  • Stereopsis only works for relatively close distances. Beyond three meters, we rely on other cues.