3. Depth Perception & Binocular Vision copy.docx

CRITICAL READING: CORNELL NOTES Depth Perception & Binocular Vision Name: Date: 1 August 2023 Section: Lecture 3 Period: Questions/Main Ideas/Vocabulary Notes/Answers/Definitions/Examples/Sentences Depth Perception The real world is 3 – dimensional. Horizontal (x), vertical (y),...

CRITICAL READING: CORNELL NOTES Depth Perception & Binocular Vision Name: Date: 1 August 2023 Section: Lecture 3 Period: Questions/Main Ideas/Vocabulary Notes/Answers/Definitions/Examples/Sentences Depth Perception The real world is 3 – dimensional. Horizontal (x), vertical (y), depth (z). But the image cast onto our retina is only 2D. Horizontal, vertical How do we judge the depth coordinates of an object? Computational goals of depth processing: Depth order: which object is nearest/furthest? Depth intervals: how much further is one object than another? Absolute distance: how far away is that object from me? Estimates of three-dimensional object shape/geometry. Depth Cues Multiple images cues to depth: Monocular Binocular Sensorimotor cues to depth: Accommodation Vergence Many cues help with depth order/depth interval. But only a few code absolute depth Monocular Cues Interposition: Provides order information, but not depth interval. Atmospheric perspective: Cue to depth order and interval. Texture gradient: Perspective (width) Compression (height) Density Cues change rapidly when close. Can be used to infer depth order, interval and 3D shape. Shadows: Can also be used to infer depth order, interval and 3D shape. Visual Angle The size of the image projected onto the retina. Measured in degrees of visual angle: Each degree can be divided into 60 minutes (60’). One minute can be divided into 60 seconds (60’’). At a distance of 57cm (arm’s length), 1cm = 1.0°. Depends on the size of the object and viewing distance. 10c coin = 2.36 diameter 5cm = 26.6°, 57cm = 2.4°, 100cm = 1.4° Moon = 3476 km diameter @ 378,000km = 0.53° 10c coin would be 0.53° when seen at 225cm Size Constancy Objects don’t appear to change size when viewed from different distances. The closer an object is, the larger image it casts on the retina. The brain corrects for this by using distance size cues. Factors Contributing to Ames Illusion Floor and ceiling tilted. Room appears square as do furnishings. Top-down processing “Rooms are level” Ponzo Illusions Exploit monocular depth cues of texture gradients/size constancy. Motion Parallax What about moving images? Relative motion is created when we move, Close objects move most, far objects move the least. We can use this cue to judge the depth of an object. Sensorimotor Cues to Depth We can also gather information about depth based on internal feedback. Accommodation Lens is stretched or relaxed to bring a target into focus. Ciliary muscles pull or push the lens. Strain on the lends is sensed by the visual system and can be used to calculate distance of an object. Accommodation Allows One to Change Focal Length Distant points: The muscles are relaxed, and the lens is at its thinnest. Near points: The muscles contract, and the lens becomes thicker. Accommodative range only covers fixations from about 15-300cm. Accommodation & Ageing As we age, the lens becomes less flexible why we get glasses. With less flexibility, it becomes harder to change focal length. Hyperopia: Refractive power is too weak. Near objects are de-focused. Myopia: Refractive power is too strong. Far objects are de-focused. Monocular Cues to Depth Image based cues: Interposition, atmospheric perspective, texture gradients, shadows, size constancy. Motion based cues > motion parallax. Sensorimotor cues > accommodation (also cues absolute depth). A Blind Spot The axons of the retinal ganglion cells leave the eye through the optic nerve, on their way to the LGN. There are no photoreceptors here. Binocular Vision Interpupillary distance = 63 mm (average). Overlapping fields of view for each eye. Two spatially displaced samples, yet one view of the world. Binocular depth cues require information to be combined from the two eyes. Vergence Binocular disparity Vergence A binocular, sensorimotor cue to depth. Angle of gaze of two eyes. Eyes can be turned towards each other or turned outwards. Again, allows one to change focal distance. The closer the point of fixation is to your eyes, the greater the angle of convergence will be. Binocular Disparity Difference in the horizontal retinal position of an image point in one eye relative to the other. Stereo vision (stereopsis). Some neurons in the visual cortex are sensitive to disparity. Receptive fields are slightly different in each eye. Disparity (Near & Far) Thresholds for Disparity (Stereo) Judgements are Comparable to Paired Width Judgements Suggests that the brain is using a similar “code: to judge the position of two locations in space, even though one is 2D and the other is 3D. Random Dot Stereograms Developed by Bela Julesz in the 1960s. Present two different images to the left and right eyes (using a stereoscope). The images are identical, apart from a small section of dots that have been displaced horizontally. Direction of shift determines whether you create a near or far disparity. Threshold is ~6’’ (1/60th cm at arms length). Disparity Can Also be Used to Simulate Depth You can see depth from disparity without any other cues present. Useful for testing stereo vision. For virtual reality and augmented reality. For movies. Binocular Disparity in Virtual Reality People use a variety of monocular and binocular cues as they interact with their environment. Virtual reality relies heavily on one cue. Binocular disparity Issues: 30-40% of people have some impairment to stereo vision. Contradiction between various depth cues can lead to cybersickness. For example, incongruence between disparity and vergence/accommodation. Strabismus Strabismic amblyopia is due to the misalignment of one eye – often caused by different muscle tension in each eye. Exotrope: Outward pointed eye. Esotrope: Inward pointing eye. Often treated with early surgery. Uncorrected: Binocular cortical neurons shift to monocular cues. Leading to stereo blindness. What is Stereoscopic Vision Good For? Fine motor control.