Vision Overview 18 September 2006

Eye: Musculature

The eye and retina

Eye: Internal Structure 2/3 focusing in first ½ mm Flexible lens fine tunes focus Myopia focuses is before retina Presbyopia focuses behind retina Cataracts are opaque flaws in lens Iris varies from 2-8mm About 17mm from lens to retina Curved image surface

Retina 120e6 rod cells (scotopic vision) 6e6 cone cells (photopic vision) Sensors operate by polarization of proteins by photons Produce a “pulse train” with rate proportional to log of intensity

Rods and cones density in the retina

Blind Spot

Eye and Brain About 1e6 nerves leave each eye Clearly a huge reduction has already been done Parts of visual field “cross over” for stereo processing Huge part of brain is devoted to processing Eyes best thought of as an extension of the brain

Retina Layers Light enters on the left! Sensors connected together into receptive fields Lateral inhibition Edge detectors, shape detectors, motion predictors Model with convolution

Dynamic Range 11 orders of magnitude! Single photons when dark adapted! Scotopic in the dark Photopic in the light Maybe only 20 levels at one point Maybe only 1000 levels at one average brightness

Visual Performance 20:20 vision corresponds to 1 arc minute Fovea: 20 minutes max density uniform, 2 degrees “rod-free” area 400 to 700 nanometer wavelength 510nm maximum rod sensitivity (green) 560nm maximum cone sensitivity (orange)

Contrast Sensitivity

Acuity away from center

Macular degeneration

A Model of Human Vision with Limited Feedback

Retinal sensors, preprocessing through LGN to V1 Receptive fields –Spatial and temporal derivatives Spatial derivatives are oriented –Other than blob sensors (Laplacian) »Intensity boundary sensors ( G u ) »Bar sensors ( G uu ) –Spatial scales (aperture size;  of Gaussian)

Receptive Fields

Combining Receptive Fields Receptive Fields are combined in many ways to produce orientation sensitive detectors

Temporal Aspects of Vision Object motion Egomotion Flicker –Minor nit with article text: When a monitor refreshes at 60Hz it flickers at 60Hz. But a conventional incandescent light driven by 60Hz AC flickers at 120Hz. Why? Saccades

Motion Detection

Perception of Brightness Affected strongly by boundariness signals of form system Determined by relative intensity changes –Weber’s law: just noticeable difference constant  I/I –Averaging within boundaries –Enhancement/sharpening at boundaries Mach effect

Weber’s Law Just noticeable differences

Mach Effect

“Simultaneous Contrast” -- Brightness Determined by Relative Luminance

Discontinuity effects

Adelson’s Shadow

Hermann grid

Perception of Color Trichromatic color mixtures Intensity + 2 chromanences –Red-green (R/(R+G)), Blue-Yellow (B/(R+G)) Affected strongly by boundariness signals of form system Determined by relative chromanence changes –Averaging within boundaries –Enhancement/sharpening at boundaries

Spectral Sensitivity

Opponent Color Model

Color blindness

Color Constancy Volunteers needed to reproduce Land’s Mondrian Experiment

What is this?

How about this?

After-Effects Stare at the center dot then switch to the next slide

After-Effects

Form System Suggests Continuations

Both inputs and expectations drive what you see

Seeing in 3D Static monocular cues –Occlusion –Relative height –Familiar size –Texture gradient –Shadow –Linear perspective Motion parallax Stereo Oculomotor feedback –Vergence –Accommodation

Stereo Random dot stereogram from Foundations of Cyclopean Perception by Bela Julesz Where must stereo correlation happen?

Types of 3D display Nice 2D rendering –perspective –shading –relative size –occlusion

Stereo Display Color Polarization Shutter Autostereo

Head-Tracked Stereo Display Stereo using any of the earlier methods Track head and update image Otherwise, object appears to warp as you move your head

Barrier Autostereo

Lenticular Autostereo

Time Multiplexed Autostereo

Head-mounted Display Present images directly to eyes Track head Update images with motion

Reimaging Display

Varifocal Mirror

Volumetric

Holograms

Pulfrich Effect Video

Volunteer opportunities Edwin Land’s Mondrian experiment (Kyle) Bela Julesz random dot stereograms Varifocal mirror

Will this be on the test? Yes! Anything is fair game But what I emphasize is more likely Unlikely: How do reimaging displays work? Likely: Apply Weber’s law to some thought experiment.