PSYCH 3420 Prelim 1
Explain why a negative lens helps a myope to see better.
- Eye is too long and image falls short of retina when object is far away (image is focusing too far forward) - Negative lens decreases extra power and increases focal length by diverging rays of light so they focus further back in the eye, at the retina
What is presbyopia? Can you avoid it? What does this do to the near and far points?
- Loss of lens accommodation with age - Can't avoid it - Near point increases, far point decreases
What is astigmatism?
- Occurs when the cornea (clear front cover of the eye) is irregularly shaped or sometimes because of the curvature of the lens inside the eye and causes blurred vision - An irregularly shaped cornea or lens prevents light from focusing properly on the retina, the light-sensitive surface at the back of the eye. Vision can become blurred at any distance, leading to eye discomfort and headaches - Frequently occurs with myopia and hyperopia (refractive errors)
Describe four applications that use eye tracking in virtual or augmented reality.
1. Foveated rendering 2. Giving a speech 3. More walking space: Basically shifting the room every time you saccade 4. Aim object by the eye (e.g. move a gun).
Describe the steps for producing anti-aliasing on a computer screen. Why is this done and what does it achieve?
1. Take high resolution images 2. Blur (remove frequencies before aliased) 3. Subsample Removes artifacts from normally sampling an image and creates an image an image that appears to have better resolution.
Describe the roles that the cornea and lens play in focusing an image.
Background: - Cornea: fixed power of ~40 diopters - Lens: variable power of 10-22 diopters (at young age) The lens focuses light through the vitreous humor, a clear gel-like substance that fills the back of the eye and supports the retina. The retina receives the image that the cornea focuses through the eye's internal lens and transforms this image into electrical impulses that are carried by the optic nerve to the brain. - The cornea functions like a window that controls and focuses the entry of light into the eye - By changing its shape, the lens changes the focal distance of the eye - it focuses the light rays that pass through it (and onto the retina) in order to create clear images of objects that are positioned at various distances
Hyperopia
Farsightedness, difficulty seeing objects that are up close
What is the illusion of competence?
Heavy multitaskers are often extremely confident in their abilities but there's evidence that those people are actually worse at multitasking than most people.
V lambda curve
visibility as a function of wavelength; scotopic light shifted left to bluer colors and photopic light shifted right to brighter colors
blindspot
spot in periphery lacking photo receptor cells caused by optic nerve; brain fills in information so we don't notice this
In terms of the power of the lens and cornea, describe the difference between someone near-sighted and far-sighted. Describe how lenses help these people.
- Myopia: too much power (eye is too long and image falls short of retina when object is far away) - Presbyopia: lens hardens with age and there is no known cure - Hyperopia: not enough power (eye is too short and the image focuses behind the retina if the object is near) Concave lenses are used for myopia -Spread light out before it reaches the convex lens in the eye, therefore letting the image focus directly on the retina Convex lenses are used for hyperopia -Focuses light in addition to the natural lens and makes the image focus on the retina
Why would late night stores blast 17,000 hertz tones?
-A frequency that mostly only teenagers can hear, this will deter underaged youths from going to such late night stores
What does the CSF say about the differences between the visual system of the infant and the adult?
-Contrast Sensitivity Function (CSF): Describes the capacity of the visual system to recognize differences in luminance (stimulus per degree) -Infants have low contrast sensitivity, grow into it (scotopic = 1 month, mesopic = 1 year, photopic = adult) (Acuity drops) -(But contrast sensitivity at medium and high spatial frequencies decrease with aging, beyond adult)
What are the conditions that produce the appearance of self-motion for a human observer? What was wrong with the drum studies? How would you design a perceptually 'efficient' automobile simulator if the observer limited his/her direction of view to straight ahead?
-Expansive motion (when you walk toward something everything expands) -Motion in the periphery -What produces best self-motion → linear in the periphery -Second best self-motion → radial or expansive flow in center of periphery -Linear flow in fovia doesn't work well -Wrong with Drum Studies: linear movement in parallel (never actually measures expansive radial) -How to Deisgn Automobile Simulator: -linear self-moving -Don't need high-res -lights in periphery to make it feel like motion
Describe the differences between efficient, inefficient and restrictive display systems.
-Limited/Restrictive: below the sensory limits of the visual system. -Efficient: Information matched to the limits of the sensory system. -Excessive/Inefficient: more information than the sensory system can handle. Not all displays need to be efficient; certain images can capture our attention could be above or below limits of visual systems (probably shouldn't be inefficient though)
Are natural scenes redundant? Describe 3 forms of statistical redundancy.Why is this important for understanding compression? Describe how the visual system takes advantage of this?
-Nearby points are highly correlated -If there is a difference in neighboring pixels, its probably an edge, edges are sparse -Sparse structure show continuity across space (edges are continuous) -Can compress -Won't look at nearby points, will look for the difference -Will know that edges will be sparse
What does it mean when we say that the visual system uses a "sparse" representation?
-Our visual system process natural scenes by finding their sparse structures -A high proportion of neurons show activity near zero, a lot of potential neurons that could fire due to stimulation, but only a small portion of them get fired
Who was Frank Rosenblatt and what is the Perceptron? What can a Perceptron learn and not learn? How is this different from 'deep' learning techniques?
-Psych Professor at Cornell and invented the Perceptron. -Linear classifier; can only draw line between two categories; if separable this way, then perceptron can solve it -Cannot do XOR or if they are "wrapped up in each other" -Different than Deep Learning: one has more layers (deep learning has more) perceptron has one layer?
What is cortical magnification? How does this relate to the eye chart of Anstis with letters of varying sizes as a function of eccentricity? (W)
-Where the number of neurons in the visual cortex responsible for processing the visual stimulus varies as a function of the location of the stimulus in the visual field ----I.e. Number of neurons processing image location of the stimulus -Chart of Anstis: When focused at the center, you can both clearly read the small letters as well as the large periphery ones, because less neurons are necessary to process larger stimulus in the periphery. Don't work when sizes are flipped since there aren't enough neurons to process the small letters
What is dithering? Why is it effective perceptually?
-high resolution with low intensity levels (just creating different densities of dots) -All colors are approximated by colors available from the palette in the intensity levels -Visual system averages all the colors (greys), it will treat the places with dots equivalently, as if it were filled
Name 4 reasons we may want to convert a non-visual data set into a visual data set (W)
1. Ability to comprehend large amounts of data 2. Perception of emergent properties that were not anticipated 3. Enables problems with data itself to become immediately apparent 4. Facilitates understanding of both large scale and small scale features of data 5. Facilitates hypothesis formation
Why is skin so hard to model in computer graphics (two reasons).
1. Bidirectional Reflection Function (BDRF): Need to use this because skin has complex reflectance function. Different layers of skin reflect light differently and sinse we are very familiar w/ skin, we are very sensitive to any faults. - BRDF is important because (1) many surfaces (e.g. faces) have complex reflectance functions and (2) the human visual system uses this function to determine material properties 2. If skin is too realistic in computer graphics, it approaches the Uncanny Valley
Describe the four stages of information visualization. What is the role of feedback in these stages (W)
1. Collection & storage of data - Feedback from human perceiver who gathers data (following up on interesting observation) 2. Preprocessing, transform data into something we can understand - Feedback from human perceiver (transform data to get meaning) 3. Display hardware & graphic algorithms produce image on screen - Feedback from human perceiver (i.e. interacting with graphic engine) 4. Human perceptual & cognitive system (perceiver processing) - Gives feedback to every other part of cycle
Describe 5 ways to make a spot appear to move.
1. Move it 2. Adapt to motion then observe the dot (motion aftereffect): Caused by an imbalance of neurons responding to motion (fatigue), so neurons will still think image is moving 3. Autokinetic: stare at a stationary point of light → eye fatigued → eye moves → perceived movement in space 4. Induced Motion: move the reference frame (eg. Watching the train leave on the next track over, who is actually moving?) 5. Strobscopic: When the stimulus is viewed in separate stages (flash lights to only show certain frames), can cause the appearance of motion
Describe the three primary stages of perceptual processing according to Ware (W).
1. Parallel processing to extract low-level properties of visual scene - Rapid parallel processing - Extraction of features, orientation, color, texture, and movement patterns - Transitory nature of information, which is briefly held in an iconic state - Bottom-up, data-driven model of processing - Serving as the basis for understanding the visual salience of elements in displays 2. Pattern perception - Slower serial processing - Top-down attention being critical to the formation of objects and patterns pulled out from the feature maps - A small # (1-3) of patterns becoming "bound" and held for a second or two under top-down attentional processes - Different pathways for object recognition and visually guided hand motion (the perception and action channels) 3. Visual working memory Direct from textbook: - Stage 1: information is processed in parallel to extract basic features of the environment - Stage 2: active processes of pattern perception pull out structures and segment the visual scene into regions of different color, texture, and motion patterns - Stage 3: the information is reduced to only a few objects held in visual working memory by active mechanisms of attention to form the basis of visual thinking
For a 1 meter wide television with 4000 pixels across, how far away should it be, to be at the highest visible resolution (assume 100 pixels/deg PPD).
100 PPD is the gold standard for resolution 4000/theta = 100, theta = 40 degrees How far back do we need to be to get a visual angle of 40 degrees? 1.37m
Provide a simple map of the pathways that take visual information from the eye to the brain. Include a discussion of the left and right visual fields and the superior colliculus.
90% of optic of LGN projects to the cortex and 10% to structures like the superior colliculus - Opposite sides mapped - Superior colliculus: direct behavioral response to visual stimuli (i.e. if someone hits a baseball you naturally follow it with your eyes) Start w/ two eyes and two halves of the brain: right visual field projects onto left side of BOTH eyes to the left side of brain; left visual field projects onto right side of BOTH eyes to right side of brain -> goes through LGN and some information goes off to the superior colliculus
Plot the relationship between scotopic and photopic spectral sensitivity and use this plot to explain the Purkinje shift and why red light or red goggles are often used under scotopic conditions.
<two sinusoidal curves with leftmost being scotopic and centering on wavelength = 505 at intensity of 1 while rightmost = photopic and centering on wavelength = 555 at intensity of 1> -Scotopic: using only rods to see (objects are visible but only in black and white) -Photopic: using cones to see and provides color -Purkinje Shift: As lighting conditions dim, the relative proportions of rod and cone contributions shift, resulting in the luminance of colors changing (reds become darker, blues become brighter). In scotopic conditions, red light helps activate the cones that aren't otherwise visible.
Are abstract visual representations based only on cultural knowledge? Explain (W)
Abstract visual representations have some grounding in cultural knowledge based on different cultures' unique perceptions of different things, but they aren't only based on cultural knowledge; it is possible to have a new semiotics based not on philosophical claims for symbols being arbitrary, but instead on scientific evidence. Deregowski: people in Zambia able to match pictures of toy animals to real animals Hochberg and Brooks: raised daughter without pictures/social input as to what pictures represented. She was still able to identify line drawings
For virtual reality glasses with a visual field of 180 degrees, how many pixels across are needed to be at the acuity limit. If the display is binocular with 120 degrees in each eye, how many pixels are required for each eye?
Acuity Limit: 180 degrees * 100 PPD = 18,000 pixels Per Eye: 120 degrees * 100 PPD = 12,000 pixel for each eye
How does one measure the contrast sensitivity function (CSF) of a human observer? Describe the contrast sensitivity function differences in adults and 3 month olds. What does this imply for information presented to children on a display?
Adjust a spatial frequency grading until you can't see it, adjust contrast, reduce until you can't see it. 3 month olds do not have acuity of an adult so they won't pick up on small details
Why is accommodation a problem for virtual reality. Why is this not a significant problem for those older than 60?
All current virtual reality headsets have a fixed focal length. This can create problems for those younger than 50 because they try to accommodate to the appropriate distance. Those older than 50 can't accommodate.
What is the Hermann grid and what is the current explanation?
An image with black boxes and white lines in between each box such that when you look at the image, you see block dots in the intersection of each black box but they aren't really there. Current Explanation: the center/surround cells that signal the white of the intersections are more suppressed than are the center/surround cells that signal the white of the lines between the intersections.
Provide an example of a stage in visual processing where there is considerable compression (hint the optic nerve). What is the magnitude of the compression?
An optic nerve has ~125 million rods and cones converging on 1.25 ganglion cells sending 1.25 million fibers down the optic nerve in a 120:1 convergence.
Describe 2 similarities and 2 differences between the processing of objects along the visual pathway and the processing of objects by deep networks.
Background: - Deep learning (also known as deep structured learning, hierarchical learning, or deep machine learning) is a branch of machine learning based on a set of algorithms that attempt to model high-level abstractions in data by using a deep graph with multiple processing layers, composed of multiple linear and non-linear transformation. - Deep neural network: 1. Begin w/ a massive amount of labeled data 2. Feed that into the network 3. Check if it gave you the right answer 4. Correct the network by "back propagating" the errors - This is NOT the way brains learn. Visual systems are much the same across primates: learn w/out labeled data. Similarities: - Both systems are fairly hierarchical (multiple-range) - Very similar in early stages (e.g. first stages detect edges) Differences: - Deep networks 1. Supervised 2. Massive amounts of labeled data 3. Requires thousands of examples 4. Feed forward after learning - Mammalian visual systems 1. Largely unsupervised 2. Very little labeled data for learning 3. Requires a handful of examples 4. Massive feedback recurrent
Provide two examples that demonstrate that shadows play a role in the interpretation of objects.
Background: - For a given surface, an observer must decide how much of the luminance is due to the illumination and how much is due to the reflectance Examples: 1. W/out shadow teapot looks like it's floating in space (shadows can help determine the location of an object: attached v. floating) 2. Changing shadow makes basketball look like it is bouncing, sliding, floating, etc. 3. Google logo: shadows can help determine the layout of an object
Explain the contrast sensitivity function under scotopic, mesopic and photopic conditions.
Background: - How far away can I recognize someone's face in daytime? Not good reliable eyewitness if saying they recognized someone across from a parking lot at 7pm - The contrast sensitivity function has the potential of adding more information about the functioning of the visual system than that given by visual acuity, because it assesses sensitivity over a wide range of spatial frequencies, while visual acuity measures primarily sensitivity at the high spatial frequencies Acuity and sensitivity drop as you go into darkness (photopic -> mesopic -> scotopic)
Under dim conditions (e.g., finding a star at night), it can be useful to look two or three degrees off the target. Why?
Background: - Rods: scotopic vision; peripheral - Cones: photopic vision; foveal Looking right at the star uses the fovea, which would make it too dim. Looking two or three degrees off the target activates the rods enough so that acuity doesn't drop; place the image of the object on the part of the retina that is rich in rods.
What will be perceived as brighter? A) A 100 watt lamp that produces most of its energy in the range of 500 to 540 nanometers or a 100 watt lamp that produce most of its energy between 650 and 700?
Background: - Scotopic vision: dim light (peaks at 500 nm) - Photopic vision: bright light(peaks at 550 nm) Answer: - 500-540 nm light because photopic vision peaks at 550 nm - Radiometric (watts): physical, total energy, whether visible or not
Describe two aspects of virtual reality that can make us sick.
Background: - Simulator sickness: Conflicting cues from visual system and vestibular system of inner ear; causes nausea. Don't expect participants to move heads from side to side while wearing a helmet-mounted display. Contributing Factors: 1. Active conflict - action does not equal vision; induced self-motion not in agreement between visual and vestibular systems 2. Update rates not high enough (slow computers) - slower than what the brain processes and causes a discord between processing and refreshing rate 3. Poor resolution - doesn't keep pace with user's head movements (expectations violated) 4. Frame rate 5. Gender (women more likely) 6. Experience w/ VR
Provide a rough estimate of the spatial frequency of the grating shown on the blackboard (in cycles/deg) and explain how you arrived at this estimate. Approximately, how many lines would there need to be to approach your acuity limit? (also tell us where you are sitting).
Background: - Spatial frequency: acuity decreases for high-frequency patterns as you increase distance. Also depends on high contrast vs low contrast. - Acuity limit: around 50 cycles/degree, or 100 cycles/degree under best conditions For a 4K display, at what distance do you need to sit for it to be at your acuity limit: - 4000 pixels/X degrees = 100 pixels/degree - x = 40 degrees - tan(theta/2) = (size/2)/distance tan(20) = (size/2)/distance distance = size/(2*tan(20)) - For a 1 meter screen distance = 1/(2*0.36) = 1.38 meters Actual answer (maybe): - 1 thumb width = 2 cycles/degree - Acuity limit = 50 cycles/degree, which is 100 lines (2 cycles/degree) = 200 lines to reach acuity limit
Suppose that you have a near point of 10 cm and a far point of 25 cm. a. What is your condition? (e.g., hyperope?) c. How should this person be corrected? d. After correction where will this person's near point and far point be located?
Background: - np = 0.1 meters -> 1/0.1 = 10 diopters of EP - fp = 0.25 meters -> 1/0.25 = 4 diopters of EP - At about 20 you have 12 D and at about 60 1 D; person is 40 A. Myopic, but also presbyopic because range is going down B. 10 D - 4 D = 6 D C. We want far point to be at infinity. -4 D lens to correct. (Aim for 0 D because 1/0 = infinity.) D. After correction, have accommodation range of 0 to 6, gives far point at infinity, near point at 1/6 D = 16.7 cm. This person can't see beyond 0.25 D (concave lens w/ 4 D too much power; give -4 D lens to get to 0). - NEW NP = 1/6 D = 0.17 cm - NEW FP = 1/0 D = infinity
Are heavy multi-taskers good or bad at multitasking? Explain.
Bad. The heaviest multitaskers are the most easily distracted. Those that rate themselves as excellent multi-taskers are those that perform the worst in an environment with distractions. There is an illusion that we feel we are processing both streams of information but evidence suggests that we process less total information than when processing a single stream. This is because there is a switching cost when alternating between tasks that slows down the about of information processing.
Ware uses the concept of 'Brain Pixels' to describe the efficiency of a display. What is a "Brain Pixel" and how might it be measured? What is an efficient system by his measure? What typically happens to efficiency with increasing display size?
Brain Pixels: image units used by brain to process space - Retinal ganglion cells best capture the brain pixel idea - Receptive field size = 0.006(e + 1) At 10 degrees of eccentricity, w/ the big screen there is an approximate match between screen pixels and brain pixels. Even though a conventional monitor covers only about 5 to 10% of our visual field when viewed normally, it stimulates almost 50% of our brain pixels. This means that even if we could have very high-resolution large screens, we would not be getting very much more information into the brain.
How does the early visual system compute ratios? Why is this a good thing?
Center-surround of Ganglion cells: inhibited and excited, compute ratios with log (Ic) - log (Is) = log (Ic/Is) This is a good thing because: - Cones have compressive relationship (log of intensity). By having cones behave w/ compressive response create ratio. - Ratios help preserve light constancy. Ratios will stay the same even with added illumination.
Explain why the dynamic range of a display depends on the amount of stray light. Provide an example.
Dynamic range: ratio of brightest pixels to darkest pixels. Light reflected back onto the screen decreases the dynamic range. Thus, not always good to have very bright and very HDR displays. E.g. Screen w/ 1,000:1 range. 1% light reflected back onto screen. Increases range to 1,010:11 = 100:1.
Provide a brief explanation of the Cornsweet illusion (W)
Edge enhancement: Lateral inhibition can be considered the first stage of an edge detection process that signals the positions and contrasts of edges in the environment. One of the consequences is that pseudoedges can be created; two areas that physically have the same lightness can be made to look differently by having an edge between them that shades off gradually to the two sides. The brain does perceptual interpolation so that the entire central region appears lighter than surrounding regions. This is called the Cornsweet effect. In the Cornsweet illusion, the region adjacent to the light part of the edge appears lighter, and the region adjacent to the dark part of the edge appears darker. In fact, they have the same brightness.
Calculate the visual angle of your thumb at arms length, the screen in front of you and your phone. Show your calculations.
Equation (large field displays): tan(theta) = size/distance theta = tan-1(size/distance) Equation (small field displays): tan (theta/2) = (size/2)/distance Step 1: measure arm length (D), and thumb length (S); triangulation Step 2: count # of thumbs across Step 3: calculate, where theta is give in degrees (2 to 3.25) Example: Square w/ lines is about 2 degrees across (measured with thumb). How many lines required to be at resolution? 2 x 100 = 200
How does the visual system factor out the amount and color of the illumination when making lightness decisions? (W)
First adaptation- changing sensitivity of the receptors and neurons in the eye helps factor out the overall level of illumination Contrast mechanisms- help achieve consistency by signaling differences in light levels, especially at the edges of objects
Describe four differences between foveal and peripheral processing.
Fovea = cones; Periphery = rods 1. Sensitivity to light: periphery is more sensitive to light 2. Wavelengths you can see: fovea is photopic (specialized for day vision) peak at 555nm; periphery is scotopic (specialized for night vision), peak at 505 nm Purkinje shift 3. Acuity: Acuity decreases rapidly as you move from fovea 4. Response to flicker: Periphery more responsive to flicker
What is foveated rendering? What are the advantages over standard rendering?
Foveated rendering: technique to put high resolution rendering (graphical detail) where the observer is looking; in foveated rendering, only have high definition in thumb area Reduces computations needed for rendering in VR but requires that the eyes can be tracked
What is the gamma of a display? Roughly what is the gamma of your visual system? How does the gamma of your visual system affect the apparent luminance of an intensity ramp? How does gamma of a digital camera and a screen help in the compression of images.
Gamma: distribution of its intensities, given by output = input^gamma (luminance = voltage) The visual system has a gamma of about 0.5 Gamma turns intensity graph into a nonlinear square root function. This compressive non-linearity reduces range of responses needed to represent an input w/ a large range of intensities (brightness = intensity^0.5). In other words, in bright regions, the visual system needs larger intensity changes to be visible. These large step sizes allow for good compression (digital images). Images are then displayed w/ a gamma of 2 to allow for a linear relation between input and output. Monitor gammas are intentionally non-linear to cancel out visual system gammas to make the monitor luminance appear to change linearly.
What is high dynamic range (HDR) imaging? How does this approach increase the apparent range of an image?
HDR imaging - capturing HDR images and creating images that look HDR on LDR displays (take pictures w/ various exposures and then use tone mapping to re-map luminance to screen's range, maintaining contrast across various areas of the image) HDR display has a huge range - efforts to stretch out apparent range by increasing contrasts can sometimes make images appear garish; can also increase range of luminances produced by display 1. Creating images w/ higher dynamic range 2. Displaying those images on displays w/ high dynamic range 3. Displaying those images on low dynamic range displays - but making them appear more like high dynamic range
Explain the illusions created by the Hermann Grid and Chevreul/Mach bands in terms of receptive field spacing. Diagrams would be useful here. (W).
Hermann Grid: more inhibition at the spaces between two squares, so they seem brighter than the regions at the intersections; the black spots that are seen at the intersections of the lines are thought to result from the fact that there is less inhibition when a receptive field is at the corner of four squares than between the edges of two squares (DoG model of concentric opponent receptive fields) Mach Bands: at the point where a uniform area meets a luminance ramp, a bright band is seen; appear where there is an abrupt change in the first derivative of a brightness profile Chevreul Illusion: when a sequence of gray bands is generated, the bands appear darker at one edge than the other, even though they are uniform; can be simulated by the application of a DoG model of the neural receptive field
Distinguish between hyperopia, myopia, and astigmatism including thesymptoms, causes, and possible remedies of each.
Hypermetropia/Hypermyopia - Symptoms: not being able to see things up close - Causes: eye is shorter than normal, cornea not curved enough, lens sits too far back; doesn't have an environmental cause, so genetic - Remedies: glasses with positive correction Myopia - Symptoms: not being able to see things far away - Causes: eye elongated; close up work, not enough bright light, genetic predisposition - Remedies: get progressive glasses to slow down progression, get negative corrective lenses Astigmatism - Symptoms: distorted vision due to eye being misshaped, light rays scattered - Causes: irregularly shaped cornea/lens - Remedies: corrective lenses/refractive surgery
What will be perceived as brighter? B) A 100 nit lamp that produces most of its energy in the range of 500 to 540 nanometers or a 100 nit lamp that produces most of its energy between 650 and 700?
If question was asked w/ nits: - Both would be perceived as having the same brightness - Photometric (candelas/meter^2 = nit): takes visual sensitivity into account
What is the relation between lens flare and perceived brightness. Why does this work?
Imperfections in lens creates a spray of light that the visual system uses as a cue to determine brightness by measuring how wide the flare is; more lens flare = brighter world This works because we are not good at perceiving high levels of brightness intensity, so instead we perceive beyond a certain threshold as the light spreading out/being wider, hence the lens flare
What is the simplest model of surface texture that creates the perception of surface shape from shading? How is this used to shade a sphere with one light source? What are two additional effects that improve how the shading looks? (W)
Lambertian shading: if we have a perfectly matte surface, how bright the surface appears depends only on the cosine of the angle between the incident light and the surface normal Lambertian shading can be used w/ specular, ambient, and cast shadows - Specular: the light that is reflected directly from a surface (highlights on glossy objects); angle of reflection = angle of incidence - Ambient: ambient light is the light that illuminates a surface from everywhere in the environment, except for the actual light sources - Cast: an object can cast shadows either on itself or on others objects; can greatly influence the perceived height of an object
What role does lateral inhibition play in lightness constancy?
Lightness constancy: observers can usually identify reflectance (brightness) of material despite changes in illumination which helps visual system factor out color and amount of illumination and signals differences in light levels especially at edges of objects - As overall illumination increases, ganglion cells more excited but also more inhibited by neighbors, creating constant perception of brightness - When looking at a uniform field, receptive field measures difference between center and surround (ratio) rather than absolute values and this ratio stays the same
Explain Weber's law regarding lightness differences. How much more luminance does a patch need to have to stand out from the background? (W)
Lightness differences in ratios: 10 vs 100, 100 vs 1000, constant ratio => similar difference
Do a news search for "eye tracking" (e.g., google news). Describe 3 recent applications of eye tracking.
Methods of tracking the eyes: 1. Contact lens 2. Infrared sensing (head mounted) 3. Electro-oculograph -Records eye movements by measuring small electrical charges with tiny electrodes attached to the skin at the inner and outer corners of the eye (sleep research, low accuracy, but unobtrusive) 4. Image processing techniques -Allows head movements. Typically overlays image with eye movement. Recent applications: 1. Medicine - reveal abnormalities in eye functioning and to conduct cognitive studies focused on learning about peoples' emotions and intentions 2. Retailer H&M is using the new Tobii Pro Sprint to better understand how online shoppers navigate their website. Set up participant w/ program and use it to record participant's eye movements. The recording shows where the user's gaze falls, potentially revealing which design features attract their attention, which they ignore, and which ones confuse them. 3. Piloting a drone - system can understand both the location and orientation of the drone and its pilot (takes instructions based on user's orientation). Tobii Pro's Glasses 2 used to track eye movements of pilot. Glasses are plugged into an NVIDIA Jetson TX2 CPU and GPU and deep neural network takes the incoming images from the glasses, crunches the numbers, and is able to calculate how far away the drone is based on its perceived size. From there, it's just a case of gazing at your chosen location and the glasses will translate that data into a vector for the drone.
Why doesn't an image normally disappear if I fixate very accurately at one point? What does one have to do to make it disappear?
Neurons in visual system only change in order to keep firing. If they get exactly the same stimulation in the same pattern, they will stop firing. You cannot make your eyes stay still - will normally give small movements or tremors (edge). You need to stabilize something very accurately on the retina (e.g. retinal blood vessels - don't see them because they move with the eye) or use a blurry object with small movements that don't create a large change in the image and thus doesn't produce much change to neurons.
Describe three cues that observers use to discriminate lightness from brightness.
Penumbra: Light sources are not point sources. Shadow produces a little bit of blur at the edge and that is the penumbra while the dark shadow is called the umbra. Multiplication across borders: Ratios of intensity maintained along illumination borders. For a given surface, an observer must decide how much of the luminance is due to the illumination and how much is due to the reflectance. If the ratio of luminances is constant across the luminance boundary, we perceive this as an illumination boundary. Co-planar hypothesis: Surfaces in the same plane are assumed to have the same illumination without other cues that signal a shadow. The "perceived" illumination (brightness) changes the interpretation of the perceived lightness (reflection).
Describe the relationship between stimulus intensity and intensity of perception that Fechner and Stevens found. How does this relate to the monitor gamma? (W)
Popularized technique known as magnitude estimation to provide a way of measuring the perceptual impact of simple sensations - S = al^n - This law states that perceived sensation S is proportional to the stimulus intensity I raised to a power n. - Perceived brightness = luminance^n The relationship of physical luminance to the input signal on a monitor is approximated by a gamma function: L = V^gamma where V is the voltage driving one of the electron guns in the monitor, L is the luminance, and gamma is an empirical constant that varies widely from monitor to monitor. This monitor non linearity is not accidental; it was created by early television engineers to make the most out of the available signal bandwidth. They made television screens nonlinear precisely because the human visual system is nonlinear in the opposite direction, as Stevens had observed.
Roughly, for a single image, what is the range of intensities found in the real world, hard copy (e.g., magazines)?
Real world - 300:1 indoors; 30,000:1 outdoors Modern LCD televisions - 5,000:1 Hard copy (oil paintings and drawings) - 30:1
What is saccadic suppression? How does this help with foveated rendering? How can the loss in sensitivity be used to increase the size of a virtual room?
Saccadic Suppression: Acuity drops tremendously during eye movement to block out the blur → large changes in object locations are not detected (blinks, saccades etc.) -Foveated rendering requires accuracy and low-latency eye tracking, but saccadic suppression allows leniency (delays) for update times -Just add to resolution over time instead -With each saccade, the system will alter the room to turn minutely so that the user is less likely to bump into objects/walk too far away, causing you to actually walk in a circle when you think you are walking straight
Describe the five different types of eye-movement and an example of when each is used (or describe the eye-movements that occur as you are running around your house trying to catch your cat - or playing tennis).
Saccadic: scanning a page Smooth Pursuit: following a baseball Fixational (Tremor, Micro-saccades, Drift): staring at something Vestibular-Ocular Reflex: eye movement when you turn your head side to side Optokinetic Nystagmus: following scenery outside of car window
Microsaccades
Small, jerk-like involuntary eye movements. Occur during prolonged fixation. Could correct eye drifting (fixational eye movement).
What is a specular reflection? What is a diffuse reflection? How can these two features combine to affect the perception of a material?
Specular reflection: screen pointed at something shiny like a phone, has high specular reflection (same direction - glossy) Diffuse reflection: when light comes at surface from one direction, will go out in all directions (evenly - matte) Phong Reflectance: pure specular will not give you a reflectance except where light in = light out; can create a combination of two features to create sense of something like a billiard ball (soft to relatively reflective material) - Early Toy Story films show a very plastic world using simple reflectance models
Describe the difference between supervised an unsupervised learning.
Supervised: discover patterns in the data that relate data attributes w/ a target (class) attribute -These patterns are then utilized to predict the values of the target attribute in future data instances Unsupervised: data have no target attribute -We want to explore the data to find some intrinsic strategies in them
The famous blue and black (or white and gold) dress is seen differently by different people. Why?
The dress can be interpreted in two ways: black and blue under a yellow-tinted illumination OR white and gold under a blue-tinted illumination Judged as different forms of illumination (brain makes assumptions); perceived illumination (brightness) impacts perceived reflectance (lightness)
What is a switching cost - and how does this apply to computers in the classroom
The switching cost is the cost of resources it takes to alternate between tasks. For example it is much easier to say the alphabet A-Z and then count 1-10 than to alternate each letter and number as you go. This switching cost applies to when trying to listen in lecture and read emails on your laptop so as a result, the first two rows of the classroom are laptop free. The goal is to limit distractions to you and your neighbor and ultimately get rid of the switching cost.
What is the Cornsweet illusion (a picture could be useful)? How has this effect been used by painters? (W)
The visual system perceives the upper surface to be a dark-colored surface exposed to bright illumination, and the lower surface to be a pale-colored surface in shadow, and this gives us the illusion that the reflectance of the lower surface is brighter than the upper surface, while in fact they have the same reflectance. The Cornsweet illusion suggests that the brain constructs surface color based largely on edge contrast information. Painters enhance edge contrast in order to make objects more clearly distinct
Describe three recommendations for children to reduce their chance of getting high myopia? What happens to the eyes of those that develop high myopia? Is myopia inherited or is it entirely environmental?
Three Recommendations: 1. More exposure to daylight/outdoors 2. Do not use negative lenses when doing close work 3. Visual correction to prevent myopia from worsening (or even progressive lenses) Myopia: Nearsightedness → Eye is becomes too long and too powerful, so the light rays will focus in front of the retina -Environmental: There are some correlations of myopia with education and urban living (Hong Kong students with their high myopia rates and the Taiwanese medical students with increased rates) --Too much close work (reading books etc.) --Lack of sufficient bright light -Genetic: myopia in offspring from myopic parents with up to 3x higher
For a 1.5 meter television screen at 1 meter, how many lines are required to be at the highest visible resolution (assume 100 PPD).
Total pixels/theta = 100 Solve for theta to get total pixels tan(𝜃) = size / distance = tan(𝜃) = 1.5/1 𝜃 = 56.3 degrees (number might be off; think i converted to degrees on Python lol) 56.3 degrees (100 PDP) = 5630 pixels
How many "bits" of luminance are available with a display that has only two intensity values (i.e., a monochrome display). How many are needed to produce photographic quality images? What are three factors that have bearing on this number.
Two intensity values have one bit of luminance (2^1). Most agree that max is 1000 intensity levels (10 bits) 3 Factors: Screen range, Particular image (smooth intensity variations require more bits), Screen gamma (optimizing gamma minimizes number of levels required)
What is the uncanny valley? Provide a graph (label the axes). Describe the perception at different points along the curve.
Uncanny Valley: when robots/other facsimiles of humans look and act almost like actual humans it causes a feeling of revulsion among human observers (effects made more dramatic if robot is moving)
Neurons are the basic unit of computation in the brain. How do neurons communicate with each other? Name one way in which they are similar to transistors, the basic unit of computation in a computer. (W)
Via electrical events called action potentials and chemical neurotransmitters. At the junction between two neurons (synapse), an action potential causes a neuron to release a chemical neurotransmitter. Neurons respond with discrete pulses of electricity. Neurons are constantly active, emitting pulses of electricity through connections with other cells. The rate of firing can be increased or decreased as the neuron is excited or inhibited.
What type of lens does a virtual reality display require if the display monitors are mounted 10 cm in front of the lens and are intended to be viewed at infinity? What type of lens is required if the observer normally requires -2 diopters of correction?
Want 10 D to put on it and make it look as if it is at infinity. What was at infinity before now looks like 0.1 m. No correction: 1/.1 + 1/infinity = 10 D lens. If you are already myopic (-2 D), can reduce by 2 D to 8 cm (-2 + 10 = 8). -2 D correction: 10 D - 2 D = 8 D. Should have an 8 D lens or just put glasses over VR display.
How can flicker above the critical fusion frequency still have an effect on a human observer?
Waving hand in front of face -> strobe (i.e. motion -> strobing) Critical fusion frequency: frequency at which flicker appears to be constant light source to observer. Related to visual persistence. Fluorescent lights above threshold, but can cause "building sickness". Incidence of headaches found to decrease when replacing 50 Hz fluorescent with 100 Hz.
Why are sinewaves often used when describing a linear system?
When you add two sine waves together you get another sine wave (put a sine wave in, always get a sine wave out with same frequency)
Is sensitivity to spatial frequency dependent on the temporal frequency of the stimulus? What does this imply about fast motion on displays?
Yes - sensitivity to spatial frequency dependent on velocity of stimulus. [Ware has an example of this complex function.] - Up to 2 degrees/second, anything slower and you have full acuity and anything faster you have lower acuity. Specific combinations of spatial/temporal frequencies can trigger epileptic seizures
Use a space time plot to explain why directional blur is sometime introduced into movies or games with fast motion. Why don't we notice the blur?
axes with space on x, time on y -If moving slowly (e.g. slow amount of space in a lot of time) -Fast (high amount of space in not a lot of time) -Once start moving fast, you get multiple flashes (blur direction of motion) gets rid of gaps in space (doesn't appear to be jumpy; smeared out) -Not an issue because our natural visual system does this anyway (don't notice we blurred it; looks like smooth motion)
bits
binary digit in terms of a display that has two intensity levels (e.g. 2^10 bits = 1024 different states)
ganglion cell
center-surround theory (on-center and off-center cells); neurons (Magno [respond faster and dominate periphery] and Parvo) that relay information from retina to brain; causes grid illusion
fovea
depression in retina where visual acuity is highest with a high concentration of cones; center of field of vision focused here
focal length
distance at which objects at infinity are focused
lateral inhibition
inhibition that neighboring neurons have on each other - fire more strongly if neighbors don't (grid illusion); receptive fields in periphery are larger (illusion there and not in center); for on-center cells, more negative space stimulated
blindsight
lesions in primary visual cortex/V1 which causes "blindness"; people can still respond to visual stimuli
troland
measure of luminance; 1 candela/meter^2 OR 1 nit seen through a 1 mm^2 pupil (common for finding units of intensity)
What causes motion sickness? How does this relate to the mismatch hypothesis? Why might motion cause sickness in the first place? Why is a passenger more likely to get sick than a driver even when they are both keeping their eyes on the road.
motion sickness caused by active conflict between the vestibular and visual system. mismatch states that there is a difference between them but there is no part that says you must be activemismatch hypothesis- conflict between vision and vestibular system.motion causes sickness because it changes the viscosity in your vesibular system that produces a slight difference- makes your body think that it has been poisoneda person riding a camel feels less oscillation than a horse. A camel has a lower frequency (closer to 1 hz). we are more sensitive to lower frequencies, therefore less sickness on a horsewhen people have stronger expectation, violations in these expectations makes a person more likely to get sick (ex: passengers in a car have a stronger expectation that the car will maintain the same direction, while the driver is paying more attention and is aware of when changes in direction are coming)passenger is not compensating for the motoin, while the driver is- they see that bump coming and can prepare -Mismatch is important but not whole story -If you strap yourself down in a bunk (don't let yourself move), tend not to get sick (visual vs. vestigial system) -What causes motion sickness: Active Conflict between vestigial and visual -Relates to Mismatch: not necessarily needing the "active part" -Why might motion: Changes motion slightly makes you think you're 'poisoned' and thus you may throw up -Camel > Horse: Frequency → camels move a little more slowly than horse -Why passenger more likely than driver → Driver has good expectation of motion of the car, passenger is not compensating correctly
Myopia
nearsightedness, eye might be too long; caused by close work or insufficient light
receptive field
particular region of sensory space in which stimulus will modify firing of neuron; measured in the world w/ electrode in brain and light in front of animal, listening to when it fires
Purkinje shift
peak luminance intensity shifts towards blue at night (lambda curve shifts); red flowers appear bright during day but black at night
visual cortex
receives information from LGN; various levels, feed forward and feedback connections; right side of brain receives information from the left eye and vice versa
mach bands
receptive field (center-surround mechanism) gives you some response and when you move right, there is less inhibition and the neurons fire more giving a slight increase before it falls again; removes inhibition at an edge and creates a little bump [false shadow on edge between light and dark object caused by lateral inhibition]
the oblique effect
relative deficiency in perceptual performance for oblique contours (slanting/diagonal) as compared to the performance for horizontal or vertical contours
diopters
unit of magnifying power of lens (1/Focal Length OR 1/Object Distance + 1/Image Distance)
lateral geniculate nucleus (LGN)
we don't really know what this does; receives sensory input from retina and relays information to primary visual cortex (connection for optic nerve to cortex)
