PSYCH 3420 Cumulative Review

Describe an example of how 'point of view' was used by Mantegna.

- Painting of Jesus from the feet to bring the observer up close to the dead body - Painting on the ceiling of figures looking down at you

What technologies use additive color mixture? Which use subtractive color mixture?

- additive color mixture: film/projection/any digital media that emits light - subtractive color mixture: hard copy media (painting, photographs etc)

What are the four main components of Virtual Reality? Provide an examlpe of each component for a VR tech like the Oculus Rift.

1. A head-mounted display. Example: the Oculus Rift has a set of goggles worn on your head to give video input to the eyes and track the user's motion 2. Tracking head position. Example: The Oculus Rift uses infrared sensors to track the user's head position in order to give the user the proper scene 3. Interactive input: The display changes based on the user's head movements. Example: As the user walks around the room or moves their head, the Oculus Rift will change the image accordingly so that the user can believe they are seeing a real environment around them 4. (optional) Interaction with the environment. Example: The Oculus Rift allows users to move objects around using the handheld remotes

Describe each system and provide at least one perceptual disadvantage for each of the following 3D display methods. 1. Anaglyph 2. Video-linked active glasses 3. Passive polarization 4. Pulfrich pendulum 5. Volumetric 6. Lenticular lenses 7. Stereoscope 8. Auto-stereogram

1. Anaglyph: A cheap 3D effect created by overlaying two differently-colored (usually blue and red) offset images onto the display. Viewers wear glasses with one eye under a blue filter and the other under a red filter so that each image goes to a different eye to produce a 3D effect. Disadvantage: there is a lot of ghosting (leakage of image from one eye to the other), and it is hard to get full-valued colors 2. Video-linked active glasses: The video is shown by alternating frames for the left and right eye. Viewers wear special glasses that block the light from one eye at a time in order for the correct frame to be seen by the correct eye. Disadvantage: Requires very precise timing with the glasses and the display, and the glasses are expensive. 3. Passive Polarization: A silver (polarized) screen displays two images for each video frame to mimic the disparity seen by our eyes. Viewers wear polarized glasses so that one eye receives one image, and the other receives an offset image. Disadvantages: the silver polarized screen is very expensive 4. Pulfrich Pendulum: The viewer wears a dark/black lens to cover one eye. Since our visual system takes longer to process dim images, it creates a sense of depth. The dimmed eye will perceive the image as being behind the other eye, creating a 3D motion effect. With the pendulum example, a pendulum swinging back and forth within a flat plane can appear to be spinning in a circle towards and away from the viewer. Disadvantage: The depth is completely dependent on the object's motion 5. Volumetric: High-speed projection of an image onto a solid, moving (usually spinning) surface, like a mirror, or moving LEDs. This produces a translucent 3D object. Disadvantages: It requires extreme precision and is hard to scale up to large displays. 6. Lenticular Lenses: A ridged sheet that has two separate images on each half of the ridges. From one side, you can then see one image, and from the other side you can see the other image. Disadvantage: There is a fixed viewing distance and area where this will work 7. Stereoscope: A small machine that displays two different, slightly offset images, to your two eyes. This is the basis for what VR goggles do, although much less complex. Disadvantage: It only works for a single viewer at a time. 8. Auto-stereogram: 'Magic Eye' illusions. Created by taking an image with a pattern and embedding an image by offsetting a set of pixels. Disadvantages: Requires you to converge/diverge your eyes (uncomfortable)

Provide examples of depth cues used: 1. At only close range 2. At only distant range 3. At all ranges

1. Close Range: - accommodation - convergence - binocular disparity - motion parallax - depth of field - height in plane 2. Distant Range - atmospheric perspective - rate of acceleration 3. All Ranges: - familiar size - linear perspective - detail perspective - interposition - shape from shading - kinetic occlusion

Describe four color phenomena that are explained by opponent color theory that are not explained by trichromatic color theory.

1. Color naming - there's no such thing as 'greenish red' or 'yellowish blues' - those are opponent colors. There are color-opponent ganglion cells which are red/green and blue/yellow as well. 2. Color Afterimages - If you stare at a colored image for a while, then look at a white, neutral surface, then you will see the opponent colors from the image you just looked at. The McCollough effect is an example - it is an aftereffect of staring at colored gratings. The colors of the gratings become associated with the grating orientations. 3. Simultaneous color contrast - color constancy - An image that is tinted appears to contain its contrast color (i.e. a shadow on a yellow background looks blue) 4. Color naming for anomylous vision - someone missing a cone can delete an opponent mechanism. For example, someone missing the red cone, the red-green opponent mechanism disappears People who are color blind in one eye only have shown that the entire opponent mechanism is missing in the color blind eye, not just one color.

Other than binocular disparity, describe two advantages of having two eyes. Use the concept of probability summation.

1. Field of view: Having two eyes increases the width of our field of view as opposed to having one eye 2. Probability summation: With two eyes, there is a higher probability of detecting an object in your field of view. For example, the probability of seeing something with two eyes is 1 - (1-P)(1-P), the probability of not seeing the object with either eye. If P = 0.8 is the probability of seeing the object with one eye, then the probability of seeing it with both eyes is 0.96 > 0.8

How does one make an Ishihara color test chart that finds protanopes and distinguishes them from deuteranopes?

1. Find two colors along a confusion line for either protanopes or deuteranopes 2. Using spots, make a number with one color surrounded by a background of the other color. 3. Randomize the intensities and sizes of the spots 4. Make a number of lower contrast that will be visible to the subject if they can't see the color difference (optional)

Describe the differences between efficient, inefficient, and restrictive display systems. Do we need all displays to be efficient?

1. Limited/restrictive: below sensory limits (too small/low res) 2. Efficient: info matches our sensory system/matches our required perception of reality 3. Excessive/inefficient: information overload/more information than is necessary for our visual systems We don't need all displays to be efficient.

Provide 2 examples that show that color symbolism is not universal. Why is this relevant to the design of websites?

1. Purple can be a 'royal' color because the pigment used to be rare/expensive, but can also be the color of death/crucifixion in Catholic Europe or prostitution in Middle Eastern countries 2. Yellow signifies jealousy and betrayal in France, and high ranking people in many African cultures, and is associated with pornography in China Websites which hope to be used by people from around the world need to make sure they are choosing colors which are having the effect that they want no matter who is viewing it.

Provide two examples of art that take advantage of the acuity limitations in the visual periphery (Livingstone)

1. The Mona Lisa smile - when you are looking directly at the Mona Lisa's mouth, it does not look like a genuine happy smile. When you look at other parts of the painting, so that her mouth is in your periphery where there is lower acuity, the smile looks genuine, making the painting look mysterious. 2. Rue Montorgueil by Monet - This painting is very spatially imprecise, with large brush strokes and jumbled details. This gives it the effect of motion, and being a snapshot in time, because the imprecision causes you to perceive it/complete the detail slightly differently every time. The style reflects the way our periphery works.

Provide two lines of evidence that we are especially sensitive to biological motion.

1. We are very good at determining the sex, age, weight, and even mood of a point light walker 2. Infants at 4 months old can differentiate biological and nonbiological motion 3. We are extremely sensitive to errors in biological motion

Name 3 display problems arising from equiluminous (isoluminant) colors? (Wade and Livingstone)

1. spatial sensitivity: Equiluminous colors tend to 'bleed' into each other and make it hard for us to see detail 2. Stereoscopic Depth: It is difficult for us to see stereoscopic depth in studio images that are equiluminous 3. Motion Sensitivity: An object moving over a background of similar luminance will appear to move more slowly than it really is 4. Form: We lose the impression of surface shape in equiluminous images

Describe two aspects of virtual reality that can make us sick.

1. too-slow update rate (if it doesn't keep up with our visual system, it can make us sick) 2. too-slow frame rate 3. screen brightness

Calculate the visual angle of your thumb at arms length, your big toe standing up and the moon. Show your calculations.

60cm = arm length 2cm = thumb width theta = arctan(2cm/60cm) ~= 2 degrees

Dichromat

A dichromat is someone who only has 2 types of cones, as opposed to the standard 3

Horopter

A horopter is a way to describe the region of space that projects to corresponding points on the two retinas in the eyes. This is a circular area, not flat. The points on a horopter are points that have no disparity between the two eyes (location on retina is the same)

What is a scale composite? Provide an example.

A scale composite is where each actor is shot separately, and the two pieces of film are put together later. This is used with blue screen so that they can be shot at different distances or one scaled down, and still be composited together. Used to make the hobbits look smaller than the elves.

Tritanope

Absent blue-sensing cone

Deuteranope

Absent green-sensing cone

Protanope

Absent red-sensing cone

Relative vs. Absolute Depth Cues

Absolute - measure of depth in units Relative - depth position compared to other objects in scene

What is display efficiency (DE)? What is visual efficiency (VE)? Why is DE never 100% with current technology? (Ware, p56)

According to Ware, Display Efficiency is the ratio of the uniquely stimulated brain pixels to the number of actual screen pixels. Since sometimes multiple brain pixels are triggered by the same signal when looking at a screen, those are counted as redundant and not included in the calculation. If there is a perfect one-to-one match, we would have a DE of 100%. This is not realistic, because screen pixels are uniformly distributed and brain pixels are not. Visual Efficiency is the ratio of the number of uniquely stimulated brain pixels to the total number of brain pixels. It tells us what fraction of our brain pixels are getting unique information.

Spectrum locus

All the colors that we can perceive (~370 to 730 nm)

Describe a color matching experiment that proves that the entire color space is 3-dimensional (can be described in terms of 3 variables).

All visible colors can be described using the following equation as long as all three of the primaries are not co-planar: AtLt = A1L1 + A2L2 + A3L3. If our primaries are all real lights, we will sometimes need 'negative light' to describe them all. If we use 'imaginary primaries', we can use the primaries to describe more than just the visible colors Experiment: That equation will always hold, even for negative colors, even if it is not physically possible to create the color. Just add the negative to both sides of the equation to make it positive

What is amblyopia? About what percentage of the population has it? Why is this relevant to designers of 3D displays?

Amblyopia (lazy eye) has two forms: 1. Strambismic amblyopia/squint: misalignment of the eyes 2. Anistropic amblyopia: one eye out of focus People with amblyopia (about 6% of the population) don't have good binocular vision, and are very dominant in one eye. Designers of 3D displays have to realize that this 6% of the population will not be able to perceive their 3D effects if they rely on binocular disparity. They should use monocular depth cues to allow people with amblyopia to see the effects

What is the difference between the standard anaglyph and Dolby 3D? What are the advantages of Dolby 3D?

An anaglyph is a standard, simple method of creating a 3D effect using a blue filter over one eye and a red filter over the other eye. The display has two images, one red and one blue, that are slightly offset. The glasses filters allow one image into one eye and not the other, creating parallax. In Dolby 3D, each eye is covered by a 3-band notch filter that allows multiple wavelengths through, which are slightly offset for each eye. Advantages of Dolby 3D: - Can get a wider range of colors - There is little ghosting, while anaglyphs have a lot

What is anamorphic art?

Anamorphic art is art that seems stretched out, but when seen at a specific angle the correct image can be seen. For example, the painting 'The Ambassadors' has a skull that can be seen properly by standing to the side of the painting.

Describe the depth cues available in painting or photography (when provided). For example, include: 1. Atmosphere 2. Perspective 3. Detail 4. Color 5. Familiar size

Atmosphere: Objects that are farther away have more air reflection between them, causing them to look more blue/gray. This can be seen in photographs of objects that are very far away, such as mountains. You can also see that in many paintings the artist has created this effect by making objects farther away appear more hazy. Perspective: In linear perspective, parallel receding lines all meet at a focal point. This is true in any photograph with receding lines (buildings, for example). In paintings, this effect was not understood until the Renaissance. During the Renaissance, artists started ensuring that buildings had lines that receded to the same vanishing points, creating more realistic images. Detail: Detail objects that are farther away appear to be smaller and closer together than the ones closer to the viewer. This can be seen in any photograph taken at an angle of some sort of detail. For example, a photograph taken of a brick wall from below will have smaller, closer together bricks at the top of the wall. Color: Color can be used to create an atmospheric effect, which makes objects that are farther away look more gray/blue and less saturated. It can also be used to create highlights, shadows, and reflections that give the viewer a sense of depth of objects and their locations int he scene. Familiar size: Artists can take advantage of how we are used to the size of certain objects in order to create the correct perception of the size of objects we may not be familiar with. This can also be used to create funny photographs that make objects appear much larger or smaller than they are based on the size of objects around them.

Benhams top

Benham's top is an image of a spinning wheel which is partially occluded and has different patterns of lines or shapes on it. If the top spins fast enough, you will start to perceive colors in the spinning colors. It is believed that different colors are associated with some temporal code, which is what causes this phenomena. This is not explained by either trichromatic color theory or opponent theory.

Binocular disparity is not incorporated into flight simulators. Why? How is motion parallax incorporated?

Binocular disparity is not needed because (1) motion parallax is a much stronger depth cue, and (2) pilots tend to look at things that are very far away (the ground), which are fairly flat relative to their position and there would not be much binocular disparity anyway. Motion parallax can be incorporated by showing objects in a 3D model of the environment that are closer to the plane moving faster than objects that are farther away. For example, the ground directly underneath the plane will move faster than the mountains in the distance. Motion parallax could also be captured in clouds that move by the plane. If a cloud is passing by the side of the plane, the pilot should observe it going quickly in the opposite direction of the plane, while the scenery in the distance should be slowly moving in the same direction as the plane.

Why is brown an odd color? (Wade and Livingstone)

Brown is a dark yellow, but is described as its own color (nobody calls it dark yellow). Brown requires some reference white area in order to be perceived as brown, and appears to be different from orange yellow.

Why does CIE use 'imaginary' primaries? What is represented by the points outside of the CIE chart?

CIE uses 'imaginary' primaries XYZ so that all visible colors can be described by positive combinations of these primaries. The points outside the CIE chart represents these 'imaginary' colors, which can still be represented mathematically by the primaries, but don't actually exist. This is preferred to the alternative of using 'negative' light. If you choose your axes to be within the set of visible colors, then in order to describe all visible colors, you would need to use negative amounts of the primaries. Equation: AtLt = A1L1 + A2L2 + A3L3

What are categorical colors? How is this related to color confusion? How is this relevant to displays (Ware)?

Categorical colors are 'ideal' colors that other colors are based off of/compared to. People tend to confuse colors within the same 'category' more often than colors in different categories. Designers should make sure to use combinations of colors from different categories when they want to make an image pop.

Describe the difference between closed loop, open loop, and partially closed loop displays.

Closed loop: Your behavior affects what you see Open loop: Your behavior does not affect what you see Partially closed loop: Not linked to natural motion, but still affects what you see (i.e. scrolling through a website with parallax, or Google street view which jumps from point to point and doesn't move smoothly/naturally)

The illumination (e.g. daylight vs fluorescent) can have a dramatic effect on the color spectra of objects. According to Livingstone (p97), why can we still identify colors under these conditions?

Color constancy allows us to identify colors in different conditions. Color constancy lets us use our knowledge of the effects of different lighting on different surfaces so that we perceive colors the same way. Even though colors might look brighter outdoors, our visual system accounts for the extra light. Under fluorescent lights, the histogram of colors is much more spiky than under natural light, but we don't really notice the difference and can still differentiate colors just as easily.

To what extent is color symbolism universal across the world?

Color symbolism is used anywhere from advertising to deciding what color to paint your kitchen. There is some universal color symbolism, but most symbolism stems from a particular culture. Examples: Purple - symbol of death/crucifixion in Catholic Europe Black - death, or class and wealth (i.e. limos and suits) White - purity/cleanliness (toilets, wedding dresses) or mourning in China Red - Blood, war, sexual connotation (red light district), stimulating products (Coke, Marlboro)

What does a tritanopic confusion line represent?

Colors not differentiated by tritanopes

Provide an example of an artificial spatial cue. Why are these used? (Ware)

Cues that are added to an image to provide a sense of relative depth/location to a fixed reference object in the image. They are not based directly on the way information is provided in the normal environment. Example: Using drop lines on a 3D plot so the viewer knows the depth of each point

Use the CIE diagram to describe the range of colors seen by a dichromat. What is represented by a confusion line?

Dichromats are missing either their red, green, or blue-sensing cones. Looking at the CIE chart, which uses R, G, and B primaries, a dichromat would be entirely missing one of the primaries. They would only be able to see the range of colors between the two primaries they are still sensitive to.

Relate Emmert's law to the perception of afterimages.

Emmert's law states: perceived size = k * perceived distance Objects with the same size will be perceived as larger if they are farther away. This applies to afterimages as well. If you stare at a light, then look at your hand, then look at a far wall, the afterimage on the wall will be perceived as larger than the afterimage on your hand, which is much closer. In reality, the afterimage is the same size in both cases.

According to Livingstone, why do equiluminant colors look unusual?

Equiluminant colors generally appear to be on the same surface (co-planar hypothesis), so they tend to blend more easily. They will appear to blend until they hit the 'border' of the shape they are in.

What is wrong with the description of some individuals as 'color blind'?

Everyone is technically 'color blind'. We only have 3 types of color receptors, while some animals have many more.

Section 508 of the Rehabilitation Act restricts the use of flicker on websites run by federal agencies. Why? What range of frequencies? (Ware and Livingstone)

Flicker can cause patter-induced epilepsy and make people convulse or even vomit blood. The flicker of helicopter blades can make some people sick as well. The range of frequencies that is most dangerous is between 2 Hz and 55 Hz

What is forced perspective? How does it apply to the filming of Frodo and Gandalf together on the cart in Lord of the Rings? Why does motion of the camera make this difficult? What is the solution?

Forced perspective is an illusion in photography and videography that makes objects look like they are in different locations and different sizes than they really are. This is used in Lord of the Rings to make Frodo look small by placing Elijah Wood farther back in the scene than Ian McKellen. The cameras are placed so that they look as though they are interacting side-by-side as opposed to different distances from the camera. This is difficult with camera motion because the props are often built to be at the correct scale for each actor and to be seen at a specific angle. The solution is to put the actors and props on tracks that move as the camera moves.

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?

Gamma is the distribution of intensities in a display. It lets us more efficiently store and present images. Our gamma is approximately 0.5. Output = Input^Gamma Since humans are more sensitive to differences in darker colors, we need more dark levels than bright intensity levels.

What is high dynamic range (HDR) imaging. How does this approach increase the apparent range of an image?

HDR imaging includes 1. capturing HDR images 2. displaying those images on HDR displays 3. creating images that look HDR on LDR displays. Tricks to make an image look HDR includes increasing contrast in very bright and dark areas of the photo (i.e. using multiple exposures) and using HDR cameras

If R and G are fixed at 100, what colors can be produced by varying the amount of B?

If R and G are fixed at 100 and B is 0, then the resulting color is orange. Adding more blue (up to the max of 100), would create a range of values of orange between a very pure orange and white.

On a graphics terminal you have created the image of a large rock on a field. The rock is intended to look 2 meters high. If the rock is perceived as being much closer than you anticipated, what will happen to the perceived size?

If the 2m tall rock is perceived as much closer than intended, it will look smaller.

You are watching a King Kong movie where a 10-story building falls in 1 second. What are you likely to see? If the original miniature was only 1 meter high but the building was intended to look 36 meters high, what do you do about the film rate?

If the building falls that quickly, you are likely to realize that it is actually a miniature. The larger an object is, the slower it should move to appear realistic. In order to make it look realistic, you should speed the filming by a factor of 6 (sqrt(36/1) = 6). This in turn will slow the projection by a factor of 6.

Probably on the test: Use projection theory to explain why a dot in depth appears to move when you move your head left or right. Also explain why perceived depth changes with viewing distance.

If we filter the image to each eye so that each eye sees an X at a different point, we will perceive the X as being in the location where the eye view directions cross. This effect will work no matter where you are sitting in the theater. If you are sitting farther away, the X will appear to be farther in front of the screen (i.e. move towards you as you walk away from the screen) Draw a picture!

What is the vergence focus problem? How does this relate to 3D movies and head mounted displays?

If you are young (under ~50), your eyes will try to accommodate for the distance at which you are seeing objects, even though they are beings projected at a fixed distance. This can cause the eyes to converge or diverge, which is uncomfortable and may contribute to VR and 3D movie sickness. This is why VR companies are trying to add different depths of field into their systems.

What is the evidence that color names are not culturally determined? What did Berlin and Kay show about the evolution of color names?

If you have a society with color concepts, you can predict what colors they will develop names for first. Berlin and Kay showed that there is a common pattern in color names. Languages with only two color names only had black and white. Languages with 3 colors added red, 4 colors added green or yellow, etc. Most languages have at least black and white color names.

Simultaneous contrast

If you see an image that is tinted a certain colors, the complement of the color will appear to be stronger than it actually is. - a green-tinged picture of strawberries will still appear to be red - Casting a shadow in white light on a blue surface will make the shadow look yellow A gray color surrounded by a color will appear to be the opponent color

What is the difference between LUV (UVW) and xyz color space?

In XYZ color space (like the CIE color chart), the colors are spread out based on their linear combination of three orthogonal axes. In LUV space, the colors are rearranged based on McAdam Ellipses, so that there is equal discriminability throughout the color space. The McAdams Ellipses, and LUV space, take into account human sensitivity to different wavelengths, and adjust the color space so we are equally sensitive to the changes in the color space.

What is the difference between additive and subtractive color mixture? Provide an example of each.

In additive color mixture is done by combining multiple sets of wavelengths to create a color. An example of additive color mixture is if you have three spotlights with different colored filters on them and you overlap the light beams. Subtractive color mixture is done by removing wavelengths from a light in order to create a color. CMYK printing is a subtractive technique because the combinations of cyan, magenta and yellow absorb some wavelengths (removing them from the light) and reflect what is left over.

Before the Renaissance, what evidence suggests that artists did not have a solid grasp of linear perspective?

In linear perspective, all parallel lines converge onto a vanishing point. Before the Renaissance, images did not follow this rule.

What is tilt-shift photography? How does it work? Why does it work? How can a change in film speed improve the effect?

In tilt-shift photography, photos/videos are taken from far away, and the top and bottom edges are blurred. The goal is to make the image look as though it is taken close-up of something very small rather than far away of something large. Increasing the speed of the video can help improve this effect, because smaller objects with less mass can move faster. The objects in the video will appear to move very fast relative to their size, just as ants do.

Explain why Johnny Lee's demonstration of depth on the Wii is so powerful.

It is a closed-loop display - the motion parallax remains correct as you move around the room

What are the advantages of filming at 60Hz or better? How does this improve the appearance over traditional techniques? What are the disadvantages?

It seems that the higher the frame rate, the smoother the resulting video will be. At 60Hz and above, the spacing between frames becomes less perceptible, so the frames look less jerky. The disadvantage of filming at higher frequencies is that it is memory-intensive to store that much more data

What does work with point light walkers tell us about our perception of biological motion?

It tells us that we are very sensitive to biological motion. A point light walker is a visual representation of biological (usually human) motion by showing dots at key points on the body (knees, hips, arms, head, etc). Small changes in the motion of these dots can make us perceive the walker differently. We can tell if it is a male or female, whether they are happy or sad, and how heavy they are by the motion of these dots.

What is the advantage of CIElab and CIEluv over the standard XYZ color space? What is the difference?

LAB and LUV color spaces take into account human sensitivity to color, and XYZ does not. This means that equally sized regions in LAB or LUV color spaces are equally discernible, unlike equally sized regions in XYZ space. The advantage with LAB and LUV color spaces is that they allow us to compare different display ranges while taking our visual sensitivity into account. [more to add?]

How can the rate of falling act as a depth and size cue? How does this relate to filming miniatures?

Larger objects will fall more slowly because they have a larger mass (F=ma, more force required to move the object). D=0.5*at^2 rate of acceleration by gravity Depth?

What is Livingstone's theory regarding the origin of the Mona Lisa elusive smile?

Livingstone's theory is that the Mona Lisa smile looks different depending on where you are looking in the painting. When her mouth is in your periphery and you can't make out the sharp details, it seems more cheerful, but when you look at it directly and get more detail with your foveal vision, it looks less like an authentic smile and looks more coy or mysterious.

A 50-foot wall is made with large bricks at the top and small bricks at the bottom. What unusual perceptions are you likely to have for objects at the top of the wall looking down? What sorts of perceptions are you likely to have looking up the wall from the bottom? Why?

Looking at the wall, we assume all bricks are the same size. Looking down from the top: The wall looks taller than 50 ft because the farther bricks are so small, and you expect them to be the same size as the bricks at the top. Looking up from the bottom: because the bricks at the top look bigger, and you expect them all to be the same size, the wall looks shorter than 50 ft.

What are the differences between luminance, lightness, and brightness (Ware and lectures)?

Luminance: intensity of light off a surface Lightness: perceived reflectance of an object Brightness: perceived illumination

McAdam Ellipses

McAdam ellipses are regions on a color chart that we can discriminate equally. Since we are more sensitive to changes in blues than greens, for example, the ellipses in the blue regions of the color chart are smaller than the ellipses in green regions of the color space.

In terms of chromaticity and luminance discuss the limitations of the range of possible colors on television, film, and the real world. Why is white similar across these media?

Most TVs and screens are limited to the RGB color space, meaning that all of the colors they can produce are limited to the linear combinations of red, green, and blue. In terms of luminance, color (except for white) in displays is created by filtering out some light. This means that luminance is tied to chromaticity. The intensity of a color on a display is limited by the intensity let through that color filter. White is similar across media, because it is always the brightest color- TV: white = 256 units of light from R,G,B- Film: white lets in the most light- Real world: white reflects the most light

Use size/distance theory to explain the moon illusion and the illusion that cars look like toys from the air.

Most people see the full moon as larger when it is closer to the horizon and farther away as it gets higher in the sky. This is because when the moon is near the horizon, we have depth cues of objects that are relatively close to us (buildings, trees, etc) that make it appear as though the moon is farther away. Because it looks farther away, it looks bigger than usual. As a strange effect, people take this a step further and actually say that the moon is closer because it looks bigger, in a sort of paradox.

Explain the concept of negative primaries in a color matching task. Imaginary primaries or negative light?

Negative light: If you choose the primaries of your color space to be within the range of visible colors, then you can describe all visible colors using combinations of these primaries. Some of the possible colors will require negative amounts of your primaries because they fall outside the convex hull described by the three primary axes. Imaginary primaries: If you choose the primaries to be outside of the color space, then they are 'imaginary' primaries, but you can use combinations of positive amounts of your primaries to create any color within your color space. You will also be able to describe other colors outside your color space with all positive amounts of your primaries.

What is the relation between the high threshold for motion and visual acuity? How fast must something move before one loses the ability to resolve spatial detail?

Past 2 degrees/second, we start to lose visual acuity of an object. The faster it moves, the less detail we can perceive.

The famous blue and black (or white and gold) dress is seen differently by different people. Why?

People see it differently because the perceived illumination affects perceived luminance. People can see it as a gold and white dress in blue light, or a blue and black dress in white/yellow light.

What is Pepper's Ghost? How is it used to bring deceased rock stars back to life?

Pepper's ghost is often called a hologram, even though it is not. It is really an effect used to make a video (usually of a person) appear to be on stage with other performers. It works by projecting the video onto a reflective screen/surface (or by directly displaying the video on the large screen). The video is then reflected onto a transparent foil, which is on the stage. The transparent foil/screen reflects the video to the audience, while allowing other performers to be seen on the stage.

Describe and provide examples of four pictorial cues, two ocular cues, and two motion cues, for the perception of depth.

Pictorial cues: - Familiar size: relates to how we are used to the size of certain objects. Example: you know how tall people tend to be, so you can estimate their distance. Another example: Disney castle plays with our expectations of house and brick sizes to make it look bigger. - Atmospheric Perspective: the air reflects some bluish light, so the more air there is between the viewer and the object, the more blue it will look. Example: As mountains get farther away, they look more light blue/gray. - Linear Perspective: parallel receding lines meet at a vanishing point. Example: MC Escher's paintings look realistic because he uses 2- and 3-point perspective to create his illusions. - Detail Perspective: things that are about the same size appear to be smaller and closer together as they get farther away. Example: The bricks on the Disney castles use this fact to make the illusion that the castle is bigger. Also bricks on a sidewalk appear to have less spacing between them and look smaller as they get farther away. - Interposition: the overlap of objects with a T junction. The top of the T appears to be the object that is in front. Example: MC Escher's illusions that make objects look like every part is in front of the other. Also hot air balloons one in front of the other have these T junctions. - Shape from Shading: the assumption that light comes from above helps give us some idea of shape. Example: Looking at a picture of a canyon from above with the light coming from below can make it look as though it is sticking out of the ground instead of cut into the ground. - Height in Plane: objects that are higher up on a surface appear to be farther away. Example: In a photograph of two people at different spots in a room, the person who is farther back will be 'higher up' on the plane of the floor. - Depth of Field: focal length; things that are at the focal length are in focus, and things in front/behind are out of focus. Example: If you hold your fingers one a few inches behind the other, the one in the middle of your view is more in focus and the other one becomes out of focus, no matter which one you look at. Temporal/Motion Cues: - Motion Parallax: most important depth cue (more than disparity). Objects that are closer to you seem to move faster and in the opposite direction of your motion, while objects that are far away appear to move slowly in the same direction as you. Example: If you walk across campus, street signs that you pass move by you fairly quickly, while buildings that are far away seem to move in the same direction but slowly. - Rate of Acceleration: the speed of an object depends on its weight and size with the rate D = 1/2 * a * t^2. Example: Godzilla is huge, so its limbs will move much more slowly than a person who is running away. In film, you would need to speed up the filming by a rate of sqrt(desired size / actual size), and then play it back at the opposite slower rate in order to create this effect. - Kinetic Occlusion: tells you the relative size of two objects. Example: ??? - Kinetic Depth: structure can come from the motion of an object. Example: a silhouette of an object may be indeterminate, but if it starts 'spinning' you can get a better idea of its structure. Ocular Cues: - Binocular Disparity: Each eye sees a slightly different image because they are at different angles. Example: If you hold up a book right in front of your face, it will look close because your eyes are seeing two images that are relatively different. If the same book is held 100 feet away, it will look far because each eye sees approximately the same image of the book. - Accommodation: The eye lens can control the focus of the image you are looking at to make it look in focus. The lens will distort different amounts to focus on an object that is nearby or far away. Example: If you look at your computer your eyes will focus on the screen, but if you look out the window your lens will change shape to accommodate the change in focal length. - Convergence: Our eyes converge (point slightly inwards) as we look at objects that are nearby. Example: This can help us see some optical illusions, like Magic Eye, which focuses our eyes in front of the image and allows two parts of the image to overlap in our visual system, showing the hidden image.

Why is pseudocoloring sometimes used for displays? What does Ware recommend (provide 2) for the choice of colors? Why can red/green pseudocoloring be a problem?

Pseudocoloring is a technique used to assign colors to a grayscale image, and is used often for display of scientific imagery/data. It is useful to help us perceive the distances in space of the image (for example, 3D data can be represented in 2D space with a color map that is brighter as it gets higher up and darker as it gets lower. These should be perceptually uniform (or close to perceptually uniform) colors. Ware recommends using CIElab and CIEluv color spaces because they are already widely used for different applications and have approximately perceptually uniform colors. Red-green pseudocoloring can be an issue because many people are red-green colorblind.

A computer screen has the standard three phosphors (RGB). Use the CIE diagram to show the approximate colors that would be produced by the following RGB outputs. What are the xyz coordinates? RGB 250 250 0 = ? RGB 125 125 0 = ? RGB 25 00 125 = ? RGB 100 00 100 = ?

RGB [250, 250, 0] = XYZ [0.45, 0.45] - yellow RGB [125, 125, 0] = XYZ [0.45, 0.45] - same yellow as before RGB [25, 0, 125] = XYZ [0.3, 0.15] - purplish-blue RGB[100, 0, 100] = XYZ [0.4, 0.2] - purple right between blue and red

Roughly, for a single image, what is the range of intensities found in the real world, modern LCD televisions and hard copy?

Real world: 300:1 LCD TV: 5000:1 Hard Copy: 30:1

Shape constancy

Seeing an object as the same shape no matter what angle it is viewed from because you expect it to be a rigid, constant shape

Describe the relationship between 'shape from shading' and the position of the light source.

Shape from shading is the assumption our visual system makes that light comes from above, because this is generally true in our environment. It assumes that the light source is above our heads, or coming from the top of an image, and so it gives us a depth cue as to which direction an image is sticking out in (i.e. into the page or out of the page)

What is the difference between protanomolous and protanopia?

Someone who is protanomolous has abnormal red-sensing cones, while someone with protanopia is completely missing their red-sensing cones.

Explain why the dynamic range of a display depends on the amount of stray light. Provide an example.

Stray light in the environment bounces back onto the display, adding to the brightness of the display and lowering the dynamic range. Example: If a display has a 1000:1 range, and 10 units of light is reflected back onto the screen, the range becomes 1010:11, or about 92:1.

Kinetic Depth Effect

Structure from motion - a static image looks 2D and may have an indeterminant 3D structure, but once it starts moving you can perceive the 3D shape

Bruneleski in 1413 described what primary aspect of linear perspective?

That parallel lines converge to a vanishing point

What are the three primary dimensions of texture? (Ware)

The 1) orientation, 2) scale, and 3) contrast of the texture

What will be perceived as brighter? 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? What about a 100 nit lamp in 500-540 nm and a 100 nit lamp in 650-700 nm?

The 500-540 nm lamp will be perceived as brighter than the 650-700 nm lamp. Green light, in the 500-540 nm range, is the peak of our visual sensitivity. 650-700 nm is red. If nits or candelas/m^2, they both have the same perceived intensity

Pulfrich effect

The Pulfrich effect is a stereo vision effect created by putting a dark, usually black, light filter over one eye to create a sense of depth. This works because our visual system takes longer to process dim images. The eye which is receiving the dimmed image 'lags' behind the other eye, so it sees the object in the image slightly behind where it really is. This is a good effect because it is cheap, doesn't require multiple images of a scene, and keeps the motion and color of the image. The cons are that the depth depends on motion (this won't work on a single still image), and that it requires motion/depth choreography.

In the video showing two lamps playing with a ball, why do the lamps look alive and seem to have personality?

The lamps look alive because they have biological motion. This is characterized by having smooth, not jerky, movements. This includes slowing down as object direction changes (i.e. move slowly at high-angle curves). For example, rather than falling flat on the ground when the lamp jumps, it bends in a smooth, continuous motion in order to break its fall, just as a human would.

According to opponent color theory what causes the perception of white?

The perception of luminance is described as one of the opponents (white vs black). A perceived white is created by balancing the opponent colors (R-G and Y-B)

What is the disappearing hand trick, and how does this relate to visual capture?

The person puts their hands into a box that has cameras inside. There is a display on the top of the box that shows images of the two hands in the box. The display shows the hands getting closer together, even though they are not. The user then drifts their hands apart to keep them from touching. The hands keep getting farther apart, while the video just shows the hands about the same distance, getting closer together every now and then to encourage the user to move them apart more. Visual capture is the phenomena that our visual input is dominant over our other sensory inputs. In the disappearing hand trick, the people really think their hands are side by side, because that is what is shown in the video, even though they could feel that their hands are far apart.

Describe the steps for making a random-dot stereogram.

The random dot stereogram was invented by Bela Julesz. It is a pair of images of random dots that are overlayed and viewed by diverging or converging your eyes, or as two separate images through a stereoscope. To make one, you would create an image of random dots, and make a copy of it. In the copy, you shift some of the dots over in the form of the image you want to be viewed. You randomize the dots within the new shape. Neither eye can see edges in the image

The sun's rays appear to diverge as they come through the clouds. Explain this illusion in terms of size and distance.

The sun's rays appear to diverge because they look wider closer to the ground and smaller closer to the clouds. The truth is that they are not diverging - the sun is way too far away from the Earth for that to happen. Really, the light is coming through the clouds towards the viewer, and so it is bigger closer to the person. Since the light is coming from so far away and the clouds are far away as well, our visual system assumes that the rays are diverging near the ground rather than getting closer to us. Rays of light come towards you like railroad tracks

For trichromatic displays explain why the full range of colors appears to be a cube in RGB space.

The three primaries (RGB) are practically perpendicular, and each one has a range of hue from 0 to 1. In 3D space, this forms a cube

Describe the relation between Panum's fusional area, diplopia and perceived depth.

There is a region of visual focus around any given point called Panum's fusional area where you can sense depth but you don't get double vision. If you focus your eyes outside of the fusional area, you will start seeing double of the object (double vision, aka diplopia) and you lose depth cues. You are sensitive to disparities within Panum's fusional area

Explain how to find a complementary color using the CIE diagram. For example, what is complementary to 600 nm (Ware and lectures)?

To find complementary colors on the CIE chart you need to look at opposite locations on the chart. You should draw a line from your color of interest through the middle of the chart (i.e. white), and the color on the other side along the edge of the chart is the complement. The complement to 600nm (reddish-orange) is about 490 nm (greenish-blue).

What is visual capture? Provide an example with simulators and VR.

Visual capture is the phenomenon that because our visual sense is dominant, it can influence your other senses. Example in VR: If you are standing in front of a roaring fireplace, you may perceive warmth even though there is nothing there. Example in a simulator: In a flight simulator, the capsule can tilt you in different directions to put pressure on your body. If you see a video of the plane doing a nosedive, you will feel as though you are facing towards the ground, even though there is not enough pressure on your body for that to be true

Under dim conditions (e.g., finding a star at night), it can be useful to look two or three degrees off the target. Why?

We have more rods outside our foveal pit, which is the center of our field of view. Looking a few degrees off-target would let you use your rods, which are better under dim lighting conditions.

Can I match a mixture of 585 nm and 500 nm (50% of each) with a color on my computer screen? What would be the RGB output?

Yes, the result would be a green with RGB values approximately [0, 90, 10]. Taking the point on the CIE chart halfway between 585 and 500 nm gives you a green on the edge of the RGB triangle . This value of green is about 90% green and 10% blue.

How does one make a 3D postcard?

You can make a 3D postcard using lenticular lenses. You can take two images that are slightly offset (or even two completely different images), and cut them into strips, alternating them as you put them on the postcard. The images have to be placed on ridges, so that one entire image can be seen by looking at the postcard from the left, and the other can be seen by looking at the postcard from the right.

How many colors can I produce with an 8 bit look up table and 6 bit DACs?

[Check answer] The lookup table can describe 2^8 = 256 possible colors, and the DACs can create 2^6 x 2^6 x 2^6=2^18 =262k colors. The lookup table is the limiting factor, so you can only produce 256 colors. Abstracted: 2^(bits in look up table) out of 2^(# of DACs * bit depth of DAC)

Metamer

two things which look the same, but are physically different. For example, a pure color purple can look the same as a purple produced on a display which uses red and blue pixels.