02 - Perception and Color Flashcards
How does a “Cathode Ray Tube (CRT)” work? What’s a technique used by the CRT?
- cathodes shoot electron beams through focusing and deflection coils
- shadow mask places beam at fluorescent coating with RGB pixels
- it uses a “frame buffer”
-> independent of computational cost
How does a “Liquid Crystal Display (LCD)” work
- two coated glass plates with liquid crystals in between
- voltage determines orientation of crystals
- liquid crystals modify polarization of light
What’s the difference between “Vector Graphics” and “Raster Graphics”
Vector Graphics
- resolution independent representation
Raster Graphics
- image buildup independent of scene complexity
- finite pixel number -> “aliasing” and “Moiré effect”
- no object access
What is “Aliasing”? What causes it?
- pattern that aren’t real in sampled images
- caused by under sampling
-> merging of highs and lows to an average value
What is the “Moiré Effect”?
- super position effect
- interference patterns
- frequency depends on angle
What are spectral colors?
- monochromatic light
- only one wavelength
-> appear bright and clear
What is a “Plane Angle”? How do you calculate it?
- angle of a curve from a point
- L/r
-> L being the arclength of the projection of the curve onto the unit circle
What is a “Solid Angle”? How do you calculate it?
- amount of field of view covered by an object from a point
- F/r^2
-> F being the projection onto the unit sphere
What’s illumination
- consist of many wavelength
- each wavelength with a certain intensity
What are the most important parts of the eye (3) and their characteristics
Retina:
- houses light sensitive cells
Fovea:
- sharpest vision
Foveola:
- sharpest vision of Fovea
What types of cells are in the eye? Where are they placed?
- cones (few, central, colors -> trichromatic)
- rods (many, peripheral, black-white-vision -> monochromatic)
What’s the “Opponent Process”? What’s the effect of this?
- brains color scheme: 1 achromatic, 2 chromatic
- red-green
- blue-yellow
- ganglion cells consist of these photoreceptors
- when both parts are activated, they cancel each other out
-> no reddish-green
-> no blueish-yellow
What are “Mach Bands”? What causes them?
- brighter or darker edges in gradient
- caused by contrasts in receptive fields that amplify contrast
What is the “Trichromatic Color Vision”? How are colors formed? How can this effect perception?
- perceptual response to different wavelength
- colors are the integrals of the cone responses
- different wavelengths / intensities can create same response
What is a “Metamerism”
- two different spectra
- cause identical response in receptors
What are the two types of “Color Mixing” and their colors? Where are each used?
Additive (e.g. screens, projectors):
- red
- green
- blue
Subtractive (e.g. printers, color pens):
- cyan
- magenta
- yellow
What is “Grassman’s Law”
- every color representable by 3 basic quantities
-> 3-dimensional quantity
How does the “RGB Color Space” work
- 3 primary colors
- C = rR + gG + bB -> (r,g,b) in [0,1]^3
How does the “CMY(K) Color Space” work
- dual space to RGB
C = 1 - R
M = 1 - G
Y = 1 - B
For CMYK -> covering + saving ink
K = min(C, M, Y)
C’ = C - K
M’ = M - K
Y’ = Y - K
How does the “HSV Color Space” work
- neither additive nor subtractive
- very intuitive
H = [0°,360°]
S = (max - min) / max
V = max(r, g, b)
Difference: “Color Model”, “Color Space”, “Tristimulus Values”
Color Model:
- mathematical model
- 3-4 tuples
Color Space:
- set of all colors describable by Color Model
Tristimulus Values:
- describe color in a Color Space
- without Color Space -> meaningless numbers
What was the “Color Matching Experiment”? What’s its result?
- try to match a given color with given primary colors
-> not all colors reproducible
-> “negative” primary colors needed
What is a “Color Matching Function”? What’s special about it?
- shows how spectral colors can be reproduced by primary colors
- can have negative values -> can’t be realized
What is the “XYZ Color Space”? What is special about it?
- nonnegative color matching function
-> oversaturated primary colors
What does the “Gamut” describe
- contains all visible colors of human perception
What are the colors and the ticks on the edge of the “Gamut”
- spectral colors -> correspond to monochromatic light
- ticks are wavelengths
What is the bottom line in the “Gamut”?
- line of purples
- set of fully saturated colors that are NOT spectral colors
What is the middle of the “Gamut”
- white point
- x, y, z = 1/3
What colors lie on the line through a color and the middle of the “Gamut”?
- color itself
- white point
- pure color (on the side of color)
- complementary color (on the opposite side of white point)
What does an area (e.g. a triangle) indicate in the “Gamut”
- representable colors of output device
What’s the “Dynamic Range”
- difference darkest black / brightest white
What are the “MacAdam Ellipses”
- all colors within ellipsis are perceived same as its center
What’s a “Raster Graphic”? What are its characteristics?
- rectangular grid of pixels
- resolution (width x height)
- color depth (number of bits per pixel)
What’s a “Framebuffer”
- storage for an raster graphic that is to be displayed by the monitor
What’s “Dithering”
- simulation of missing colors
- perception of mixed colors from available colors
What’s the typical resolution and color depth of modern displays
- 1920 x 1080 pixel
- 24 Bit color Depth (3 x 8 -> RGB each 2^8 = 256)
What’s the “Weber-Fechner Law”
- subjectively perceived intensity
- proportional to logarithm of physical intensity
What is the “Gamma Channel”
- declares opacity
- 8 bits
Why does “Aliasing” happen? What are its result?
- sample frequency lower than frequency of signal
-> frequencies in reconstructed signal that are not present in original signal
What does the “Nyquist-Shannon Sampling Theorem” say
- f_sample > 2 x f_max
-> exact reconstruction possible
What are “Jaggies”
- type of aliasing at edges of objects
What is “Image Manipulation”? What two types are there?
- modification of pixel based on current color at this position
- modification pf pixel also based on neighbors
What is a “Convolution” in the Computer Graphics sense? How does it work?
- image function
- assigns color to each position
- use a kernel matrix to sum over the products of the image and the kernel
What is the “Kernel” for a “Convolution” that blurs
- entries sum up to 1
- most basic: 1/9 for 3x3 kernel
What is the “Kernel” for a “Convolution” that detects edges
- negative around middle (N, E, S, W)
- positive in middle
-> sum total: 0
What is the “Kernel” for a “Convolution” that sharpenes
- negative around middle (N, E, S, W)
- positive in middle
-> sum total: >0
What are “Morphological Operations”
- structurally changing operations
How does a “Dilation” work
- at least one 1 in B overlaps with a 1 in A -> 1
How does an “Erosion” work
- ALL 1s in B overlap with a 1 in A -> 1
How does an “Opening” work
- smoothing of contour
- removing bridges and protrusions (bumps)
- Erosion
- Dilation
How does a “Closing” work
- smoothing of contour
- closes smaller gaps
- Dilation
- Erosion
