CS248 Midterm Review

1
CS248 Midterm Review
2
CS248 Midterm

• Mon, November 3, 7-9 pm, Gates B01
• Mostly short answer questions
• Keep your answers short and sweet!
• Covers lectures up to Tuesday, Oct 28
• plus taxonomy from start of last lecture
• Review session slides available from class
  website
• Exam is closed book, closed notes

3
Raster Displays, Resolution, Perception

• Measures of spatial resolution
• physical vs. addressable resolution
• CRTs
• 3 phosphors for red, green, and blue
• Triads and shadow mask

4
Human spatial frequency sensitivity

• Sensitivity highest in fovea
• Frequency sensitivity
• Phase sensitivity (Vernier acuity)
• Temporal sensitivity
• Flicker (50-70Hz)
• Perceived motion
• 12 Hz cartoons, 24 Hz film, 60 Hz video

5
Raster Displays, Resolution, Perception

• Human intensity sensitivity
• Response to intensity is nonlinear
• Gamma in cameras, CRTs
• Gamma correction
• Dithering
• Trade off spatial resolution for intensity res.

6
Raster Displays, Resolution, Perception

• Sample (easy) questions
• A scene is photographed with a TV camera with
  gamma0.5 and displayed on a CRT with gamma2.4.
  If we want system gamma to be 1.0, we should do
  gamma correction with what exponent?
• You are doing clustered-dot dithering on an image
  for output on a 300 dpi laser printer. If you
  need at least 75 dpi effective resolution, how
  many shades of gray can you output?

7
Color

• Perception of color
• Humans are trichromat
• Three cones sensitive to red, green, and
  blue
• Overlapping response curves
• Know their general shapes!
• Color matching
• Color matching experiment

8
Color spaces

• Linear colorspaces
• ?, ?, ? space (perceptual stimulus)
• R, G, B space
• X, Y, Z space
• Non-linear colorspaces
• HSV
• Spectral locus
• Gamut of reproducible colors

9
Color

• Sample questions
• Can your printer necessarily reproduce all colors
  that your monitor can reproduce if they both use
  RGB primaries?
• You are wearing glasses that block all light from
  the lower third of the visible spectrum, what
  color does the world look tinted?

10
Color

• Sample questions
• Circle those colors that are not reproducible
  with a single monochromatic light
• Red
• Yellow
• Blue
• Magenta
• White
• Green

11
Color

• Sample questions
• 4. If you had a special CRT that could produce
  pure spectral colors, how many spectral colors
  would you need to represent a normal RGB color
  gamut? How about the spectral locus?

12
Digital Compositing

• The compositing approximation
• Conditions for validity
• Compositing algebra
• Know the operations and their effect on an image

13
Digital Compositing

• Sample question
• You are doing the special effects for a movie,
  and need to composite a computer generated object
  over a live background. Why should you use an
  8-bit alpha matte rather than a binary (1-bit)
  matte, even if the computer-generated object is
  fully opaque?

14
Rasterization

• Rasterization of lines
• DDA, incremental algorithm
• Rasterization of polygons
• Only pixels in the polygon
• Supersampling
• Patterns understand its effect on the image

15
Rasterization

• Sample question
• If you rasterized this line using DDA, which
  pixels would get turned on?

16
Transformations

• Homogeneous coordinates why?
• Matrices rotation, translation, scale, shear in
  2D, 3D
• Know the form of each kind
• Geometric properties preserved/changed by each
  kind
• Composing transformations
• multiply matrices in reverse order

17
Transformations

• Sample questions
• Compute the 2D transform that translates an
  object centered at (-3,4) to the origin, then
  rotates it by 45o, then translates it to (10,5).
• What sequence of transforms would cause the
  triangle to change as shown below ?

18
Sampling and Antialiasing

• The sampling and reconstruction pipeline
• Prefiltering
• Sampling
• Resampling
• Reconstruction
• Aliasing in the frequency domain
• Filtering and convolution
• Duality F(x)G(x) ltgt F(w)G(w)

19
Sampling and Antialiasing

• Prefiltering vs. postfiltering
• Desirable filters for antialiasing
• Box, pyramid, gaussian, sinc
• Methods of antialiasing
• Supersampling regular vs. stochastic
• Analytical antialiasing

20
Sampling and Antialiasing

• Sample questions
• What is the result of convolving a 1-D box filter
  with itself?
• Which of the following would affect your choice
  of a reconstruction filter?
• Pixel shape
• choice of prefilter
• actual size of display

21
Sampling and Antialiasing

• Sample questions
• 3. Image
• filter

0 0 0 0 0 0 0 10 20 30 40 0 0 20
0 0 50 0 0 30 0 0 60 0 0 40 60 70 80
0 0 0 0 0 0 0
-1 0 1 -2 0 2 -1 0 1
22
GOOD LUCKANDHAPPY HALLOWEEN!