CS248 Midterm Review
Michael Green and Sean Walker
(based on the work of previous TAs)
CS248 Midterm
• Mon, November 1, 7-9 pm, Gates B01
• Mostly “short answer” questions
– Keep your answers short and sweet!
• Covers lectures up to Tuesday, Oct 26
– plus taxonomy from start of last lecture
• Review session slides available from
class website
• Exam is closed book, closed notes
Raster Displays, Resolution,
Perception
• CRTs
– 3 phosphors for “red”, “green”, and “blue”
– Triads and shadow mask
• Measures of spatial resolution
– physical vs. addressable resolution
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
Raster Displays, Resolution,
Perception
• Human intensity sensitivity
– Response to intensity is nonlinear
– Gamma in cameras, CRTs
– Gamma correction
Raster Displays, Resolution,
Perception
• Sample (easy) question:
1. A scene is photographed with a TV
camera with gamma=0.5 and displayed
on a CRT with gamma=2.4. If we want
system gamma to be 1.0, we should do
gamma correction with what exponent?
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
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
Color
Sample questions:
1. Circle those colors that are not
reproducible with a single monochromatic
light
1.
2.
3.
4.
5.
6.
Red
Yellow
Blue
Magenta
White
Green
Color
Sample questions:
2. 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?
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) <=> F(w)G(w)
Sampling and Antialiasing
• Prefiltering vs. postfiltering
• Desirable filters for antialiasing
– Box, pyramid, gaussian, sinc
• Methods of antialiasing
– Supersampling: regular vs. stochastic
– Analytical antialiasing
Sampling and Antialiasing
Sample questions:
1. What is the result of convolving a 1-D box
filter with itself?
2. Which of the following would affect your
choice of a reconstruction filter?
a) pixel shape
b) choice of prefilter
c) actual size of display
Rasterization
• Rasterization of lines
– DDA, incremental algorithm
• Rasterization of polygons
– Only pixels in the polygon
• Supersampling
– Patterns: understand its effect on the
image
Rasterization
• Sample question:
– If you rasterized this line using DDA, which
pixels would get turned on?
Digital Compositing
• What is compositing?
• The compositing approximation
– Conditions for validity
Compositing algebra
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?
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
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 ?
GOOD LUCK
AND
HAPPY HALLOWEEN!