CS248 Midterm Review
Derek Chan and Ethan Dreyfuss
CS248 Midterm
• Mon, Oct 27, 7-9 pm, 370-370
• Mostly multiple choice and
short answer questions
– Keep your answers short and sweet!
• Covers lectures up to Tuesday, Oct 21
(Transforms and Taxonomy of Mappings)
• Exam is closed book, closed notes
• Slides will be up tonight
Administrivia
• Assignment 2 Late Grading
– If you have not received an email from
staff, we have not arranged a late demo
• Derek extra office hours
3-4pm Sat in Gates Basement
Class Topics
• Approximate class coverage
– Perception, Color (2 Lectures)
– Sampling (2 Lectures)
– Rasterization (1 Lecture)
– Transformations (2 Lectures)
– Digital Compositing (1 Lecture)
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. 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 polygons
– Only pixels in the polygon
• Supersampling
– Patterns: understand its effect on the
image
Digital Compositing
• What is compositing?
– Method for combining 2+ images to
approximate the intervisibility of the scenes
that gave rise to those images
• Compositing Algebra
– Porter-Duff algebra vs Colors and Alphas
Compositing algebra
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
Transformations
• Consider composing two 2D transformations from
among the set consisting of translation (T), uniform
scaling (S), and rotation (R). There are six unique
pairs listed below. For which of these six pairs can
the order of applying the two constituent
transformations be switched without affecting the
result?
• TT
TS
TR
SS
SR
RR
Transformations
Sample questions
•
What sequence of transforms would cause the
triangle to change as shown below ?