CS248 Midterm Review
CS248 Midterm
• Mon, November 4, 7-9 pm, Terman Aud
• Mostly “short answer” questions
– Keep your answers short and sweet!
• Covers lectures up to Tuesday, Oct 29
– plus corrections and taxonomy from start of
last lecture
• Review session slides available from
class website
Raster Displays, Resolution,
Perception
• Measures of spatial resolution
– physical vs. addressable resolution
• CRTs
– 3 phosphors for “red”, “green”, and “blue”
– Triads and shadow mask
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
• Dithering
– Trade off spatial resolution for intensity res.
Raster Displays, Resolution,
Perception
•
Sample (easy) questions:
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?
2. 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?
Color
• Perception of color
– Humans are trichromat
• Three cones sensitive to “red”, “green”, and “blue”
– Overlapping response curves
• Color matching
– Color matching experiment
Color spaces
• Linear colorspaces
– , ,  space (perceptual stimulous)
– R, G, B space
– X, Y, Z space
• Non-linear colorspaces
– HSV
• Spectral locus
• Gamut of reproducible colors
Color
Sample questions:
1. Can your printer necessarily reproduce all
colors that your monitor can reproduce if
they both use RGB primaries?
2. You are wearing glasses that block all
light from the lower third of the visible
spectrum, what color does the world look
tinted?
Digital Compositing
• The compositing approximation
– Conditions for validity
• Deriving alpha mattes
– Blue screen
– Computing while rendering
• 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?
Rasterization
• Rasterization of lines
– Definition of “one-pixel-thick” line – which
points get rasterized?
• Rasterization of polygons
– Only pixels in the polygon
• Supersampling
– Patterns
Rasterization
• Sample question:
– If you rasterized this line, which pixels
would get turned on?
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 ?
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:
• 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
GOOD LUCK!