CS248 Midterm Review

CS248 MidtermCS248 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

Raster Displays, Raster Displays, Resolution, PerceptionResolution, Perception

• Measures of spatial resolution– physical vs. addressable resolution

• CRTs– 3 phosphors for “red”, “green”, and

“blue”– Triads and shadow mask

Human spatial frequency Human spatial frequency sensitivitysensitivity

– 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, Raster Displays, Resolution, PerceptionResolution, 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, Raster Displays, Resolution, PerceptionResolution, 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?

ColorColor

• 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 spacesColor spaces

• Linear colorspaces– , , space (perceptual stimulus)– R, G, B space – X, Y, Z space

• Non-linear colorspaces– HSV

• Spectral locus• Gamut of reproducible colors

ColorColor

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?

ColorColor

Sample questions:3. Circle those colors that are not

reproducible with a single monochromatic light1. Red2. Yellow3. Blue4. Magenta5. White6. Green

ColorColor

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?

Digital CompositingDigital Compositing

• The compositing approximation– Conditions for validity

• Compositing algebra– Know the operations and their effect

on an image

Digital CompositingDigital 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?

RasterizationRasterization

• Rasterization of lines– DDA, incremental algorithm

• Rasterization of polygons– Only pixels in the polygon

• Supersampling– Patterns: understand its effect on the

image

RasterizationRasterization

• Sample question:– If you rasterized this line using DDA,

which pixels would get turned on?

TransformationsTransformations

• 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

TransformationsTransformations

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 AntialiasingSampling 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 AntialiasingSampling and Antialiasing

• Prefiltering vs. postfiltering• Desirable filters for antialiasing

– Box, pyramid, gaussian, sinc

• Methods of antialiasing– Supersampling: regular vs. stochastic– Analytical antialiasing

Sampling and AntialiasingSampling 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?• Pixel shape• choice of prefilter• actual size of display

Sampling and AntialiasingSampling and Antialiasing

Sample questions:3. Image:

filter:

0 0 0 0 0 0 0 10 20 30 40 0 0 20 0 0 50 00 30 0 0 60 00 40 60 70 80 00 0 0 0 0 0

-1 0 1-2 0 2-1 0 1

GOOD LUCKANDHAPPY HALLOWEEN!