CS248 Midterm Review Michael Green and Sean Walker (based on the work of previous TAs)

CS248 Midterm Review

Michael Green and Sean Walker

(based on the work of previous TAs)

CS248 MidtermCS248 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, Raster Displays, Resolution, PerceptionResolution, Perception

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

“blue”– Triads and shadow mask

• Measures of spatial resolution– physical vs. addressable resolution

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


• Human intensity sensitivity– Response to intensity is nonlinear– Gamma in cameras, CRTs – Gamma correction


• 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?

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. Circle those colors that are not

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

ColorColor

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 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)


• Prefiltering vs. postfiltering• Desirable filters for antialiasing

– Box, pyramid, gaussian, sinc

• Methods of antialiasing– Supersampling: regular vs. stochastic– Analytical 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 shapeb) choice of prefilterc) actual size of display

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?

Digital CompositingDigital Compositing

• What is compositing?• The compositing approximation

– Conditions for validity

Compositing algebraCompositing algebra

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?

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 ?

GOOD LUCKANDHAPPY HALLOWEEN!

CS248 Midterm Review Michael Green and Sean Walker (based on the work of previous TAs)

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