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.Red 2.Yellow 3.Blue 4.Magenta 5.White 6.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 +45 o, then translates it to (10,5). What sequence of transforms would cause the triangle to change as shown below ?

GOOD LUCK AND HAPPY HALLOWEEN!