CS248 Midterm Review Derek Chan and Ethan Dreyfuss

CS248 Midterm Mon, Oct 27, 7-9 pm, 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 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? TTTSTRSSSRRR

Transformations Sample questions What sequence of transforms would cause the triangle to change as shown below ?