CS248 Midterm Review

1. CS248 Midterm Review

2. 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

3. Raster Displays, Resolution, Perception • Measures of spatial resolution • physical vs. addressable resolution • CRTs • 3 phosphors for “red”, “green”, and “blue” • Triads and shadow mask

4. 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

5. 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.

6. Raster Displays, Resolution, Perception • Sample (easy) questions: • 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? • 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?

7. Color • Perception of color • Humans are trichromat • Three cones sensitive to “red”, “green”, and “blue” • Overlapping response curves • Color matching • Color matching experiment

8. Color spaces • Linearcolorspaces • , ,  space (perceptual stimulous) • R, G, B space • X, Y, Z space • Non-linear colorspaces • HSV • Spectral locus • Gamut of reproducible colors

9. Color Sample questions: • Can your printer necessarily reproduce all colors that your monitor can reproduce if they both use RGB primaries? • You are wearing glasses that block all light from the lower third of the visible spectrum, what color does the world look tinted?

10. Digital Compositing • The compositing approximation • Conditions for validity • Deriving alpha mattes • Blue screen • Computing while rendering • Compositing algebra

11. 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?

12. Rasterization • Rasterization of lines • Definition of “one-pixel-thick” line – which points get rasterized? • Rasterization of polygons • Only pixels in the polygon • Supersampling • Patterns

13. Rasterization • Sample question: • If you rasterized this line, which pixels would get turned on?

14. 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

15. 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 ?

16. 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)

17. Sampling and Antialiasing • Prefiltering vs. postfiltering • Desirable filters for antialiasing • Box, pyramid, gaussian, sinc • Methods of antialiasing • Supersampling: regular vs. stochastic • Analytical antialiasing

18. 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

19. GOOD LUCK!