Post on 21-Dec-2015
transcript
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
Raster Displays, Raster Displays, Resolution, PerceptionResolution, Perception
• Human intensity sensitivity– Response to intensity is nonlinear– Gamma in cameras, CRTs – Gamma correction
Raster Displays, Raster Displays, Resolution, PerceptionResolution, 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?
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)
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?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
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 ?