Digitization Informatics INFO I101 January 26, 2004 John C.Paolillo.
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Transcript of Digitization Informatics INFO I101 January 26, 2004 John C.Paolillo.
Digitization
Informatics INFO I101January 26, 2004John C.Paolillo
Mars “Anomalies”
• Are the photos from various Mars missions faked?
• Do the existing photos show evidence of civilized life?
• Do the photos show evidence of martians?
Etc.
http://rush.digitalchainsaw.com/marspath.html
Raster Image Digitizaztion
• Grid– Resolution
• Quantization– Bit-depth– Color
cf. Vector Graphics
1280 960
Great Sand Dunes, Colorado (IRS satellite image; www.spaceimaging.com)
640 480
320 240
160 120
80 60
256 Grays
16 Grays
4 Grays
Black and White
Digital Artifacts
• Pixelation (“jaggies”)– From discretization of the analog signal
• shape• color/gray level
• Resolution mismatches– cause geometric distortions as error accumulates
• Fix: digital interpolation – dithering– anti-aliasing (especially with fonts)
Color
Color Perception
3 Electron guns, aimed at 3 different colors of phosphor dots — analog signals
3 types of retinal sensor cells, sensitive to 3 different bands of light
Color: Response Patterns
red conesgreen cones blue cones
Wavelength
The Eight-Color World
• Eight colors: black, yellow, magenta, red, cyan, green, blue, white
• Three color tubes on a TV monitor: Red, Green, Blue23=8
• Additive color relations: red+green+blue=white
A Psycho-Physical Encoding
Wavelength
RGB
101 100 110 010 011 001 101
000 111
More Colors
• Recognize more levels in each channel– 2 bits per channel: 26 = 64 colors– 4 bits per channel: 212 = 4096 colors– 8 bits per channel: 224 = 16,777,216 colors
• Except for 3-bit and 24 bit colors, most standard colors are not in multiples of 3 bits– 8 bits (256 colors)– 16 bits (32,768 colors)(8 bits is a convenient storage unit)
The Color Table
• A table of some convenient number of values– 4, 16, 256, etc.
• Each location in the table is mapped to some higher resolution color value (24 bit)– Some locations may be unused (mapped to black)
• A monitor typically uses only one color table at a time
Signal Levels
• Intensity is analog• Levels are digital
How do we convert analog intensity to digital levels?
• Quantization: convert analog signals to digital numbers
Quantization1
0
1
0
1
0
1
0
1
0
1
0
1
0Black
White
Med Gray
1. Evenly divide signal levels2. Assign a unique binary
number to each recognized level
3. Match signal with recognized levels and round any intermediate signal level to the nearest recognized level
4. Report the signal as a list of binary numbers000
111
100
011
010
001
110
101
Counting in Binary
• Two values: 0, 1• Each digit is a power of 2
– 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, ...– Fractions: 0.5, 0.25, 0.125, 0.0625, 0.03125, ...– positive, negative, rational, real, imaginary...We’ll stick to whole numbers for now
Binary counting• Start with zero: 00000• Add 1: 00001• Adding 1 more carries: 00010• Add 1: 00011• Adding 1 more carries 2x: 00100• Add 1: 00101 Etc. OR:• Divide the full range into 2 halves, 0 (low) and 1• Divide each range again for each next bit• Stop with the last bit
End