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Digitization

Digitization. Informatics INFO I101 January 26, 2004 John 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.

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Digitization

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  1. Digitization Informatics INFO I101 January 26, 2004 John C.Paolillo

  2. 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.

  3. http://rush.digitalchainsaw.com/marspath.html

  4. Raster Image Digitizaztion • Grid • Resolution • Quantization • Bit-depth • Color cf. Vector Graphics

  5. 1280  960 Great Sand Dunes, Colorado (IRS satellite image; www.spaceimaging.com)

  6. 640  480

  7. 320  240

  8. 160  120

  9. 80  60

  10. 256 Grays

  11. 16 Grays

  12. 4 Grays

  13. Black and White

  14. 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)

  15. Color

  16. 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

  17. green cones blue cones red cones Color: Response Patterns Wavelength 

  18. The Eight-Color World • Eight colors: black, yellow, magenta, red, cyan, green, blue, white • Three color tubes on a TV monitor: Red, Green, Blue 23=8 • Additive color relations: red+green+blue=white

  19. A Psycho-Physical Encoding RGB 101 100 110 010 011 001 101 110 000 111 Wavelength 

  20. 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)

  21. 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

  22. Signal Levels • Intensity is analog • Levels are digital How do we convert analog intensity to digital levels? • Quantization: convert analog signals to digital numbers

  23. 1 1 1 1 1 1 1 White 111 110 101 Med Gray 100 0 0 0 0 0 0 0 011 010 Black 001 000 Quantization • Evenly divide signal levels • Assign a unique binary number to each recognized level • Match signal with recognized levels and round any intermediate signal level to the nearest recognized level • Report the signal as a list of binary numbers

  24. 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

  25. 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

  26. End

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