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CS 111 – Sept. 3

CS 111 – Sept. 3. More data representation Review hex notation Text ASCII and Unicode Sound and images Commitment: For Wednesday: Please read pp. 46-57 Quiz next Friday. Numbers in a byte. A byte is 8 bits So, how big can an 8-bit binary number be? Hexidecimal shorthand

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CS 111 – Sept. 3

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  1. CS 111 – Sept. 3 More data representation • Review hex notation • Text • ASCII and Unicode • Sound and images • Commitment: • For Wednesday: Please read pp. 46-57 • Quiz next Friday

  2. Numbers in a byte • A byte is 8 bits • So, how big can an 8-bit binary number be? • Hexidecimal shorthand • 8/4 = 2 hexidecimal digits per byte • What do the letters ‘a’ – ‘f’ mean? a = 10, b = 11, c = 12, d = 13, e = 14, f = 15 • Example: 010111102 = 5e in hex. • Try this one: 1110002 = ______ in hex. • Try this one: a4c in hex = ________ in binary.

  3. Text • Fundamental unit is the character. • Each character of a text document is given a numerical code. • ASCII code • Contiguous (make it easy to alphabetize) • Case sensitive • One byte per character • ASCII table (p. 597) • ‘A’ = 65 ‘a’ = 97 ‘0’ = 48 • Try encoding the word: “Dog”

  4. Unicode • To support foreign alphabet and misc. symbols. • Extension of ASCII • 16 bits per character, rather than 8 • unicode.org has code charts • Codes are given in hex.

  5. Sampling • “Real” sound and visual data are continuous, constantly changing • Sampling means to take rapid snapshots • Video: 30 images a second is good enough for our eyes • Real sound is in the form of a wave (p. 43) • Sampling sound means finding points along the curve. • Music CD: take a reading 44,100 times a second, and store as a 16-bit number… How much data is captured in 1 hour? • MIDI (= Musical Instrument Digital Interface) uses far less space, though does not sound like an actual recording.

  6. Images • Fundamental unit is the pixel • Usually 8 bits (1 byte) per pixel • This means each pixel is assigned a value from 0 to 255 • What do these numbers mean? Depends on color system • Grayscale = system for B/W images • Image dimensions are (horiz x vert) • Ex. 400 x 300  120,000 pixels • Aspect ratio • When changing size, this should not change.

  7. Resolution • Resolution – total number of pixels in image • “hi res” takes up more space • “lo res” means pixels become more obvious, pixelated

  8. Dynamic range • Dynamic range – how many colors / how many shades of gray • High dynamic range: more bits per pixel • Low dynamic range: may obscure features

  9. B/W vs. color • B/W: usually 1 byte per pixel • Each pixel = grayscale number 0-255 • Ex. 180 is a brighter shade of gray • Color: usually 3 bytes (24 bits) per pixel • Each pixel has 3 values, each 0-255 • Ex. (200, 50, 128) = ? • Most common scheme is RGB, where each pixel has a red #, green #, and blue #.

  10. RGB system • Based on primary colors for light • (red, green, blue) • Examples • Black = (0, 0, 0) • Purple = (75, 0, 100) • White = (255, 255, 255) • How about (x, x, x) or (0, 0, x) ?

  11. RGB examples

  12. Indexed color • Do we really need 24 bits to represent color of one pixel? • This means we allocate 16,777,216 colors! • About 200 would be more practical • Indexed color is a “compressed” RGB • 6 values of each primary color, not 256 • Use hex values 00, 33, 66, 99, cc, ff • This is the color system used on the Web. • 1 byte per pixel instead of 3 • Use “dithering” to simulate in-between colors.

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