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– 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.

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”

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.

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.

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.

Resolution

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

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

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 #.

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) ?

RGB examples

Color R G B

Black 0 0 0

White 255 255 255

Red 255 0 0

Green 0 255 0

Blue 0 0 255

Cyan 0 255 255

Magenta 255 0 255

Yellow 255 255 0

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.