CS 111 – Sept. 3 More data representation Review hex notation Text –ASCII and Unicode Sound and images Commitment: –For Wednesday: Please read pp –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: = 5e in hex. –Try this one: = ______ 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 –Ex. 180 is a brighter shade of gray Color: usually 3 bytes (24 bits) per pixel –Each pixel has 3 values, each –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 ColorRGB Black000 White255 Red25500 Green02550 Blue00255 Cyan0255 Magenta2550 Yellow255 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.