CS 101 – Sept. 16

• Finish color representation– RGB √

– CMY

– HSB

– Indexed color

• Chapter 4 – how computers think– Begin with basic building blocks

Color rep’ns

• RGB – system based on light

• CMY – based on printing

• HSB – based on art

• Indexed color – a swatch to save space

RGB system

• Based on primary colors for light

• Each pixel has (red, green, blue) values.

• Examplesblack = (0, 0, 0)

purple = (75, 0, 100)

white = (255, 255, 255)

• How about (x, x, x) or (0, 0, x) ?

RGB examplesColor 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

CMY system

• Based on primary colors of printing

• Each pixel has (cyan, magenta, yellow) values

• In contrast to RGB:white = (0, 0, 0)

black = (255, 255, 255)

CMY examplesColor C M Y

white 0 0 0

black 255 255 255

cyan 255 0 0

magenta 0 255 0

yellow 0 0 255

red 0 255 255

green 255 0 255

blue 255 255 0

Practical notes

• Printout may look different to screen

• Ex. RGB blue = (0, 0, 255)

but CMY blue = (255, 255, 0)

In other words, in color printer, 2 different toners required to produce blue.

• CMY, a.k.a. CMYK

HSB system

• From artistic standpoint, neither RGB nor CMY makes much sense to people

• More intuitive color definition:– Hue = what color you want– Saturation = how much of that color– Brightness

HSB geometry• Hue = which direction

on color wheel

• Saturation = how far from center

• Brightness = how far up or down

hue

saturation

brightness

Trade-off between saturation and

brightness

Indexed color

• Do we really need 16,777,216 colors?– ~ 200 is more practical

• Indexed color is like RGB:– 6 values of each primary color, not 256– Hex values: 00, 33, 66, 99, cc, ff

• 1 byte per pixel instead of 3

• Dithering to simulate in-between colors

Chapter 4

Begin chapter on computer organization

• Logic gates– Used to perform math operations

• Later: finite automata– basic model of computation

Logic Gates

• Basic building blocks• Usually 2 inputs• X, Y could be 0 or 1.

1 = true0 = false

• By combining 2+ gates, you get more sophisticated functions

‘AND’ and ‘OR’

AND

X Y ans

1 1 1

1 0 0

0 1 0

0 0 0

OR

X Y ans

1 1 1

1 0 1

0 1 1

0 0 0

Adder

• We can teach the computer how to add using just a few logic gates.

• However, we need to look at one more gate, the XOR.

Exclusive or (XOR)

• XOR basically says “either, but not both”

• The output is 1 if both inputs are different.

XOR

X Y Ans

1 1 0

1 0 1

0 1 1

0 0 0

Adder

• Here is the logic to add, one bit at a time.