Digital Media

Dr. Jim Rowan

ITEC 2110

Color Part 2

Color Models other than RGB

• CMYK– cyan– magenta– yellow– key (black)

• HSV, HSB, HSL– Hue– Saturation – value. brightness, lightness

• First, CMYK ==>

CMYK• Colored light shining on a white surface

• One red light, one blue light and one green light

• Arranged to overlap...

• Creating Cyan, Magenta and Yellow: the complementary colors

w

g

r

b

c

y

m

CMYK

w

g

r

b

c

y

mCyan = Green + BlueCyan = White - Red

Yellow = Red + GreenYellow = White - Blue

Magenta = Red + BlueMagenta = White - Green

CMYK is an Additive model (like RGB) and appropriate for use with light

CMYK also provides the Subtractive model thatis appropriate for use with ink and paint... thingsthat absorb light

CMYK

white paper

white paper cyan ink

magenta ink

white light

removes green

removes red

white light

white light

removes green

removes red

leavingblue light

magentaleaving

light

magentaleaving

lightblueleaving

light

CMYK & Ink

• You can make black by printing the all three color inks but...– it isn’t a very good black...

• Cyan, Magenta and Yellow inks aren’t perfect• They are chemical compounds • Some Red is reflected by cyan ink• Some Blue is reflected by yellow ink• Some Green is reflected by magenta ink

– takes a long time to dry

• Many printing processes are supplemented by black ink

The HSV model

• RGB and CMYK make sense theoretically (based on cones in the eye) but...

• They don’t correspond to the way we experience color in the real world

• You don’t look at a patch of Cyan color and think about the mixture of green and blue light in it

• You probably relate to Cyan to other blues you know (hue), how pale it is (saturation) and its brightness (intensity-V)

HSV

• Three dimensions H S V • Hue - is the dominant frequency

– expressed as degrees from red

• Saturation - how pale the hue is– adding white to paint reduces the saturation

• V - The color’s value, brightness or lightness– adding black to paint reduces the brightness

Historical Trivia and Color• Consider the B&W tv and the Color tv• B&W just needs intensity changes to make white-gray-black

– intensity varies as the electron beam is drawn across the screen

• Color is RGB+Intensity for each– could transmit each color with its intensity and it would work for color tv but

– can’t use the same signal model for color because the B&W tv’s wouldn’t work– Solution: Transmit the necessary color information during the retrace.– Color is transmitted as luminance and two colors

– B&W tv’s simply ignore it because the beam is off

Channels and color images

• Each of the RGB can be shown as a grey scale image

• Each of the RGB is stored in its own array

• Each can be manipulated individually

RGB RGB RGB

RGB RGB RGB

RGB RGB RGB

B B B

B B B

B B B

G G G

G G G

G G G

R R R

R R R

R R R

Channels RGB

• Allows the use of the brightness and contrast of each individual color to be manipulated using levels and curves

• Allows a very fine level of control

• Very time consuming• Color balance, hue and

saturation adjustments on the whole image is a shortcut

Color consistency?

• Different devices use different phosphors• Physical devices are not usually linear

– Red, Green and Blue phosphors each react differently and

– They do not react linearly... this means• 10 times as much excitation will not produce 10 times as

much emitted light• In the end, it is all a compromise

• Phosphors and excitation circuitry age

Questions?