Digital Media Dr. Jim Rowan ITEC 2110 Color Part 2

Question You have a blue ball and walk into a room lit with a red light. What color is the ball while in the room?

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 m Cyan = Green + Blue Cyan = White - Red Yellow = Red + Green Yellow = White - Blue Magenta = Red + Blue Magenta = White - Green CMYK is an Additive model (like RGB) and appropriate for use with light CMYK also provides the Subtractive model that is appropriate for use with ink and paint... things that absorb light

CMYK white paper cyan ink magenta ink white light removes green removes red white light removes green removes red leaving blue light magenta leaving light magenta leaving light blue leaving 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 R and the G and the B can be shown as a grey scale image Think of each as being stored in its own array That way, each can be manipulated individually

RGB BBB BBB BBB GGG GGG GGG RRR RRR RRR 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? Probably not. 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 age and change Excitation circuitry age and change

Questions?