Digital Media Dr. Jim Rowan ITEC 2110 Color

COLOR Is a mess It’s a subjective sensation PRODUCED in the brain Color differs for light and paint/ink Printing is different than viewing a monitor –a monitor EMITS light of a specific wavelength (or a combination of them) –print is like paint... it absorbs all the colors EXCEPT the color that you see which is reflected by the paint –a ball that is painted yellow and is viewed in a room that is lit by a light that is completely blue will look black... Why?

Light Is electromagnetic radiation (just like microwaves and radio signals... just different wavelengths) Visible light has a wavelength that is between 400 and 700 nanoMeters A nanoMeter is 1 billionth of a meter... –HINT: It’s a very short wave

/wiki/Electromagnetic_ spectrum

Light Visible light is a mix of different wavelengths of light at different intensities The sun has one Spectral Power Distribution, fluorescent light another, a camera flash another and an LED light another

Color To reproduce it electronically or manipulate it digitally we need a way to model color. i.e. we need a way to convert a subjective sensation to a reproducible physical phenomenon

A model of color: roughly based on the eye Rods(night vision, B&W) Cones (3 kinds, one for red, one for blue and one for green) RGB –tri-stimulus theory: the theory that states any color can be completely specified with just 3 values

RGB Color is specified by 3 numbers –one for red –one for green –one for blue Color is displayed on a monitor by 3 different colored things –one for red –one for green –one for blue

The 3 colored things Phosphor for a CRT and Flat panel displays (aka SED or surface conductor electron emitter display) Pockets of fluorescent gas for Plasma a bunch of other stuff too... All of them have the ability to adjust the intensity of each of the three things resulting in the display of most of the visible colors

RGB... Good but not all visible colors In truth, the 3 different cones in the eye are cross connected in very complex ways This keeps the model, which assumes (wrongly) that each is strictly sensing R or G or B, from being able to completely reproduce the visual stimulus

RGB Pure red –(100%, 0%, 0%) –(255,0,0) Pure green –(0%, 100%,,0%) –(0,255,0) Pure blue –(0%, 0%, 100%) –(0,0,255)

Mixing the Color of Light Is an additive process –monitors emit light Is not like mixing paint which is a subtractive process –paint absorbs light

How many colors? Different cultures have different ideas about when 2 colors differ People individually differ in their ability to distinguish between two colors With the range of (which can be encoded in one byte) for each of the Red, Green and Blue there are 16.8 million possibilities

Color Depth Usually expressed in bits One byte for each of the RGB => 24 bits Back to binary... –1 bit => 2 1 => 2 choices –2 bits => 2 2 => 4 choices –4 bits => 2 4 => 16 choices –8 bits => 2 8 => 256 choices

Color and Grey RGB (0, 0, 0) is black RGB (255, 255, 255) is white When R = G = B you get grey –RGB (25, 25, 25) is dark grey –RGB (200, 200, 200) is light grey

Color at 16 bit Color Depth RGB at 24 bits –24 bits => 3 bytes –3 bytes, 3 colors => one byte per color RGB at 16 bits –16 bits => 2 bytes –2 bytes, 3 colors... –16/3 = 5 bits with one left over... –HMMMmmmm... –What to do?

16 bit color 16/3 = 5 bits with one left over... What to do with the extra bit? Go back to human perception –Humans do not discriminate Blue as well as they do Green vision –Evolutionary roots? Lots of blue light in daylight Lots of green to discriminate Assign 5 bits to R & B, and 6 bits to G –allows twice as many greens as blues

Why more than 16.8 million? 24 bits is plenty for human vision and 48 bits are WAY more than needed for human vision... If you scan at 48 bit color there is a lot of information buried in the image than we can distinguish BUT... This information can be used by the program to make extremely fine distinctions during image manipulation (edge finding for example) (Failed rocket engine example)

Why worry about color depth? One reason: file size Any reduction in color depth has a 3-fold effect on the final image size A 100X100 RGB image –at 24 bit color => 30,000 bytes uncompressed –at 16 bit color => 20,000 bytes uncompressed –1 byte => 1/3 reduction of size

Why is too big bad? It wastes valuable computer resources –hard drive disk space –vram space –data transit time VRAM internet monitor main memory hard drive Sure, computers are really fast and big now BUT... Consider Video...

Indexed color We’ve seen this before... the color table Used by a number of file formats –tiff, png, bmp, gif, Use a table (color palete) to store colors Use a map of logical colors to reference the color map

Indexed color vs. 8 bit color 8 bit color defines only 256 colors Indexed color allows 256 different colors What’s the difference? –8 bit defines 256 colors---whether they are used or not –Indexed color allows 256 different colors that exist in the image

Indexed color vs. 8 bit color 8 bit direct color... an example –suppose... Red gets 3 bits ==> 000, 001, 010, ==> 8 values Green gets 3 bits ==> 000, 001, 010, ==> 8 values Blue gets 2 bits ==> 00, 01, 10, 11 ==> 4 values total different colors ==> 8 x 8 x 4 = 256 different colors But images in nature have a narrower range of colors... a palete With indirect color you can store 256 different colors that are actually found in the image –results in an image that more closely mimics the image

Indexed color Even though it allows for a closer-to- real-life image, some colors must be modified How to do this? –use the nearest color –optical mixing... dithering

Nearest color Loss of some detail Distorted color Generates artifacts –Banding or posterization

Optical mixing: Dithering Dithering uses a group of colors to approximate the desired color Works well for high resolution images Works poorly for low resolution images See figure 6.8, p169 for dithering effects

Questions?