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Digital Media

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)

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Digital Media

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  1. Digital Media Dr. Jim Rowan ITEC 2110 Color

  2. 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?

  3. 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

  4. http://en.wikipedia.org/wiki/Electromagnetic_spectrum

  5. http://en.wikipedia.org/wiki/Electromagnetic_spectrum

  6. Light • Visible light is a mix of different wavelengths of light at different intensities • Light isn’t just light • The sun has one Spectral Power Distribution, fluorescent light another, a camera flash another and an LED light another

  7. Color • We need to reproduce it electronically and manipulate it digitally • So… we need a way to model color. • i.e. we need a way to convert a subjective sensation to a reproducible physical phenomenon

  8. One 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

  9. 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

  10. The 3 colored things • Phosphor for a CRT and some Flat panel displays • Pockets of fluorescent gas for Plasma panel • …plus a bunch of other varieties... • All of them have the ability to adjust the intensity of each of the three colored things resulting in the display of most of the visible colors http://en.wikipedia.org/wiki/Computer_display

  11. 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 • ==> RGB cannot completely reproduce the visual stimulus

  12. 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) • Gray? R = G = B • (25,25,25) • (150,150,150)

  13. Mixing the Color of Light • …is an additive process • monitors emit light • …is not like mixing paint • mixing paint is a subtractive process • paint absorbs light

  14. 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 0-255 • which can be encoded in 1 byte (8 bits) • The combinations of Red, Green and Blue results in 16.8 million possibilities with 4 binary digits 2**4 = 16 with 3 digits that can have 256 different values 256**3 = 16,777,216

  15. Color Depth • Usually expressed in bits • One byte for each of the RGB => 24 bits • Back to binary... • 1 bit => 21 => 2 choices • 2 bits => 22 => 4 choices • 4 bits => 24 => 16 choices • 8 bits => 28 => 256 choices

  16. 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

  17. Color at 16 bit Color Depth16 bits total to represent a color • 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?

  18. …16 bit color • 16/3 = 5 bits with one bit 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? • Our environment is green • Lots of green to discriminate • Assign 5 bits to R & B, and 6 bits to G • allows twice (how?) as many greens as blues

  19. Why more than 16.8 million? • 24 bits is plenty for human vision... • 30 and 48 bit color 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 see 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)

  20. 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

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

  22. 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 palate) to store colors • Use a map of logical colors to reference the color map

  23. 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

  24. Indexed color vs. 8 bit direct color • 8 bit direct color... an example • suppose... • Red gets 3 bits ==> 000, 001, 010, 011... 111 ==> 8 values • Green gets 3 bits ==> 000, 001, 010, 011... 111 ==> 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 palate • 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

  25. Indexed colorwith 256 colors in palate • 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

  26. Nearest color • Loss of some detail • Distorted color • Generates artifacts • Banding or posterization • Example ===>

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

  28. Questions?

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