Digital Media

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Digital Media

• Dr. Jim Rowan
• ITEC 2110
• Color

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Question!

• Inside Photoshop and Gimp there are image filters
  that, among other things, allow you to blur the
  image. To do this these filters use a complex
  mathematical process. What is the mathematical
  process called?
• Send answer to
• dr.jimrowan_at_gmail.com

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COLOR

• Is a mess
• Its 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?

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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 Its a very short wave

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http//en.wikipedia.org/wiki/Electromagnetic_spect
rum
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Light

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

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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

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But first Bird eye trivia

• Eagles have 600,000 cones per square mm of
  retina, humans have 150,000
• The chestral (a bird) has cones that can see UV
  light (we dont)
• Owls see with their ears
• they have flaps that are offset in front of the
  ear openings to detect vertical positioning

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One model of colorroughly based on the eye

• Rods(night vision, BW)
• Cones (3 kinds, one for red, one for blue and one
  for green)
• gt RGB
• tri-stimulus theory the theory that states any
  color can be completely specified with just 3
  values

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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

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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 (Not All!) of
  the visible colors
• Why not all colors gt

http//en.wikipedia.org/wiki/Computer_display
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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
• gt RGB cannot completely reproduce the visual
  stimulus

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Gamut The complete range or scope of something
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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)

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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

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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 24 16
• with 3 digits that can have 256 different values
• 2563 16,777,216

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Color Depth

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

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Color and Grey

• RGB (0, 0, 0) is black
• Macs and PCs are opposite go figure!
• 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

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Color at 16 bit Color Depth16 bits total to
represent a color

• RGB at 24 bits
• 24 bits gt 3 bytes
• 3 bytes, 3 colors gt one byte per color
• RGB at 16 bits
• 16 bits gt 2 bytes
• 2 bytes, 3 colors...
• 16/3 5 bits with one left over...
• HMMMmmmm...
• What to do?

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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

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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)

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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 gt 30,000 bytes uncompressed
• at 16 bit color gt 20,000 bytes uncompressed
• 1 byte gt 1/3 reduction of size

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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
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Indexed color

• Weve 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

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Indexed (indirect) color vs. 8 bit (direct) color

• 8 bit color defines only 256 colors
• Indexed color allows 256 different colors
• Whats the difference?
• 8 bit defines 256 colors---whether they are used
  or not
• Indexed color allows 256 different colors that
  exist in the image

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Indexed color vs. 8 bit direct color

• 8 bit direct color... an example
• suppose...
• Red gets 3 bits gt 000, 001, 010, 011... 111
  gt 8 values
• Green gets 3 bits gt 000, 001, 010, 011... 111
  gt 8 values
• Blue gets 2 bits gt 00, 01, 10, 11 gt 4 values
• total different colors gt 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

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Indexed (indirect) 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

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Nearest color

• Loss of some detail
• Distorted color
• Generates artifacts
• Banding or posterization
• Example gt

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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?)

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Questions?