Digital Media

Dr. Jim Rowan

ITEC 2110

Color

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?

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dr.jimrowan@gmail.com

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

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

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

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

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

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

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 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 ==>

http://en.wikipedia.org/wiki/Computer_display

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

Gamut: The complete range or scope of something

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)

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

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 = 16with 3 digits that can have 256 different values256**3 = 16,777,216

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

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

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?

…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

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)

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

mainmemory

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 palate) to store colors

• Use a map of logical colors to reference the color map

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

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

Nearest color

• Loss of some detail

• Distorted color

• Generates artifacts – Banding or posterization– Example ===>

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

Questions?