Digital Image Fundamentals Chapter 2. KEY CONCEPTS Sampling and quantizing in digitizing images...

Digital Image Fundamentals

Chapter 2

KEY CONCEPTS

• Sampling and quantizing in digitizing images• Pixels and image resolution• Image bit depth

KEY CONCEPTS

• How pixels, image resolution, and bit depth are related to sampling and quantizing

• Color representation in digital images• Bit-mapped images versus vector graphics

Natural Image (pretend)

Capture: Scanner or Digital Camera

Sampling

How many pixels do you capture?

Area the might have much detail

Is reduced to one pixel“Averaging” effect

Sampling

Each Sample Point is translated into a pixel

Real world image:Infinite detail

Single Pixel:Finite detail

Quality Loss illuminated

• Once the digital image is captured….• There is no way to “restore” the original

detail.

Analog / Real World Images

• In the real world, we can perceive almost infinite detail

• By…– Moving closer– Using a microscope– Nano-visualization

Sampling: Number of Pixels

Original 7500 pixels (100 X 75)

Sampling

Scanners• Up to 4800 pixels per inch• 8 X 10” photograph • 38,400 pixels X 48,000 • 1,843,200,000

• 1.7 Gigapixels * 32-bit color• 6.8 Gigabytes

Digital Cameras• Up to 12 Megapixels• 4000 X 3000 pixels

• Consider 300dpi printing• 13 X 10’ looks great

• Consider 150dpi printing• 26 X 20” looks pixelated.

Megapixel

• Associated with digital cameras• Camera manufactures do NOT follow the

base-2 standard so – a Megapixel is literally 1 million pixels – not 220 pixels.

• A 3000 × 2000 pixel digital image is 6,000,000 pixels– Referred to as six megapixels.

Pixels and Dots

Pixel• Just the base unit of

storage.• Does NOT have a set size,

i.e., 1 millimeter X 1 millimeter– Pixel size depends on the

monitor or projector

• May not be a square– Could be a circle of light

Dot• Smallest unit that a printer

can print.• Smallest unit that a scanner

can detect

Over sampling

Scanner• 4800dpi

• 4800/300 = 16

• 16 X 16

Printer• 300dp

• Printed as one dot

Under sampling

Scanner

• 300dpi

• One dot

Printer• 600dpi• 600/300 = 2• Printed using 2 X 2

• NOT taking full advantage of the printers capabilities.

Quantization: Number of Colors

Original (16.7 million colors) 2 colors (1-bit)

Quantization

• A natural image is colored in continuous tones– theoretically has an infinite number of colors.

• Binary representation restricts the reproduction colors and shades. – 8 bits can only encode 256 different color values

• In image capturing, the process of encoding an infinite number of possible colors into a finite number list of colors is called quantization.

Color selection

• Quantization limits how many colors you can represent in a digital image, but

• Not which colors you decide to encode.

• Although this image has only 8 colors

• The colors are very specific to the original image.

Color PalettesImage formats (like GIF) can encode different palettes - depends on colors in the original image.

If you chose the colors wisely the image looks more realistic.

Color Translation

Meta data “maps” each 3-bit code to a universal 32-bit palette

000 10101010101010101010101001010101001 10101010101010100010010010010101010 11010101001000111101010101010101011 00010101010101010101111010100011100 11101010111010111011010010100010101 10101001000100010011110101010000110 10101010100010110001111001101011111 00100010001111010010000001111111

Red Green Blue• A single 8-bit byte can be used to represent one of 256 different

possible values. • The values range from 0 to 255. • An RGB (red, green, blue) color can be represented in three 8-bit

bytes• Example:

– Red: 255 (maximum red) 8-bits– Green: 0 (minimum green) 8-bits– Blue: 127 (50% blue) 8-bits

24-bits total

• What color is this?

Red Green Blue

• Although 24-bit color is sufficient for human vision, 48-bit RGB can be used when applications need to analyze images beyond the spectrum visible to humans.

• 48-bit RGB color is represented using 16 bits per component of R, G, and B.

32-bit color

• A 32-bit image is basically 24-bit RGB with an additional 8-bit alpha channel.

• The alpha channel is used to specify the level of transparency.

• Unlike 24-bit images that are fully opaque, 32-bit images can be smoothly blended with other images.

Alpha-channel

• Refers to the transparency of a color.

• Transparency is a powerful feature because it better models real world entities.

• Consider a digital image of a tinted window.

Questions

• How many possible colors can be represented with 48-bit color depth?

• How many possible levels of red can be represented with 48-bit RGB color?

• How many times would a file size increase by going from 24-bit to 48-bit?

Bitmapped Images

• Hopefully, lab helped illustrate this…

Bitmapped Images

But now consider that instead of these being 1’s and 0’s, they are 24-bit, 32-bit, or 48-bit color codes.

Bitmaps Raster Graphics

• Bitmapped images also are called raster graphics

• “rastering” refers to the way most video displays translate the images into a series of horizontal lines on the screen.

Bitmaps

• Bitmap image formats are the most commonly used in image-editing applications.

• However, bitmap appearance depends on the resolution of the output device– Bitmapped images can appear jagged and lose

detail when they’re scaled onscreen or printed.

Jagged images (or Jaggies)

• A picture is worth a thousand words, right?

Alternative to Bitmaps

• Besides pixel by pixel representation, what other way could we store images digitally?

• Hmmm? Think about it.

• Math Power!

Vector Graphics

• Vector graphics is the use of geometrical primitives such as – points, – lines, – curves, and– polygon(s)

• …to represent images.

Vectors

• points, • lines, • curves, and• polygon(s)• are all based upon mathematical equations.

x, y coordinate

x1, y1

x2, y2

Splines (not in book)

• A spline is a curve defined by piecewise polynomial functions.

Fonts – The first vector images

(a) vectors still appear smooth at higher magnification

(b) rasterized graphic (c) rasterized graphic with

anti-aliasing

Fonts

Font magnification example:

g

Rasterization

Stored as vector• Vectors are literally made

up of mathematical formulas

• No pixels at all• In principle, vectors can be

rendered at limitless resolution

Displayed as scalable raster• Monitors and projects still

display using pixels.• To display a vector image,

software has to convert the vector information into a temporary raster image.

• Called rendering or rasterization

Aliasing (formal word for jagged)

• Rasterized images will always appear jagged– You just have to zoom in.

• This jagged effect is called aliasing

• caused by under-sampling or over-maginfication.

Anti-Aliasing

• Pixels with intermediary shades can be used to soften the jaggedness

• This technique is called anti-aliasing.

• Technique to make rasterization more smooth.

Raster (bitmap) and Vector formats

Raster• GIF• JPEG• PICT• TIFF

Vector• SWF (Shockwave Flash)• SVG (Scalable Vector Graphic)• EPS (Encapsulated Postscript)• AI (Adobe Illustrator)

Combo-format (not in book)

PNG (Portable Network Graphic) – Raster format but includes vector information when applicable.

File Size

• How much file space does a 6-megapixel 24-bit color image take up uncompressed.

• 6,000,000 pixels × 24 bits per pixel• = 144,000,000 bits• File size in bytes: • 144,000,000 bits/(8 bits per byte) • = 18,000,000 bytes

Obvious Compression

• Reduce sampling– Lower the resolution– 3000 X 2000 (6 megapixels) 1500 X 1000 (3

megapixels• Lower the color depth– 24-bit color 16-bit color– Maybe there are only 20,000 different colors

present

Clever Compression

• Run-length encoding (RLE) • runs of data are stored as a single data value

and count, rather than as the original run.• 0000000111110000000000000 (25 bits)• (0,7) (1,5) (0, 13)• (0, 0111) (1, 0101) (0, 1101)• 001111010101101 (15 bits)• Patterns instead of runs can be counted.

Clever Compression

• Huffman coding• Find most frequent color, use smallest

representation– 1000 white pixels (1)– 80 black pixels (01)– 24 pink pixels (001)– 8 red pixels (0001)– 3 blue pixels (00001)

• 111101001010001

LZW Compression

• Lempel-Ziv-Welch (LZW) is a universal lossless data compression algorithm

• created by Abraham Lempel, Jacob Ziv, and Terry Welch.

• It was published by Welch in 1984• Used in GIF compression.

DEFLATE Compression

• Deflate is a lossless data compression algorithm that uses a combination of the LZW algorithm and Huffman coding.

• Used by the PNG format.

JPEG

• The name "JPEG" stands for Joint Photographic Experts Group, – the name of the committee that created the standard.

• Standardized format published in 1992• most widely-used format for storing digital

photographs• Lossy compression algorithm• Reduces pixel blocks (typically 8X8) into a single

representation.

Representing Color

Color is a light wave

The wavelengths of visible light range from about 380 to 700 nm (nanometers)—creating a continuous spectrum of rainbow color, from the violet end (380 nm) to the red end (700 nm).

Color is a light wave

The retina of the human eye has two categories of light receptors: rods and cones. Rods are active in dim light but have no color sensitivity. Cones are active in bright light and have color sensitivity.

Color is a light waveThere are three types of cones: one type is sensitive to red (ρ) wavelengths, one to green (γ), and one to blue (β). The curves represent the relative sensitivity of these three receptors for the normal human eye

• Color is how our eyes perceive different forms of energy.

• Energy moves in the form of waves.

What is color?

What is a wave?

• Think of a fat guy (Dr. Breimer) doing a cannonball into a pool.

• The incredible energy created by my fat ass hitting the water is transfer and dispersed into the pool in the form of a wave

Why does energy move in waves?

• I don’t f***ing know. Are you 4-years old? you have to ask a million stupid questions?

• Seriously, there is some complex physics behind the reason, but here is a simple way to explain it….

Why does energy move in waves?

• Q: How does a snake move without legs?• A: By going “swish swish”• Similarly, the “swish swish”

of a wave allows energy to move even in a vacuum.

Why does energy need to move anyway?

• To get a 40oz beverage from the liquor store

Where were we anyhow?

• Short wavelength • Long wavelength

• Light is a form of energy that travels in a wave pattern.

• The length of the wave can vary

The Human Eye…

• has Cones and Rods (like nerves) that can detect different wavelengths of light…

• and send signals to the brain.

• We can only see a very limited range of wave lengths.

• What would it be like if we could see microwaves?

Visible Energy

What microwaves might look like

Spectrum of visible light

What is your favorite color?

• Can you guess mine?– Infared

• My son’s favorite color is yellow, red, black, white, blue, purple, brown (poop color), khaki (light poop color), and orange.– This is his way of saying he hates pink

Tristimulus Theory

• Any color can be produced by mixing different amounts of three additive primaries

Monitors

• Create color by using a combination of red green and blue light (RGB)

How do TVs and Computer Monitors create color?

How do TVs and Computer Monitors create color?

• The same way our eyes detect color.• By mixing the three wavelengths your eyes

can detect.

Red, Yellow and Blue (NOT!)

• In kindergarten, we all learned that the primary colors were:

• Red, Yellow, and Blue, right?• Well, that was a lie.• Just, like in 1st grade when they told you there was a

giant vacuum in space.• There is NO giant vacuum in space.• Microwaves are NOT invisible.• And, Yellow is NOT a primary color!

Yellow

• Yellow is ONLY considered primary when mixing paint or ink

• Mixing paint is different than mixing light• More colors =

darker color• Red + Green is too

dark (brownish, notyellow)

Green

• Mixing light is different than mixing paint.• It is an additive and synergistic process

• More color = lighter color• Red + Green = bright yellow.• Red + Green + Blue = white!

Back to TVs and Monitors

• The surface is black, no light equals black.• Each pixel is created from three separate light

signals.• Two models:– RGB: Red, Green, Blue– CMYK: • Cyan• Magenta• Yellow• Key (level of intensity – bright to dark)

Pixel Components• If you put colors close enough together, the

eye perceives them as one color.

TVs and Monitors

• Light signals can be generated in many different ways

• The key is that you want the pixel to be very small and bright.

• Three technologies:

1. CRT: Cathode Ray Tube2. LCD: Liquid Crystal Display3. Plasma

CRT: Cathode Ray Tube• Glass tube containing an electron gun and a

fluorescent screen

LCD: Liquid Crystal Display• Each pixel consists of a layer of molecules

aligned between transparent electrodes, and polarizing filters

Plasma TV

• Cells between two panels of glass hold neon and xenon gas. Gas is electrically turned into a plasma which excites phosphors to emit light.

RGB vs. Wavelength

• Technologically, it is easier to control color by emitting three different colors RGB, rather than vary the wavelength to create a “pure” color.

• Similar to Binary– Can encode any number in binary– Can encode any color with RGB combination

RGB vs. Wavelength

• In fact, the cones and rods in the eyes detect only three colors.

• We see more than three because the cones and rods send “mixed” or synergistic signals to the brain.

• Humans have a hard time distinguishing RGB mixtures from “pure colors” because we sense color as RGB mixtures anyway.

RGB is great but not perfect

• You can NOT reproduce all the visible color wavelengths using RGB combinations

• But, you can get pretty close.

RGB vs. CMYK

• RGB is NOT suitable for printing on paper.• Color printers can NOT produce

Yellow (Red+Green) because ink does not have the same synergistic properties of light.

• Thus, Yellow has to be a primary pigment.

• The color wheel gets turned.

RGB vs. CMYK

• CMYK: Cyan, Magenta, Yellow, and K (Key) which is really black.

• RGB is used almost exclusively for TVs/Monitors (where the surface is Black), you don’t need Key/Black

• Because CMYK is also for print (where paper is typically white), you need Black (C+M+Y = purplish brown).

• How do you get White with RGB?

•Subtract additive primary from white gives its complement–Equivalently, add other two additive primaries

•C = G+B = W-R

•M = R+B = W-G

•Y = R+G = W-B

•Cyan, magenta and yellow are subtractive primary colours (mixing ink/paint)

Complementary Colours170–171

CMYK

• CMYK encoding is used for applications that focus on printing: Photo Developing software and publishing software like QuarkXpress, Framemaker, etc.

• Applications that use RGB must convert to CMYK for printing

• Some RGB colors (on the monitor) can be perfectly matched using CMYK.

RGB vs. CMYK

Digital Color

• Operating Systems and applications encode color using bits.

• Very early color systems only used 2 bits (4 colors).• Dr. B’s first computer (IBM 8086) supported only 4

colors CMYK.• As process speeds increased and graphics hardware

improved• 8 bit color and 16 bit color became the standard

(1988-1994)

Data Color

• Assume a four color encoding (2 bits)• Assume a monitor with 640 X 480 pixels• Monitor refreshes 60 times per second – (60 Hertz)

• The operating system must send…• 640 X 480 X 2 X 60 bits per second.• = 36 million bit per second.

Data Color: Hardware• Monitor plugs into a video/graphics card.• The video card converts the bit pattern into an

electrical signal.• Monitors and graphics cards

work together because of international standards.– For example,

VGA standard

Monitor

• The electrical signal triggers the pixel color. • CRT and LCD technology has a limit on– How small a pixel can be.– How bright it can be– How often it can be refreshed• 60-90 Hertz is the typical range

Data Color: Software

• The graphics card actually plugs into the mother board of the computer.

• The bit pattern travels across the motherboard.• A device driver is used so that the operating system

can communicate with the graphics card.• A device driver is just small program…still written

directly in assembly language.

Graphics Cards

• Old graphics card were just signal converters• New graphics cards have memory (RAM) and

processors– Takes the burden off of the computer’s processor.– Enables 24-bit color at resolutions as high as 2560x1600.– Plus graphics card can also do things like render vectors

(geometry computations).

• http://www.nvidia.com/page/geforce_8800.html

http://www.nvidia.com/page/geforce_8800.html

RGB Color Depth

• Choose number of bits for each of R, G and B• More bits per color means more total colors,

but image files will be larger• 8 bits per color is not the standard: 24-bit

color, 16.7 million colors

0 255 218

RGB Color Depth

• 8 bits (1 byte) per component means that you have 256 different “levels”

• If R = G = B, color is a shade of gray.• Human eye can distinguish 256 shades of gray• So, while 16.7 million colors is beyond what

the human eye can distinguish.• 24-bit RGB is under quantized for gray.• But for Gray only.

Practical Technique:Color Palettes

• Choose 256 most important colors in an image to store in its palette

• When 24-bit image is reduced to indexed color, some colors may be missing form the palette– Replace missing color by nearest, may lead to posterization– Dither – use pattern of dots and optical mixing

• Web-safe palette – 216 colors guaranteed to reproduce accurately on all platforms and browsers

•Alternative way of specifing colour•Hue (roughly, dominant wavelength)•Saturation (purity)•Value (brightness)•Model HSV as a cylinder: H angle, S distance from axis, V distance along axis•Basis of popular style of colour picker

HSV173–176

Digital Image Fundamentals Chapter 2. KEY CONCEPTS Sampling and quantizing in digitizing images...

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Transcript of Digital Image Fundamentals Chapter 2. KEY CONCEPTS Sampling and quantizing in digitizing images...