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: Number of Pixels Original 1900 pixels (50 X 38)

Sampling: Number of Pixels Original 300 pixels (20 X 15)

Sampling Scanners Digital Cameras 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 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 Dot 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 Smallest unit that a printer can print. Smallest unit that a scanner can detect

Over sampling Scanner Printer 4800dpi 4800/300 = 16 16 X 16 300dp Printed as one dot

Under sampling Scanner Printer 300dpi One dot 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: Number of Colors Original (16.7 million colors) 4 colors (2-bit)

Quantization: Number of Colors Original (16.7 million colors) 8 colors (3-bit)

Quantization: Number of Colors Original (16.7 million colors) 16 colors (4-bit)

Quantization: Number of Colors Original (16.7 million colors) 32 colors (5-bit)

Quantization: Number of Colors Original (16.7 million colors) 64 colors (6-bit)

Quantization: Number of Colors Original (16.7 million colors) 256 colors (8-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 Palettes Image 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  10101010101010101010101001010101 001  10101010101010100010010010010101 010  11010101001000111101010101010101 011  00010101010101010101111010100011 100  11101010111010111011010010100010 101  10101001000100010011110101010000 110  10101010100010110001111001101011 111  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 x2, y2 points, lines, curves, and polygon(s) are all based upon mathematical equations. x, y coordinate x1, y1

Splines (not in book) A spline is a curve defined by piecewise polynomial functions.

Fonts – The first vector images vectors still appear smooth at higher magnification rasterized graphic rasterized graphic with anti-aliasing

Fonts Font magnification example: g

Rasterization Displayed as scalable raster Stored as vector Vectors are literally made up of mathematical formulas No pixels at all In principle, vectors can be rendered at limitless resolution 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 GIF JPEG PICT TIFF 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 color depth 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 wave There 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

What is color? Color is how our eyes perceive different forms of energy. Energy moves in the form of waves.

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? Light is a form of energy that travels in a wave pattern. The length of the wave can vary Short wavelength Long wavelength

The Human Eye… has Cones and Rods (like nerves) that can detect different wavelengths of light… and send signals to the brain.

Visible Energy We can only see a very limited range of wave lengths. What would it be like if we could see microwaves?

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, not yellow)

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: CRT: Cathode Ray Tube LCD: Liquid Crystal Display 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?

Complementary Colours 170–171 Complementary Colours 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)

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

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 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

HSV Alternative way of specifing colour 173–176 HSV 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