Chapter 7 Digital Images and Video Information Technology in Theory By Pelin Aksoy and Laura...

Chapter 7Digital Images and Video

Information Technology in Theory

By Pelin Aksoy and Laura DeNardis

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Objectives

• Learn how images are digitized • Understand the technical features and mathematical

calculations that determine the bit size of a digital image

• Understand the process by which digital video is made

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Objectives (continued)

• Identify the factors that affect the quality of digital images

• Learn about popular digital image and video file formats

• Understand the technical attributes of different types of display technologies

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Imaging Technologies

• Traditional camera– Invented more than a century ago

– Captures images with a few simple components:

• Lens

• Shutter system

• Photographic medium such as film

– Light from the object is focused by the lens, the quick shutter action exposes the film to light, and the image is permanently captured on film

– The film is then developed and printed on photographic paper

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Digitizing Images and Video

• A digital image is an electronic file made up of a string of 1s and 0s when stored on a medium

• When the image file is accessed and shown on a display screen or monitor, it is called a pixelized image

• Composed of many elementary units called pixels (short for picture elements), each containing a single unit of color information

• A pixelized image is similar to a mosaic

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Mosaic Image

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Pixelized Images

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A Bitmap Image

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Vector Graphics

• Images are represented in the form of mathematical expressions

• Parameters can include attributes such as orientation, length, width, and color

• Takes up less storage space than bitmap images• Can be easily scaled without degradation in quality

and resolution• Useful for representing geometric shapes but not

effective for photographs

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Capturing Color Images Using Digital Cameras

• Lens system• Sensor array• Filter system• Memory• Processor• Software• Viewing screen

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Capturing Color Images Using Digital Cameras

• Lens system is used to focus the image onto a small sensor array called a CCD (charge-coupled device) array

• These sensors detect light and produce electricity proportional to the intensity of that light

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The CCD Array

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The Additive Color Model

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The Additive Color Model

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Spinning Disk Filter

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Image Resolved Into Basic Color Components

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Color Palette and Corresponding Binary Codes

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Color Gamut

• The color gamut is directly related to the number of bits assigned per pixel

• The number of bits allocated to each pixel is called the bit depth

• Commercial cameras usually have a bit depth of 24, meaning that 224, or roughly 16.7 million, colors can be captured by the camera; “true color”

• This number of bits is necessary because our eyes are sensitive enough to detect approximately 10 million colors

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Spinning Disk Filter vs. Bayer Filter

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Calculating Digital CameraImage Sizes

Calculate the number of megabytes in a color image captured by a digital camera that can produce images with 2272 x 1704 pixels with a bit depth of 24

1. Calculate the total number of pixels:Number of pixels = 2272 x 1704 = 3,871,488

2. Calculate the number of bits: 3,871,488 x 24 = 92,915,712

3. Convert bits to bytes (8 bits = 1 byte):92,915,712 bits / 8 = 11,614,464

4. Convert bytes to megabytes (1MB = 220 bytes = 1,048,576):11,614,464 bytes / 1,048,576 = 11.07MB

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Scanning Color Images

• Lens system• Sensor array• Filter system• Processor• Mirrors• High-intensity lamp

Scanner

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Scanning Color Images (continued)• Object to be scanned is placed on a glass plate• The object is illuminated with a high-intensity light• A scanning head that carries the CCD array is

positioned on the object and slides over it as scanning takes place

• The image is filtered through RGB filters and focused on the CCD array by way of mirrors and lenses

• The image is quantized and converted to a binary number, then stored as an electronic file

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The CCD Array of a Scanner

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CCD Array Positioned on a Scanner

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Calculating Scanned Image SizesCalculate the number of megabytes produced at the output of a 1200 x 2400 dpi scanner that has scanned an 8.5 x 11-inch photograph in true color

1. Calculate the total number of pixels:Number of pixels = (1200 x 8.5) x (2400 x 11) = 269,280,000

2. Calculate the number of bits (true color corresponds to 24 bits): 269,280,000 x 24 = 6,462,720,000

3. Convert bits to bytes (8 bits = 1 byte):6,462,720,000 bits / 8 = 807,840,000

4. Convert bytes to megabytes (1MB = 220 bytes = 1,048,576):807,840,000 bytes / 1,048,576 = 770.4MB

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Some Facts on Resolution

• Two characteristics determine the quality of digital images:– Spatial resolution

– Brightness resolution

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Spatial Resolution

• Spatial resolution– An indicator of the number of pixels per unit length

– The more pixels per unit length, the higher the spatial resolution

– The higher the spatial resolution, the better the image quality

– Affected by image size, viewing distance

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Images with Different Spatial Resolutions

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Brightness Resolution

• Related to bit depth and thus the number of colors present in the image

• Low brightness resolution = fewer colors• Changing the brightness resolution changes the

number of colors visible in the image• Human eye can discern about 24 bits worth of

information• Dependent upon the device used to capture the

image

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Monochrome Image

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Images with Different Brightness Resolutions

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Digital Video

• Motion is emulated by showing images in rapid succession

• Visual persistence causes us to see an image for a split second after closing our eyes

• Human eyes can process approximately 20 images per second before losing the ability to distinguish the transition of one image to another

• Motion picture industry standard uses 24 frames per second (fps) to create a continuous motion effect

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Digital Video (continued)

A stick figure drawn in different positions can be brought to life by flipping the pages in rapid succession

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Digital Image and Video Formats

• Digital images and videos could not be captured, stored, and disseminated without the existence of standard formats for compression

• These formats provide the sets of rules, or protocols, required to compress and view images on display devices such as televisions, computer screens, cellular phones, and PDAs

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Image Formats

• JPEG: Joint Photographic Experts Group• GIF: Graphics Interchange Format• PNG: Portable Network Graphics• BMP: Bitmap Picture• TIFF: Tagged Image File Format

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Video Formats

• MPEG: Moving Pictures Experts Group• AVI: Audio Video Interleaved• WMV: Windows Media Video• MOV: Apple (Quicktime)

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Display Technologies

• CRT: Cathode Ray Tube• LCD: Liquid Crystal Display• LED: Light Emitting Diode• Plasma Display

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CRT Displays

• CRT– Some computer monitors– Traditional television sets

• Cathode ray tube emits three electron beams– Strength and direction of travel is controlled

electromagnetically by a beam deflection mechanism

– Follows a raster scan format

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Components of a CRT Display

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LCD Displays• LCD displays

– Laptop computers– Digital camcorders– Gaming devices– Cellular phone

• Use liquid crystal materials, whose properties can be varied by electrical signals

• An LCD system contains two glass plates that sandwich a liquid with tiny crystals filled in small cells, two polarizing filters on each side of the glass plates, a color filter, and backlighting for illumination from one face of the plates

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Components of an LCD Display

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LED Displays

• LED displays – Present both still and time-varying digital

images– Large billboards, concert hall marquees,

and sports events – An array of tiny light emitting diodes,

similar to small bulbs, which emit light of a certain color

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Components of an LED Display

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Plasma Displays

• Use an ionized gas that is trapped inside cells between two glass plates

• Each cell acts like a miniature fluorescent bulb

• The intensity of light emitted by each cell is controlled with electrical signals, and various phosphors that glow as red, green, and blue painted on one of the glass plates as light from the cells impinges on them

• The combination of red, green, and blue phosphors forms the color of each pixel in the display

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Technical Characteristics of Display Devices

• Spatial resolution• Screen size• Pixel/dot pitch• Brightness resolution (bit depth)

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Measurement of Screen Size

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Pixel/Dot Pitch

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Projection Systems

• Film-based projectors• LCD-based projectors• Digital light processing (DLP) based projectors

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Components of a DLP Projector

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Summary

• A digital image file is made up of a string of ones and zeros that represent the pixelized image

• A pixelized image is made up of rows and columns of pixels, each with a unique color associated with it; a binary code expresses each pixel color

• Two characteristics that determine the quality of images are spatial resolution and brightness resolution

• Spatial resolution indicates the number of pixels per unit length (such as inches or centimeters) contained in an image

• Brightness resolution describes the bit depth, and is the number of different colors inherently present in an image

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Summary (continued)• Digital video is based on the principle of motion

emulation• Because the human eye can distinguish at most 20

images per second, the frame rate standard used in the motion picture industry is at least 24 frames per second (fps), which creates the illusion of a perfect continuous-motion effect

• Formats such as JPEG, GIF, BMP, PNG, TIFF, MPEG, AVI, WMV, and MOV provide the rules for structuring and compressing files that allow us to share and display images and video

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Summary (continued)

• The most common devices for displaying digital images are plasma displays, LCDs, CRT displays, and LED displays

• The design features that contribute to the quality of display devices are spatial resolution, screen size, pixel pitch, and brightness resolution (also called bit depth)

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