Information Technology in Theory
By Pelin Aksoy and Laura DeNardis
Chapter 7
Digital Images and Video
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
Information Technology in Theory
2
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
Information Technology in Theory
3
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
Information Technology in Theory
4
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
Information Technology in Theory
5
Mosaic Image
Information Technology in Theory
6
Pixelized Images
Information Technology in Theory
7
A Bitmap Image
Information Technology in Theory
8
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
Information Technology in Theory
9
Capturing Color Images
Using Digital Cameras
•
•
•
•
•
•
•
Lens system
Sensor array
Filter system
Memory
Processor
Software
Viewing screen
Information Technology in Theory
10
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
Information Technology in Theory
11
The CCD Array
Information Technology in Theory
12
The Additive Color Model
Information Technology in Theory
13
The Additive Color Model
Information Technology in Theory
14
Spinning Disk Filter
Information Technology in Theory
15
Image Resolved Into Basic Color
Components
Information Technology in Theory
16
Color Palette and Corresponding
Binary Codes
Information Technology in Theory
17
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
Information Technology in Theory
18
Spinning Disk Filter vs. Bayer Filter
Information Technology in Theory
19
Calculating Digital Camera
Image 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
Information Technology in Theory
20
Scanning Color Images
Scanner
•
•
•
•
•
•
Information Technology in Theory
Lens system
Sensor array
Filter system
Processor
Mirrors
High-intensity lamp
21
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
Information Technology in Theory
22
The CCD Array of a Scanner
Information Technology in Theory
23
CCD Array Positioned on a
Scanner
Information Technology in Theory
24
Calculating Scanned Image Sizes
Calculate 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
Information Technology in Theory
25
Some Facts on Resolution
• Two characteristics determine the quality of
digital images:
– Spatial resolution
– Brightness resolution
Information Technology in Theory
26
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
Information Technology in Theory
27
Images with Different Spatial
Resolutions
Information Technology in Theory
28
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
Information Technology in Theory
29
Monochrome Image
Information Technology in Theory
30
Images with Different Brightness
Resolutions
Information Technology in Theory
31
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
Information Technology in Theory
32
Digital Video (continued)
A stick figure drawn in different positions can be brought
to life by flipping the pages in rapid succession
Information Technology in Theory
33
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
Information Technology in Theory
34
Image Formats
•
•
•
•
•
JPEG: Joint Photographic Experts Group
GIF: Graphics Interchange Format
PNG: Portable Network Graphics
BMP: Bitmap Picture
TIFF: Tagged Image File Format
Information Technology in Theory
35
Video Formats
•
•
•
•
MPEG: Moving Pictures Experts Group
AVI: Audio Video Interleaved
WMV: Windows Media Video
MOV: Apple (Quicktime)
Information Technology in Theory
36
Display Technologies
•
•
•
•
CRT: Cathode Ray Tube
LCD: Liquid Crystal Display
LED: Light Emitting Diode
Plasma Display
Information Technology in Theory
37
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
Information Technology in Theory
38
Components of a CRT Display
Information Technology in Theory
39
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
Information Technology in Theory
40
Components of an LCD Display
Information Technology in Theory
41
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
Information Technology in Theory
42
Components of an LED Display
Information Technology in Theory
43
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
Information Technology in Theory
44
Technical Characteristics of
Display Devices
•
•
•
•
Spatial resolution
Screen size
Pixel/dot pitch
Brightness resolution (bit depth)
Information Technology in Theory
45
Measurement of Screen Size
Information Technology in Theory
46
Pixel/Dot Pitch
Information Technology in Theory
47
Projection Systems
• Film-based projectors
• LCD-based projectors
• Digital light processing (DLP) based projectors
Information Technology in Theory
48
Components of a DLP Projector
Information Technology in Theory
49
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
Information Technology in Theory
50
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
Information Technology in Theory
51
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)
Information Technology in Theory
52