Download Multimedia - School of Computer Science

School of Computer Science &
Information Technology
G6DPMM - Lecture 4
Graphics & Still Image Representation
Analogue vs Digital
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Analogue information
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Continuously variable signal
Physical phenomena
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Waveform
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Sound/light/temperature/position/pressure
Electromagnetic (e.g. light)
Pressure (e.g. sound)
Information conveyed by amplitude and frequency
Digital information

Discrete values
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Smoke signals / Morse code / Binary electronic
Sampling of analogue information
Analogue to Digital Conversion (A2D) is sampling
Analogue Media
 We
see an analogue world
 Analogue image storage technologies:
 Paint
/ Chemical film / Photocopier / Video
 Analogue
systems all have “noise”
 Random
variations
 Hence sequential copies deteriorate
 Analogue
media is hard to manipulate by
computer
 Generally involves
computer-controlled devices
Digital Media
 Digital
media is very much easier to
manipulate by software
 Digitisation is never perfect
 A2D
Sampling is an approximation
 Quality is dependent upon the amount of sampling
done
 High
quality digital media tends to be large
 Lots
of bits needed to store samples!
 Compression
is a major issue
Types of Graphics

Computer graphics fall into two categories:
 Vector Graphics
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Used for computer generated images, line
drawings, cartoons etc.
Bitmap (Raster) Graphics

Used for photographs
Vector Graphics
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Mathematical definitions of lines
 Scaleable
 Not suitable for photographs
 Examples
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Postcript
CGM
WMF
HPGL
DXF
Edited using “drawing” software
Bitmap Graphics
Matrix of ‘pixels’
 Difficult to re-size
 Suitable for photographs

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Examples
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BMP (DIB)
GIF
PCX
TIFF
TARGA
JPEG
PNG
Edited using painting software (eg Photoshop)
Bitmap Graphics
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Colour Depth
Bits per Pixel
No. Colours
1
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16
...
24
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8
16
32
64
128
256
512
1,024
2,048
2,096
8,192
16,384
32,768
65,536
16,777,216
Colour Depth
Colour
Depth
1-bit
4-bit
8-bit
16-bit
24-bit
Maximum
Colours
2
16
256
65,536
16,777,216
Demonstration
File
38.46 Kb
153.72 Kb
308.28 Kb
615.5 Kb
921.65 Kb
Colourmapping
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8-bit colour depth - pixels contain a reference to a
“palette” (ie 24-bit values)
High quality 8-bit (256 colour) images
16-bit colourmapping (32,768 colours)
Reasons for colourmapping
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Hardware may require it
 Some software manipulation requires it
 Some compression techniques require it
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Optimised vs System palette
Display Mode and Palette Flashing
Vector / Bitmap Conversion
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Vectors Bitmap
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Easy
Perfect representation - scaling issue
Bitmap Vector
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Much harder - autotrace
 Poor quality
 Highly lossy
The Need for Compression
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Graphics tend to be big!
 Consider the following:
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1024x768 24-bit image
1024 x 768 = 786,432 pixels
786,432 x 24 bits = 18,874,368 (c. 18.4 Mb)
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Approximately 40 images per 750 Mb CD-ROM!
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Data compression is essential
Image Compression
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Lossless Compression
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Decompressed image is a perfect copy of the original
Example File Format: GIF
Lossy Compression
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Decompressed image is an imperfect approximation
of the original
Example File Format: JPEG
Lossless Algorithms
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Run Length Encoding
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LZ77
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Lempel-Ziv substitutional compression (1977)
Keeps track of a “window” of data – if repetition is seen it replaces this with
a reference.
Many applications – including Huffman (LZH) and Zip
LZW (Lempel-Ziv-Welch)
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Sequences of “runs” of repeated data is replaced by a single data item, and
the length of the run.
Used by TIFF, DIB/BMP
Derived from LZ77
Developed (and patented) by Unisys – licensed for Compuserve
Used by GIF
Deflate
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Derived from LZ77
Used by PNG
Lossy Algorithms
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Common algorithms all operate on the waveform
 Fourier Transform
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DCT (Discrete Cosine Transform)
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A type of Fourier transform
Waveform is expressed as a weighted sum of cosines
Used by JPEG
Wavelets
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A technique for expressing a waveform as a weighted series of
sines and cosines
An alternative to Fourier transform
Signals converted into a series of rough-edged wavelets
Mostly used for specialised purposes (e.g. for fingerprints)
Fractal Compression
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Fractal theory
Not (currently) widely used
JPEG Compression
Original Image - 285 K
JPEG Compression
50% Compression - 15 K
JPEG Compression
70% Compression - 10.8 K
JPEG Compression
90% Compression - 6.9 K
JPEG Compression
95% Compression - 5.3 K
JPEG Compression
99% Compression - 2.6 K
Image File Formats
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Vendor Defined Formats
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OS Vendors (eg Microsoft / Apple)
 Application Vendors (eg Adobe)
 May be open, (ie published specifications) or closed
(protected by IPR)
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Vendor Neutral Formats
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Usually defined by standards organisations
Apple Macintosh Formats
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PICT
Very versatile
May contain bitmap and vector graphics, and
metadata.
May be compressed or uncompressed using
various algorithms.
Can be ported to other platforms, but some
features usually lost.
Now rarely used – even on Macintosh!
Windows Formats
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Microsoft DIB (Device Independent Bitmap)
 .BMP .DIB .RLE
 1, 8 or 24 bit bitmap - optional RLE compression
 Microsoft PAL (Palette)
 Palette for 8 bit images
 Microsoft RIFF
(Resource Interchange File Format)
 Embedded DIB
 Other media types
 Windows Metafiles (WMF)
 Usually used for vectors, but can contain almost anything!
Adobe Formats
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Photoshop PDF
 Bitmap format (mostly)
 Uncompressed
 Supports various colour models
 Supports all features of Photoshop (eg layers, channels
etc).
 Version issues
 Postcript
 Page description language for printers
 Encapsulated postcript (EPF)
 Primarily used for Vectors, but can contain embedded
bitmaps.
Truevision Targa
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Truevision - graphics hardware & software company
Targa (TGA)
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Bitmap (1 to 32 bits), with optional RLE
Multiple images (eg different resolutions)
Metadata
Advanced features (eg alpha channels and gamma values)
Developer definable data
Very widely used for storage of high quality
(24 or 32 bit) images.
Aldus/Adobe TIFF
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TIFF (.TIF) Tag Image File Format
 Formerly Aldus - now maintained by
Adobe
 24 bit bitmap format
 Supports a wide range of compression
algorithms (including RLE, LZW, JPEG
and many others)
 Extensive metadata capability
CompuServe GIF
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8 bit LZW compressed bitmap
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Supports transparency
Supports multiple images & animation
 Widely used on WWW
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Licence problems
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CompuServe patent
 Unisys policy
PNG
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Portable Network Graphics
 Designed to replace GIF
 Supports greyscale, colourmapped or
truecolour images (up to 48 bit!)
 Supports alpha channels and gamma
correction
 Lossless CRC-32 compression
 No multiple image support
 No patent problems!
JPEG
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Joint Picture Expert Group
Both an algorithm and a file format!
Lossy Truecolour compression (DCT)