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Image Formats. Image Formats. To be able save image data for future manipulation and display we need to be able to store it in a consistent manner Formats enable standardisation and the use of graphical data by more than one application. Standards. - PowerPoint PPT Presentation
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Image FormatsImage Formats
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Image FormatsImage Formats
To be able save image data for future manipulation and display we need to be able to store it in a consistent manner
Formats enable standardisation and the use of graphical data by more than one application
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StandardsStandards
Unfortunately, when the first graphics applications appeared there were no set graphics formats
Different applications stored graphical data in the most suitable way for that application e.g.Targa (.TGA from Truevision)Scitex (.SCT)
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Graphical InformationGraphical Information
There are two principle methods of representing graphical dataVectors
Bitmaps (raster or pixel maps)
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Vector GraphicsVector Graphics
Vector images are made up of many individual, scalable objects
These objects are defined by mathematical equations rather than pixels
Objects may consist of lines, curves, and shapes with editable attributes such as colour
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Vector GraphicsVector Graphics
Unsuitable for producing photo-realistic imagery
(1,1)
(4,2)
(2,3) Radius 1
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BitmapsBitmaps
Bitmap images - also known as raster images - are made up of pixels in a grid
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BitmapsBitmaps
Bitmaps work well with the type of sampled images that are produced by real-world applications
They are good at representing complex variations in colours, shades and shapes
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GIFGIF
CompuServe developed the Graphics Interchange Format to enable graphical data to be transferred via their network
One of the most widely used image file formats
Two main versions of the format:the original definition, published in 1987
(GIF87A) enhanced version was released in 1989
(GIF89A)
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GIFGIF
It is an efficient format, but is limited to a palette of only 256 colours
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GIFGIF
One of the two main formats supported by Internet browsers
Best format for line-art images, such as icons, graphs and line-art logos
Uses LZW lossless compressionSupports the sequencing or overlay of
multiple images – Animated GIFsSupports transparent backgrounds
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GIF FormatGIF Format
GIF Signature
Screen Descriptor
Image Descriptor
Global Colour Map
Local Colour Map
Image Data
GIF Terminator
Repeated 1 to n times
Identifies GIF and format
GIF87A or GIF89A
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GIF Screen DescriptorGIF Screen Descriptor
Describes the overall parameters for all GIF images in the file
Screen Width
Screen Height
Background
Aspect ratio
M CR S Size
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M = 1, Global colour map follows Descriptor
CR = bits per pixel in image
S = whether the Global Colour Table is sortedSize = Size of Global Colour Table
Background = Index in the global colour table for the background colour
Aspect ratio of pixels
Bytes
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GIF Global Colour MapGIF Global Colour Map
This block contains a colour table, which is a sequence of bytes representing red, green, blue colour triplets
Used by images without a Local Colour Table
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GIF Image DescriptorGIF Image Descriptor
Defines the placement of the following image within the space defined in the Screen DescriptorAlso defined are flags to indicate the presence
of a local colour map and to define the pixel display sequence
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GIF Local Colour MapGIF Local Colour Map
This block contains a colour table, which is a sequence of bytes representing red, green, blue colour triplets
Used for the image data directly following it
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Image DataImage Data
Contains raster image index values for the image compressed using LZW lossless compression
Included in the image data is graphic control information for animated GIFs such as time between consecutive images
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Application ExtensionsApplication Extensions
Application Extensions allow for blocks of data to be inserted in the GIF for specific programs to act upon e.g.To loop the imagesTo set how many times an image
should loop
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Animated GIFsAnimated GIFs
Produced by defining several images within a GIF file and displaying them sequentiallyProvides simple low quality animationNot very good for large or long duration
animations
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Transparent GIFsTransparent GIFs
The GIF89a file format allows you to pick one colour from the colour map of the GIF to be transparent
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JPEGJPEG
Developed by the Joint Photographic Experts Group, a working group set up jointly by ISO and CCITT
The dominant format for high quality true colour images
JPEG images are full-colour images (24-bit, or true colour), unlike GIFs that are limited to a maximum of 256 colours in an image
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JPEGJPEG
Typically achieves compression ratios of 30:1 with true colour imagesbut both compression and unpacking are
relatively slow Most widely used of the sophisticated
image storage/compression formats such as wavelet and fractal
Can use many incompatible coding schemes, however baseline lossy method normally used
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A Word About CompressionA Word About Compression
There are two general types of compression that can be applied to digital media such as audio, video, text etc.Lossless compressionLossy compression
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Lossless CompressionLossless Compression
A very basic method of lossless compression is Run Length Encoding 11111111111111111100000111111111
• 4 Bytes long18,5,9
• 3 Bytes long
Compresses data by storing it in a more appropriate way
Rarely gives compression better than 2:1
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Lossy CompressionLossy Compression
Provides compression by removing parts of the data
Takes advantage of the properties of the human sensory system – vision and hearing
Tries to removes data without it being noticed e.g.The eye is relatively poor in distinguish
differences in chrominance (changes in colour)
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CompressionCompression
As lossless and lossy compression work in different ways they can both be applied to the same data
Compression is often most effective when both lossless and lossy compression methods are used together
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JPEG StagesJPEG Stages
Conversion from RGB to Luminance, Brightness and ChrominanceThe human eye is much less sensitive to
changes in colour (chrominance) than brightness
Colour information is subsampled (lossy)
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JPEG StagesJPEG Stages
Discrete cosine transform is then appliedIntensity data converted to frequency data
This frequency data is then quantized (lossy)
Quantized data is then Huffman compressed (lossless)
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JPEG CompressionJPEG Compression
JPEG compression efficiency as a function of approximate quality setting
Quality setting true colour image
95% 4.5 : 1
75% (default) 12 : 1
50% 19 : 1
20% 41 : 1
10% 59 : 1
5% 92 : 1
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JPEG CompressionJPEG Compression
Original image After 31:1 compression
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JPEG CompressionJPEG Compression
Magnified views of Red Sea / Gulf of Aden region
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PNGPNG
Portable Network Graphics is a new bit-mapped graphics format similar to GIFApproved as a standard by the World Wide
Web consortium to replace GIF because GIF uses a patented data compression algorithm called LZW
Uses lossless compressionHandles true colour images
Gives better quality reproduction than GIF, but no animation
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TIFFTIFF
Tag Image File Format - developed jointly by Microsoft and Aldus to exchange image data between desktop publishing and related packages
Versatile format that can handle almost any kind of image efficientlyA variety of alternative compression options
are supported. Unfortunately this makes it hard to implement decoding software
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Sun RasterfileSun Rasterfile
Developed by Sun Microsystems for storing images displayed on graphics workstations
The format is well supported by Sun, but rarely used on other systems
Easy to code and decode
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BMP DIBBMP DIB
The Microsoft Device Independent Bitmap format is the principal image format used by Microsoft Windows applications on IBM-compatible PCs
Like the Sun Rasterfile format it is easy to implement but rarely used on other systems
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