1 Image Formats

2  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

3 Standards  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)

4 Graphical Information  There are two principle methods of representing graphical data  Vectors  Bitmaps (raster or pixel maps)

5 Vector 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

6 Vector Graphics  Unsuitable for producing photo-realistic imagery (1,1) (4,2) (2,3) Radius 1

7 Bitmaps  Bitmap images - also known as raster images - are made up of pixels in a grid

8 Bitmaps  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

9 GIF  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)

10 GIF  It is an efficient format, but is limited to a palette of only 256 colours

11 GIF  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 compression  Supports the sequencing or overlay of multiple images – Animated GIFs  Supports transparent backgrounds

12 GIF 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

13 GIF Screen Descriptor  Describes the overall parameters for all GIF images in the file Screen Width Screen Height Background Aspect ratio M CR SSize M = 1, Global colour map follows Descriptor CR = bits per pixel in image S = whether the Global Colour Table is sorted Size = Size of Global Colour Table Background = Index in the global colour table for the background colour Aspect ratio of pixels Bytes

14 GIF 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

15 GIF Image Descriptor  Defines the placement of the following image within the space defined in the Screen Descriptor  Also defined are flags to indicate the presence of a local colour map and to define the pixel display sequence

16 GIF 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

17 Image 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

18 Application 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 images  To set how many times an image should loop

19 Animated GIFs  Produced by defining several images within a GIF file and displaying them sequentially  Provides simple low quality animation  Not very good for large or long duration animations

20 Transparent GIFs  The GIF89a file format allows you to pick one colour from the colour map of the GIF to be transparent

21 JPEG  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

22 JPEG  Typically achieves compression ratios of 30:1 with true colour images  but 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

23 A Word About Compression  There are two general types of compression that can be applied to digital media such as audio, video, text etc.  Lossless compression  Lossy compression

24 Lossless Compression  A very basic method of lossless compression is Run Length Encoding  Bytes long  18,5,9 3 Bytes long  Compresses data by storing it in a more appropriate way  Rarely gives compression better than 2:1

25 Lossy 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)

26 Compression  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

27 JPEG Stages  Conversion from RGB to Luminance, Brightness and Chrominance  The human eye is much less sensitive to changes in colour (chrominance) than brightness  Colour information is subsampled (lossy)

28 JPEG Stages  Discrete cosine transform is then applied  Intensity data converted to frequency data  This frequency data is then quantized (lossy)  Quantized data is then Huffman compressed (lossless)

29 JPEG Compression  JPEG compression efficiency as a function of approximate quality setting Quality settingtrue colour image 95%4.5 : 1 75% (default)12 : 1 50%19 : 1 20%41 : 1 10%59 : 1 5%92 : 1

30 JPEG Compression Original image After 31:1 compression

31 JPEG Compression Magnified views of Red Sea / Gulf of Aden region

32 PNG  Portable Network Graphics is a new bit- mapped graphics format similar to GIF  Approved as a standard by the World Wide Web consortium to replace GIF because GIF uses a patented data compression algorithm called LZW  Uses lossless compression  Handles true colour images  Gives better quality reproduction than GIF, but no animation

33 TIFF  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 efficiently  A variety of alternative compression options are supported. Unfortunately this makes it hard to implement decoding software

34 Sun 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

35 BMP 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

36 Fin