Image Formats

1. Image Formats

2. 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

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 (2,3) Radius 1 (4,2) (1,1)

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 Identifies GIF and format GIF87A or GIF89A Screen Descriptor Global Colour Map Image Descriptor Local Colour Map Repeated 1 to n times Image Data GIF Terminator

13. GIF Screen Descriptor • Describes the overall parameters for all GIF images in the file 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 1 Screen Width 2 3 Screen Height 4 M S Size CR 5 Background 6 Aspect ratio 7

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 • 11111111111111111100000111111111 • 4 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 setting true 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 Graphicsisa 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 Fin