Understanding Graphics and Image Representation: Analogue vs Digital

1. G6DPMM - Lecture 4 Graphics & Still Image Representation

2. Analogue vs Digital • Analogue information • Continuously variable signal • Physical phenomena • Sound/light/temperature/position/pressure • Waveform • Electromagnetic (e.g. light) • Pressure (e.g. sound) • Information conveyed by amplitude and frequency • Digital information • Discrete values • Smoke signals / Morse code / Binary electronic • Sampling of analogue information • Analogue to Digital Conversion (A2D) is sampling

3. 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

4. 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

5. Types of Graphics • Computer graphics fall into two categories: • Vector Graphics • Used for computer generated images, line drawings, cartoons etc. • Bitmap (Raster) Graphics • Used for photographs

6. Vector Graphics • Mathematical definitions of lines • Scaleable • Not suitable for photographs • Examples • Postcript • CGM • WMF • HPGL • DXF • Edited using “drawing” software

7. Bitmap Graphics • Matrix of ‘pixels’ • Difficult to re-size • Suitable for photographs • Examples • BMP (DIB) • GIF • PCX • TIFF • TARGA • JPEG • PNG • Edited using painting software (eg Photoshop)

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9. Colour Depth

10. Colour Depth

11. Colourmapping • 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 • Hardware may require it • Some software manipulation requires it • Some compression techniques require it • Optimised vs System palette • Display Mode and Palette Flashing

12. Vector / Bitmap Conversion • Vectors Bitmap • Easy • Perfect representation - scaling issue • Bitmap Vector • Much harder - autotrace • Poor quality • Highly lossy

13. The Need for Compression • Graphics tend to be big! • Consider the following: • 1024x768 24-bit image • 1024 x 768 = 786,432 pixels • 786,432 x 24 bits = 18,874,368 (c. 18.4 Mb) • Approximately 40 images per 750 Mb CD-ROM! • Data compression is essential

14. Image Compression • Lossless Compression • Decompressed image is a perfect copy of the original • Example File Format: GIF • Lossy Compression • Decompressed image is an imperfect approximation of the original • Example File Format: JPEG

15. Lossless Algorithms • Run Length Encoding • Sequences of “runs” of repeated data is replaced by a single data item, and the length of the run. • Used by TIFF, DIB/BMP • LZ77 • 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) • Derived from LZ77 • Developed (and patented) by Unisys – licensed for Compuserve • Used by GIF • Deflate • Derived from LZ77 • Used by PNG

16. Lossy Algorithms • Common algorithms all operate on the waveform • Fourier Transform • A technique for expressing a waveform as a weighted series of sines and cosines • DCT (Discrete Cosine Transform) • A type of Fourier transform • Waveform is expressed as a weighted sum of cosines • Used by JPEG • Wavelets • 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 • Fractal theory • Not (currently) widely used

17. JPEG Compression Original Image - 285 K

18. JPEG Compression 50% Compression - 15 K

19. JPEG Compression 70% Compression - 10.8 K

20. JPEG Compression 90% Compression - 6.9 K

21. JPEG Compression 95% Compression - 5.3 K

22. JPEG Compression 99% Compression - 2.6 K

23. Image File Formats • Vendor Defined Formats • OS Vendors (eg Microsoft / Apple) • Application Vendors (eg Adobe) • May be open, (ie published specifications) or closed (protected by IPR) • Vendor Neutral Formats • Usually defined by standards organisations

24. Apple Macintosh Formats • 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!

25. Windows Formats • 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!

26. Adobe Formats • 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.

27. Truevision Targa • Truevision - graphics hardware & software company • Targa (TGA) • 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.

28. Aldus/Adobe TIFF • 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

29. CompuServe GIF • 8 bit LZW compressed bitmap • Supports transparency • Supports multiple images & animation • Widely used on WWW • Licence problems • CompuServe patent • Unisys policy

30. PNG • 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!

31. JPEG • Joint Picture Expert Group • Both an algorithm and a file format! • Lossy Truecolour compression (DCT)