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Graphics File Formats Graphics File Formats Terminology Bitmaps PBM Graphics File Formats • Why have graphics file formats? • What to look for when choosing a file format • A sample tour of different file formats, including – bitmap-based formats – vector-based formats – metafiles – proprietary formats – fractal compression formats The Graphics File Format Page The Graphics File Formats FAQ (see online references) 1 2 Graphics File Formats Terminology • Need to store and retrieve graphical data in an (…or what to look out for in file formats) efficient and logical way • Data stored according to specific format conventions When choosing which is more appropriate for your • Formats are immortal - technology evolves, new purpose, some common factors to consider: formats appear, but the old ones will still be there! • Lossy or lossless compression? • No one universal format - different formats for • What is the compression ratio in this case? different purposes • 8-bit or 7-bit (binary or text)? • You (probably) won’t need to access the formats in • Is it stored at a fixed resolution? detail • How many images per file (static or animated)? – there is library code to input/output/convert images for you! – useful to understand what is going on “behind the scenes”, • ….amongst other things for making the best image format choices 3 4 Bitmaps PBM, PGM, PPM Simplified structure of a bitmap file: • Family of portable bitmaps • Designed to be as simple as possible! • Written in ASCII • No compression • 1 image per file In practice there are many format variations 5 6 1 PBM PGM • Portable Bit Map • Portable Grey Map • For monochrome images P1 P2 # feep.pgm # feep.pbm 24 7 24 7 15 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 0 1 1 1 1 0 0 1 1 1 1 0 0 1 1 1 1 0 0 3 3 3 3 0 0 7 7 7 7 0 0 11 11 11 11 0 0 15 15 15 15 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 1 0 0 3 0 0 0 0 0 7 0 0 0 0 0 11 0 0 0 0 0 15 0 0 15 0 0 1 1 1 0 0 0 1 1 1 0 0 0 1 1 1 0 0 0 1 1 1 1 0 0 3 3 3 0 0 0 7 7 7 0 0 0 11 11 11 0 0 0 15 15 15 15 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 3 0 0 0 0 0 7 0 0 0 0 0 11 0 0 0 0 0 15 0 0 0 0 0 3 0 0 0 0 0 7 7 7 7 0 0 11 11 11 11 0 0 15 0 0 0 0 0 1 0 0 0 0 0 1 1 1 1 0 0 1 1 1 1 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 7 8 PPM BMP • Standard bitmap storage format for Microsoft • Portable Pix Map Windows (also supported by some other non- Microsoft applications) – Never meant to be portable – Nor meant for a data interchange format across P3 different operating systems # sq.ppm • 1 image per file 4 4 • Uses RLE (run-length encoding) 15 – therefore uses lossless compression 0 0 0 0 0 0 0 0 0 0 0 15 – original bitmap did not support compression! 0 0 0 0 15 0 0 0 0 0 0 0 0 0 0 0 0 0 0 15 0 0 0 0 • Huge file sizes! 0 0 15 0 0 0 0 0 0 0 0 0 – e.g. sample 470x682 photo stored as 40.3K JPG file is a 963K BMP file 9 10 BMP GIF • Graphics Interchange Format (1987 and 1989 versions) – The format specification says it is to be pronounced “jif” • Uses a palette of up to 256 colours – not good for photographs, but great for text/diagrams • Uses LZW compression of the bitmap data – causes copyright problems (Unisys patent)! – reasonably easy to read and decompress • Supports multiple images (animation!!) • 1989 version supports transparency and interlacing – At most one colour in the palette may be declared transparent 11 12 2 Interlacing of Pixel Data GIF Row Number Interlace Pass 0 ----------------------------------------- 1 1 ----------------------------------------- 4 2 ----------------------------------------- 3 3 ----------------------------------------- 4 4 ----------------------------------------- 2 5 ----------------------------------------- 4 6 ----------------------------------------- 3 7 ----------------------------------------- 4 8 ----------------------------------------- 1 9 ----------------------------------------- 4 10 ----------------------------------------- 3 11 ----------------------------------------- 4 12 ----------------------------------------- 2 13 ----------------------------------------- 4 14 ----------------------------------------- 3 15 ----------------------------------------- 4 16 ----------------------------------------- 1 17 ----------------------------------------- 4 18 ----------------------------------------- 3 19 ----------------------------------------- 4 13 14 Interlacing PNG • Portable Network Graphics – informally known as “PNG Not GIF” – pronounced “ping” • Successor to GIF (lossless but free) • Not a restricted palette - up to 48-bit truecolour • Supports transparency, interlacing, but not animation – animation available in a related format, MNG • Transparency through an alpha channel – also supports anti-aliasing • Compression marginally better than GIFs, based on LZ77 (earlier than LZW) 15 16 JFIF TIFF • Tag Image File Format • JFIF - JPEG File Interchange Format • Designed primarily for desktop publishing and related • There is no JPEG format! applications • JPEG more commonly refers to the compression • Contains lots of tags in the file, identifying different algorithms used types of content – typically between 10:1 and 20:1 for photographs • Rationale behind using tags is that it is easier to • Basically a data stream with a few restrictions and incorporate new features in future versions of the identifying markers format without confusing old software, or making old TIFF images useless • Very complex! 17 18 3 FIG FIG Example • Facility for Interactive Generation of figures #FIG 3.2 FIG: 227bytes • Proprietary vector format for xfig drawing program Landscape GIF: 1.49kb Center • ASCII text file PNG: 2.12kb Inches JPG: 6.42kb Letter 100.00 Single -2 1200 2 1 3 0 1 6 6 10 0 20 0.000 1 0.0000 3976 2992 1665 1665 3976 2992 5641 2992 2 2 0 1 4 4 100 0 20 0.000 0 0 -1 0 0 5 600 675 4500 675 4500 3450 600 3450 600 675 19 20 CGM POVRay • Computer Graphics Metafile • POVRay is a 3D computer graphics rendering • Three different encodings program – One for minimal size/ease of transmission – it has its own proprietary 3D scene-based format, looks like program code – Binary encoded for speed of access – e.g. – Clear text encoding for human readability/editing • Primarily vector graphics, but also bitmap data • An ISO standard 21 22 global_settings { assumed_gamma 2.2 } VRML plane { y, 0 • Virtual Reality Modelling Language #include "colors.inc" pigment { Tan } #include "shapes.inc" finish { • Interactive 3D worlds: browser plugins available #include "textures.inc" crand 0.015 #include "stones.inc" ambient 0.12 diffuse 0.8 camera { } location <0, 45, -40> } direction <0, 0, 1.5> look_at <0, 0, 0> cylinder { } <0, -4, 0>, <0, 4, 0>, 2.5 light_source { translate <-7, 4, -8> <30, 20, -10> texture { T_Stone18 } color White } area_light <4, 0, 0>, <0, 0, 4>, 9, 9 } 23 24 4 VRML Code FIF #VRML V1.0 ascii Separator { DirectionalLight { • FIF (Fractal Image Format) direction 0 0 -1 # Light shining from viewer into scene } • Barnsley (and later, Jacquin) developed the PerspectiveCamera { compression technique position -8.6 2.1 5.6 orientation -0.1352 -0.9831 -0.1233 1.1417 • Commercial format by Iterated System Inc. focalDistance 10.84 } • Suffers from over-hype and commercialization Separator { # The red sphere Separator { # The blue cube – claims of several thousand to one compression ratios Material { Material { diffuseColor 1 0 0 # Red diffuseColor 0 0 1 # Blue • Realistic compression ratios range from 4:1 to 100:1 } } Translation { translation 3 0 1 } Transform { • FIF good for most pictures, particularly Sphere { radius 2.3 } translation -2.4 .2 1 photographs/scenes, with elements of self-similarity } rotation 0 1 1 .9 } (not difficult!) Cube {} } 25 26 Comparison of Images Comparison Table raw data 100% 100% 100% PPM 407% 406% 406% BMP 100% 33% 33% 624 ´ 453 pixels GIF 19% 18% 1% PNG 32% 16% 1% 1024 ´ 768 pixels TIFF (LZW) 111% 18% 1% JPEG 5% 5% 10% FIF 5% 6% 3% 600 x 401 pixels 27 28 End of Lecture Next three lectures will look at sound (lecture on compression and file formats for moving images will be delivered later!) 29 5.
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