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Multimedia Compression

Multimedia Compression. John Hall SMDC Training June, 2006. Concepts. Lossy vs. Lossless Compression Container Format vs. Compression Format Constant vs. Variable Bit Rate Exposure to different compression and container formats. Examples. JPEG Image Compression MPEG-1 Video Compression

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Multimedia Compression

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  1. Multimedia Compression John Hall SMDC Training June, 2006

  2. Concepts • Lossy vs. Lossless Compression • Container Format vs. Compression Format • Constant vs. Variable Bit Rate • Exposure to different compression and container formats

  3. Examples • JPEG Image Compression • MPEG-1 Video Compression • MPEG-2 Video Compression • MPEG-4 Video Compression • Containers: (MP4, MOV, AVI) • Audio Compression (AAC, MP3)

  4. Demos • Exploring Compression Artifacts • Exporting from QuickTime Pro

  5. Lossless Compression • Run Length Encoding (RLE): • aaaaaaabbbb • 7a4b • abababababa • 1a1b1a1b1a1b1a1b1a1b1a • Lossless compression relies on input being non-random to achieve compression.

  6. Lossy Compression • Removes information • Does so “intelligently” • For media files, remove what is least noticeable by the senses.

  7. Things Image/Video Compression Has Problems With • Edges • High Motion • Transparency (Smoke, Shadow) • Subtle gradations of color • Small details • Quick Cuts • Fading In ./ Out

  8. JPEG Compression • Lossy (JPEG-2000 can be lossless) • JFIF file format and JPEG compression • 8x8 blocks matched tocombinations of 64 basic patterns • ‘Quality’ determineshow hard it tries to match

  9. JPEG Compression

  10. Video Compression • Series of compressed images (JPEG) • But can make smaller by compressing what’s common between frames • Only save the information that’s changed between frames • Match macroblocks to previous and possibly next frames

  11. MPEG-1 • 3 frame types: • I frame - Independent, full image frame • P frame - depends on previous frame • B frame - depends on last and next frame I P B I

  12. MPEG-1 • Group of Pictures (GOP) • Pattern of I,P and B frames • Implications for editing • Implications for errors I P B I

  13. Things Image/Video Compression Has Problems With • Edges • High Motion • Transparency (Smoke, Shadow) • Subtle gradations of color • Small details • Quick Cuts • Fading In ./ Out

  14. MPEG-2 • Designed for DVD and Satellite TV • Generally higher quality and larger than MPEG-1 • No free players • Commecial disks are encrypted

  15. MPEG-4 • Video compression is Part 2 of standard • Used in podcasts • Scalable - great for web at low quality • Many parts to standard aren’t used • Container / File is Part 10 of standard

  16. When to use Which? • MPEG2 - Must use for Video-DVD • MPEG1 - If distributing on CD for older computers • MPEG4 - For web downloading • Real - For streaming (contact UMS)

  17. Containers / Files • MP4, AVI and MOV are container & file formats • They can contain a variety of audio and video and specify the codecs to decode them • For instance: • A MOV file can contain MPEG-4 video with AAC audio • OR • A MOV file can contain Sorenson video with MPEG-3 audio • OR… • MP4 (M4V) is MPEG-4 Part 14

  18. Container Formats • Comparison of Container Formats

  19. Example - Quicktime Pro

  20. Audio Compression • Uncompressed audio is usually contained in a .wav or .aiff file • MP3 and AAC are compressed • MP3 is MPEG-1 Layer 3 audio, not MPEG-3 • AAC is MPEG-4 audio, better sounding for same size but not yet as universal

  21. Bit Rate • How much information is decoded per second • MP3 typically 128Kbps - 192Kbps • MPEG-4 Video: 64Kbps, 256Kbps… • Quality depends on bit rate AND encoder quality • Variable bit rate

  22. MP3 File Format

  23. For More Information • Image Compression • JPEG: http://www.faqs.org/faqs/jpeg-faq/part1/ • JPEG: http://en.wikipedia.org/wiki/JPEG • Video Compression • MPEG-4 pt. 2: http://en.wikipedia.org/wiki/MPEG-4_Part_2 • MPEG-4 pt. 10/H.264/AVC: http://en.wikipedia.org/wiki/H.264/MPEG-4_AVC • File Formats • General info: http://en.wikipedia.org/wiki/File_format • Containers: http://en.wikipedia.org/wiki/Container_format • MPEG-4 pt. 14: http://en.wikipedia.org/wiki/.m4a • Codec list: http://www.fourcc.org/ • Audio • MP3: http://en.wikipedia.org/wiki/MP3

  24. Sources and Attribution • DCT Basis Function Image: http://en.wikipedia.org/wiki/Image:Dctjpeg.png, GNU licensed • JPEG Example Image: http://en.wikipedia.org/wiki/Image:Phalaenopsis_JPEG.jpg , by Ilmari Karonon at Wikipedia, Creative Commons Attribution-ShareAlike 2.5 License • MP3 File Image: http://en.wikipedia.org/wiki/Image:Mp3filestructure.jpg

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