MPEG-4 Part 2
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MPEG-4 Part 2 is a video compression technology developed by MPEG. It belongs to the MPEG-4 ISO/IEC standard (ISO/IEC 14496-2). It is a discrete cosine transform compression standard, similar to previous standards such as MPEG-1 and MPEG-2. Several popular codecs including DivX, Xvid and Nero Digital are implementations of this standard. Note that MPEG-4 Part 10 defines a different format than MPEG-4 Part 2 and should not be confused with it. MPEG-4 Part 10 is commonly referred to as H.264 or AVC, and was jointly developed by ITU-T and MPEG.
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[edit] Profiles
To address various applications ranging from low-quality, low-resolution surveillance cameras to high definition TV broadcasting and DVDs, many video standards group features into profiles and levels. MPEG-4 Part 2 has approximately 21 profiles, including profiles called Simple, Advanced Simple, Main, Core, Advanced Coding Efficiency, Advanced Real Time Simple, etc. The most commonly deployed profiles are Advanced Simple and Simple, which is a subset of Advanced Simple.
Most of the video compression schemes standardize the bitstream (and thus the decoder) leaving the encoder design to the individual implementations. Therefore, implementations for a particular profile (such as DivX or Nero Digital which are implementations of Advanced Simple Profile and Xvid that implements both profiles) are all technically identical on the decoder side. A point of comparison would be that an MP3 file can be played in any MP3 player, whether it was created through iTunes, Windows Media Player, LAME or the common Fraunhofer encoder.
[edit] Simple Profile (SP)
Simple Profile is mostly aimed for use in situations where low bit rate and low resolution are mandated by other conditions of the applications, like network bandwidth, device size etc. Examples are cell phones, some low end video conferencing systems, surveillance systems etc.
[edit] Advanced Simple Profile (ASP)
Advanced Simple Profile 's notable technical features relative to the Simple Profile, which is roughly similar to H.263, include:
- Support for "MPEG"-style quantization
- Support for interlaced video
- Support for B pictures (a.k.a. B Frames)
- Quarter Pixel motion compensation (Qpel)
- Global motion compensation (GMC)
The MPEG quantization and interlace support are designed in basically similar ways to the way it is found in MPEG-2 Part 2. The B picture support is designed in a basically similar way to the way it is found in MPEG-2 Part 2 and H.263v2.
The quarter-pixel motion compensation feature of ASP was innovative, and was later also included (in somewhat different forms) in MPEG-4 Part 10 and VC-1. Some implementations omit support for this feature, because it has a significantly harmful effect on speed and it is not always beneficial for quality.
The global motion compensation feature is not actually supported in most implementations although the standard officially requires decoders to support it. Most encoders do not support it either, and some experts say that it does not ordinarily provide any benefit in compression. When used, ASP's global motion compensation has a large unfavorable impact on speed and adds considerable complexity to the implementation.
[edit] Criticisms
Despite being generally well received, MPEG-4 Part 2 has drawn some criticism. FFmpeg's maintainer Michael Niedermayer has criticised MPEG-4 for lacking an in loop deblocking filter, GMC being too computationally intensive, and OBMC being defined but not allowed in any profiles among other things. [1] Microsoft's Ben Waggoner states "Microsoft (well before my time) went down the codec standard route before with MPEG-4 part 2, which turns out to be a profound disappointment across the industry - it didn't offer that much of a compression advantage over MPEG-2, and the protracted license agreement discussions scared off a lot of adoption. I was involved in many digital media projects that wouldn't even touch MPEG-4 in the late '90s to early '00s because there was going to be a 'content fee' that hadn't been fully defined yet." [2]
[edit] Popular software implementations
[edit] External links
[edit] Notes
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