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Video Compression Tutorial
Video Compression Technology
At its most basic level, compression is performed when
an input video stream is analyzed and information that is indiscernible
to the viewer is discarded. Each event is then assigned a code - commonly
occurring events are assigned few bits and rare events will have codes
more bits. These steps are commonly called signal analysis, quantization
and variable length encoding respectively. There are four methods for
compression, discrete cosine transform (DCT), vector quantization (VQ),
fractal compression, and discrete wavelet transform (DWT).
Discrete cosine transform is a lossy compression
algorithm that samples an image at regular intervals, analyzes the frequency
components present in the sample, and discards those frequencies which
do not affect the image as the human eye perceives it. DCT is the basis
of standards such as JPEG, MPEG, H.261, and H.263. We covered the definition
of both DCT and wavelets in our tutorial on Wavelets
Theory.
Vector quantization is a lossy compression that looks
at an array of data, instead of individual values. It can then generalize
what it sees, compressing redundant data, while at the same time retaining
the desired object or data stream's original intent.
Fractal compression is a form of VQ and is also a lossy
compression. Compression is performed by locating self-similar sections
of an image, then using a fractal algorithm to generate the sections.
Like DCT, discrete wavelet transform mathematically
transforms an image into frequency components. The process is performed
on the entire image, which differs from the other methods (DCT), that
work on smaller pieces of the desired data. The result is a hierarchical
representation of an image, where each layer represents a frequency
band.
Compression Standards
MPEG stands for the Moving
Picture Experts Group. MPEG is an ISO/IEC working group, established
in 1988 to develop standards for digital audio and video formats. There
are five MPEG standards being used or in development. Each compression
standard was designed with a specific application and bit rate in mind,
although MPEG compression scales well with increased bit rates. They
include:
MPEG-1
Designed for up to 1.5 Mbit/sec
Standard for the compression of moving pictures
and audio. This was based on CD-ROM video applications, and is a popular
standard for video on the Internet, transmitted as .mpg files. In
addition, level 3 of MPEG-1 is the most popular standard for digital
compression of audio--known as MP3. MPEG-1 is the standard of compression
for VideoCD, the most popular video distribution format thoughout
much of Asia.
MPEG-2
Designed for between 1.5 and 15 Mbit/sec
Standard on which Digital Television set
top boxes and DVD compression is based. It is based on MPEG-1, but
designed for the compression and transmission of digital broadcast
television. The most significant enhancement from MPEG-1 is its ability
to efficiently compress interlaced video. MPEG-2 scales well to HDTV
resolution and bit rates, obviating the need for an MPEG-3.
MPEG-4
Standard for multimedia and Web compression. MPEG-4 is based on object-based
compression, similar in nature to the Virtual Reality Modeling Language.
Individual objects within a scene are tracked separately and compressed
together to create an MPEG4 file. This results in very efficient compression
that is very scalable, from low bit rates to very high. It also allows
developers to control objects independently in a scene, and therefore
introduce interactivity.
MPEG-7 -
this standard, currently under development, is also called the Multimedia
Content Description Interface. When released, the group hopes the
standard will provide a framework for multimedia content that will
include information on content manipulation, filtering and personalization,
as well as the integrity and security of the content. Contrary to
the previous MPEG standards, which described actual content, MPEG-7
will represent information about the content.
MPEG-21
- work on this standard, also called the Multimedia Framework, has
just begun. MPEG-21 will attempt to describe the elements needed to
build an infrastructure for the delivery and consumption of multimedia
content, and how they will relate to each other.
JPEG stands for Joint
Photographic Experts Group. It is also an ISO/IEC working group,
but works to build standards for continuous tone image coding. JPEG
is a lossy compression technique used for full-color or gray-scale images,
by exploiting the fact that the human eye will not notice small color
changes.
JPEG 2000 is an initiative that will provide
an image coding system using compression techniques based on the use
of wavelet technology.
DV is a high-resolution digital video format used with
video cameras and camcorders. The standard uses DCT to compress the
pixel data and is a form of lossy compression. The resulting video stream
is transferred from the recording device via FireWire (IEEE 1394), a
high-speed serial bus capable of transferring data up to 50 MB/sec.
H.261 is an ITU standard designed for two-way communication
over ISDN lines (video conferencing) and supports data rates which are
multiples of 64Kbit/s. The algorithm is based on DCT and can be implemented
in hardware or software and uses intraframe and interframe compression.
H.261 supports CIF and QCIF resolutions.
H.263 is based on H.261 with enhancements that improve
video quality over modems. It supports CIF, QCIF, SQCIF, 4CIF and 16CIF
resolutions.
DivX Compression
DivX is a software application that uses the MPEG-4
standard to compress digital video, so it can be downloaded over a
DSL/cable modem connection in a relatively short time with no reduced
visual quality. The latest version of the codec, DivX 4.0, is being
developed jointly by DivXNetworks
and the open source community. DivX works on Windows 98, ME, 2000,
CE, Mac and Linux.
Terms
Lossy compression - reduces a file by permanently eliminating
certain redundant information, so that even when the file is uncompressed,
only a part of the original information is still there.
ISO/IEC
International
Organization for Standardization - a non-governmental organization
that works to promote the development of standardization to facilitate
the international exchange of goods and services and spur worldwide
intellectual, scientific, technological and economic activity.
International Electrotechnical
Commission - international standards and assessment body for the
fields of electrotechnology
Codec - A video codec is software that can compress a
video source (encoding) as well as play compressed video (decompress).
CIF - Common Intermediate Format - a set of standard video
formats used in videoconferencing, defined by their resolution. The
original CIF is also known as Full CIF (FCIF).
QCIF - Quarter CIF (resolution 176x144)
SQCIF - Sub quarter CIF (resolution 128x96)
4CIF - 4 x CIF (resolution 704x576)
16CIF - 16 x CIF (resolution 1408x1152
Additional sources of information*
TECH
Online Review - Video Compression Overview
DataCompression.info
IGM - Desktop
Video - Compression Standards
*The WAVE Report is not responsible for content on additional
sites. |
