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Video compression method
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A wide variety of methods are used to compress video streams. Video
data contains spatial and temporal redundancy, making uncompressed
video streams extremely inefficient. Broadly speaking, spatial
redundancy is reduced by registering differences between parts of a
single frame; this task is known as intraframe compression and is
closely related to image compression. Likewise, temporal redundancy
can be reduced by registering differences between frames; this task
is known as interframe compression, including motion compensation
and other techniques. The most common modern standards are MPEG-2,
used for DVD and satellite television, and MPEG-4, used for home
video. |
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Video compression deals with the compression of digital video data.
Video compression is necessary for efficient coding of video data in
video file formats and streaming video formats. Compression is a
conversion of data to a format that requires fewer bits, usually
performed so that the data can be stored or transmitted more
efficiently. If the inverse of the process, decompression, produces
an exact replica of the original data then the compression is
lossless. Lossy compression, usually applied to image data, does not
allow reproduction of an exact replica of the original image, but it
is more efficient. While lossless video compression is possible, in
practice it is virtually never used, and all standard video data
rate reduction involves discarding data. Video is basically a three-dimensional array of color pixels. Two dimensions serve as spatial (horizontal and vertical) directions of the moving pictures, and one dimension represents the time domain. A frame is a set of all pixels that (approximately) correspond to a single point in time. Basically, a frame is the same as a still picture. However, in interlaced video, the set of horizontal lines with even numbers and the set with odd numbers are grouped together in fields. The term "picture" can refer to a frame or a field. However, video data contains spatial and temporal redundancy. Similarities can thus be encoded by merely registering differences within a frame (spatial) and/or between frames (temporal). Spatial encoding is performed by taking advantage of the fact that the human eye is unable to distinguish small differences in color as easily as it can changes in brightness and so very similar areas of color can be "averaged out" in a similar way to jpeg images (JPEG image compression FAQ, part 1/2). With temporal compression only the changes from one frame to the next are encoded as often a large number of the pixels will be the same on a series of frames (About video compression). Video compression typically reduces this redundancy using lossy compression. Usually this is achieved by image compression techniques to reduce spatial redundancy from frames (this is known as intraframe compression or spatial compression) and motion compensation and other techniques to reduce temporal redundancy (known as interframe compression or temporal compression). Formats such as DV avoid interframe compression to allow easier non-linear editing. Today, nearly all video compression methods in common use (e.g., those in standards approved by the ITU-T or ISO) apply a discrete cosine transform (DCT) for spatial redundancy reduction. Other methods, such as fractal compression, matching pursuits, and the use of a discrete wavelet transform (DWT) have been the subject of some research, but are typically not used in practical products (except for the use of wavelet coding as still-image coders without motion compensation). Interest in fractal compression seems to be waning, due to recent theoretical analysis showing a comparative lack of effectiveness to such methods. The use of most video compression techniques (e.g., DCT or DWT based techniques) involves quantization. The quantization can either be scalar quantization or vector quantization; however, nearly all practical designs use scalar quantization because of its greater simplicity. In broadcast engineering, digital television (DVB, ATSC and ISDB ) is made practical by video compression. TV stations can broadcast not only HDTV, but multiple virtual channels on the same physical channel as well. It also conserves precious bandwidth on the radio spectrum. Nearly all digital video broadcast today uses the MPEG-2 standard video compression format, although H.264/MPEG-4 AVC and VC-1 are emerging contenders in that domain. Bit rate (digital only)Bit rate is a measure of the rate of information content in a video stream. It is quantified using the bit per second (bit/s) unit or Megabits per second (Mbit/s). A higher bit rate allows better video quality. For example VideoCD, with a bit rate of about 1 Mbit/s, is lower quality than DVD, with a bit rate of about 5 Mbit/s. HDTV has a still higher quality, with a bit rate of 10 Mbit/s.Variable bit rate (VBR) is a strategy to maximize the visual video quality and minimize the bit rate. On fast motion scenes, a variable bit rate uses more bits than it does on slow motion scenes of similar duration yet achieves a consistent visual quality. For real-time and non-buffered video streaming when the available bandwidth is fixed, e.g. in videoconferencing delivered on channels of fixed bandwidth, a constant bit rate (CBR) must be used. StereoscopicStereoscopic video requires either two channels — a right channel for the right eye and a left channel for the left eye or two overlayed color coded layers. This left and right layer technique is occasionally used for network broadcast, or recent "anaglyph" releases of 3D movies on DVD. Simple Red/Cyan plastic glasses provide the means to view the images discreetly to form a stereoscopic view of the content. New HD DVD and HD Blu-ray disks will greatly improve the 3D effect, in color coded stereo programs. The first commercially available HD players are expected to debut at the 2006 NAB Show in Las Vegas in April. See articles Stereoscopy and 3-D film |
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