Video compression

Video compression deals with the compression of digital video data.

Related Topics:
Compression - Digital video - Data

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Video compression is necessary for efficient coding of video data in video file formats and streaming video formats. To be accurate, what people call Video compression is more accurately referred to as video data rate reduction - the methods used usually require discarding data (lossy compression) rather than just reducing the required bandwidth (lossless compression.) While lossless video compression is possible, in practice it is virtually never used, and all standard video data rate reduction involves discarding data.

Related Topics:
Efficient - Coding of video data - Video file formats - Streaming video formats - Lossy compression - Lossless compression

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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.

Related Topics:
Color - Pixel - Time domain

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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.

Related Topics:
Frame - Interlace - Field

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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).

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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.

Related Topics:
Lossy compression - Image compression - Motion compensation - DV - Non-linear editing

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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.

Related Topics:
ITU-T - ISO - Discrete cosine transform - Fractal compression - Matching pursuits - Discrete wavelet transform

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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.

Related Topics:
Quantization - Vector quantization

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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 and VC-1 are emerging contenders in that domain.

Related Topics:
Broadcast engineering - Digital television - DVB - ATSC - ISDB - TV station - Broadcast - HDTV - Bandwidth - Radio spectrum - MPEG-2 - H.264 - VC-1

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